Sheahan Diamond Literature Technical Reference Compilation 2019
The Sheahan Diamond Literature Reference Compilation
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcementscalled the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Resource Center
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
The SDLRC provides 3 types of references identified in the reference code. DS for scientific article, DM for a media article, and DC for a corporate announcement. Consider DS0512-0001. The DS stands for "diamond scientific". 05 stands for 2005, the year the reference was posted. 12 represents the month the reference was posted. For all years prior to 2015 the default month is 12. -0001 is the reference's identifier and it does not mean anything. The number below the refence code, ie 2015, is the year the article was published. Note that the posted year may sometimes be later than the published year.
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Abstract: Diamonds from Juina, Brazil, are well-known examples of superdeep diamond crystals formed under sublithospheric conditions and evidence would indicate their origins lie as deep as the Earth’s mantle transition zone and the Lower Mantle. Detailed characterization of these minerals and of inclusions trapped within them may thus provide precious minero-petrogenetic information on their growth history in these inaccessible environments. With the aim of studying non-destructively the structural defects in the entire crystalline volume, two diamond samples from this locality, labelled JUc4 and BZ270, respectively, were studied in transmission mode by means of X-ray Diffraction Topography (XRDT) and micro Fourier Transform InfraRed Spectroscopy (µFTIR). The combined use of these methods shows a good fit between the mapping of spatial distribution of extended defects observed on the topographic images and the µFTIR maps corresponding to the concentration of N and H point defects. The results obtained show that both samples are affected by plastic deformation. In particular, BZ270 shows a lower content of nitrogen and higher deformation, and actually consists of different, slightly misoriented grains that contain sub-grains with a rounded-elongated shape. These features are commonly associated with deformation processes by solid-state diffusion creep under high pressure and high temperature.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 265-6.
Abstract: The effect of fluorescence on the appearance of diamonds has been a subject of debate for many years (Moses et al., 1997). In the trade, fluorescence is generally perceived as an undesirable characteristic. Nearly 80% of diamonds graded at HRD Antwerp receive a “nil” fluorescence grade, while the remainder are graded as “slight,” “medium,” and “strong,” their value decreasing with level of fluorescence. To understand how fluorescence might change diamond appearance, a selection of 160 round brilliant-cut diamonds were investigated in detail. This study focused on the effect of thetic samples, it is possible that some of the observed phosphorescence does not involve boron impurities. In this paper we report on the results of combined fluorescence, phosphorescence, thermoluminescence, and quantitative charge transfer investigations undertaken on both HPHT and CVD synthetic diamond, with the objective of identifying which defects are involved in the fluorescence and phosphorescence processes.
Earth and Planetary Science Letters, Vol. 505, pp. 162-172.
Abstract: The isopycnicity hypothesis states that the lithospheric mantle of ancient platforms has a unique composition such that high density due to low lithosphere temperature is nearly compensated by low-density composition of old cratonic mantle. This hypothesis is supported by petrological studies of mantle xenoliths hosted in kimberlite magmas. However, the representativeness of the kimberlite sampling may be questioned, given that any type of magmatism is atypical for stable regions. We use EGM2008 gravity data to examine the density structure of the Siberian lithospheric mantle, which we compare with independent constraints based on free-board analysis. We find that in the Siberian craton, geochemically studied kimberlite-hosted xenoliths sample exclusively those parts of the mantle where the isopycnic condition is satisfied, while the pristine lithospheric mantle, which has not been affected by magmatism, has a significantly lower density than required by isopycnicity. This discovery allows us to conclude that our knowledge on the composition of cratonic mantle is incomplete and that it is biased by kimberlite sampling which provides a deceptive basis for the isopycnicity hypothesis.
Russian Geology and Geophysics, Vol. 59, 11, pp. 1389-1409.
Abstract: Precambrian cratons cover about 70% of the total continental area. According to a large volume of geomorphological, geological, paleontological, and other data for the Pliocene and Pleistocene, these cratons have experienced a crustal uplift from 100-200 m to 1000-1500 m, commonly called the recent or Neotectonic uplift. Shortening of the Precambrian crust terminated half a billion years ago or earlier, and its uplift could not have been produced by this mechanism. According to the main models of dynamic topography in the mantle, the distribution of displacements at the surface is quite different from that of the Neotectonic movements. According to seismic data, there is no magmatic underplating beneath most of the Precambrian cratons. In most of cratonic areas, the mantle lithosphere is very thick, which makes its recent delamination unlikely. Asthenospheric replacement of the lower part of the mantle lithosphere beneath the Precambrian cratons might have produced only a minor part of their Neotectonic uplifts. Since the above mechanisms cannot explain this phenomenon, the rock expansion in the crustal layer is supposed to be the main cause of the recent uplift of Precambrian cratons. This is supported by the strong lateral nonuniformity of the uplift, which indicates that expansion of rocks took place at a shallow depth. Expansion might have occurred in crustal rocks that emerged from the lower crust into the middle crust with lower pressure and temperature after the denudation of a thick layer of surface rocks. In the dry state, these rocks can remain metastable for a long time. However, rapid metamorphism accompanied by expansion of rocks can be caused by infiltration of hydrous fluids from the mantle. Analysis of phase diagrams for common crustal rocks demonstrates that this mechanism can explain the recent crustal uplift of Precambrian cratons.
Earth and Planetary Science Letters, Vol. 505, pp. 162-172.
Abstract: Some high-Mg eclogite xenoliths, entrained by kimberlites from the mantle lithospheres of ancient continental cores, and rare orogenic eclogites and ophiolites, exhumed or obducted during the closure of palaeo-ocean basins, have elemental and isotopic compositions indicative of protoliths that formed as little-differentiated melts erupted in ancient ocean floors. Despite metamorphism and, in part, partial melt loss, these samples of ancient mid-ocean ridge basalt and picrite retain a memory of the chemical and physical state of their protoliths' ambient convecting mantle sources. Published data show that, when filtered to exclude specimens with cumulate protoliths or showing evidence for later enrichment (metasomatism), the samples lack Y or Al 2 O 3 depletion relative to TiO 2 and MgO. This indicates melt segregation of the protolith predominantly from a garnet-free peridotite source and implies intersection of the solidus at low pressures (=3 GPa). Given the dependence of melt composition and volume on source composition (assumed to be similar to modern depleted mantle) and mantle potential temperature (T P), we calculate moderate average melt fractions F (~0.22 ± 0.01) from the Ti contents of the least differentiated samples in three sample suites with 2.6 to 2.9 Ga ages. This converts to T P of ~1410 ± 10 • C assuming a final pressure of melting of 0.5 GPa, melt productivity of 10%/GPa and mantle adiabat of 0.4 • C/km, and using a mantle solidus parameterisation. Though model-dependent, the results are in agreement with recent work advocating moderate Archaean mantle T P. Estimates drop to F = 0.19 and T P = 1380 • C at 1.9 Ga and F = 0.12 and T P = 1310 at 0.6 Ga, corresponding to a decrease in T P of only ~100 • C over the last 3 Ga. A less depleted mantle source yields higher F and T P , but the above estimates are in better agreement with qualitative evidence from Al 2 O 3 and Y, and with Nd-Hf and Sr isotope compositions of orogenic eclogite and granulite suites and mantle eclogites, respectively, which indicate that portions of the Meso-to Neoarchaean mantle were depleted. Moderate T P supports early plate strengthening and a possible transition to plate tectonics in the Mesoarchaean if not earlier. Moreover, moderate temperatures in Archaean subduction zones may have facilitated deep recycling of volatiles that would otherwise have been lost from subducting slabs at shallow depths.
Doklady earth Sciences, Vol. 483, 1, pp. 1427-1430.
Abstract: Experimental studies were performed in the Fe3C-SiO2-(Mg,Ca)CO3 system (6.3 GP?, 1100-1500°C, 20-40 h). It is established that the carbide-oxide-carbonate interaction leads to the formation of ferrosilite, fayalite, graphite, and cohenite (1100 and 1200°?), as well as a Fe-C melt (1300°?). It is determined that the main processes in the system are decarbonation, redox-reactions of cohenite and a CO2-fluid, extraction of carbon from carbide, and crystallization of metastable graphite (± diamond growth), as well as the formation of ferriferous silicates. The interaction studied can be considered as a simplified model of the processes that occur during the subduction of oxidized crustal material to reduced mantle rocks.
Abstract: Of great importance in the problem of redox evolution of mantle rocks is the reconstruction of scenarios of alteration of Fe°- or Fe3C-bearing rocks by oxidizing mantle metasomatic agents and the evaluation of stability of these phases under the influence of fluids and melts of different compositions. Original results of high-temperature high-pressure experiments (P = 6.3 GPa, T = 1300-1500°?) in the carbide-oxide-carbonate systems (Fe3C-SiO2-(Mg,Ca)CO3 and Fe3C-SiO2-Al2O3-(Mg,Ca)CO3) are reported. Conditions of formation of mantle silicates with metallic or metal-carbon melt inclusions are determined and their stability in the presence of CO2-fluid representing the potential mantle oxidizing metasomatic agent are estimated. It is established that garnet or orthopyroxene and CO2-fluid are formed in the carbide-oxide-carbonate system through decarbonation, with subsequent redox interaction between CO2 and iron carbide. This results in the formation of assemblage of Fe-rich silicates and graphite. Garnet and orthopyroxene contain inclusions of a Fe-C melt, as well as graphite, fayalite, and ferrosilite. It is experimentally demonstrated that the presence of CO2-fluid in interstices does not affect on the preservation of metallic inclusions, as well as graphite inclusions in silicates. Selective capture of Fe-C melt inclusions by mantle silicates is one of the potential scenarios for the conservation of metallic iron in mantle domains altered by mantle oxidizing metasomatic agents.
Abstract: In this study, we present a number of experiments on the transformation of graphite, diamond, and multiwalled carbon nanotubes under high pressure conditions. The analysis of our results testifies to the instability of diamond in the 55-115 GPa pressure range, at which onion-like structures are formed. The formation of interlayer sp3-bonds in carbon nanostructures with a decrease in their volume has been studied theoretically. It has been found that depending on the structure, the bonds between the layers can be preserved or broken during unloading.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 275.
Abstract: Diamond is often romanticized as a symbol of purity and perfection, with values that exceed all other gemstones. However, even the most flawless and colorless natural diamonds have atomic-level imperfections. Somewhat ironically, the rarest and most valuable gem diamonds are those that contain abundant impurities or certain atomic defects that produce beautiful fancy colors such as red, blue, or green—stones that can sell for millions of dollars per carat. Atomic defects can consist of impurities such as nitrogen or boron that substitute for carbon atoms in the diamond atomic structure (resulting in classifications such as type Ia, type Ib, type IIa, and type IIb) or missing or misaligned carbon atoms. Some defects are created during diamond growth, while others are generated over millions to billions of years as the diamond sits deep in the earth at high temperatures and pressures. Defects may be created when the diamond is rapidly transported to the earth’s surface or by interaction with radioactive fluids very near the earth’s surface. Each defect selectively absorbs different wavelengths of light to produce eye-visible colors. Absorptions from these color-producing defects (or color centers) are detected and identified using the gemological spectroscope or more sensitive absorption spectrometers such as Fouriertransform infrared (FTIR) or ultraviolet/visible/near-infrared (UV-Vis-NIR; figure 1). Some defects not only absorb light but also produce their own luminescence, called fluorescence. For example, the same defect that produces “cape” yellow diamonds also generates blue fluorescence when exposed to ultraviolet light. In some cases, the fluorescence generated by defects can be strong enough to affect the color of gem diamonds. With the exception of most natural white and black diamonds, where the color is a product of inclusions, colored diamonds owe their hues to either a single type of defect or a combination of several color centers. More than one type of defect can produce a particular color, however. Table 1 provides a list of the most common causes of color in diamond. Subtle differences in atomic defects can drastically affect a diamond’s color. For example, isolated atoms of nitrogen impurities usually produce strong yellow color (“canary” yellow diamonds). If those individual nitrogen atoms occur together in pairs, no color is generated and the diamond is colorless. If instead the individual nitrogen atoms occur adjacent to missing carbon atoms (vacancies), the color tends to be pink to red. Rearrangement of diamond defects is the foundation of using treatments to change the color of diamond. Identification of treatments and separation of natural and synthetic diamond requires a thorough understanding of the atomic-level imperfections that give rise to diamond color and value.
Abstract: Deep crustal and mantle rocks are exhumed in core complex mode of extension in three types of structures: metamorphic core complexes, oceanic core complexes and magma poor passive margins. Using available analogue and numerical models and their comparison with natural examples, the present paper reviews the mechanical processes involved in these different types of extensional setting. Three main aspects are considered: i) the primary role of lithosphere rheology, ii) the lithosphere-scale patterns of progressive deformation that lead to the exhumation of deep metamorphic or mantle rocks and iii) the initiation and development of detachment zones. Crustal core complexes develop in continental lithospheres whose Moho temperature is higher than 750 °C with “upper crust-dominated” strength profiles. Contrary to what is commonly believed, it is argued from analogue and numerical models that detachments that accommodate exhumation of core complexes do not initiate at the onset of extension but in the course of progressive extension when the exhuming ductile crust reaches the surface. In models, convex upward detachments result from a rolling hinge process. Mantle core complexes develop in either the oceanic lithosphere, at slow and ultra-slow spreading ridges, or in continental lithospheres, whose initial Moho temperature is lower than 750 °C, with “sub-Moho mantle-dominated” strength profiles. It is argued that the mechanism of mantle exhumation at passive margins is a nearly symmetrical necking process at lithosphere scale without major and permanent detachment, except if strong strain localization could occur in the lithosphere mantle. Distributed crustal extension, by upper crust faulting above a décollement along the ductile crust increases toward the rift axis up to crustal breakup. Mantle rocks exhume in the zone of crustal breakup accommodated by conjugate mantle shear zones that migrate with the rift axis, during increasing extension.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 303.
Abstract: A group of natural diamonds known as chameleon diamonds change color from green to yellow based on their exposure to light and heat. These diamonds also emit long-lived phosphorescence after UV excitation. We have observed the optical response of these diamonds to optical and thermal excitation and developed a model to explain the observed phenomena. A principal element of the model is the proposal of an acceptor state (figure 1), which should be observable in the near-infrared (NIR) region. Subsequently, we have observed the NIR absorption to this acceptor state, supporting our model of charge-transfer processes in these diamonds.
Abstract: Dr. Laurent Cartier and Dr. Saleem Ali of the Knowledge Hub recently co-authored an overview article on traceability in the gem and jewellery industry. This paper was published in the Journal of Gemmology and is entitled 'Blockchain, Chain of Custody and Trace Elements: An Overview of Tracking and Traceability Opportunities in the Gem Industry'. Recent developments have brought due diligence, along with tracking and traceability, to the forefront of discussions and requirements in the diamond, coloured stone and pearl industries. This article provides an overview of current trends and developments in the tracking and traceability of gems, along with an explanation of the terms used in this context. Further, the article discusses current initiatives in the sector and provides an introduction blockchain concepts.
Abstract: Models of the volume of continental crust through Earth history vary significantly due to a range of assumptions and data sets; estimates for 3?Ga range from <10% to >120% of present day volume. We argue that continental area and thickness varied independently and increased at different rates and over different periods, in response to different tectonic processes, through Earth history. Crustal area increased steadily on a pre-plate tectonic Earth, prior to ca. 3?Ga. By 3?Ga the area of continental crust appears to have reached a dynamic equilibrium of around 40% of the Earth's surface, and this was maintained in the plate tectonic world throughout the last 3?billion?years. New continental crust was relatively thin and mafic from ca. 4-3?Ga but started to increase substantially with the inferred onset of plate tectonics at ca. 3?Ga, which also led to the sustained development of Earth's bimodal hypsometry. Integration of thickness and area data suggests continental volume increased from 4.5?Ga to 1.8?Ga, and that it remained relatively constant through Earth's middle age (1.8-0.8?Ga). Since the Neoproterozoic, the estimated crustal thickness, and by implication the volume of the continental crust, appears to have decreased by as much as 15%. This decrease indicates that crust was destroyed more rapidly than it was generated. This is perhaps associated with the commencement of cold subduction, represented by low dT/dP metamorphic assemblages, resulting in higher rates of destruction of the continental crust through increased sediment subduction and subduction erosion.
South African Journal of Geology, Vol. 121, pp. 253-260.
Abstract: The definition of a lopolith as a large planar-convex (downward) intrusion was coined by F.F. Grout one hundred years ago for the proposed shape of the Duluth gabbro. Subsequent research has challenged the concept that it is a single body, and that it has the lateral extent (under Lake Superior) and shape originally proposed. Other large basic intrusions have shapes, especially for their lower contacts, that are difficult to constrain, and none can be convincingly shown to be of the proposed shape of a lopolith. Their inferred or proposed shapes range from wedge to funnel to planar shaped, with a rarely exposed vertical feeder, and with angles to the sides that vary from minimal to very steep, and variably contorted surface plan. If no intrusions fit the definition, should the term lopolith be discontinued?
Russian Geology and Geophysics, Vol. 59, 11, pp. 1450-1460.
Abstract: We consider a hypothesis for the origin of PGE-bearing ultramafic rocks of the Inagli massif (Central Aldan) through fractional crystallization from ultrabasic high-potassium magma. We studied dunites and wehrlites of the Inagli massif and olivine lamproites of the Ryabinovy massif, which is also included into the Central Aldan high-potassium magmatic area. The research is focused on the chemistry of Cr-spinels and the phase composition of Cr-spinel-hosted crystallized melt inclusions and their daughter phases. Mainly two methods were used: SEM-EDS (Tescan Mira-3), to establish different phases and their relationships, and EPMA, to obtain precise chemical data on small (2-100 µm) phases. The obtained results show similarity in chromite composition and its evolutionary trends for the Inagli massif ultramafites and Ryabinovy massif lamproites. The same has been established for phlogopite and diopside from crystallized melt inclusions from the rocks of both objects. Based on the results of the study, the conclusion is drawn that the ultramafic core of the Inagli massif resulted from fractional crystallization of high-potassium melt with corresponding in composition to low-titanium lamproite. This conclusion is consistent with the previous hypotheses suggesting an ultrabasic high-potassium composition of primary melt for the Inagli ultramafites.
Contributions to Mineralogy and Petrology, Vol. 174, 4, doi.org/10. 1007/s00410-018-1530-x 13p.
Abstract: Here we present an experimental study of the distribution of a broad range of trace elements between carbonatite melt, calcite and fluorite. The experiments were performed in the CaCO3 + CaF2 + Na2CO3 ± Ca3(PO4)2 synthetic system at 650-900 °C and 100 MPa using rapid-quench cold-seal pressure vessels. Starting mixtures were composed of reagent-grade oxides, carbonates, Ca3(PO4)2 and CaF2 doped with 1 wt% REE-HFSE mixture. The results show that the distribution coefficients of all the analyzed trace elements for calcite and fluorite are below 1, with the highest values observed for Sr (0.48-0.8 for calcite and 0.14-0.3 for fluorite) and Y (0.18-0.3). The partition coefficients of REE gradually increase with increasing atomic number from La to Lu. The solubility of Zr, Hf, Nb and Ta in the synthetic F-rich carbonatitic melts, which were used in our experiments, is low and limited by crystallization of baddeleyite and Nb-bearing perovskite.
Abstract: Maohokite, a post-spinel polymorph of MgFe2O4, was found in shocked gneiss from the Xiuyan crater in China. Maohokite in shocked gneiss coexists with diamond, reidite, TiO2-II, as well as diaplectic glasses of quartz and feldspar. Maohokite occurs as nano-sized crystallites. The empirical formula is (Mg0.62Fe0.35Mn0.03)2+Fe3+2O4. In situ synchrotron X-ray microdiffraction established maohokite to be orthorhombic with the CaFe2O4-type structure. The cell parameters are a = 8.907 (1) Å, b = 9.937(8) Å, c = 2.981(1) Å; V = 263.8 (3) Å3; space group Pnma. The calculated density of maohokite is 5.33 g cm-3. Maohokite was formed from subsolidus decomposition of ankerite Ca(Fe2+,Mg)(CO3)2 via a self-oxidation-reduction reaction at impact pressure and temperature of 25-45 GPa and 800-900 °C. The formation of maohokite provides a unique example for decomposition of Fe-Mg carbonate under shock-induced high pressure and high temperature. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2017-047). The mineral was named maohokite after Hokwang Mao, a staff scientist at the Geophysical Laboratory, Carnegie Institution of Washington, for his great contribution to high pressure research.
Abstract: The CIM Mineral Exploration Best Practice Guidelines (the Exploration Guidelines) have been prepared to assist professional geoscientists and engineers to conduct consistently high-quality work in order to maintain public confidence. The Exploration Guidelines are meant to assist professional geoscientists and exploration practitioners in planning, supervising, and executing exploration programs. In Canada, there are generally two types of public resource reporting: “Disclosure”, as defined by NI 43-101, is the reporting of technical information to the public and market participants for securities legislation purposes where a Qualified Person (QP) must be involved, and reporting of exploration information for governmental agencies to support obligations under laws including the Mining Acts of each of the Provinces and Territories. The Exploration Guidelines are also relevant where the results will not be publicly reported but are intended for internal company use. While this document is intended as guidance for work conducted or supervised by geoscientists in Canada, many of the practices described herein can be adapted to mineral exploration activities in other countries. The Exploration Guidelines are not intended to inhibit original thinking, or to prevent the application of new approaches that may develop into fundamental components of successful mineral exploration programs. Rather than provide prescriptive solutions to specific issues, they include general guidelines for current professional practice and to demonstrate and defend the merits of new methods. These guidelines do not preclude individuals and companies from developing more detailed guidelines specific to their own requirements. The initial version of the Exploration Guidelines was prepared by the Canadian Institute of Mining and Metallurgy and Petroleum (CIM) Exploration Best Practices Committee and adopted by CIM Council on August 20, 2000. On January 9, 2018, CIM Council formed a new committee, the Mineral Resources and Reserves Committee (CIM MRMR Committee) with a mandate to, among other things, update the Exploration Best Practice Guidelines. The mandate for the committee was accepted by CIM Council on March 2, 2018. The new Exploration Guidelines document was adopted by the CIM Council on November 23, 2018.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 276.
Abstract: Diamond characterization is carried out via a wide variety of gemological and chemical analyses. An important analytical tool for this purpose is spectroscopic characterization utilizing both absorption and emission measurements. The main techniques are UV-visible and infrared spectroscopy, though Raman as well as cathodoluminescence spectroscopy are also used. We have used electron paramagnetic resonance (EPR) spectroscopy to compare the properties of treated colored diamonds to the pretreated stones. The colors studied were blue, orange, yellow, green, and pink. The EPR technique determines radicals (atoms with unpaired electrons) and is very sensitive, capable of measuring concentrations as low as ~1 × 10–17 radicals/cm3. The results, shown in table 1, indicate that all the carbon radicals determined are affected by adjacent nitrogen atoms, with the spectra showing a hyperfine structure attributed to the presence of nitrogen. The highest concentration of radicals and hyperfine structures is observed in pink and orange treated diamonds. The results concerning nitrogen concentration were correlated with the infrared spectra, which determine the absorption peaks of the diamonds as well as those of the nitrogen contamination in their crystal structure.
Abstract: The existence of undulations of the geoid, gravity and bathymetry in ocean basins, as well as anomalies in heat flow, point to the existence of small scale convection beneath tectonic plates. The instabilities that could develop at the base of the lithosphere are sufficiently small scale (< 500 km) that they remain mostly elusive from seismic detection. We take advantage of 3D spherical numerical geodynamic models displaying plate-like behavior to study the interaction between large-scale flow and small-scale convection. We find that finger-shaped instabilities develop at seafloor ages > 60 Ma. They form networks that are shaped by the plate evolution, slabs, plumes and the geometry of continental boundaries. Plumes impacting the boundary layer from below have a particular influence through rejuvenating the thermal lithosphere. They create a wake in which new instabilities form downstream. These wakes form channels that are about 1000 km wide, and thus are possibly detectable by seismic tomography. Beneath fast plates, cold sinking instabilities are tilted in the direction opposite to plate motion, while they sink vertically for slow plates. These instabilities are too small to be detected by usual seismic methods, since they are about 200 km in lateral scale. However, this preferred orientation of instabilities below fast plates could produce a pattern of large-scale azimuthal anisotropy consistent with both plate motions and the large scale organisation of azimuthal anisotropy obtained from recent surface wave models.
Contributions to Mineralogy and Petrology, Vol. 173, 12, pp. 106-
Abstract: Multiphase solid inclusions in minerals formed at ultra-high-pressure (UHP) provide evidence for the presence of fluids during deep subduction. This study focuses on barian mica, which is a common phase in multiphase solid inclusions enclosed in garnet from mantle-derived UHP garnet peridotites in the Saxothuringian basement of the northern Bohemian Massif. The documented compositional variability and substitution trends provide constraints on crystallization medium of the barian mica and allow making inferences on its source. Barian mica in the multiphase solid inclusions belongs to trioctahedral micas and represents a solid solution of phlogopite KMg3(Si3Al)O10(OH)2, kinoshitalite BaMg3(Al2Si2)O10(OH)2 and ferrokinoshitalite BaFe3(Al2Si2)O10(OH)2. In addition to Ba (0.24-0.67 apfu), mica is significantly enriched in Mg ( X Mg 0.85 to 0.95), Cr (0.03-0.43 apfu) and Cl (0.04-0.34 apfu). The substitution vector involving Ba in the I-site which describes the observed chemical variability can be expressed as BaFeIVAlClK-1Mg-1Si-1(OH)-1. A minor amount of Cr and VIAl enters octahedral sites following a substitution vector VI(Cr,Al)2?VI(Mg,Fe)-3 towards chromphyllite and muscovite. As demonstrated by variable Ba and Cl contents positively correlating with Fe, barian mica composition is partly controlled by its crystal structure. Textural evidence shows that barian mica, together with other minerals in multiphase solid inclusions, crystallized from fluids trapped during garnet growth. The unusual chemical composition of mica reflects the mixing of two distinct sources: (1) an internal source, i.e. the host peridotite and its garnet, providing Mg, Fe, Al, Cr, and (2) an external source, represented by crustal-derived subduction-zone fluids supplying Ba, K and Cl. At UHP-UHT conditions recorded by the associated diamond-bearing metasediments (c. 1100 °C and 4.5 GPa) located above the second critical point in the pelitic system, the produced subduction-zone fluids transporting the elements into the overlying mantle wedge had a solute-rich composition with properties of a hydrous melt. The occurrence of barian mica with a specific chemistry in barium-poor mantle rocks demonstrates the importance of its thorough chemical characterization.
Abstract: The sinking remnant of a surface plate crosses and interacts with multiple boundaries in Earth's interior. Here, we specifically investigate the prominent dynamic interaction of the sinking plate portion with the upper-mantle transition zone and its corresponding surface elevation signal. We unravel, for the first time, that the collision of the sinking slab with the transition zone induces a sudden, dramatic downward tilt of the upper plate towards the subduction trench. Unraveling this crucial interaction was only possible thanks to state-of-the-art numerical modelling and post-processing. The new model that is introduced here to study the dynamically self-consistent temporal evolution of subduction features accurate subduction-zone topography, robust single-sided plate sinking, stronger plates close to laboratory values, an upper-mantle phase transition, and simple continents at a free surface. To distinguish the impact of the new physical model features, three different setups are used: the simplest model setup includes a basic high-viscosity lower mantle, the second adds a 660-km phase transition, and the third includes, additionally, a continental upper plate. Common to all models is the clear topographic signal upon slab-transition-zone interaction: the upper plate tilts abruptly towards the subduction trench by about 0.05° and over around 10 Ma. This dramatic increase in upper-plate tilt can be related to the slab-induced excitation of the high-viscosity lower mantle, which introduces a wider flow pattern. A large change in horizontal extent of inundation of up to 900 km is observed as a direct consequence of the upper-plate tilting. Such an abrupt variation in surface topography and inundation extent should be clearly visible in temporal records of large-scale surface elevation and might explain continental tilting as observed in Australia since the Eocene and North America during the Phanerozoic.
Abstract: The breakup of continents and their subsequent drifting plays a crucial role in the Earth's periodic plate aggregation and dispersal cycles. While continental aggregation is considered the result of oceanic closure during subduction, what drives sustained divergence in the following stages remains poorly understood. In this study, thermo-mechanical numerical experiments illustrate the single contribution of subduction and coupled mantle flow to the rifting and drifting of continents. We quantify the drag exerted by subduction-induced mantle flow along the basal surface of continental plates, comparing models of lithospheric slab stagnation above the upper-lower mantle boundary with those where slabs penetrate into the lower mantle. When subduction is upper-mantle confined, divergent basal tractions localise at distances comparable to the effective upper mantle thickness (~ 500 km), causing the opening of a marginal basin. Instead, subduction of lithosphere in the lower mantle reorganises the flow into a much wider cell localising extensional stresses at greater distances from the trench (~ 3000 km). Sub-continental tractions are higher and more sustained over longer time periods in this case, and progressively increase as the slab sinks deeper. Although relatively low, basal-shear stresses when integrated over large plates, generate tension forces that may exceed the strength of the continental lithosphere, eventually leading to breakup and opening of a distal basin. The models illustrate the emergence of a similar mechanism, which results in the formation of back-arc basins above upper-mantle confined subduction, and scales to much larger distances for deeper subduction. Examples include the Atlantic Ocean formation and drifting of the South and North American plates during the Mesozoic-Cenozoic Farallon plate subduction.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 305-6.
Abstract: Laboratory-grown diamonds are created using either high-pressure, high-temperature (HPHT) or chemical vapor deposition (CVD). With the influx of manmade diamonds on the market over the past few years, instrument producers and labs have launched screening and detection instruments to help dealers and jewelers spot HPHTor CVD-grown specimens. Most standard instruments are inaccurate testers or just type I and type II screening devices that do not give a definite answer about diamond genesis. Over the last four annual Mediterranean Gemmological and Jewellery Conferences and more than 30 workshops given in 17 countries, we have assembled a portable new Synthetic Diamond Identification Kit. The kit comprises two portable instruments and two booklets: 1) A PL inspector (mini UV lamp with magnifier) to inspect laboratory-grown, treated, and natural diamonds using long- and short-wave fluorescence and phosphorescence 2) A 2017 handbook with images and explanation of longand short-wave reactions of diamonds of all types 3) A mini foldable polariscope with portable light to separate natural diamonds using characteristic birefringence patterns from HPHT and CVD diamonds 4) A 2010 handbook with images and explanations of crosspolarized filter reactions of diamonds of all types The combination of this kit with professional training could identify all HPHT-grown diamonds and most CVD-grown diamonds on the market, loose or mounted. Also available are melee and jewelry inspectors consisting of larger UV lamps with magnifiers designed for identification of small loose or mounted diamonds. Different diamond types and subtypes can exhibit different birefringence under cross-polarized filters. A clear majority of natural diamonds exhibit some degree of internal strain, with type II natural diamonds showing a weak “tatami” pattern. HPHTgrown diamonds are free of such strain, and CVD-grown diamonds show mostly coarse columnar patterns. Most natural diamonds have a strong reaction to long-wave UV; this reaction is usually weaker (mostly blue) at shorter wavelengths. Laboratory-grown diamonds generally exhibit more intense fluorescence with short-wave UV compared to long-wave UV, with a chalky coloring tinged with green or yellow. Most HPHT-grown diamonds also phosphoresce. If a diamond is free of inclusions, fluorescence is a reliable screening test to flag suspicious stones that should be further checked under cross-polarized filters (figure 1). In the case of some rare near-colorless clean CVD-grown diamonds that do not show fluorescence or have a birefringence pattern that is coarse but resembling tatami in type IIa and weak patterns in natural Ia diamonds, additional tests using advanced spectroscopy and strong short-wave UV light to observe growth patterns are needed to confirm diamond genesis.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 273-4.
deposit - Karowe
Abstract: In November 2015, Lucara Diamond’s operation at the Karowe mine in Botswana gained notoriety due to the extraction of a series of large colorless diamonds, including the 1,109 ct Lesedi La Rona and the 812 ct Constellation. The Lesedi La Rona marks the largest gem diamond recovered since the Cullinan (3,106 ct) in 1905. The Constellation, considered to be the seventh-largest recorded diamond, attained the highest price ever paid for a rough, selling for $63.1 million ($77,649 per carat). Additionally, three other significant colorless diamonds were recovered during the same period, weighing 374, 296, and 183 ct. Due to the similarity in their external characteristics— which include cleavage faces—as well as their extraction locations and dates, it was suspected that these stones might have originated from a larger rough that had broken. Lucara demonstrated that the 374 ct diamond and the Lesedi La Rona fit together, yet a large cleavage plane is still unaccounted for. GIA was able to study several rough and/or faceted pieces of these five diamonds using a range of spectroscopic and imaging techniques to gain insight into the presence and distribution of point defects in these diamonds. Diamonds are commonly classified according to their nitrogen content measured by Fourier-transform infrared (FTIR) spectroscopy: Type I diamonds contain nitrogen in either isolated (Ib) or aggregated (IaAB) forms, while type II diamonds do not contain detectable nitrogen concentrations (IIa) but may contain boron (IIb). Analysis of faceted stones cut from the Lesedi La Rona indicates that the rough is a mixed-type diamond, containing both type IIa and pure type IaB regions. These types of diamonds, though exceedingly unusual, have been observed at GIA and reported by Delaunay and Fritsch (2017). The Constellation and the 374, 296, and 183 ct diamonds were determined to be type IaB, containing 20 ± 4 ppm B-aggregates (N4V), in agreement with the concentration for the type IaB pieces of the Lesedi La Rona. Pure type IaB diamonds such as these are actually quite rare, accounting for only 1.2% of a random suite of 5,060 large (>10 ct) D-to-Z diamonds submitted to GIA, whereas 24.6% were type II. Photoluminescence spectra further confirmed analogous defect content for the five large Karowe diamonds, with emissions from H4 (N4V2 0, 496 nm), H3 (NVN0, 503 nm), 505 nm, NV– (637 nm), and GR1 (V0, 741 nm) defects showing similar relative intensities and peak widths. Even for diamonds of the same type, parallel defect content and characteristics across such a variety of defects is unlikely for unrelated stones. The external morphologies of the diamonds showed primary octahedral, resorbed, and fractured faces, with the Constellation and the 296 ct diamond featuring fractures containing metallic inclusions and secondary iron oxide staining. Deep UV fluorescence (< 230 nm) imaging elucidated the internal growth structures of the samples. For the Constellation and the 374, 296, and 183 ct diamonds, at least two growth zones with differing blue fluorescence intensities were observed within single pieces. Combined with the spectroscopic data, these results provide compelling evidence that the Lesedi La Rona, the Constellation, and the 374, 296, and 183 ct diamonds from Karowe had comparable growth histories and likely originated from the same rough, with a combined weight of at least 2,774 ct.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 265.
Abstract: Diamond, known for its splendor in exquisite jewelry, has been synthesized since the 1950s. In the last six decades, the perfection of laboratory-grown single-crystal diamond has vastly improved through the research and development of two main synthesis techniques. One replicates Earth’s natural process, where the diamond is grown in the laboratory under conditions of diamond stability at high temperature and high pressure (HPHT). The other technique relies on the dissociation of methane (or other carbon-containing source gas) and hydrogen and the subsequent deposition of diamond at low pressures from the gaseous phase in a process known as chemical vapor deposition (CVD). In the latter case, diamond is not the stable form of carbon, but the kinetics in the CVD process are such that diamond wins out. Large gem-quality synthetic diamonds are now possible, and a 6 ct CVD (2018) and a 15.32 ct HPHT (2018) have been reported. It is of course possible to differentiate laboratory-grown from natural diamond based on how extended and point defects are incorporated into the crystal. Furthermore, treated diamond can be identified utilizing knowledge of how defects are produced and how they migrate and aggregate in both natural and synthetic diamond samples. Room-temperature confocal photoluminescence microscopy can be used to image the emission of light from defects in diamond with a spatial resolution limited only by the diffraction limit; a lateral spatial resolution approaching 300 nm is routinely achieved (figure 1). It is possible with this tool to identify point defects with concentrations less than 1 part per trillion (1011 cm–3). This talk will outline the experimental setup, how this tool has been used to identify the decoration of dislocations with point defects in CVD lab-grown diamond, and how different mechanisms for defect incorporation operate at growth sector boundaries in HPHT synthetic diamond.
Journal of Geophysical Research: Solid Earth, Vol. 123, 10, pp. 8459-8473.
Abstract: Olivine is the most abundant and among the weakest phases in Earth's upper mantle, and thus, its rheological properties play a critical role in governing thermal structure and convective flow in the upper mantle. A persistent obstacle to constraining the in situ flow properties of olivine by laboratory experiment has been the difficulty in resolving the effect of pressure, which is weak within the 0- to ~2-GPa pressure range of conventional laboratory deformation instruments but potentially strong over the 1- to ~14-GPa range of the upper mantle. Using a deformation-DIA, one of a new generation of bonafide deformation devices designed for operation to =10 GPa, we have deformed dry, polycrystalline San Carlos olivine in high-temperature creep with the singular intent of providing the best achievable measurement of activation volume V* and a comprehensive statement of uncertainty. Under strictly dry conditions, at constant temperature (1,373 K) and strain rate (1 × 10-5 s-1), varying only pressure (1.8 to 8.8 GPa), we measure V* = 15 ± 5 cm3/mol. We have reproduced the well-known mechanism change from -slip to -slip near 5 GPa and determined that, whatever the change in V* associated with the change in slip system, the effective value of 15 ± 5 cm3/mol is still accurate for modeling purposes in the 2- to 9-GPa pressure range. This is a substantial pressure effect, which in the absence of a temperature gradient would represent a viscosity increase from the top to bottom of the upper mantle of 5 ± 2 orders of magnitude.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 269-270..
Abstract: While chemical vapor deposition (CVD) diamond growth technology has progressed significantly in recent years, with improvements in crystal size and quality, the use of these goods in the jewelry trade is still limited. Not all CVD-grown gem diamonds are submitted to GIA for grading reports, and they only account for about 0.01% of GIA’s annual diamond intake (both D–Z equivalents and fancy color; Eaton-Magaña and Shigley, 2016). The CVD process involves diamond growth at moderate temperatures (700–1300°C) but very low pressures of less than 1 atmosphere in a vacuum chamber (e.g., Angus and Hayman, 1988; Nad et al., 2015). This presentation summarizes the quality factors and other characteristics of the CVD-grown material submitted to GIA (e.g., figure 1) and discusses new research and products. Today the CVD process is used to produce high-color (as well as fancy-color) and high-clarity type II diamonds up to several carats in size. The majority of the CVD material seen at GIA consists of near-colorless (G–N equivalent) with colorless (D–F equivalent) and various “pink” hues. Additionally, CVD material is constantly setting new size milestones, with the announcement of an approximately 6 ct round brilliant earlier this year (Davis, 2018). However, the attainable sizes among CVD products are dwarfed by those from the HPHT process, with 15.32 ct as the current record for a faceted gem (Ardon and Eaton-Magaña, 2018). One particular challenge for gemologists (albeit very rarely encountered) comes from the lab-grown/natural hybrids (figure 2) that have been submitted to and documented by gemological laboratories (e.g., Moe et al., 2017; Tang et al., 2018). In these specimens, the grower places a natural diamond into the CVD reactor as the seed plate, with both components retained in the faceted gem. If the manufacturer is using a colorless natural type Ia diamond as a seed plate for near-colorless CVD growth, the hybrid cannot undergo any post-growth HPHT treatment, as this would radically alter the natural seed by turning the natural diamond yellow. If the manufacturer is creating a CVD overgrowth layer on a faceted natural diamond, the intent is to either add weight to a diamond that may be near a weight boundary or to achieve a color change, typically to blue. These hybrid products also make it more difficult to infer a diamond’s history based solely on its diamond type. The CVD process has also created some unique gems that have not been duplicated among natural, treated, or HPHT-grown diamonds. These include CVD-grown diamonds with a high concentration of silicon impurities, which create a pink to blue color shift. In those samples, a temporary effect was activated by UV exposure, which precipitated a charge transfer between negative and neutral silicon-vacancy centers (D’Haenens-Johansson et al., 2015). Also recently seen are type IIb CVD goods. Some that were submitted by clients had a low boron concentration (3 ppb, with G-equivalent color and 1.05 carat weight). Meanwhile, some research samples produced by a manufacturer in China and fashioned as flat plates had dark bluish coloration and very high boron concentration (2500 ppb and higher). Also among that suite of flat-plate CVD samples was one with a black color caused by extremely high amounts of nitrogen-vacancy centers. Although new CVD products are continually being manufactured and introduced to the trade, the laboratory-grown diamonds examined to date by GIA can be readily identified.
Conflict, Security and Develoment, Vol. 18, 6, pp. 463-492.
Africa, Sierra Leone
Abstract: Shortcomings in natural resource governance leading to economic mismanagement, political clientelism, underdevelopment and civil conflict, have caused an increase in global norms of ‘good governance’ of natural resource sectors. As a result, a growing number of global governance initiatives are targeting conflict-prone natural resource sectors. Whether these regulatory efforts stand a chance of being successful depends on their implementation in producer countries. As the transnational regulatory framework aimed at curbing the trade in conflict minerals is expanding, this article investigates the local translation of global norms of resource governance. Drawing on the ‘local-to-global’ research perspective developed in this special issue and norm diffusion theories, the article examines one of the most prominent cases of governance reform targeting conflict-affected natural resource sectors: The Sierra Leonean diamond market. Based on extensive field research, the article analyses the implementation of KPCS requirements on the national and subnational level of governance institutions. It evaluates the accomplishments, the challenges and the local adaption to and (formal and informal) interpretation of KPCS norms.
Earth and Planetary Science Letters, Vol. 506, pp. 493-506.
Abstract: Most diamonds found in kimberlites show complex patterns of growth and dissolution (resorption) surface features. Populations of diamonds from within single kimberlite bodies commonly contain a large diversity of diamond surface forms, some of which are a result of dissolution in kimberlite magma and others are inherited from the mantle. Morphological studies of natural diamonds differentiated features produced during dissolution in kimberlite magma and during mantle metasomatism. The former features were experimentally reproduced at 1 3 GPa and used to infer the presence and composition of magmatic fluid in different kimberlites. However, the mantle-derived resorption features have not been reproduced experimentally and the composition and origins of their formative solvents are unknown. Here we report the results of diamond dissolution experiments conducted in a multi-anvil apparatus at 6 GPa and 1200 to 1500 °C in synthetic CaO MgO SiO2 CO2 H2O system. The experiments produced very different diamond resorption morphologies in COH fluid, in silicate-saturated fluid, and in silicate and carbonate melts. Dissolution in SiO2-free COH fluid developed rounded crystal forms with shallow negative trigons, striations and hillocks, which are commonly observed on natural diamonds and are similar in 6 GPa and in 1 3 GPa experiments. However, silicate-saturated fluid produced very different resorption features that are rarely observed on natural diamonds. This result confirms that natural, SiO2-poor fluid-induced resorption develops under the comparatively low-pressures of kimberlite ascent, because at mantle pressures the high content of SiO2 in fluids would produce features like those from the silicate-saturated experiments. Comparison of the experimental products from this study to natural diamond resorption features from the literature suggests that natural diamonds show no record of dissolution by fluids during mantle metasomatism. Diamond resorption morphologies developed in experiments with silicate carbonate melts closely resemble many of the mantle-derived resorption features of natural diamonds, whose diversity can result from variable SiO2 concentration in carbonatitic melts and temperature variation. The experimental results imply that metasomatism by fluids does not dissolve diamond, whereas metasomatism by melts is diamond-destructive. The repetitive growth-dissolution patterns of natural diamonds could be due to diamond growth from fluids in harzburgitic lithologies followed by its dissolution in partial melts.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, Fall 1p. Abstract p. 263-4
Abstract: The dream of growing synthetic diamonds existed for many centuries before it was achieved in the 1950s. The development of techniques to identify synthetic diamonds and enable their reliable separation from natural diamonds has not had the luxury of centuries to work with. Since the early reports on the characteristics of laboratory-grown stones, scientists have been working steadily to establish and improve the means of detection. For many years the De Beers Group has been developing equipment for rapidly screening and testing for potential synthetic and treated diamonds as part of a strategy aimed at maintaining consumer confidence in natural untreated diamonds. This work has been underpinned by extensive research into defects in natural and synthetic diamond, either conducted within De Beers’ own facilities or through financial and practical support of research in external institutions. Key to any detection technique for synthetic diamonds is a fundamental understanding of the differences between them and natural diamonds. This could take the form of differences in the atomic impurity centers or differences in the spatial distributions of these centers brought about by very significant distinctions in the growth environments. The former was used in the development of the Dia - mondSure instrument that, among other things, detects variance differences in the absorption spectra due to the presence or absence of the N3 feature. This absorption is from a nitrogen-related defect that is usually only produced in nitrogen-containing diamonds by extended periods at relatively high temperatures—that is, conditions generally experienced by natural diamonds. Growth-related differences in impurity distributions can be very accurately imaged using the DiamondView instrument. Short-wave ultraviolet (UV) light is used to excite luminescence from a very thin layer of diamond near the surface to give images free from the blurring encountered with more common longer-wavelength excitation sources. DiamondView has, since its launch, provided the benchmark for the detection of synthetic diamonds. A number of approaches involving absorption features have been developed, including the use of almost complete absorption in the ultraviolet region of the spectrum to indicate that a diamond is not synthetic. The UV absorption is produced by the A center (two adjacent nitrogen atoms) and is rarely encountered in as- grown synthetic diamonds. The main form of nitrogen in synthetic diamonds is a single substitutional nitrogen atom that absorbs in both the ultraviolet and visible regions to produce yellow color. The combination of UV absorption and no strong yellow color is therefore restricted to natural diamond. However, treatment of nitrogen-containing synthetic diamonds is capable of generating A centers, but generally does not produce a colorless stone. This effect accounts for the careful color ranges often applied to instruments relying on UV absorption for screening. This also highlights one of the limitations of absorption spectroscopy: When smaller stones are tested, the amount of absorption decreases and the technique becomes less reliable. In recent years we have seen a shift to smaller sizes (below 0.01 ct) in the synthetic diamonds being offered for sale to the jewelry market, and screening techniques have had to evolve to address this situation and the limitations of absorption-based approaches. Testing melee-sized diamonds, as well as introducing technical challenges around the measurement technique, has also led to the introduction of greater automation. In 2014 the De Beers Group introduced the first automated melee screening instrument (AMS1), which combined the measurement technique from Dia - mondSure with automated feeding and dispensing of stones in the range of 0.20 to 0.01 ct. While this instrument was well received and effectively addressed concerns around synthetic melee-sized stones in the trade at the time, there soon came calls for improvements— a faster instrument capable of measuring smaller stones, no restrictions on cut, and a lower referral rate for natural diamonds. These requirements proved impossible to meet with the limitations imposed by absorption measurements, and a new technique based on time-resolved spectroscopy was developed. This resulted in the AMS2 instrument, launched in March 2017. The AMS2 processes stones at a speed of one stone per second, 10 times faster than the AMS1. It measures round brilliants down to 0.003 ct (0.9 mm dia - meter) and can be used on other cuts for stones of 0.01 ct and above. The measurement technique itself has been incorporated into the SYNTHdetect (figure 1, left), an instrument launched in September 2017 that allows manual observation of the time-resolved emission. Besides providing the same testing capability as AMS2 (figure 1, right) for loose stones, various holders allow testing of mounted stones in a wide range of configurations. The benefit of this approach is that stones tested loose using AMS2 will generate a broadly consistent result when mounted on SYNTHdetect. Changes in growth processes for synthetic diamonds have also led to the gradual introduction of new characteristics. High - pressure, high-temperature (HPHT) synthetics have tended to be fairly consistent in their growth-related luminescence patterns, while significant variations in the features associated with chemical vapor deposition (CVD) synthetics have been observed. These continue to be well documented and have led to the gradual evolution of the DiamondView instrument and the way in which it is used. Post-growth treatment of synthetic diamonds can be applied for a number of reasons: improvement in the color, modification of the atomic defects to make the stones look more like a natural diamond, and removal of a characteristic that could be used to identify a synthetic diamond. The motivation for the latter two treatments can only be described as fraudulent. The challenge in developing detection instruments and techniques is to ensure that they are as robust as possible in the face of such challenges. Treatment techniques will rarely have any effect on the growth patterns associated with synthetic diamonds, and it is therefore very difficult to treat synthetics in a way that would make them undetectable using the DiamondView. Screening instruments tend to be based on a single technique, and it is important that the approach adopted not be vulnerable to simpler forms of treatment. This has been of primary concern to the De Beers Group in the development of our own screening instruments. It has also been necessary in certain cases to withhold detailed information about detection techniques where disclosure of this would lead to undermining of the detection technique itself. The De Beers Group continues to invest heavily in growth and treatment research in order to develop the next generation of instruments and techniques that will assist the trade in maintaining detection capability to support consumer confidence. The Group is uniquely placed in the industry to address these challenges due to its collaboration with Element Six (world leaders in synthesis of diamond for industrial and technical applications) and its indepth knowledge of the properties of natural diamonds with known provenance from its own mines.
Abstract: Carbonate-rich intrusions in contact with felsic rocks theoretically should show the effects of interaction between the two rock types, due to their contrasting compositions. In reality, though, such interaction is rarely reported at kimberlite contacts. We present the first documented case of lithological and mineralogical zonation at the margin of a kimberlite, the Snap Lake dyke, in contact with the wall-rock granitoid. Our detailed petrographic, mineralogical and geochemical study shows that the fresh hypabyssal kimberlite consists of olivine macrocrysts and microcrysts, and phlogopite macrocrysts set in a groundmass of serpentinized monticellite, phlogopite, spinel, perovskite and apatite, with interstitial lizardite and calcite. This typical Group I kimberlite mineralogy does not match the bulk-rock composition, which resembles a Group II micaceous kimberlite. The mismatch between the chemical and mineralogical properties is ascribed to contamination by granitoid xenoliths and metasomatic reactions with the felsic country rocks, the Snap Lake kimberlite has extremely low bulk-Ca compared to other documented Group I kimberlites. Reaction with deuteric H2O and CO2 has led to Ca removal, serpentinization of olivine, replacement of calcite by dolomite, alteration of perovskite and decomposition of apatite. Adjacent to the contact with the host granitoid and in haloes around granitoid clasts, poikilitic phlogopite and lizardite are replaced by subsolidus phlogopite and a multiphase phyllosilicate composed of phlogopite+?lizardite+?chlorite+?talc. A modified isocon analysis accounts for felsic xenolith assimilation and isolates metasomatic changes. Enrichment of altered kimberlites in Si owes solely to xenolith incorporation. The metasomatic ingress of granitoid-derived Al for a limited distance inside the dyke was counteracted by a flux of Mg and Fe to the granitoid. Metasomatic changes in K and Ca tend to be positive in all lithologies of kimberlite and in the granitoids implying distal transport. The combination of xenolith digestion with metasomatic element transport is expected in hybrid zones where kimberlite magmas interact with felsic wall-rocks.
Abstract: As subducting plates reach the base of the upper mantle, some appear to flatten and stagnate, while others seemingly go through unimpeded. This variable resistance to slab sinking has been proposed to affect long-term thermal and chemical mantle circulation. A review of observational constraints and dynamic models highlights that neither the increase in viscosity between upper and lower mantle (likely by a factor 20-50) nor the coincident endothermic phase transition in the main mantle silicates (with a likely Clapeyron slope of -1 to -2 MPa/K) suffice to stagnate slabs. However, together the two provide enough resistance to temporarily stagnate subducting plates, if they subduct accompanied by significant trench retreat. Older, stronger plates are more capable of inducing trench retreat, explaining why backarc spreading and flat slabs tend to be associated with old-plate subduction. Slab viscosities that are ~2 orders of magnitude higher than background mantle (effective yield stresses of 100-300 MPa) lead to similar styles of deformation as those revealed by seismic tomography and slab earthquakes. None of the current transition-zone slabs seem to have stagnated there more than 60 m.y. Since modeled slab destabilization takes more than 100 m.y., lower-mantle entry is apparently usually triggered (e.g., by changes in plate buoyancy). Many of the complex morphologies of lower-mantle slabs can be the result of sinking and subsequent deformation of originally stagnated slabs, which can retain flat morphologies in the top of the lower mantle, fold as they sink deeper, and eventually form bulky shapes in the deep mantle.
Abstract: Intraplate continental magmatism represents a fundamental mechanism in Earth's magmatic, thermal, chemical and environmental evolution. It is a process intimately linked with crustal development, large-igneous provinces, metallogeny and major global environmental catastrophes. As a result, understanding the interactions of continental magmas through time is vital in understanding their effect on the planet. The interaction of mantle plumes with the lithosphere has been shown to significantly affect the location and form of continental magmatism, but only at modern mantle conditions. In this study, we perform numerical modelling for Late Archean (1600 °C), Paleoproterozoic (1550 °C), Meso-Neoproteroic (1500 °C) and Phanerozoic (1450 °C) mantle potential temperatures (Tp) to assess the time-space magmatic effects of ambient-mantle- and plume- lithosphere interaction over Earth's thermal history. Within these experiments, we impinge a mantle plume, with a time-appropriate Tp, onto a ‘step-like’ lithosphere, to evaluate the effect of craton margins on continental magmatism through time. The results of this modelling demonstrate that lithospheric architecture controls the volume and location of continental magmatism throughout Earth history, irrespective of ambient mantle or plume Tp. In all plume models, mantle starting plumes (diameter 300 km) impinge on the base of the lithosphere, and spread laterally over > 1600 km, flowing into the shallowest mantle, and producing the highest volume magmas. In ambient-mantle only models, Archean and Paleoproterozoic Tp values yield significant sub-lithospheric melt volumes, resulting in ‘passive’ geodynamic emplacement of basaltic magmatic provinces, whereas no melts are extracted at > 100 km for Meso-Neoproterozoic and Phanerozoic Tp. This indicates a major transition in non-subduction related continental magmatism from plume and ambient mantle to a plume-dominated source around the Mesoproterozoic. While the experiments presented here show the variation in plume-lithosphere interaction through time, the consistency in melt localisation indicates the lithosphere has been a first-order control on continental magmatism since its establishment in the Mesoarchean.
South African Journal of Geology, Vol. 121, pp. 271-286.
Abstract: Major, trace, radiogenic isotope and stable isotope data from lavas along the northeastern coast of Mozambique are described. The whole rock composition data demonstrate that the rocks are dominantly andesitic with compositions typical of calc-alkaline volcanic rocks from arc environments. SHRIMP U/Pb data from zircons indicate that the zircons are xenocrystic, having ages of between 500 Ma and 660 Ma, with the age of the lava constrained by Rb/Sr data at ~184 Ma. Strontium, Nd and Pb radiogenic isotope data support an interpretation of extensive mixing between a Karoo age basaltic magma (dolerite) from Antarctica and continental crust similar in composition to the Mozambique basement. Oxygen isotope data also imply a significant crustal contribution to the lavas. Possible tectonic settings for the lavas are at the margin of a plume or from a locally restricted compressional setting during Gondwana breakup processes.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 277-8.
Abstract: Most colored gemstones form near the earth’s surface in a wide range of different environments; for example, they can crystallize from igneous magmas or hydrothermal solutions, or via the recrystallization of preexisting minerals during metamorphism. The specific environment determines the types of gem minerals that form, as well as their physical and chemical properties. Field studies of colored gem deposits provide the basis for the scientific understanding of natural gemstone formation and, in turn, the basis for criteria for gem identification. Gem deposits are of scientific interest because they represent unusual geologic and geochemical conditions; for example, emeralds are rare because they require beryllium and chromium (and/or vanadium), which generally travel in very different geochemical circles. Scientists study gem deposits by collecting rock and mineral samples in the field, mapping geological formations and structures, documenting the environment in which the gems occur, and examining the collected samples back in the laboratory. Such examination yields information on the chemical, temperature, and pressure conditions of gem formation, the associated minerals (often found as distinctive inclusions in the gems themselves), and the age of the deposit. Determining the origin of a gem deposit usually requires a small amount of very specific data. The results are published in publicly available peerreviewed publications. Such field studies provide clues that can be used to explore for similar types of gem deposits. Challenges include the remoteness of locations that have not been previously studied by geologists, the small size of deposits that precludes study by large mining companies, and the rarity of the gems themselves. There is much left to do in gem deposit research. For example, despite its growing popularity as a gemstone, there are few studies of gem spinel deposits, especially cobalt-blue spinel (figure 1), for which only one deposit has been studied. To date we know little about what factors control spinel genesis and color. Recently there has been another reason to study gem deposits: gem fingerprinting, in which modern methods are used to obtain characteristic information. This information is then compared to information obtained from stones from known localities to estimate where a stone with no locality information originated. Modern fingerprinting methods analyze the chemistry of the stones (using electron probe microanalysis, isotopic analysis, laser ablation-inductively coupled plasma-mass spectrometry) and/or their solid and fluid inclusions. We know that the chemistry of the stones must reflect the chemistry of the host rock environment; for example, the chromophore in emerald from Lened in Canada is vanadium, and not the typical chromium, because there are no chromium-bearing rocks in the area. With respect to solid inclusions, rubies from Aappaluttoq in Greenland have phlogopite mica inclusions because they recrystallized in a rock at pressures and temperatures where phlogopite is the stable potassium-bearing phase. An example of diagnostic fluid inclusions is the three-phase variety seen in Colombian emeralds (and now also observed elsewhere). New is the use of ICP-MS on fluid inclusions to define part of the fluid assemblage from which the stones were formed; this tells us about the environment of formation, but also may assist in defining a fingerprint for the stone. Where scientific studies require only very specific data, the more data available from stones of known origin, and the more representative those stones are of the full range of compositions and inclusions found in a specific deposit or country of origin, the more accurate the estimation should be. Unfortunately, these data are generally not made public, so every lab doing fingerprinting is essentially working independently, and there is no way to know how accurate their data and the resulting country- or deposit-oforigin estimates are. We also note that a serious problem in origin determination is that some of the best gemstones will be lacking diagnostic inclusions altogether, which then restricts the tools and observations can be used.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 306-7.
Abstract: Nitrogen is one of the most common impurities in diamond, and its aggregation styles have been used as criteria for diamond classification. Pure type IaB diamonds (with 100% nitrogen in B aggregation) are rather rare among natural diamonds. The occurrence of the B center is generally associated with high temperature and a long residence time of the host diamond, which would potentially provide information on the earth’s deep interior. Seawater circulation is the unique process that shapes the surface of our planet and potentially has a profound effect on its interior due to slab subduction. In about 50 type IaB diamonds with detectable micro-inclusions submitted to GIA for screening, we found that more than 70% of them contained a typical mineral assemblage from the sublithosphere. Jeffbenite (TAPP), majorite garnet, enstatite, and ferropericlase have been observed, which could be retrograde products of former bridgmanite. CaSiO3-walstromite with larnite and titanite is the dominant phase present in approximately 40% of all diamond samples. Direct evidence from oxygen isotope ratios measured by secondary ion mass spectrometry, or SIMS, (d18OVSMOWin the range +10.7 to +12.5‰) of CaSiO3-walstromite with coexisting larnite and titanite that retrograde from CaSiO3-perovskite suggest that hydrothermally altered oceanic basalt can subduct to depths of >410 km in the transition zone. Incorporation of materials from subducted altered oceanic crust into the deep mantle produced diamond inclusions that have both lower mantle and subduction signatures. Ca(Si,Al)O3-perovskite was observed with a high concentration of rare earth elements (>5 wt.%) that could be enriched under P-Tconditions in the lower mantle. Evidence from ringwoodite with a hydroxide bond, coexisting tuite and apatite, precipitates of an NH3phase, and cohenite with trace amounts of Cl imply that the subducted brines can potentially introduce hydrous fluid to the bottom of the transition zone. In the diamonds with subducted materials, the increasing carbon isotope ratio from the core to the rim region detected by SIMS (d13C from -5.5‰ to -4‰) suggests that an oxidized carbonate-dominated fluid was associated with recycling of the subducted hydrous material. The deep subduction played an important role in balancing redox exchange with the reduced lower mantle indicated by precipitated iron nanoparticles and coexisting hydrocarbons and carbonate phases.
Abstract: During flat subduction, material is scraped off the base of the continental mantle lithosphere, building a migrating keel. This testable mechanism for flat subduction recreates features of the Laramide orogeny.
Abstract: The fraction of radioactive heat production in Earth’s mantle to convective heat loss has decreased with the aging of Earth, as more of its nuclear fuel became spent and more of its heat was lost to space. Earth was therefore hotter in its past, but there is no consensus as to how much higher the mantle’s temperature was in early Earth compared to the present. This is an important problem to understand because it is expected to have imposed secular changes in the formation of oceanic lithosphere at ridges and its cycling at subduction zones (Herzberg and Rudnick, 2012; Foley, 2018). In a hotter early Earth, the ambient mantle melted more extensively, to make thicker basaltic oceanic crust and residual mantle peridotite, the latter of which was depleted in chemical elements that entered the magmas. Sometime later, the basaltic oceanic crust became hydrated by seawater, and it in turn melted to make silicic continental crust. As discussed in more detail below, this transformation led to the juxtaposition of continental crust on top of oceanic lithospheric mantle (Herzberg and Rudnick, 2012). The original “oceanic mantle lithosphere” is now called "continental mantle lithosphere" because it is located below continental crust in cratons. This hypothesis is explored by Servali and Korenaga (2018, p. 1047 in this issue of Geology), and is the reason why they entitle their paper an "oceanic origin of continental mantle lithosphere".
Abstract: The formation, storage and chemical differentiation of magma in the Earth’s crust is of fundamental importance in igneous geology and volcanology. Recent data are challenging the high-melt-fraction ‘magma chamber’ paradigm that has underpinned models of crustal magmatism for over a century, suggesting instead that magma is normally stored in low-melt-fraction "mush reservoirs". A mush reservoir comprises a porous and permeable framework of closely packed crystals with melt present in the pore space1,10. However, many common features of crustal magmatism have not yet been explained by either the ‘chamber’ or ‘mush reservoir’ concepts. Here we show that reactive melt flow is a critical, but hitherto neglected, process in crustal mush reservoirs, caused by buoyant melt percolating upwards through, and reacting with, the crystals. Reactive melt flow in mush reservoirs produces the low-crystallinity, chemically differentiated (silicic) magmas that ascend to form shallower intrusions or erupt to the surface. These magmas can host much older crystals, stored at low and even sub-solidus temperatures, consistent with crystal chemistry data. Changes in local bulk composition caused by reactive melt flow, rather than large increases in temperature, produce the rapid increase in melt fraction that remobilizes these cool- or cold-stored crystals. Reactive flow can also produce bimodality in magma compositions sourced from mid- to lower-crustal reservoirs. Trace-element profiles generated by reactive flow are similar to those observed in a well studied reservoir now exposed at the surface. We propose that magma storage and differentiation primarily occurs by reactive melt flow in long-lived mush reservoirs, rather than by the commonly invoked process of fractional crystallization in magma chambers.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 268.
Abstract: China is the world’s largest producer of HPHT-grown industrial diamonds. Its 2016 production of about 20 billion carats accounted for 98% of the global supply. Since the beginning of 2015, meleesized colorless HPHT synthetic diamonds have been tested at the National Gemstone Testing Center’s (NGTC) Shenzhen and Beijing laboratories in parcels submitted by different clients, which means that colorless HPHT synthetic diamonds have entered the Chinese jewelry market and may be mistaken for natural diamonds. CVD synthesis technology has grown rapidly in recent years. Large colorless and colored (blue, pink) CVD-grown diamonds have been entering the market, and a few have been fraudulently sold as natural diamonds. China has independently developed gem-grade HPHT synthetic diamond production technology since 2002, and can grow gem-grade type Ib, IIa, and IIb and high-nitrogen-content synthetic diamonds in volume, depending on market needs. Gemgrade type Ib, IIa, and IIb HPHT synthetic diamonds have been grown using the temperature gradient method, under a cubic press at high pressure (e.g., 5.4 GPa) and high temperature (1300-1600°C). Driven by a specific temperature gradient, the carbon source from high-purity graphite (>99.9%) located at the high-temperature zone can diffuse into the seed crystals in the cubic press, resulting in the crystallization of synthetic diamonds. Chinese production of melee-sized colorless to near-colorless HPHT synthetic diamonds accounts for about 90% of the global output. Gem-grade type IIa and IIb CVD synthetic diamonds are grown using the microwave plasma chemical vapor deposition (MPCVD) and direct current (DC) arc plasma methods. Faceted colorless CVD diamonds can be grown in sizes up to 6 ct by at least two Chinese companies (table 1). After testing and analyzing thousands of natural and synthetic diamonds collected directly from the Chinese companies, NGTC independently developed the GV5000, PL5000, DS5000, and ADD6000 instruments for rapidly screening and identifying the diamonds based on the gemological characteristics obtained. Besides HPHT and CVD synthetic diamonds, a thickly layered hybrid diamond consisting of both natural and CVD material was identified at the NGTC Beijing laboratory (figure 1). The identification features and properties of regrown CVD synthetic diamonds using natural type Ia diamond crystals as seeds will be reported. The current status and features of colored stones examined at NGTC laboratories, including several cases studies, will be discussed.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 267.
Abstract: Pink diamond is extremely popular among fancy-color diamonds, which has prompted numerous attempts to produce pink diamond artificially. Pink CVD synthetic diamonds appeared on the gem market around 2010. Their color was produced by a multistep process combining post-growth HPHT treatment to remove the brown hue and subsequent electron irradiation, followed by low-temperature annealing. Pink CVD synthetic diamonds treated only with low pressure and high temperature (LPHT), without additional post-growth irradiation, have also been reported but are rarely seen on the market. Recently, a loose pink stone (figure 1) was submitted to the Central Gem Laboratory in Tokyo for grading purposes. Our examination revealed that this 0.192 ct brilliant-cut marquise was a CVD synthetic diamond that had been LPHT treated. Visually, this diamond could not be distinguished from natural diamonds with similar color. However, three characteristics of CVD origin were detected: 1. C-H related absorption peaks between 3200 and 2800 cm-1, located with infrared spectroscopy 2. A luminescence peak at 737 nm, detected with photoluminescence (PL) spectroscopy 3. A trace of lamellar pattern seen in the DiamondView However, irradiation-related peaks such as at 1450 cm-1 (H1a), 741.1 nm (GR1), 594.3 nm, or 393.5 nm (ND1) that are seen in the pink CVD diamonds treated with common multi-step processes were not detected. The presence of four peaks at 3123, 2901, 2870, and 2812 cm-1 between 3200 and 2800 cm-1 suggests this stone was LPHT treated; the following observations indicate that it was not HPHT treated: 1) The 3123 cm-1 peak presumably derived from NVH0 disappears after a normal HPHT treatment. 2) The 2901, 2870, and 2812 cm-1 peaks are known to shift toward higher wavenumbers as the annealing temperature rises. Our own HPHT treatment experiments on CVDgrown diamonds proved that the 2902 and 2871 cm-1 peaks detected after 1600°C annealing shifted to 2907 and 2873 cm-1 after 2300°C annealing. The peak shift of 2901, 2870, and 2812 cm-1 is also related to the pressure during the annealing, as these peaks shifted to 2902, 2871, and 2819 cm-1 at the higher pressure of 7 GPa compared to 2900, 2868, and 2813 cm-1 at the ambient pressure under the same annealing temperature of 1600°C. 3) Absorption peaks at 7917 and 7804 cm-1 in the infrared region and at 667 and 684 nm in the visible range were also detected, which coincide with the features seen in LPHTtreated stones. From the combination of the intensity ratios of optical centers such as H3 and NV centers that were detected with PL measurement, this sample is presumed to have been treated with LPHT annealing at about 1500- 1700°C as a post-growth process. In recent years, CVD synthetic diamonds have been produced in a wider range of colors due to progress in the crystal growth techniques and post-growth treatments. Although HPHT treatment has been employed mainly to improve the color in a diamond, LPHT annealing may become widespread as the technique is further developed. Gemologists need to have deep knowledge about the optical defects in such LPHT-treated specimens.
European Journal of Mineralogy, Vol. 30, 6, pp. 1083-1094.
deposit - Dalydyn
Abstract: The spatial distribution patterns of Mg-bearing ilmenite (Ilm) composition were studied on 54 kimberlite bodies of the Daldyn field in the Yakutian kimberlite province. The representativity of the ilmenites sampled in this study is ensured by analysing ca. 100 grains from each kimberlite body. The major conclusions are as follows: (1) ilmenites from neighbouring pipes within the same linear cluster have similar average compositions and compositional fields on the MgO-Cr2O3 plots; (2) ilmenites from different clusters of pipes show different average compositions and compositional fields on the MgO-Cr2O3 plots. (3) regardless of belonging to different clusters, low-Mg Ilm across the whole Daldyn field is characterized by a direct correlation between Al2O3 and MgO; (4) significant changes of MgO content are observed in high-Mg Ilm, while Al2O3 content remains at the same level. The similarity of Ilm compositions across the kimberlite field, as shown by the MgO-Al2O3 plots, is due to a common asthenospheric source. The similar Ilm compositions in different bodies within cluster of pipes is accounted for by a single supply of magma via a lithospheric mantle channel for all pipes of the cluster. The composition of the kimberlite melts can be altered owing to the incorporation and assimilation of lithospheric mantle rocks rich in Mg and Cr. These changes of the melt cause corresponding changes in the Ilm macrocryst composition, both during and after crystallization of Ilm. Thus, the Ilm macrocryst composition follows a trend from low-Mg/low-Cr for Ilm crystallizing in the asthenosphere, to high-Mg/high-Cr at higher levels in the lithosphere. The key conclusion of this study is that Ilm can be used to decipher the structure of kimberlite fields. This can provide a reliable geological criterion for grouping an association of pipes together in clusters, which were previously identified only through subjective considerations of the spatial proximity of kimberlite bodies.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, Fall 1p. Abstract p. 258
Africa, Namibia, South America, Brazil
Abstract: Laser- and ultraviolet-excited luminescence spectroscopy and imaging are important techniques for gemstone testing, as they are among the most sensitive spectroscopic methods (see Hainschwang et al., 2013). They are able to identify optically active crystallographic defects such as vacancies and substitutions that are present in such small amounts that they cannot be detected by any other analytical method. Photoluminescence (PL) analysis became particularly important in the last decade for the separation of natural from synthetic diamonds and the detection of treatments. Today the availability of specially designed and reasonably priced portable equipment enables the rapid in situ identification of mounted and unmounted natural diamonds. Although PL spectroscopy is most commonly used for diamond identification, it can also be applied to colored stones. Some stones exhibit unique luminescence patterns, which can be used to identify the material. Other examples are the separation of natural from synthetic spinel and the detection of heat-treated spinel. Since chromium is a typical PL-causing trace element, it is also possible to separate chromium-colored gems such as ruby and jadeite from their artificially colored counterparts. The color authenticity of specific types of corals and pearls can also be determined. The rare earth elements (REE) are among the main substituting luminescence centers in Ca2+-bearing minerals (Gaft et al., 2005). Recently, REE photoluminescence has been observed in cuprian liddicoatite tourmalines from Mozambique (Milisenda and Müller, 2017). When excited by a 785 nm laser, the stones showed a series of bands at 861, 869, 878, 894, and 1053 nm, consistent with the PL spectra of other calcium-rich minerals (Chen and Stimets, 2014). LA-ICP-MS analysis confirmed the REE enrichment in this type of tourmaline compared to cuprian elbaites from Brazil and Nigeria. As a result, photoluminescence can be used as a further criterion for origin determination of Paraíba-type tourmalines. We have extended our research on other calcium-rich gems, including various grossular garnet varieties such as hessonite and tsavorite (figure 1), uvarovite garnet, apatite, titanite, and scheelite, as well as a number of high-refractive-index glasses and colorchange glasses, respectively.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 307-8.
Abstract: Type Ib-dominant mixed-type diamonds (Ib-IaA) can be formed by multiple growth events (Titkov et al., 2015; Smit et al., 2018). In this study, we report on a 0.41 ct Fancy Dark brown gem - quality diamond that formed in a single growth event. It is a type Ib-IaA with a C defect (single-substitutional nitrogen atom) concentration up to 21 ppm. The Fourier-transform infrared (FTIR) peaks of the H1a and H1b defects (figure 1, left) suggest that this diamond was irradiated and annealed to achieve a Fancy color grade. The cuboctahedral structure can be observed in the DiamondView images (figure 1, right), which show reddish orange submitted to GIA for screening, we found that more than 70% of them contained a typical mineral assemblage from the sublithosphere. Jeffbenite (TAPP), majorite garnet, enstatite, and ferropericlase have been observed, which could be retrograde products of former bridgmanite. CaSiO3-walstromite with larnite and titanite is the dominant phase present in approximately 40% of all diamond samples. Direct evidence from oxygen isotope ratios measured by secondary ion mass spectrometry, or SIMS, (d18OVSMOWin the range +10.7 to +12.5‰) of CaSiO3-walstromite with coexisting larnite and titanite that retrograde from CaSiO3-perovskite suggest that hydrothermally altered oceanic basalt can subduct to depths of >410 km in the transition zone. Incorporation of materials from subducted altered oceanic crust into the deep mantle produced diamond inclusions that have both lower mantle and subduction signatures. Ca(Si,Al)O3-perovskite was observed with a high concentration of rare earth elements (>5 wt.%) that could be enriched under P-Tconditions in the lower mantle. Evidence from ringwoodite with a hydroxide bond, coexisting tuite and apatite, precipitates of an NH3phase, and cohenite with trace amounts of Cl imply that the subducted brines can potentially introduce hydrous fluid to the bottom of the transition zone. In the diamonds with subducted materials, the increasing carbon isotope ratio from the core to the rim region detected by SIMS (d13C from -5.5‰ to -4‰) suggests that an oxidized carbonate-dominated fluid was associated with recycling of the subducted hydrous material. The deep subduction played an important role in balancing redox exchange with the reduced lower mantle indicated by precipitated iron nanoparticles and coexisting hydrocarbons and carbonate phases.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 304.
Abstract: Gem-quality laboratory-grown diamonds are manufactured in large quantities. With frequent reports of the mixing of meleesized synthetic diamonds with natural stones, demand for melee diamond screening is increasing. During melee diamond screening at GIA’s Tokyo lab, two notable types of samples with uncommon characteristics have been found. 1. Natural melee diamonds with silicon and nickel defects. Luminescence peaks derived from Si- and Ni-related defects are often observed in colorless melee grown by the HPHT method. The silicon-related defect, once considered proof of CVD-grown diamond, is now known to exist naturally as well (Breeding and Wang, 2008). Several colorless melee diamonds having both silicon- and nickel-related emissions have been found in GIA’s Tokyo lab; olivine inclusions were found in one of these samples. Spectroscopic and gemological features confirmed that the samples were grown in nature. 2. Irradiated laboratory-grown diamond melee found among irradiated natural melee diamonds. Several thousand greenish blue melee diamonds have been submitted by various clients to the Tokyo lab for testing. Each diamond’s color was attributed to a strong GR1 defect caused by irradiation treatment. Fourier-transform infrared (FTIR), photoluminescence (PL), and DiamondView analysis revealed that most of them were irradiated natural diamonds. Eight were irradiated CVD-grown diamonds, and one was an irradiated HPHT-grown specimen. The infrared spectrum of all the CVD samples showed a peak at 3123 cm-1, while their PL spectrum showed a doublet peak at 596/597 nm. Those peaks are specific to as-grown CVD diamonds, as annealing removes the peaks. From their spectra, these CVD specimens were considered irradiated without pre-annealing.
Abstract: In the present study, four samples of natural melilites were characterized using electron microprobe analysis, powder X-ray diffraction, FTIR, and Raman spectroscopy, and their thermodynamic properties were measured with a high-temperature heat-flux Tian-Calvet microcalorimeter. The enthalpies of formation from the elements were determined to be: -3796.3 ± 4.1 kJ/mol for Ca1.8Na0.2(Mg0.7Al0.2Fe2+0.1?)Si2O7, -3753.6 ± 5.2 kJ/mol for Ca1.6Na0.4(Mg0.5Al0.4Fe2+0.1?)Si2O7, -3736.4 ± 3.7 kJ/mol for Ca1.6Na0.4(Mg0.4Al0.4Fe2+0.2?)Si2O7, and -3929.2 ± 3.8 kJ/mol for Ca2(Mg0.4Al0.6)[Si1.4Al0.6O7]. Using the obtained formation enthalpies and estimated entropies, the standard Gibbs free energies of formation of these melilites were calculated. Finally, the enthalpies of the formation of the end-members of the isomorphic åkermanite-gehlenite and åkermanite-alumoåkermanite series were derived. The obtained thermodynamic properties of melilites of different compositions can be used for quantitative modeling of formation conditions of these minerals in related geological and industrial processes.
Diamond & Related Materials, Vol. 91, pp. 207-212.
Abstract: We report the results of a study of the polycrystalline powder of the diamond-lonsdaleite from the Popigai crater (Siberia) using UV micro-Raman spectroscopy and high-resolution synchrotron X-ray diffraction. By subtracting two experimental Raman spectra of diamond-lonsdaleite samples with close amounts of diamond and lonsdaleite, we were able to identify the polytypic composition of impact diamonds in contrast to the method of X-ray diffraction. We have managed to get for the first time the spectrum of “pure” lonsdaleite. Its deconvolution has allowed us to identify all the three Raman - active vibrational modes E2g, A1g, and E1g whose positions agree well with the results of ab initio calculations.
Abstract: This special issue is intended to serve as a multidisciplinary forum covering broad aspects of the science, technology, and application of synthetic and natural diamonds. This special issue contains 12 papers, which highlight recent investigations and developments in diamond research related to the diverse problems of natural diamond genesis, diamond synthesis and growth using CVD and HPHT techniques, and the use of diamond in both traditional applications, such as mechanical machining of materials, and the new recently emerged areas, such as quantum technologies. The results presented in the contributions collected in this special issue clearly demonstrate that diamond occupies a very special place in modern science and technology. After decades of research, this structurally very simple material still poses many intriguing scientific questions and technological challenges. It seems undoubted that diamond will remain the center of attraction for many researchers for many years to come.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 286-7.
Abstract: Many gemologists know that there are important technological applications for laboratory-grown diamonds; however, it is less understood how broad the nongemological uses really are or why diamond is the ideal material for each use. This presentation will review modern industrial applications of laboratory-grown diamonds, including surgical tools, tumor detection, orthopedic implants, water purification, industrial tooling, compound refractive energy focusing, Fresnel lenses, high-pressure anvils (figure 1), sound reproduction, deep space communication, high-power electronics, quantum computing, long-term data storage, AC/DC conversion, and electrical vehicle efficiency. These applications are rooted in the less frequently discussed gemological properties of diamonds that make it a “supermaterial.” The biological, thermal, mechanical, optical, acoustic, and electrochemical properties of diamond will be introduced. Specific properties discussed will include thermal conductivity, Young’s modulus, breakdown field, band gap, and saturated electron drift velocity. Furthermore, the utility of diamond defects such as nitrogen vacancies and boron will be explored. In addition to discussions about functional monocrystal diamonds, two unnatural forms of functional diamond will be discussed: polycrystalline diamond (PCD) and diamond-like carbon (DLC). Many of the functional diamonds discussed, including PCD and DLC, will be available for hands-on examination as part of the presentation.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 270.
Abstract: For the past 50 years, the majority of diamond research has focused on diamonds derived from the lithospheric mantle root underpinning ancient continents. While lithospheric diamonds are currently thought to form the mainstay of the world’s economic production, the continental mantle lithosphere reservoir comprises only ~2.5% of the total volume of Earth. Earth’s upper mantle and transition zone, extending from beneath the lithosphere to a depth of 670 km, occupy a volume approximately 10 times larger. Diamonds from these deeper parts of the earth—“superdeep diamonds”—are more abundant than previously thought. They appear to dominate the high-value large diamond population that comes to market. Recent measurements of the carbon and nitrogen isotope composition of superdeep diamonds from Brazil and southern Africa, using in situ ion probe techniques, show that they document the deep recycling of volatile elements (C, N, O) from the surface of the earth to great depths, at least as deep as the uppermost lower mantle. The recycled crust signatures in these superdeep diamonds suggest their formation in regions of subducting oceanic plates, either in the convecting upper mantle or the transition zone plus lower mantle. It is likely that the deep subduction processes involved in forming these diamonds also transport surficial hydrogen into the deep mantle. This notion is supported by the observation of a high-pressure olivine polymorph—ringwoodite—with close to saturation levels of water. Hence, superdeep diamonds document a newly recognized, voluminous “diamond factory” in the deep earth, likely producing diamonds right up to the present day. Such diamonds also provide uniquely powerful views of how crustal material is recycled into the deep earth to replenish the mantle’s inventory of volatile elements. The increasing recognition of superdeep diamonds in terms of their contribution to the diamond economy opens new horizons in diamond exploration. Models are heavily influenced by the search for diamonds associated with highly depleted peridotite (dunites and harzburgites). Such harzburgitic diamonds were formed in the Archean eon (>2.5 Ga) within lithospheric mantle of similar age. It is currently unclear what the association is between these ancient lithospheric diamonds and large, high-value diamonds, but it is likely a weak one. In contrast, the strong association between superdeep diamonds and these larger stones opens up a new paradigm because the available age constraints for superdeep diamonds indicate that they are much younger than the ancient lithospheric diamonds. Their younger age means that superdeep diamonds may be formed in non-Archean mantle, or mantle that has been strongly overprinted by post-Archean events that would otherwise be deemed unfavorable for the preservation of ancient lithospheric diamonds. An additional factor in the search for new diamond deposits is the increasing recognition that major diamond deposits can form in lithospheric mantle that is younger than—or experienced major thermal disruption since—the canonical 2.5 billion years usually thought to be most favorable for diamond production. This talk will explore these new dimensions in terms of the potential for discovering new diamond sources in “unconventional” settings.
Abstract: Mechanisms of Precambrian orogeny and their contribution to the origin of ultrahigh temperature granulites, granite-greenstone terranes and net crustal growth remain debatable. Here, we use 2D numerical models with 150 °C higher mantle temperatures compared to present day conditions to investigate physical and petrological controls of Precambrian orogeny during forced continental plates convergence. Numerical experiments show that convergence between two relatively thin blocks of continental lithosphere with fertile mantle creates a short-lived cold collisional belt that later becomes absorbed by a long-lived thick and flat ultra-hot accretionary orogen with Moho temperatures of 700-1100 °C. The orogen underlain by hot partially molten depleted asthenospheric mantle spreads with plate tectonic rates towards the incoming lithospheric block. The accretionary orogeny is driven by delamination of incoming lithospheric mantle with attached mafic lower crust and invasion of the hot partially molten asthenospheric wedge under the accreted crust. A very fast convective cell forms atop the subducting slab, in which hot asthenospheric mantle rises against the motion of the slab and transports heat towards the moving orogenic front. Juvenile crustal growth during the orogeny is accompanied by net crustal loss due to the lower crust subduction. Stability of an ultra-hot orogeny is critically dependent on the presence of relatively thin and warm continental lithosphere with thin crust and dense fertile mantle roots subjected to plate convergence. Increased thickness of the continental crust and subcontinental lithospheric mantle, pronounced buoyancy of the lithospheric roots, and decreased mantle and continental Moho temperature favor colder and more collision-like orogenic styles with thick crust, reduced magmatic activity, lowered metamorphic temperatures, and decreased degree of crustal modification. Our numerical modeling results thus indicate that different types of orogens (cold, mixed-hot and ultra-hot) could be created at the same time in the Early Earth, depending on compositional and thermal structures of interacting continental blocks.
Geochimica et Cosmochimica Acta, Vol. 242, pp. 165-190.
United States, Arizona
Abstract: Scientists have known for a long time that various types of rock conduct current differently and that these differences are even more pronounced as the temperatures and pressures increase farther beneath Earth’s surface. They also know that unusual changes in electrical conductivity can signal activity down below, like migrating magma or a release of trapped fluids. Thus, electrical measurements can uncover clues about the events that trigger earthquakes and volcanic eruptions here on the surface. They can also give clues to the mantle’s structure and dynamics. However, interpreting these signals is far from straightforward. Earth scientists increasingly use electrical observations made in the field to image Earth’s crust and mantle, in particular, at subduction zones and mid-ocean ridges. An effective means of interpreting these electrical images and placing them into context with other geological observations is key to translating raw data into usable knowledge. Such knowledge includes assessing potential hazards by investigating, for example, links between fluid release and earthquake generation or the production and transport of magmatic melt from its source region to an eventual eruption. SIGMELTS is a freely available app that helps to characterize electrically conductive or resistive features detected at depth using electromagnetic observations. The objective of this Web application is to facilitate the elaboration of models of the electrical properties of crust and mantle materials, which, in turn, is used to improve the interpretation of field electromagnetic observations. A new version of SIGMELTS is now available.
Contributions to Mineralogy and Petrology, Vol. 173, 12, pp. 106-
Russia, Kola Peninsula
deposit - Afrikanda
Abstract: Perovskite is a common accessory mineral in a variety of mafic and ultramafic rocks, but perovskite deposits are rare and studies of perovskite ore deposits are correspondingly scarce. Perovskite is a key rock-forming mineral and reaches exceptionally high concentrations in olivinites, diverse clinopyroxenites and silicocarbonatites in the Afrikanda alkaline-ultramafic complex (Kola Peninsula, NW Russia). Across these lithologies, we classify perovskite into three types (T1-T3) based on crystal morphology, inclusion abundance, composition, and zonation. Perovskite in olivinites and some clinopyroxenites is represented by fine-grained, equigranular, monomineralic clusters and networks (T1). In contrast, perovskite in other clinopyroxenites and some silicocarbonatites has fine- to coarse-grained interlocked (T2) and massive (T3) textures. Electron backscatter diffraction reveals that some T1 and T2 perovskite grains in the olivinites and clinopyroxenites are composed of multiple subgrains and may represent stages of crystal rotation, coalescence and amalgamation. We propose that in the olivinites and clinopyroxenites, these processes result in the transformation of clusters and networks of fine-grained perovskite crystals (T1) to mosaics of more coarse-grained (T2) and massive perovskite (T3). This interpretation suggests that sub-solidus processes can lead to the development of coarse-grained and massive perovskite. A combination of characteristic features identified in the Afrikanda perovskite (equigranular crystal mosaics, interlocked irregular-shaped grains, and massive zones) is observed in other oxide ore deposits, particularly in layered intrusions of chromitites and intrusion-hosted magnetite deposits and suggests that the same amalgamation processes may be responsible for some of the coarse-grained and massive textures observed in oxide deposits worldwide.
Geochimica et Cosmochimica Acta, Vol. 243, pp. 133-148.
Abstract: Chemical exchange between seawater and the oceanic crust is thought to play a significant role in the regulation of the global magnesium (Mg) cycle, yet relatively little is known about the rates and mechanisms of Mg exchange in these crustal environments. In this study we experimentally characterize the extent, and nature, of Mg isotope fractionation during the carbonation and serpentinization of olivine (one of the principal minerals found in ultramafic rocks) under hydrothermal conditions. Olivine alteration was found to be incongruent, with the reactant fluid composition varying according to the extent of olivine dissolution and the precipitation of secondary minerals. In mildly acid water (pH?~?6.5), olivine dissolved to form Mg-Fe carbonate solid solutions and minor chrysotile. Upon carbonation and a decrease of CO2 in the water, the pH increased to >8, with chrysotile and brucite becoming the dominant alteration minerals. The Mg-rich carbonates preferentially incorporated lighter Mg isotopes, resulting in a ~0.5‰ increase of the d26Mg composition of the fluid relative to olivine during the initial carbonation and serpentinization reactions. This was followed by a decrease in d26Mg under higher pH conditions associated with the formation of brucite. Our experimental and modeling results therefore demonstrate that the d26Mg composition of fluids involved in olivine alteration reflect the type and quantity of secondary Mg minerals formed, which in turn depend on the pH and CO2 concentration of the water. Comparison of these results with natural groundwaters and geothermal waters from basaltic terrains indicate that the d26Mg composition of natural waters are likely to also be controlled by mafic rock dissolution and the preferential incorporation of isotopically light Mg into carbonates and isotopically heavy Mg into Mg-Si minerals. Together, these findings improve our understanding of Mg isotope systematics during water-rock interaction, and suggest that d26Mg may be a useful tool for tracing reactions that are critical to geological CO2 sequestration.
Abstract: Yellow cuboid diamonds are commonly found in diamondiferous alluvial placers of the Northeastern Siberian platform. The internal structure of these diamonds have been studied by optical microscopy, X-Ray topography (XRT) and electron backscatter diffraction (EBSD) techniques. Most of these crystals have typical resorption features and do not preserve primary growth morphology. The resorption leads to an evolution from an originally cubic shape to a rounded tetrahexahedroid. Specific fibrous or columnar internal structure of yellow cuboid diamonds has been revealed. Most of them are strongly deformed. Misorientations of the crystal lattice, found in the samples, may be caused by strains from their fibrous growth or/and post-growth plastic deformation.
Abstract: Isotopic and trace element variations within single diamond crystals are widely known from both natural stones and synthetic crystals. A number of processes can produce variations in carbon isotope composition and nitrogen abundance in the course of diamond crystallization. Here, we present evidence of carbon and nitrogen fractionation related to the growing surfaces of a diamond. We document that difference in the carbon isotope composition between cubic and octahedral growth sectors is solvent-dependent and varies from 0.7‰ in a carbonate system to 0.4‰ in a metal-carbon system. Ab initio calculations suggest up to 4‰ instantaneous 13C depletion of cubic faces in comparison to octahedral faces when grown simultaneously. Cubic growth sectors always have lower nitrogen abundance in comparison to octahedral sectors within synthetic diamond crystals in both carbonate and metal-carbon systems. The stability of any particular growth faces of a diamond crystal depends upon the degree of carbon association in the solution. Octahedron is the dominant form in a high-associated solution while the cube is the dominant form in a low-associated solution. Fine-scale data from natural crystals potentially can provide information on the form of carbon, which was present in the growth media.
Abstract: Earth's continents drift in response to the force balance between mantle flow and plate tectonics and actively change the plate-mantle coupling. Thus, the patterns of continental drift provide relevant information on the coupled evolution of surface tectonics, mantle structure and dynamics. Here, we investigate rheological controls on such evolutions and use surface tectonic patterns to derive inferences on mantle viscosity structure on Earth. We employ global spherical models of mantle convection featuring self-consistently generated plate tectonics, which are used to compute time-evolving continental configurations for different mantle and lithosphere structures. Our results highlight the importance of the wavelength of mantle flow for continental configuration evolution. Too strong short-wavelength components complicate the aggregation of large continental clusters, while too stable very long wavelength flow tends to enforce compact supercontinent clustering without reasonable dispersal frequencies. Earth-like continental drift with episodic collisions and dispersals thus requires a viscosity structure that supports long-wavelength flow, but also allows for shorter-wavelength contributions. Such a criterion alone is a rather permissive constraint on internal structure, but it can be improved by considering continental-oceanic plate speed ratios and the toroidal-poloidal partitioning of plate motions. The best approximation of Earth's recent tectonic evolution is then achieved with an intermediate lithospheric yield stress and a viscosity structure in which oceanic plates are ~ 103 × more viscous than the characteristic upper mantle, which itself is ~ 100-200 × less viscous than the lowermost mantle. Such a structure causes continents to move on average ~ (2.2 ± 1.0) × slower than oceanic plates, consistent with estimates from present-day and from plate reconstructions. This does not require a low viscosity asthenosphere globally extending below continental roots. However, this plate speed ratio may undergo strong fluctuations on timescales of several 100 Myr that may be linked to periods of enhanced continental collisions and are not yet captured by current tectonic reconstructions.
Abstract: The original connections of Archean cratons are becoming traceable due to an increasing amount of paleomagnetic data and refined magmatic barcodes. The Uauá block of the northern São Francisco craton may represent a fragment of a major Archean craton. Here, we report new paleomagnetic data from the 2.62 Ga Uauá tholeiitic mafic dyke swarm of the Uauá block in the northern São Francisco craton, Eastern Brazil. Our paleomagnetic results confirm the earlier results for these units, but our interpretation differs. We suggest that the obtained characteristic remanent magnetization for the 2.62 Ga swarm is of primary origin, supported by a provisionally-positive baked contact test. The corresponding paleomagnetic pole (25.2°N, 330.5°E, A95 = 8.1° N = 20) takes the present northern part of the São Francisco craton to moderate latitudes. Based on the comparison of the paleolatitudes of cratons with high-quality paleomagnetic data and magmatic barcodes, we suggest that the northern part of the São Francisco craton could have been part of the proposed Supervaalbara supercraton during the Archean. Supervaalbara is proposed as including (but not limited to) the part of the São Francisco craton as well as the Superior, Wyoming, Kola + Karelia, Zimbabwe, Kaapvaal, Tanzania, Yilgarn, and Pilbara cratons.
Earth and Planetary Science Letters, Vol. 506, pp. 1-7.
Abstract: We present a theoretical investigation, based on ab initio calculations and the quasi-harmonic approximation, on the stability properties of magnesium (MgCO3) and calcium (CaCO3) carbonates at high temperatures and pressures. The results indicate that those carbonates should be stable in the Earth's lower mantle, instead of dissociating into other minerals, in chemical environments with excess of SiO2, MgO, or MgSiO3. Therefore, considering the lower mantle chemical composition, consisting mostly of the MgSiO3 and MgO minerals, calcium and magnesium carbonates are the primary candidates as carbon hosts in that region. For the thermodynamic conditions of the mantle, the results also indicate that carbon should be primarily hosted on MgCO3, contrasting with what was found by other theoretical studies, which neglected temperature effects. Finally, the results indicate that carbon, in the form of free CO2, is unlikely in the lower mantle.
Journal of Metamorphic Geology, doi.org/10.1111/jmg.12465
Abstract: Partial melting of continental crust and evolution of granitic magmas are inseparably linked to the availability of H2O. In the absence of a free aqueous fluid, melting takes place at relatively high temperatures by dehydration of hydrous minerals, whereas in its presence, melting temperatures are lowered, and melting need not involve hydrous minerals. With the exception of anatexis in water-saturated environments where anhydrous peritectic minerals are absent, there is no reliable indicator that clearly identifies the presence of a free aqueous fluid during anatexis. Production of Ab-rich magmas or changes in LILE ratios, such as an increase in Sr and decrease in Rb indicating increased involvement of plagioclase, are rough guidelines to the presence of aqueous fluids. Nevertheless, all of them have caveats and cannot be unequivocally applied, allowing for the persistence of a bias in the literature towards dehydration melting. Investigation of mineral equilibria modelling of three metasedimentary protoliths of the Kangaroo Island migmatites in South Australia, shows that the main indicator for the presence of small volumes of excess water under upper amphibolite to lower granulite facies conditions (660-750°C) is the melt volume produced. Melt composition, modal content or chemical composition of peritectic minerals such as cordierite, sillimanite or garnet are relatively insensitive to the presence of free water. However, the mobility of melt during open system behaviour makes it difficult to determine the melt volume produced. We therefore argue that the presence of small volumes of excess water might be much more common than so far inferred, with large impact on the buffering of crustal temperatures and fertility, and therefore rheology of the continental crust.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 276-7.
Abstract: Natural diamonds generally exhibit a very wide range of spectra. In polished stones, absorption along with proportions and size define perceived diamond color and thus beauty. In rough diamonds, the quantitative absorption spectrum (the “reference spectrum” in the context of this article) can be measured using an optical spectrometer through a set of parallel windows polished on a stone, so the diamond can be considered a planeparallel plate with known thickness. Polished diamonds lack the parallel facets that might allow plane-parallel plate measurement. That is why polished diamond colorimetry uses one of two approaches that have certain limitations for objective color estimation: 1) Qualitative spectrum assessment with an integrating sphere. Suppose three diamonds are polished from a yellow rough with even coloration: a round (with short ray paths), a cushion (with high color uniformity and long ray paths), and a “bow tie” marquise (with both long and short ray path areas). The spectra captured from these three stones by an integrating sphere will be completely different because the ray paths are very different. However, the quantitative absorption spectrum will be the same for all three stones, since they are cut from the same evenly colored rough. Therefore, spectrum assessment with an integrating sphere has very limited accuracy and is practical for qualitative estimations only. 2) Analysis of multiple images of a diamond made by color RGB camera. This method has low spectral resolution defined by digital camera color rendering. The camera has a smaller color gamut than the human eye, so most fancycolor diamonds are outside the color-capturing range of a digital camera. However, quantitative absorption data is very valuable for: 1) Color prediction and optimization for a new diamond after a recut process 2) Objective color assessment and description of a polished diamond This paper presents a new technology based on spectral lightemitting diodes (LEDs) and high-quality ray tracing, which together allow the reconstruction of a quantitative absorption spectrum for a polished diamond. The approach can be used for any transparent polished diamond. The recent technology prototype has a resolution of 20–60 nm, which is practical for color assessment. Figure 1 (top) presents three photorealistic diamond images: A is based on the reconstructed absorption spectrum collected from a polished diamond, B uses the reference spectrum collected in the rough stage through a pair of parallel windows, and C uses the averaged reference spectrum. Figure 1 (bottom) shows both measured quantitative absorption and reconstructed absorption spectra. This technology has the potential to ensure very close to objective color estimation for near-colorless and fancy-color polished diamonds. The reconstructed spectrum resolution can be enhanced to 10–15 nm in future devices.
Abstract: The ~1100?Ma CC2 and P13 lamproite dykes in the Wajrakarur Kimberlite Field (WKF), Eastern Dharwar Craton, and ~65?Ma Kodomali and Behradih lamproite diatremes in the Mainpur Kimberlite Field (MKF), Bastar Craton share a similar mineralogy, although the proportions of individual mineral phases vary significantly. The lamproites contain phenocrysts, macrocrysts and microcrysts of olivine set in a groundmass dominated by diopside and phlogopite with a subordinate amount of spinel, perovskite, apatite and serpentine along with rare barite. K-richterite occurs as inclusion in olivine phenocrysts in Kodomali, while it is a late groundmass phase in Behradih and CC2. Mineralogically, the studied intrusions are classified as olivine lamproites. Based on microtextures and compositions, three distinct populations of olivine are recognised. The first population comprises Mg-rich olivine macrocrysts (Fo89-93), which are interpreted to be xenocrysts derived from disaggregated mantle peridotites. The second population includes Fe-rich olivine macrocrysts (Fo82-89), which are suggested to be the product of metasomatism of mantle wall-rock by precursor lamproite melts. The third population comprises phenocrysts and overgrowth rims (Fo83-92), which are clearly of magmatic origin. The Mn and Al systematics of Mg-rich olivine xenocrysts indicate an origin from diverse mantle lithologies including garnet peridotite, garnet-spinel peridotite and spinel peridotite beneath the WKF, and mostly from garnet peridotite beneath the MKF. Modelling of temperatures calculated using the Al-in-olivine thermometer for olivine xenocrysts indicates a hotter palaeogeotherm of the SCLM beneath the WKF (between 41 and 43?mW/m2) at ~1100?Ma than beneath the MKF (between 38 and 41?mW/m2) at ~65?Ma. Further, a higher degree of metasomatism of the SCLM by precursor lamproite melts has occurred beneath the WKF compared to the MKF based on the extent of CaTi enrichment in Fe-rich olivine macrocrysts. For different lamproite intrusions within a given volcanic field, lower Fo olivine overgrowth rims are correlated with higher phlogopite plus oxide mineral abundances. A comparison of olivine overgrowth rims from the two fields shows that WKF olivines with lower Fo content than MKF olivines are associated with increased XMg in spinel and phlogopite and vice versa. Melt modelling indicates relatively Fe-rich parental melt for WKF intrusions compared to MKF intrusions. The Ni/Mg and Mn/Fe systematics of magmatic olivines indicate derivation of the lamproite melts from mantle source rocks with a higher proportion of phlogopite and/or lower proportion of orthopyroxene for the WKF on the Eastern Dharwar Craton compared to those for the MKF on the Bastar Craton. This study highlights how olivine cores provide important insights into the composition and thermal state of cratonic mantle lithosphere as sampled by lamproites, including clues to elusive precursor metasomatic events. Variable compositions of olivine rims testify to the complex interplay of parental magma composition and localised crystallisation conditions including oxygen fugacity variations, co-crystallisation of groundmass minerals, and assimilation of entrained material.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 272-3.
Africa, Sierra Leone
Abstract: Diamond ages are obtained from radiogenic isotopic analysis (Rb-Sr, Sm-Nd, Re-Os, and Ar-Ar) of mineral inclusions (garnet, pyroxene, and sulfide). As diamonds are xenocrysts that cannot be dated directly, the ages obtained on mineral inclusions provide a unique set of interpretive challenges to assure accuracy and account for preexisting history. A primary source of geological/mineralogical uncertainty on diamond ages is any process affecting protogenetic mineral inclusions before encapsulation in the diamond, especially if it occurred long before diamond formation. In practical application, the isotopic systems discussed above also carry with them inherent systemic uncertainties. Isotopic equilibrium is the essential condition required for the generation of a statistically robust isochron. Thus, isochron ages from multiple diamonds will record a valid and accurate age when the diamond-forming fluid promotes a large degree of isotopic equilibrium across grain scales, even for preexisting (“protogenetic”) minerals. This clearly can and does occur. Furthermore, it can be analytically tested for, and has multiple analogues in the field of dating metamorphic rocks. In cases where an age might be suspect, an age will be valid if its regression uncertainties can encompass a known and plausible geological event (especially one for which an association exists between that event and the source of diamond-forming fluids) and petrogenetic links can be established between inclusions on the isochron. Diamonds can be dated in six basic ways: 1. model ages 2. radiogenic daughter Os ages (common-Os-free) 3. single-diamond mineral isochrons 4. core to rim ages 5. multiple single-diamond isochron/array ages 6. composite isochron/array ages Model ages (1) are produced by the intersection between the evolution line for the inclusion and a reference reservoir such as the mantle. The most accurate single-diamond age is determined on a diamond with multiple inclusions (3). In this case an internal isochron can be obtained that not only establishes equilibrium among the multiple grains but also unequivocally dates the time of diamond growth. With extreme luck in obtaining the right diamond, concentric diamond growth zones visible in UV fluorescence or cathodoluminescence can sometimes be shown to constrain inclusions to occur in the core of the diamond and in the exterior at the rim. These single grains can be extracted to give a minimum growth time (4) for the diamond. In optimal situations, multiple inclusions are present within single growth zones, in single diamonds, allowing internal isochrons to be constructed for individual growth zones in single diamonds. If enough diamonds with inclusions can be obtained for study, valid ages for diamond populations can be obtained on multiple single-diamond ages that agree (5) or on composited, mineralogically similar inclusions to give an average age (6).
Abstract: Apatite can host significant levels of trace elements, including REE, within its crystal lattice, making it particularly useful for deciphering geological events and processes. This study employs hyperspectral cathodoluminescence (CL) and in situ microchemical techniques to identify and characterize various generations of apatite occurring in the phoscorites, carbonatites, and fenites of the Gifford Creek Carbonatite Complex (GCCC), Western Australia. Hyperspectral CL revealed that apatite crystals in all samples have complex internal zoning, including multiple distinct generations, with zones of relatively bright CL generally having more complex spectra compared to darker CL zones. Most of the CL spectra have prominent sharp peaks at ~1.4 eV and ~2.l eV as well as a broad peak between 2.3 eV and 3.5 eV. We relate these different peaks to individual REE activators and groups of activators, in particular Nd3+, Eu3+, Sm3+, and Ce3+. Trace element analyses of apatite confirm the relative enrichment of REE in the CL brighter zones. Most apatite generations exhibit concave-down to sinusoidal REY patterns lacking Eu anomalies, but often feature distinct negative Y anomalies. The depletion in LREE is interpreted to be due to LREE sequestration into monazite, which is relatively abundant in most of the samples. Most apatite samples contain very low Si contents, but appreciable Na, so REE incorporation into apatite was primarily via a coupled substitution of REE + Na replacing 2Ca, which is consistent with the highly alkaline, low SiO2 environment under which the apatite formed. Based on the combined trace-element signatures and CL textures, we interpret the multiple generations of apatite to reflect magmatic growth from alkaline magmas followed by recrystallization during subsequent metamorphic/hydrothermal events. The notable exception is the apatite core domains from a fenite sample that contain relatively high Si and Mn contents, low Sr, and relatively HREE-enriched REY patterns with distinct negative Eu anomalies. This apatite is interpreted to be relict from the granitic precursor to fenitization. The apatite samples also show systematic compositional variations across the GCCC, with apatite from phoscorite samples from the southeast part of the complex containing higher Sr, lower Gd/Ce, and lower ?3 values (normalized REE pattern inflections) compared to apatite from the northwest part of the complex. Recognition of these spatial variations in apatite compositions from the intra-grain micro-scale through to the district scale demonstrates the utility of combining advanced petrographic methods, such as hyperspectral CL, with micro-chemical analysis to reveal complex geological records preserved in apatite. As apatite is a common accessory mineral, these techniques may be more broadly applicable to igneous source tracing, understanding metamorphic and/or metasomatic processes, provenance studies from detrital mineral records, and studies of the evolution of ore systems.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 307.
Africa, Sierra Leone
deposit - zimmi
Abstract: Type Ib diamonds from Zimmi, Sierra Leone, have 500 My mantle residency times whose origin is best explained by rapid tectonic exhumation after continental collision to shallower depths in the mantle prior to kimberlite eruption (Smit et al., 2016). Here we present spectroscopic data for a new suite of Zimmi sulfide-bearing type Ib diamonds that allow us to evaluate the link between their rare Fancy yellow colors, the distribution of their spectroscopic features, and their unusual geological history. Cathodoluminesence (CL) imaging revealed irregular patterns with abundant deformation lamellae, associated with the diamonds’ tectonic exhumation (Smit et al., 2018). Vacancies formed during deformation were subsequently naturally annealed to form vacancy clusters, NV0/- centers, and H3 (NVN0). The brownish yellow to greenish yellow colors observed in Zimmi type Ib diamonds result from visible absorption by a combination of isolated nitrogen and deformation-related vacancy clusters (Smit et al., 2018). Color-forming centers and other spectroscopic features can all be attributed to the unique geological history of Zimmi type Ib diamonds and their rapid exhumation after formation.
South African Journal of Geology, Vol. 121, pp. 227-236.
Africa, South Africa
Abstract: Rocky reaches of the southeast African coastline are characterized mainly by log-spiral and headland-bound bays. Extensive fieldwork was carried out to investigate both documented and new exposures of Cretaceous beds on the southern KwaZulu-Natal and upper Eastern Cape (Transkei) coasts. Our results suggest that geological inheritance plays an important role in the contemporary rocky coast geomorphology. We offer evidence that post-Gondwana break-up mass flow channels play an important role in the present southeast African coastline morphology. Mass flow channels contain fills of incompetent Cretaceous rocks which are being preferentially eroded by prevailing marine and fluvial processes to form headland-bound embayment landforms. This study has identified an important geomorphic process for the development of the current southeast African coastline.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 274.
Africa, South Africa, Angola
deposit - Cullinan, Lulo
Abstract: Many of the world’s largest and most valuable gem diamonds exhibit an unusual set of physical characteristics. For example, in addition to their conspicuously low nitrogen concentrations, diamonds such as the 3,106 ct Cullinan (type IIa) and the Hope (type IIb, boron bearing) tend to have very few or no inclusions, and in their rough state they are found as irregular shapes rather than as sharp octahedral crystals. It has long been suspected that type IIa and IIb diamonds form in a different way than most other diamonds. Over the past two years, systematic investigation of both type IIa and IIb diamonds at GIA has revealed that they sometimes contain rare inclusions from unique geological origins. Examination of more than 130 inclusion-bearing samples has established recurring sets of inclusions that clearly show many of these diamonds originate in the sublithospheric mantle, much deeper in the earth than more common diamonds from the cratonic lithosphere. We now recognize that type IIa diamonds, or more specifically, diamonds with characteristics akin to the historic Cullinan diamond (dubbed CLIPPIR diamonds), are distinguished by the occurrence of ironrich metallic inclusions. Less frequently, CLIPPIR diamonds also contain inclusions of majoritic garnet and former CaSiO3perovskite that constrain the depth of formation to within 360–750 km. The inclusions suggest that CLIPPIR diamonds belong to a unique paragenesis with an intimate link to metallic iron in the deep mantle (Smith et al., 2016, 2017). Similarly, findings from type IIb diamonds also place them in a “superdeep” sublithospheric mantle setting, with inclusions of former CaSiO3 perovskite and other high-pressure minerals, although the iron-rich metallic inclusions are generally absent (Smith et al., 2018). Altogether, these findings show that high-quality type II gem diamonds are predominantly sourced from the sublithospheric mantle, a surprising result that has refuted the notion that all superdeep diamonds are small and nongem quality. Valuable information about the composition and behavior of the deep mantle is cryptically recorded in these diamonds. CLIPPIR diamonds (figure 1) confirm that the deep mantle contains metallic iron, while type IIb diamonds suggest that boron and perhaps water can be carried from the earth’s surface down into the lower mantle by plate tectonic processes. In addition to being gemstones of great beauty, diamonds carry tremendous scientific value in their unique ability to convey information about the interior of our planet.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 271-2.
Abstract: Through research on inclusions in diamonds over the past 50 years, a detailed picture has emerged of the mineralogical and chemical composition of diamond substrates in Earth’s mantle and of the pressure-temperature conditions during diamond formation. The exact diamond-forming processes, however, are still a subject of debate. One approach to constrain diamond-forming processes is through model calculations that aim to obtain the speciation and the carbon content of carbon-hydrogen-oxygen (CHO) fluids at particular O/(O+H) ratios and pressure-temperature conditions (using GFluid of Zhang and Duan, 2010, or other thermodynamic models of fluids). The predictions of such model calculations can then be tested against carbon and nitrogen stable isotopes and nitrogen content fractionation models, based on in situ analyses across homogenously grown diamond growth layers. Based on this approach, Luth and Stachel (2014) proposed that diamond precipitation occurs predominantly from cooling or ascending CHO fluids, composed of water with minor amounts of CO2 and CH4 (which in response to decreasing temperature may react to form diamond: CO2+ CH4 ? 2C + 2H2O). The second approach focuses on constraining the diamondforming medium by studying submicrometer fluid inclusions in fibrous-clouded and, more recently, gem diamonds. Such studies established the presence of four compositional end members of inclusions: hydrous-saline, hydrous-silicic, high-Mg carbonatitic, and low-Mg carbonatitic (e.g., Navon et al., 1988; Weiss et al., 2009). Although these fluid inclusions only depict the state of the diamond-forming medium after formation, they nevertheless provide unique insights into the major and trace-element composition of such fluids that otherwise could not be obtained. The apparent dichotomy between the two approaches—models for pure CHO fluids and actual observation of impure fluids (socalled high-density fluids) in clouded and fibrous diamonds—relates to the observation that in high-pressure and high-temperature experiments close to the melting temperature of mantle rocks, hydrous fluids contain 10–50% dissolved solid components (e.g., Kessel et al., 2015). Although at this stage the impurity content in natural CHO fluids cannot be included in numerical models, the findings for clouded and fibrous diamonds are not in conflict with the isochemical diamond precipitation model. Specifically, the fact that observed high-density inclusions are often carbonate bearing is not in conflict with the relatively reducing redox conditions associated with the O/(O+H) ratios of modeled diamond-forming CHO fluids. The model for the minimum redox stability of carbonate - bearing melts of Stagno and Frost (2010) permits fluid carbonate contents of up to about 30% at such redox conditions. Although additional data need to be obtained to build a thermodynamic model for CHO fluids with dissolved silicates and to better characterize the major and trace-element composition of high-density CHO fluids in equilibrium with typical diamond substrates (the rock types peridotite and eclogite), we already see sufficient evidence to suggest that the two approaches described above are converging to a unified model of isochemical diamond precipitation from cooling or ascending high-density CHO fluids.
Abstract: The outermost layer of the solid Earth consists of relatively rigid plates whose horizontal motions are well described by the rules of plate tectonics. Yet, the thickness of these plates is poorly constrained, with different methods giving widely discrepant results. Here a recently developed procedure to derive lithospheric thickness from seismic tomography with a simple thermal model is discussed. Thickness is calibrated such that the average as a function of seafloor age matches the theoretical curve for half-space cooling. Using several recent tomography models, predicted thickness agrees quite well with what is expected from half-space cooling in many oceanic areas younger than ˜ 110 Myr. Thickness increases less strongly with age for older oceanic lithosphere, and is quite variable on continents, with thick lithosphere up to ˜ 250 km inferred for many cratons. Results are highly correlated for recent shear-wave tomography models. Also, comparison to previous approaches based on tomography shows that results remain mostly similar in pattern, although somewhat more variable in the mean value and amount of variation. Global correlations with and between lithosphere thicknesses inferred from receiver functions or heat flow are much lower. However, results inferred from tomography and elastic thickness are correlated highly, giving additional confidence in these patterns of thickness variations, and implying that tomographically inferred thickness may correlate with depth-integrated strength. Thermal scaling from seismic velocities to temperatures yields radial profiles that agree with half-space cooling over large parts of their depth range, in particular for averaged profiles for given lithosphere thickness ranges. However, strong deviations from half-space cooling profiles are found in thick continental lithosphere above depth ˜ 150 km, most likely due to compositional differences.
Earth and Planetary Science Letters, Vol. 490, 1, pp. 161-169.
Abstract: In this study, we have determined the phase boundary between Mg0.735Fe0.21Al0.07Si0.965O3-Bm and PPv and the thermal equations of state of both phases up to 202 GPa and 2600 K using synchrotron X-ray diffraction in laser heated diamond anvil cells. Our experimental results have shown that the combined effect of Fe and Al produces a wide two-phase coexistence region with a thickness of 26 GPa (410 km) at 2200 K, and addition of Fe lowers the onset transition pressure to 98 GPa at 2000 K, consistent with previous experimental results. Furthermore, addition of Fe was noted to reduce the density (?) and bulk sound velocity () contrasts across the Bm-PPv phase transition, which is in contrast to the effect of Al. Using the obtained phase diagram and thermal equations of state of Bm and PPv, we have also examined the effect of composition variations on the ? and
profiles of the lowermost mantle. Our modeling results have shown that the pyrolitic lowermost mantle should be highly heterogeneous in composition and temperature laterally to match the observed variations in the depth and seismic signatures of the D? discontinuity. Normal mantle in a pyrolitic composition with ~10% Fe and Al in Bm and PPv will lack clear seismic signature of the D? discontinuity because the broad phase boundary could smooth the velocity contrast between Bm and PPv. On the other hand, Fe-enriched regions close to the cold slabs may show a seismic signature with a change in the velocity slope of the D? discontinuity, consistent with recent seismic observations beneath the eastern Alaska. Only regions depleted in Fe and Al near the cold slabs would show a sharp change in velocity. Fe in such regions could be removed to the outer core by strong core-mantle interactions or partitions together with Al to the high-pressure phases in the subduction mid ocean ridge basalts. Our results thus have profound implication for the composition of the lowermost mantle.
Dyke Swarms of the World: a modern perspective Ed. Srivastava et al. Springer , Chapter pp. 111-154. available
South America, Guiana, Brazil
Abstract: We review geochronological data including U-Pb baddelyite ages of Proterozoic mafic dyke swarms and sills of the Amazonian Craton, as well as their geochemical character and geological settings, in order to arrive at an integrated tectonic interpretation. The information together with the characteristics of coeval volcanic-plutonic suites indicates a cyclicity of the mafic-felsic activity through time and space. At least four LIP/SLIP events are apparent, and each one appears to accompany the stepwise accretionary crustal growth of Amazonia. The oldest two, the Orocaima (1.98-1.96 Ga) and Uatumã (c. 1.89-1.87 Ga) SLIPs, comprise calc-alkaline I-type and subordinate A-type plutonic and volcanic rocks. Synchronous mafic intraplate activity occurs across the Guiana and Central-Brazil Shields. These two events may be caused by interaction between subduction-related processes and mantle plumes with synchronous lithosphere extension during the two time periods. The Avanavero (1.79 Ga) LIP event mostly consists of mafic dykes and sills which are intrusive into the Roraima platform cover, in the Guiana Shield. They show tholeiitic chemistry and similarities with E-MORB and subcontinental lithospheric mantle-derived basalts, whereas the REE pattern suggests affinity with intraplate settings. The age of the Avanavero rocks is identical to the Crepori Diabase, located ca. 1800 km away to the south (Central-Brazil Shield). The youngest LIP event (1.11 Ga), the Rincón del Tigre-Huanchaca, has the Rio Perdido Suite as a component in the Rio Apa Terrane, which is ca. 300 km away from the Rincón del Tigre Complex, located in the SW portion of the Amazonian Craton. Furthermore, the Central-Brazil and Guiana Shields boasts widespread intraplate mafic activity, highlighted by the Mata-Matá (1.57 Ga), Salto do Céu (1.44 Ga) and Nova Floresta (1.22 Ga) mafic sills and the Cachoeira Seca Troctolite (1.19 Ga). Contemporaneous A-type, rapakivi granites with roughly similar ages also occur elsewhere. These particular episodes are extension specific steps of the Mesoproterozoic Amazonia, and the quite large distribution is consistent with LIP events. In a broader perspective, the intermittent Proterozoic intracratonic activity has a barcode that matches LIP/SLIP events in Columbia and Rodinia.
Abstract: Archean tectonics was capable of producing virtually indestructible cratonic mantle lithosphere, but the dominant mechanism of this process remains a topic of considerable discussion. Recent geophysical and petrological studies have refuelled the debate by suggesting that thickening and associated vertical movement of the cratonic mantle lithosphere after its formation are essential ingredients of the cratonization process. Here we present a geodynamical study that focuses on how the thick stable cratonic lithospheric roots can be made in a thermally evolving mantle. Our numerical experiments explore the viability of a cratonization process in which depleted mantle lithosphere grows via lateral compression into a > 200-km thick, stable cratonic root and on what timescales this may happen. Successful scenarios for craton formation, within the bounds of our models, are found to be composed of two stages: an initial phase of tectonic shortening and a later phase of gravitational self-thickening. The initial tectonic shortening of previously depleted mantle material is essential to initiate the cratonization process, while the subsequent gravitational self-thickening contributes to a second thickening phase that is comparable in magnitude to the initial tectonic phase. Our results show that a combination of intrinsic compositional buoyancy of the cratonic root, rapid cooling of the root after shortening, and the long-term secular cooling of the mantle prevents a Rayleigh-Taylor type collapse, and will stabilize the thick cratonic root for future preservation. This two-stage thickening model provides a geodynamically viable cratonization scenario that is consistent with petrological and geophysical constraints.
Earth and Planetary Science Letters, Vol. 490, 1, pp. 88-99.
Abstract: Advances in whole waveform seismic tomography have revealed the presence of broad mantle plumes rooted at the base of the Earth's mantle beneath major hotspots. Hotspot tracks associated with these deep mantle plumes provide ideal constraints for inverting absolute plate motions as well as testing the fixed hotspot hypothesis. In this paper, 27 observed hotspot trends associated with 24 deep mantle plumes are used together with the MORVEL model for relative plate motions to determine an absolute plate motion model, in terms of a maximum likelihood optimization for angular data fitting, combined with an outlier data detection procedure based on statistical tests. The obtained T25M model fits 25 observed trends of globally distributed hotspot tracks to the statistically required level, while the other two hotspot trend data (Comores on Somalia and Iceland on Eurasia) are identified as outliers, which are significantly incompatible with other data. For most hotspots with rate data available, T25M predicts plate velocities significantly lower than the observed rates of hotspot volcanic migration, which cannot be fully explained by biased errors in observed rate data. Instead, the apparent hotspot motions derived by subtracting the observed hotspot migration velocities from the T25M plate velocities exhibit a combined pattern of being opposite to plate velocities and moving towards mid-ocean ridges. The newly estimated net rotation of the lithosphere is statistically compatible with three recent estimates, but differs significantly from 30 of 33 prior estimates.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, Fall 1p. Abstract p. 262-3
Africa, Sierra Leone
deposit - Zimmi
Abstract: Isolated nitrogen is one of the major defects in producing yellow color in natural diamonds. In regular type Ib yellow diamonds, isolated nitrogen is normally the dominant form, with limited aggregations in A centers (nitrogen pairs). Type Ib diamonds normally experienced strong plastic deformations. In addition to vacancy clusters, many other optic centers were introduced during annealing over their long geological history, such as GR1, NV, and H3 centers. Diamonds from the Zimmi area of West Africa are a typical example (Smit et al., 2016). As a result, clear brownish and greenish hues are common among these diamonds, so most do not possess true “canary” yellow color. Here we studied more than 2,000 diamonds with real canary yellow color. Their color origin and relationship with type Ib diamonds were explored. Sizes of the studied diamonds ranged from 0.01 to about 1.0 ct. They showed pure yellow color, with grades of Fancy Intense or Fancy Vivid yellow. Infrared absorption analysis showed that they were all type IaA with very high nitrogen concentrations, but a very weak absorption from isolated nitrogen at 1344 cm–1 was detected in all samples. Concentration of isolated nitrogen was estimated at ~2–3 ppm. This isolated nitrogen created smooth absorption in the ultraviolet-visible (UV-Vis) region, increasing gradually to the high-energy side. No other defects were detected using UV-Vis absorption spectroscopy, which explained the pure yellow color we observed. Fluorescence imaging revealed multiple nucleation centers with dominant green color, which was attributed to the S3 defects confirmed through photoluminescence analysis. Compared with natural type Ib diamonds, an outstanding feature of the studied samples is the absence of plastic deformation. For this reason, other vacancy-related defects were not introduced to these diamond lattices over the geological period after their formation. Sulfide inclusions are common in type Ib diamonds, but they were not observed in these canary stones. Instead, some calcite inclusions were observed. All the observations from this study indicated that the canary diamond samples were formed in a different geological environment than type Ib diamonds.
Abstract: Cratons are strong and their preservation demonstrates that they resist deformation and fragmentation. Yet several cratons are rifting now, or have rifted in the past. We suggest that cratons need to be weakened before they can rift. Specifically, metasomatism of the depleted dehydrated craton mantle lithosphere is a potential weakening mechanism. We use 2D numerical models to test the efficiency of simulated melt metasomatism and coeval rehydration to weaken craton mantle lithosphere roots. These processes effectively increase root density through a parameterized melt-peridotite reaction, and reduce root viscosity by increasing the temperature and rehydrating the cratonic mantle lithosphere. The models are designed to investigate when a craton is sufficiently weakened to undergo rifting and is no longer protected by adjacent standard Phanerozoic lithosphere. We find that cratons only become vulnerable to rifting following large-volume melt metasomatism (~ 30% by volume) and thinning of the gravitationally unstable cratonic lithosphere from > 250 km to ~ 100 km; at which point its residual crustal strength is important. Furthermore, our results indicate that rifting of cratons depends on the timing of extension with respect to metasomatism. An important effect in the large-volume melt models is the melt-induced increase in temperature which must have time to reach peak values in the uppermost mantle lithosphere before rifting. Release of water stored in the transition zone at the base of a big mantle wedge may provide a suitable natural setting for both rehydration and refertilization of an overlying craton and is consistent with evidence from the eastern North China Craton. An additional effect is that cratons subside isostatically to balance the increasing density of craton mantle lithosphere where it is moderately metasomatized. We suggest that this forms intracratonic basins and that their subsidence and subsequent uplift, and cratonic rifting constitute evidence of progressive metasomatism of cratonic mantle lithosphere.
Abstract: Besides the intrinsic rheological layering of the lithosphere and its thermal structure, inherited heterogeneities may play an important role in strain localization during continental extension. This is similar to the role that defects play in the failure and necking of other materials. Here, we consider both inherited small-scale weak zones and the effects of lateral juxtaposition of two lithospheres with differing properties as mechanisms to localize deformation and initiate necking instabilities. Using 2D finite-element models that contain lateral lithospheric boundaries, in combination with smaller scale heterogeneities, we illustrate that two controls determine how necking instabilities grow and thus lead to varying styles of rifting: Control 1, the stiff/pliable nature of the lithosphere and Control 2, the background strain rate in the lithosphere. Control 1 depends on the lithospheric rheology, such that necking instabilities grow faster in materials with high power-law creeping flow exponents (stiff, brittle lithosphere) than in those with low power-law creeping flow exponents (pliable, viscous lithosphere). Control 2 prevails in lithosphere where background strain rates are highest. This happens because necking amplifies the background strain rate in power-law materials, leading to faster necking where strain rates are highest. The model results show that Control 2 determines the location of localization, unless the background strain rate is equal or near equal in both lithospheres, in which case Control 1 wins. These results explain why rifting does not localize in cratons even though they contain heterogeneities. The results also provide a mechanism for the formation of asymmetric rifted margins.
Abstract: The late Mesoproterozoic Ngualla carbonatite complex in southwest Tanzania comprises a central magnesiocarbonatite plug surrounded sequentially by an annular calcite carbonatite intrusion and fenitised felsic igneous country rocks. The calcite carbonatite contains phlogopite-rich (glimmerite) enclaves interpreted as fenitised wallrock xenoliths that have contributed silicate minerals, apatite and magnetite through dispersal and interaction, mainly within the calcite carbonatite magma. Ultramafic magmas were emplaced into the magnesiocarbonatite magma chamber before complete solidification of the magnesiocarbonatite. Contemporaneity allowed the two magmas to mingle. Rounded enclaves of hematite-barite in the magnesiocarbonatite are tentatively attributed to magma immiscibility. Following complete solidification of the calcite carbonatite, and overlapping late crystallization of the magnesiocarbonatite plug, late magnesiocarbonatite dikes and ultramafic dikes were emplaced, some of the latter as diatremes. Crystallization of ferroan dolomite in the magnesiocarbonatite plug resulted in residual magmatic concentration of Si, Ba, F and rare earth elements (REE), and crystallization of barite, quartz, calcite, fluorite and REE fluorocarbonates in miarolitic cavities. Concentrations of (total) rare earth oxides (TREO) in the unweathered magnesiocarbonatite are 1 to 2%. REE ore with 3 to 6% TREO resulted from weathering, during which CaCO3 and MgCO3 were leached from ferroan dolomite leaving a porous goethite-rich residue containing barite and bastnaesite, the latter having replaced primary synchesite. Other commodities with potential economic significance include phosphate and niobium, both of which were enriched by residual accumulation over the calcite carbonatite as a result of karstic weathering. Although weathering was a critical factor in the formation of REE ore at Ngualla, the primary proto-ore resulted mainly from in situ igneous processes. This genetic model is different from that used to account for many carbonatite-hosted ore bodies, which result from late-stage hydrothermal processes. Examples of hydrothermal rare earth deposits include those of the late Jurassic to early Cretaceous Chilwa Province, located 800?km south of Ngualla. The differences in ore-forming processes may reflect the relative ages of the carbonatites and a deeper level of erosion at Ngualla.
Journal of Geophysical Research: Planets, doei:10.1029/ 2018JE005698
Abstract: People have long had curiosity in the origin of Earth's water (equivalently hydrogen). Solar nebula has been given the least attention among existing theories, although it was the predominating reservoir of hydrogen in our early solar system. Here we present a first model for Earth's water origin that quantifies contribution from the solar nebula in addition to that from chondrites, the primary building blocks of Earth. The model considers dissolution of nebular hydrogen into the early Earth's magma oceans and reaction between hydrogen and iron droplets within the magma ocean. Such processes not only delivered countless hydrogen atoms from the mantle to the core but also generated an appreciable difference in hydrogen isotopic composition (2H/1H ratio) between the mantle and core. Fitting the model to current knowledge about Earth's hydrogen produces best combinations of nebular and chondritic contributions to Earth's water. We find that nearly one out of every 100 water molecules on Earth came from the solar nebula. Our planet hides majority of its water inside, with roughly two oceans in the mantle and four to five oceans in the core. These results suggest inevitable formation of water on sufficiently large rocky planets in extrasolar systems.
Abstract: Mantle-derived carbonatites provide a unique window in the understanding of mantle characteristics and dynamics, as well as insight into the assembly and breakup of supercontinents. As a petrological indicator of extensional tectonic regimes, Precambrian carbonatites provide important constraints on the timing of the breakup of ancient supercontinents. The majority of the carbonatites reported worldwide are Phanerozoic, in part because of the difficulty in recognizing Precambrian carbonatites, which are characterized by strong foliation and recrystallization, and share broad petrologic similarities with metamorphosed sedimentary lithologies. Here we report the recognition of a ~1.85?Ga carbonatite in Chaihulanzi area of Chifeng in north China based on systematic geological, petrological, geochemical, and baddeleyite U-Pb geochronological results. The carbonatite occurs as dikes or sills emplaced in Archean metasedimentary rocks and underwent intense deformation. Petrological and SEM/EDS results show that calcite and dolomite are the dominant carbonate minerals along with minor and varied amounts of Mg-rich mafic minerals, including forsterite (with Fo?>?98), phlogopite, diopside, and an accessory amount of apatite, baddeleyite, spinel, monazite, and ilmenite. The relatively high silica content together with the non-arc and OIB-like trace element signatures of the carbonatite indicates a hot mantle plume as the likely magma source. The depleted Nd isotopic signatures suggest that plume upwelling might be triggered by the accumulation of recycled crust in the deep mantle. As a part of the global-scale Columbia supercontinent, the Proteozoic tectonic evolution of the North China Craton (NCC) provides important insights into the geodynamics governing amalgamation and fragmentation of the supercontinent. The Paleo-Mesoproterozoic boundary is the key point of tectonic transition from compressional to extensional settings in the NCC. The newly-identified ~1.85?Ga carbonatite provides a direct link between the long-lasting supercontinental breakup and plume activity, which might be sourced from the “slab graveyard”, continental crustal slabs subducted into asthenosphere, beneath the supercontinent. The carbonatite provides a precise constraint of the initiation of the continental breakup at ~1.85?Ga.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 304-5.
Abstract: In diamond grown by the CVD method, nitrogen behaves differently than it does in natural and HPHT-grown diamond. The most striking peculiarities are low efficiency of doping, formation of unique optical centers over a wide spectral range from the ultraviolet (UV) to the IR regions, and formation of unusual defects related to aggregated nitrogen. In order to gain a better insight into this problem, several nitrogen-doped specimens grown in GIA’s CVD diamond lab and a few commercial yellow CVD-grown diamonds have been studied in their as-grown (asreceived) state and after electron irradiation and annealing at temperatures up to 1900°C (low-pressure, high-temperature treatment). We found that the brightest pink color of electron-irradiated nitrogen-doped CVD-grown diamond is produced by the NV– center after annealing at temperatures of about 1000°C. Annealing at temperatures over 1600°C destroys the irradiation-induced pink color (figure 1). The most prominent optical centers in the IR spectral region (figure 2, left) produced absorptions at 2828, 2874, 2906, 2949, 3031, 3107, 3123, and 3310 cm–1 (latter two not shown). These are ascribed to nitrogen-hydrogen complexes. Two characteristic absorption features at 1293 and 1341 cm–1 (figure 2, right) are unique to CVD diamond. They are tentatively ascribed to a modified form of nitrogen A-aggregates. In the visible and NIR spectral ranges, characteristic nitrogenrelated centers have zero-phonon lines (ZPLs) at 457, 462, 489, 498, 647, 722.5, 852.5, 865.5, 868.5, 908, 921.5, and 924.5 nm. The 489 nm feature is a major color center of electron-irradiated, nitrogen-doped CVD-grown diamond. This center, together with the GR1 center, is responsible for the green color in this material. An assumption is made that N atoms may form clusters in highly nitrogen-doped CVD-grown diamonds. These clusters may result in broad-band luminescence at wavelengths of 360, 390, 535, and 720 nm and a strong broadening of the ZPLs of many optical centers
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 266.
Abstract: Photoluminescence (PL) and phosphorescence underpin many of the discrimination techniques used to separate natural from synthetic diamond. PL is at the heart of many new quantum technologies based on color centers in lab-grown diamonds. In HPHT synthetic diamond, the phosphorescence observed is explained in terms of donor-acceptor pair recombination. The thermal activation of electrons to neutral boron acceptors shows that boron plays a key role in the phosphorescence process. However, there are a number of things we struggle to explain. For example, the phosphorescence peak positions are not fully explained, and there is no conclusive link between the emission and charge transfer involving the substitutional nitrogen donor. Secondly, the origin of the phosphorescence observed in some synthetic diamond samples grown by the CVD process is unclear. Although we now have evidence for unintentional boron impurity incorporation at stop-start growth boundaries in some CVD syn- thetic samples, it is possible that some of the observed phosphorescence does not involve boron impurities. In this paper we report on the results of combined fluorescence, phosphorescence, thermoluminescence, and quantitative charge transfer investigations undertaken on both HPHT and CVD synthetic diamond, with the objective of identifying which defects are involved in the fluorescence and phosphorescence processes.
Doklady earth Sciences, Vol. 483, 1, pp. 1473-1474.
Abstract: The experimental data on diamond growth in the Fe-Ni-S-C and Fe-S-C systems with a sulfur content of 5-14 wt % at 5.5 GPa and 1300-1350°C are reported. Colorless and light yellow diamond crystals with a weight of 0.1-0.8 ct were synthesized. It is shown in the Fe-S-C system that at 5.5. GPa diamond may crystallize in a very narrow temperature range, from 1300 to 1370°C. Based on comparative analysis of the experimental data and the results of the study of native iron inclusions in natural diamonds from kimberlite pipes, it is suggested that diamond genesis may be partly controlled by the pre-eutectic (by the concentration of sulfur in relation to metal) metal-sulfide melt.