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The Sheahan Diamond Literature Reference Compilation - Scientific and Media Articles based on Major Keyword - Garnet
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 announcements called 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 Keyword Index
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
Each article reference in the SDLRC is tagged with one or more key words assigned by Pat Sheahan to highlight the main topics of the article. In an effort to make it easier for users to track down articles related to a specific topic, KRO has extracted these key words and developed a list of major key words presented in this Key Word Index to which individual key words used in the article reference have been assigned. In most of the individual Key Word Reports the references are in crhonological order, though in some such as Deposits the order is first by key word and then chronological. Only articles classified as "technical" (mainly scientific journal articles) and "media" (independent media articles) are included in the Key Word Index. References that were added in the most recent monthly update are highlighted in yellow.
A Garnet is a semi-precious silicate mineral used for jewelry and as an abrasive. Almandine garnets are the common variety formed within metamorphic rocks such as schist, but the garnets relevant to diamonds are the pryope garnets which form in ultramafic rocks under high pressure conditions within the mantle such as peridotite (harzburgite and lherzolite) and eclogite. Statistical studies have demonstrated that certain relative composition ranges of chromium and calcium within pyrope garnets correlate with temperature-pressure conditions that foster diamond formation. This has allowed pyrope garnets derived from kimberlite pipes to serve as diamond indicator minerals.
Stability Field of Knorringite Mg3 Chromium 2 Si3 012 at High Pressure and its implication to the Occurrence of Chromium Rich Pyrope in the Upper Mantle.
Physics of The Earth And Plan. Interiors, Vol. 27, PP. 263-272.
Stability Field of Knorringite Mg3 Chromium 2 Si3 012 at High Pressure and its implication to the Occurrence of Chromium Rich Pyrope in the Upper Mantle.
Physics of The Earth And Plan. Interiors, Vol. 27, PP. 263-272.
Alfred Wegener Conference.... Geophysical, Geochemical and Petrological Evidence on Deformation and Composition of the Continental Subcrustal Lithosphere.
A Comparative Study of the Composition and Properties of Garnets from The alkali Basalt Pipes of the Minusinsk Basin And kimberlites of Yakutia.(russian)
Mineral. Zhurn., (Russian), Vol. 7, No. 4, pp. 18-29
A note on the appearance of inclusions in garnets supposedly from India and problems relating to disclosure of origin: using this as an elementary example.
The South African Gemologist, Vol. 6, No. 2, June, pp. 11-13.
Kimberlite xenocrysts from the Porcupine Dome diatremes east-centralMontana: evidence for multiple sources of garnet and evaluation of diamondpotential
Geological Society of America Annual Abstract Volume, Vol. 25, No. 6, p. A98 abstract only
Metasomatism of the southern African lithosphere as recorded by mantlegarnets.
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 127-132.
Can the garnet signature in Mid Ocean Ridge Basalt (MORB) be derived from garnet pyroxenites in Mid Ocean Ridge Basalt (Mid Ocean Ridge Basalt (MORB))source regions?
Geological Society of America (GSA) Abstract Volume, Vol. 26, No. 7, ABSTRACT only p. A38.
An AEM study of garnet clinopyroxenite from the Sulu ultrahigh pressure terrane: formation mechanisms of oriented ilmenite, spinel, magnetite, amphibole and
Contributions to Mineralogy and Petrology, in press available, 14p.
Topotaxic relationships between spinel and pyroxene in kelphite after garnet in mantle derived peridotites and their implications to reaction mechanism and kinetics.
Mineralogy and Petrology, Vol. 101, 3-4, pp. 217-224.
Mineralogy of heavy minerals concentrates from the unconsolidated deposits of Eeldoi and Pello Hill volcanic cones (Gelai volcano, northern Tanzania) prel.
Peralk-Carb 2011... workshop June 16-18, Tubingen, Germany, Abstract p.111-112.
Mineralogy of heavy minerals concentrates from the unconsolidated deposits of Eeldoi and Pello Hill volcanic cones (Gelai volcano, northern Tanzania) prel.
Peralk-Carb 2011... workshop June 16-18, Tubingen, Germany, Abstract p.111-112.
Dymshits, A., Litasov, K., Sharygin, I., Shatskiy, A., Ohtani, E.
Mineral physics of high pressure garnets.
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept. 22-26, 2p. Abstract
Schulze, D.J., Davis, D.W., Helmstaedt, H., Joy, B.
Timing of the Cenozoic " Great Hydration" event beneath the Colorado Plateau: Th-Pb dating of monazite in Navajo volcanic field metamorphic eclogite xenoliths.
Geology, Vol. 43, pp. 727-730.
United States, Colorado Plateau
Diatremes - Moses Rock, Mule's Ear, Garnet Ridge, Cane Valley, Red Mesa, Buell Park, Green Knobs
Abstract: The article focuses on the study of composition of garnets of the lherzolitic and harzburgitic parageneses and the conditions of peridotite. As per the study, reconstruction of the conditions of metasomatism of peridotitic sources of kimberlite is possible in the evolution of garnet. It mentions the importance of dry and hydrous carbonatitic melt upon alteration of peridotitic sources of kimberlite as it acted as an another heat source.
43rd Annual Yellowknife Geoscience Forum Abstracts, abstract p. 47.
Canada, Northwest Territories
Garnet chemistry
Abstract: In diamond exploration, the use of compositional data to identify diamond-related peridotitic xenocrysts has long been a widely used and powerful tool. In contrast, the application of similar methods to eclogitic garnet chemistry remains a challenge. The inability to unequivocally classify certain “eclogitic” garnet compositions as either mantle- or crust-derived implies that a high abundance of lower-crustal garnets will increase diamond-exploration expenditures by introducing a number of “false positives.” Revising existing classification schemes (e.g., Schulze, 2003) to reduce the abundance of “false positives” may, however, increase the number of “false negatives” through the misclassification of mantle-derived garnets as crustal. This study presents new geochemical and petrographical data for garnet and clinopyroxene from 724 kimberlite-hosted, crust- and mantle-derived xenoliths from localities worldwide, with a focus on samples whose lithology is constrained petrographically, rather than single mineral grains from concentrate. Mantle samples are primarily eclogitic and pyroxenitic, as constrained by mineral assemblage and garnet and clinopyroxene mineral chemistry, while crustal samples are dominantly plagioclase-bearing garnet-granulites. For those localities where an established geothermal gradient is available from literature resources, garnet-clinopyroxene pairs are employed in the estimation of pressure-temperature conditions of equilibration through the iterative coupling of the Krogh (1988) geothermometer and the relevant geothermal gradient. Our preliminary results suggest that closure temperatures for Fe-Mg exchange exceed the temperatures of residence of many lower-crustal samples, as geotherm-based calculated pressures of equilibration exceed the apparent stability of plagioclase (see Green and Ringwood, 1972). Comparison of equilibration pressures with sodium contents in garnet for mantle-derived samples (the diamond-facies criterion of Gurney, 1984) shows a positive correlation at localities for which an adequate range of pressures is observed (e.g., the Diavik mine). Other populations, such as mantle eclogitic garnets from Roberts Victor, plot at a much more restricted range of pressures and hence fail to demonstrate this correlation; instead, these samples may reflect the influence of a broader range of bulk-compositions, providing varying amounts of sodium to their constituent garnets. The results presented here demonstrate clearly that garnets from mantle- and crust-derived samples show significant overlap in geochemical character, for example in garnet Ca# vs. Mg# space (discrimination diagram of Schulze, 2003), where approximately 66% of our crust-derived garnet analyses plot in the “mantle” field. This percentage varies among locations. A selection of particularly high-Mg#, low-Ca# garnets derived from crustal, plagioclase-bearing lithologies in this study highlights the potential for crust-mantle confusion, as these garnets have Mg# in-excess of many mantle-derived eclogitic/pyroxenitic garnets. As a consequence, Fe-Mg-Ca-based classifications alone cannot reliably discriminate mantle and crustal garnets. The next step in this project will be to obtain trace element data for the entire sample suite. This will allow us to test the Li-geobarometer of Hanrahan et al. (2009) for eclogites and to search for trace element signatures that can be used as robust indicators of a diamond-facies origin of eclogitic garnets. Trace element data will also be employed in the refinement of the crust/mantle division discussed above.
Abstract: A type of pink pyrope garnet containing vanadium and chromium, believed to have been mined in Tanzania, appeared at the 2015 Tucson shows. The material shows a noticeable color difference from purplish pink under incandescent light (A) to purple under daylight-equivalent light (D65). This study reports a quantitative analysis of the difference in color between the two lighting conditions, based on the use of high-quality visible absorption spectroscopy to calculate CIELAB 1976 colorimetric coordinates. L*, a*, and b* colorimetric parameters were calculated for a wide range of path lengths as extrapolated from visible absorption spectra of thinner samples. Using this method, the path length of light through the stone that produces the optimal color difference can be calculated. This path length can then be used to determine the optimal depth range to maximize color change for a round brilliant of a specific material. The pink pyrope studied here can be designated as "color-change" garnet according to certain classification schemes proposed by other researchers. In many of these schemes, however, the material fails to exceed the minimum requirements for quantitative color difference and hue angle difference to be described as "color-change." Nonetheless, there is no simple solution to the problem of applying color coordinates to classify color-change phenomena. Also presented is a method by which spectra can be corrected for reflection loss and accurately extrapolated to stones with various path lengths.
Abstract: A type of pink pyrope garnet containing vanadium and chromium, believed to have been mined in Tanzania, appeared at the 2015 Tucson shows. The material shows a noticeable color difference from purplish pink under incandescent light (A) to purple under daylight-equivalent light (D65). This study reports a quantitative analysis of the difference in color between the two lighting conditions, based on the use of high-quality visible absorption spectroscopy to calculate CIELAB 1976 colorimetric coordinates. L*, a*, and b* colorimetric parameters were calculated for a wide range of path lengths as extrapolated from visible absorption spectra of thinner samples. Using this method, the path length of light through the stone that produces the optimal color difference can be calculated. This path length can then be used to determine the optimal depth range to maximize color change for a round brilliant of a specific material. The pink pyrope studied here can be designated as "color-change" garnet according to certain classification schemes proposed by other researchers. In many of these schemes, however, the material fails to exceed the minimum requirements for quantitative color difference and hue angle difference to be described as "color-change." Nonetheless, there is no simple solution to the problem of applying color coordinates to classify color-change phenomena. Also presented is a method by which spectra can be corrected for reflection loss and accurately extrapolated to stones with various path lengths.
Abstract: This article reviews the development of mineralogy as a science by focusing largely on the common silicate garnets of general formula {X3}[Y2](Si3)O12. It tells of important discoveries, analyses, and proposals by various scientists relating to crystallography, crystal structures, isomorphism, and solid solution starting in Europe in the late 1700s. The critical recognition of the importance of ionic size of atoms in determining crystal-chemical properties and solid-solution behavior is emphasized. The two garnet species “pyralspite” and “(u)grandite,” which were considered to represent two independent solid-solution series, were introduced by N.H. Winchell and A.N. Winchell (1927) in their well-known book Elements of Optical Mineralogy. Critical comments on the assumptions behind the classification scheme have been pointed out for at least 50 yr, but it remains in use. There is more, though, behind this garnet classification scheme than just simple terminology. There are a long series of scientific discoveries and advances that are largely forgotten by the broader mineralogical community. They begin, here, with the work of the “father of crystallography,” René-Just Haüy, concerning the microscopic nature of crystals around 1780 and include later discoveries and proposals by Mitscherlich, Beudant, Wollaston, and Kopp relating to isomorphism and solid-solution behavior all before 1850. A second key era started with the discovery of X-ray diffraction in 1912 that allowed the atomic structures of crystals and, furthermore, atomic and ion radii to be determined. In terms of isomorphism and solid solution, the proposals and studies of Vegard, Zambonini, Wherry, A.N. Winchell, and the “father of crystal chemistry” Goldschmidt are briefly discussed. The recognition of the sizes of atoms and ions, along with an understanding of chemical bonding behavior in crystals, was critical in the establishment of what can be termed “modern mineralogy,” a quantitative science as it is largely understood today that emerged by the mid-1930s. The silicate garnet system pyrope-almandine-spessartine-grossular-andradite-uvarovite shows extensive homovalent substitutional solid solution over two structural sites and complete compositional variation between “pyralspite species” and “ugrandite species” has been documented. Thus, the prerequisites behind the terms “pyralspite” and “(u)grandite,” as originally formulated and often accepted even today, are incorrect and use of this classification is not recommended. Diffraction determinations of the volumes of garnet end-members and volumes of mixing of garnet solid solutions give physical insight into solid-solution behavior. Today, investigations of local structural and crystal-chemical properties, together with determinations of lattice strain and thermodynamic mixing properties, of silicate solid solutions are leading to an ever more quantitative understanding of mineral behavior from the microscopic to macroscopic level.
Abstract: Abundant multiphase solid inclusions (MSI) were found in garnet in an ultrahigh-pressure (UHP) paragneiss from the Kokchetav complex, Kazakhstan. The MSI are composed of mineral associations that include rock-forming and accessory minerals, which crystallized during exhumation. We present experimental and analytical protocols for how such inclusions can be homogenized to glass and analysed for major and trace elements. After homogenization we identified two types of glass. One type is present in garnet porphyroblasts in the melanocratic part of the sample and represents a high-pressure melt formed close to peak conditions of >45 kbar, 1000°C. These inclusions are characterized by high concentrations of light rare earth elements (LREE), Th and U. Extraction of these melts resulted in a pronounced depletion of the Kokchetav gneisses in those elements. Measured partition coefficients of large ion lithophile elements (LILE) between phengite inclusions and melt inclusions are DRb?=?1•9-2•5, DBa?=?1•1-6•9 and DCs?=?0•6-0•8, resulting in limited depletion of these elements during partial melting in the presence of phengite. The Nb concentration in melts (27?ppm) is about double that in the restite (15?ppm), indicating slightly incompatible behaviour during UHP anatexis, despite the presence of residual accessory rutile and phengite. A second type of inclusion occurs in garnet from the leucocratic part of the rock and represents a late-stage melt formed during exhumation at 650-750°C and crustal pressures. These inclusions are characterized by low LREE and Nb and high U. Zircon domains formed during high-temperature melting are characterized by high Ti content (100-300?ppm) and unfractionated Th/U (0•4-0•8), whereas the low-temperature domains display low Ti (10?ppm) and Th/U (0•08). The composition of UHP melts with moderate enrichment in LILE, no depletion in Nb and extreme enrichment in LREE and Th is remarkably different from the trace element signature of arc basalts, arguing against involvement of this type of melting in the generation of arc crust. The composition of the UHP melt inclusions is similar to that of melt inclusions from HP crustal xenoliths from Pamir and also to some shoshonites from Tibet. UHP anatexis, as observed in the Kokchetav massif, might be related to the formation of shoshonitic alkaline igneous rocks, which are common in collisional settings.
Physics and Chemistry of Minerals, in press available 9p.
Technology
Garnet morphology
Abstract: We present a systematic experimental study on the phase transition, lattice microstrain, and order-disorder of cations for garnets in the majorite-pyrope system. Polycrystalline gem-quality garnets were synthesized at high pressure and high temperature using a Kawai-type multi-anvil apparatus. A phase transition from a cubic to tetragonal structure is clearly observed for garnets with the majorite content of more than 74 mol % through X-ray diffraction (XRD) and Raman scattering studies. Microstrain of garnets, evaluated with the Williamson-Hall plot on XRD profiles, shows a nonlinear dependence of the garnet compositions. The variation of the XRD peak broadening suggests the lattice microstrain of these garnets may be associated with the local structural heterogeneities due to the substitution of different cations via the coupled substitution (Mg2+ + Si4+ = 2Al3+) in the garnet structure. The width variation of Raman scattering peaks indicates that cation disorder occurs in the garnet structure for intermediate compositions. It is found that intermediate garnets and end-members have a minimum of microstrain, while those between end-members and intermediate compositions possess a larger microstrain.
Abstract: The widespread occurrence of pyrope garnet in Archean lithospheric mantle remains one of the ‘holy grails’ of mantle petrology. Most garnets found in peridotitic mantle equilibrated with incompatible-trace-element-enriched melts or fluids and are the products of metasomatism. Less common are macroscopic intergrowths of pyrope garnet formed by exsolution from orthopyroxene. Spectacular examples of these are preserved in both mantle xenoliths and large, isolated crystals (megacrysts) from the Kaapvaal craton of southern Africa, and provide direct evidence that some garnet in the sub-continental lithospheric mantle initially formed by isochemical rather than metasomatic processes. The orthopyroxene hosts are enstatites and fully equilibrated with their exsolved phases (low-Cr pyrope garnet ±± Cr-diopside). Significantly, P-TP-T estimates of the post-exsolution orthopyroxenes plot along an unperturbed conductive Kaapvaal craton geotherm and reveal that they were entrained from a large continuous depth interval (85 to 175 km). They therefore represent snapshots of processes operating throughout almost the entire thickness of the sub-cratonic lithospheric mantle. New rare-earth element (REE) analyses show that the exsolved garnets occupy the full spectrum recorded by garnets in mantle peridotites and also diamond inclusions. A key finding is that a few low-temperature exsolved garnets, derived from depths of ~90 km, are more depleted in light REEs than previously observed in any other mantle sample. Importantly, the REE patterns of these strongly LREE-depleted garnets resemble the hypothetical composition proposed for pre-metasomatic garnets that are thought to pre-date major enrichment events in the sub-continental lithospheric mantle, including those associated with diamond formation. The recalculated compositions of pre-exsolution orthopyroxenes have higher Al22O33 and CaO contents than their post-exsolution counterparts and most likely formed as shallow residues of large amounts of adiabatic decompression melting in the spinel-stability field. It is inferred that exsolution of garnet from Kaapvaal orthopyroxenes may have been widespread, and perhaps accompanied cratonization at ~ 2.9 to 2.75 Ga. Such a process would considerably increase the density and stability of the continental lithosphere.
Abstract: The widespread occurrence of pyrope garnet in Archean lithospheric mantle remains one of the ‘holy grails’ of mantle petrology. Most garnets found in peridotitic mantle equilibrated with incompatible-trace-element-enriched melts or fluids and are the products of metasomatism. Less common are macroscopic intergrowths of pyrope garnet formed by exsolution from orthopyroxene. Spectacular examples of these are preserved in both mantle xenoliths and large, isolated crystals (megacrysts) from the Kaapvaal craton of southern Africa, and provide direct evidence that some garnet in the sub-continental lithospheric mantle formed initially by isochemical rather than metasomatic processes. The orthopyroxene hosts are enstatites and fully equilibrated with their exsolved phases (low-Cr pyrope garnet?±?Cr-diopside). Significantly, P-T estimates of the post-exsolution orthopyroxenes plot along an unperturbed conductive Kaapvaal craton geotherm and reveal that they were entrained from a large continuous depth interval (85 to 175?km). They therefore represent snapshots of processes operating throughout almost the entire thickness of the sub-cratonic lithospheric mantle. New rare-earth element (REE) analyses show that the exsolved garnets occupy the full spectrum recorded by garnets in mantle peridotites and also diamond inclusions. A key finding is that a few low-temperature exsolved garnets, derived from depths of ?90?km, are more depleted in light rare-earth elements (LREEs) than previously observed in any other mantle sample. Importantly, the REE patterns of these strongly LREE-depleted garnets resemble the hypothetical composition proposed for pre-metasomatic garnets that are thought to pre-date major enrichment events in the sub-continental lithospheric mantle, including those associated with diamond formation. The recalculated compositions of pre-exsolution orthopyroxenes have higher Al2O3 and CaO contents than their post-exsolution counterparts and most probably formed as shallow residues of large amounts of adiabatic decompression melting in the spinel-stability field. It is inferred that exsolution of garnet from Kaapvaal orthopyroxenes may have been widespread, and perhaps accompanied cratonization at ?2.9 to 2.75 Ga. Such a process would considerably increase the density and stability of the continental lithosphere.
Abstract: It is now well established that the cratonic sub-continental lithospheric mantle (SCLM) represents a residue of extensively melted fertile peridotite. The widespread occurrence of garnet in the Archaean SCLM remains a paradox because many experiments agree that garnet is exhausted beyond c. 20% melting. It has been suggested that garnet may have formed by exsolution from Al-rich orthopyroxene [1,2,3]. However, the few examples of putative garnet exsolution in cratonic samples remain exotic and have not afforded a link to garnet that occurs as distinct grains in granular harzburgite. We present crystallographic (EBSD), petrographic and chemical (SEM-EDS and LA-ICP-MS) data for an exceptionally well-preserved orthopyroxene megacryst juxtaposed against granular harzburgite. Garnet lamellae within the megacryst show crystallographic continuity and have a strong fabric relative to the host orthopyroxene, strongly indicating that the megacryst formed by exsolution. Garnet lamellae are sub-calcic Cr-pyropes with sinusoidal rare earth element patterns, while the orthopyroxene host is high-Mg enstatite; the reconstructed precursor is clinoestatite. The megacryst shows evidence for disintegrating into granular peridotite, and garnet and orthopyroxene within the granular peridotite are texturally and chemically identical to equivalent phases in the megacryst. Collectively, this evidence supports a common origin for the granular and exsolved portions of the sample. The compositions of the exsolved Cr pyrope and enstatite are typical of harzburgites and depleted lherzolites from the SCLM. Furthermore, garnet inclusions within orthopyroxene in several granular peridotites exhibit the same fabric as those in the exsolved megacryst. We hypothesise that clinoenstatite was a common phase in cratonic SCLM and that exsolution is the likely origin of many sub-calcic garnets in depleted peridotites.
Abstract: Jeffbenite, ideally Mg3Al2Si3O8, previously known as tetragonal-almandine-pyrope-phase (‘TAPP’), has been characterized as a new mineral from an inclusion in an alluvial diamond from São Luiz river, Juina district of Mato Grosso, Brazil. Its density is 3.576 g/cm3 and its microhardness is ?7. Jeffbenite is uniaxial (-) with refractive indexes ??=?1.733(5) and ??=?1.721(5). The crystals are in general transparent emerald green. Its approximate chemical formula is (Mg2.62Fe2+0.27)(Al1.86Cr0.16)(Si2.82Al0.18)O12 with very minor amounts of Mn, Na and Ca. Laser ablation ICP-MS showed that jeffbenite has a very low concentration of trace elements. Jeffbenite is tetragonal with space group I4¯2d, cell edges being a?=?6.5231(1) and c?=?18.1756(3) Å. The main diffraction lines of the powder diagram are [d (in Å), intensity, hkl]: 2.647, 100, 2 0 4; 1.625, 44, 3 2 5; 2.881, 24, 2 1 1; 2.220, 19, 2 0 6; 1.390, 13, 4 2 4; 3.069, 11, 2 0 2; 2.056, 11, 2 2 4; 1.372, 11, 2 0 12. The structural formula of jeffbenite can be written as (M1)(M2)2(M3)2(T1)(T2)2O12 with M1 dominated by Mg, M2 dominated by Al, M3 dominated again by Mg and both T1 and T2 almost fully occupied by Si. The two tetrahedra do not share any oxygen with each other (i.e. jeffbenite is classified as an orthosilicate). Jeffbenite was approved as a new mineral by the IMA Commission on New Minerals and Mineral Names with the code IMA 2014-097. Its name is after Jeffrey W. Harris and Ben Harte, two world-leading scientists in diamond research. The petrological importance of jeffbenite is related to its very deep origin, which may allow its use as a pressure marker for detecting super-deep diamonds. Previous experimental work carried out on a Ti-rich jeffbenite establishes that it can be formed at 13 GPa and 1700 K as maximum P-T conditions.
Journal of Geochemical Exploration, Vol. 186, pp. 24-35.
Mantle
garnet diamond exploration
Abstract: In diamond exploration, the accurate distinction between garnets from the crust or mantle, or from those having a cognate origin with kimberlite (low-Cr megacrysts), is important for the assessment of indicator mineral samples; misclassifications potentially result in costly misdirection of exploration efforts. Existing literature databases and graphical classification schemes for garnets suffer from a paucity of craton-derived, lower-crustal garnets that - as shown here - are among the most difficult to distinguish from garnets of mantle origin. To improve this situation, a large database of new and literature garnet major element analyses has been compiled. Using this dataset, it is shown that the conventionally used Mg# (Mg/(Mg + Fe)) vs. Ca# (Ca/(Mg + Ca)) plot (Schulze, 2003) for discrimination of crust and mantle garnets results in significant overlap (39.2% crustal failure rate using our dataset). We propose a new graphical classification scheme that uses the parameters ln(Ti/Si) and ln(Mg/Fe) to discriminate low-Cr garnets of crust origin from those of a mantle eclogite-pyroxenite origin with an error rate of 10.1 ± 2.1% at the 95% confidence level (assessed via K-fold cross-validation with ten random test datasets), significantly lower than existing methods. Multivariate statistical solutions, which incorporate a wide selection of geochemical variables, represent additional possibilities for discrimination. Using our new database, logistic regression (LR) and linear discriminant analysis (LDA) approaches are evaluated and new crust-mantle garnet discrimination schemes derived. The resulting solutions, using a wide variety of cations in garnet, provide lower misclassification rates than existing literature schemes. Both LR and LDA are successful discrimination techniques with error rates for the discrimination of crust from mantle eclogite-pyroxenite of 7.5 ± 1.9% and 8.2 ± 2.3%, respectively. LR, however, involves fewer stipulations about the distribution of training data (i.e., it is more "robust") and can return an estimate for probability of classification certainty for single garnets. New data from diamond exploration programs can be readily classified using these new graphical and statistical methods. As the discrimination of low-Cr megacrysts from mantle eclogite-pyroxenite is not the focus of this study, we recommend the method of Schulze (2003) or Grütter et al. (2004) for low-Cr megacryst discrimination to identify megacrysts in the "mantle" suite. Runstreams for our LDA and LR approaches using the freeware "R" are provided for quick implementation.
From subcalcic pyropes to uvarovites: experimental crystalllization of Cr-rich garnets in ultramafic systems with presence of Ca bearing hydrous fluid.
PluS One, Vol. 13, 8, doi:10.1371/journal.pone.0200351
Asia, Thailand
garnets
Abstract: Garnets from disparate geographical environments and origins such as oxidized soils and river sediments in Thailand host intricate systems of microsized tunnels that significantly decrease the quality and value of the garnets as gems. The origin of such tunneling has previously been attributed to abiotic processes. Here we present physical and chemical remains of endolithic microorganisms within the tunnels and discuss a probable biological origin of the tunnels. Extensive investigations with synchrotron-radiation X-ray tomographic microscopy (SRXTM) reveal morphological indications of biogenicity that further support a euendolithic interpretation. We suggest that the production of the tunnels was initiated by a combination of abiotic and biological processes, and that at later stages biological processes came to dominate. In environments such as river sediments and oxidized soils garnets are among the few remaining sources of bio-available Fe2+, thus it is likely that microbially mediated boring of the garnets has trophic reasons. Whatever the reason for garnet boring, the tunnel system represents a new endolithic habitat in a hard silicate mineral otherwise known to be resistant to abrasion and chemical attack.
Geophysical Research Abstracts EGRU2019-3091, Vol. 21, 3091, 1p.
Europe, Alps
garnet
Abstract: Garnet is the paradigmatic cubic mineral of metamorphic and igneous rocks, and is generally regarded as optically isotropic. Nonetheless, evident birefringence is observed, particularly in the rare CaFe 3+ hydrogarnets, which is attributed to the coexistence of two or more cubic phases. A weak birefringence, with rare examples of optical sector zoning, has also been documented in much more common Fe 2+-Mg-Mn garnets, but an adequate explanation for its cause is, so far, lacking. Here we show that optically anisotropic garnets are much more widespread than previously thought, both in blueschists and blueschist-facies rocks, as well as in lower greenschist-facies phyllites, but they are frequently overlooked when working with conventional, 30-µm-thick thin sections. Utilizing a multi-technique approach including optical microstructural analysis, BSEM, EMPA, EBSD, FTIR, TEM, EDT and single-crystal XRD, we demonstrate here that the birefringence in these garnets is related to their tetragonal symmetry, that it is not due to strain, and that crystals are twinned according to a merohedral law. We also show that the birefringent garnets from blueschists and phyllites are anhydrous, lacking any hydrogarnet component, and have compositions dominated by almandine (58-79%) and grossular (19-30%) with variable spessartine (0-21%) and very low pyrope (1-7%). Considering the widespread occurrence of optically anisotropic OH-free garnets in blueschists and phyllites, their common low-grade metamorphic origin, and the occurrence of optically isotropic garnets with similar Ca-rich almandine composition in higher-grade rocks, we conclude that garnet does not grow with cubic symmetry in low-temperature rocks (< 400 • C). The tetragonal structure appears to be typical of Fe-Ca-rich compositions, with very low Mg contents. Cubic but optically sector-zoned garnet in a lower amphibolite-facies metapelite from the eastern Alps suggests that preservation of tetragonal garnet is favored in rocks which did not progress to T> ?500 • C, where transition to the cubic form, accompanied by change of stable chemical composition, would take place. Our data show that the crystal-chemistry of garnet, its thermodynamics and, in turn, its use in unravelling petrogenetic processes in cold metamorphic environments need to be reassessed.
Abstract: The thermoelastic parameters of Ca3Cr2Si3O12 uvarovite garnet were examined in situ at high pressure up to 13 GPa and high temperature up to 1100 K by synchrotron radiation energy-dispersive X-ray diffraction within a 6-6-type multi-anvil press apparatus. A least-square fitting of room T data to a third-order Birch-Murnaghan (BM3) EoS yielded K0 = 164.2 ± 0.7 GPa, V0 = 1735.9 ± 0.3 Å3 (K’0 fixed to 4.0). P-V-T data were fitted simultaneously by a modified HT-BM3 EoS, which gave the isothermal bulk modulus K0 = 163.6 ± 2.6 GPa, K’0 = 4.1 ± 0.5, its temperature derivative (?K0,T/?T)P = -0.014 ± 0.002 GPa K?1, and the thermal expansion coefficients a0 = 2.32 ± 0.13 ×10?5 K?1 and b0 = 2.13 ± 2.18 ×10?9 K?2 (K’0 fixed to 4.0). Our results showed that the Cr3+ enrichment in natural systems likely increases the density of ugrandite garnets, resulting in a substantial increase of mantle garnet densities in regions where Cr-rich spinel releases chromium through a metasomatic reaction.
Abstract: The effect of aluminum (Al) on the elasticity of majorite-pyrope garnets was investigated by means of ultrasonic interferometry measurements on well-fabricated polycrystalline specimens. Both velocities and elastic moduli increase almost linearly with increasing Al content within analytical uncertainty. No significant variation of the velocities and elastic moduli is observed across the tetragonal-to-cubic phase transition at majorite with the pyrope content up to 26 mol% along the majorite-pyrope system. The elasticity variation of majorite-pyrope garnets is largely dominated by the Al content, while the phase transition as a result of cation ordering/disordering of Mg and Si via substitution of Al on octahedral sites cannot significantly affect elastic properties. Seismic velocity variations of a garnet-bearing mantle transition zone are therefore dominated by garnet composition (e.g., Al, Fe, Ca, and Na) rather than the tetragonal-to-cubic phase transition because of cation ordering/disordering.
Abstract: Garnet is the archetypal cubic mineral, occurring in a wide variety of rock types in Earth’s crust and upper mantle. Owing to its prevalence, durability and compositional diversity, garnet is used to investigate a broad range of geological processes. Although birefringence is a characteristic feature of rare Ca-Fe3+ garnet and Ca-rich hydrous garnet, the optical anisotropy that has occasionally been documented in common (that is, anhydrous Ca-Fe2+-Mg-Mn) garnet is generally attributed to internal strain of the cubic structure. Here we show that common garnet with a non-cubic (tetragonal) crystal structure is much more widespread than previously thought, occurring in low-temperature, high-pressure metamorphosed basalts (blueschists) from subduction zones and in low-grade metamorphosed mudstones (phyllites and schists) from orogenic belts. Indeed, a non-cubic symmetry appears to be typical of common garnet that forms at low temperatures (<450?°C), where it has a characteristic Fe-Ca-rich composition with very low Mg contents. We propose that, in most cases, garnet does not initially grow cubic. Our discovery indicates that the crystal chemistry and thermodynamic properties of garnet at low-temperature need to be re-assessed, with potential consequences for the application of garnet as an investigative tool in a broad range of geological environments.
Abstract: The geochemistry and distribution of garnets in the southern Slave Province could have considerable implications for drift prospecting and diamond potential. Presented here is a study interpretting geochemistry in dispersal trains of the Slave Province. Over one-hundred-thousand garnets have been sampled from the northern Slave Province with quantitative analyses conducted on each sample, and the data has been compiled for public release (NTGS Data Hub, 2018). A smaller subset of samples have been collected in the southern Slave Province by this study and the NTGS within recent years. Data from the NTGS is used in this study to construct regional maps showing dispersal trains of indicator minerals and chemistry of indicator garnets throughout the region. The variation in dispersal train pattern, size, mineralogy, and chemistry are being utilized to assess the southern Slave for it’s kimberlite potential. The geochemistry of garnets is used to make further observations into the diamond potential of the area using the garnet classifications G3D, G4D, G5D, and G10D (Grutter et al., 2004). It has been observed that there is an abundance of Na2O rich (>0.07 wt %) garnets in the northern Slave Province and a deficit of Na2O (<0.07 wt %) in garnets of the south. There is also a visible discrepency in olivine in the north and south, with the north Slave showing olivine in dispersal trains and the south lacking any olivine. These discrepancies in Na2O could be indicative of pressure/temperature conditions that coincide with diamond formation in the north (Grutter et al., 2004). The olivine dispersal may be the product of glacial dispersal in conjunction with the facies/mineralogy of kimberlites in the immediate area.
Abstract: The phases of CaAl2O4 and Ca2Al6O11 were synthesized at 15 GPa and 1600 °C. Microprobe data gave formulae Ca1.003Al1.998O4 and Ca2.05Al5.97O11, on the basis of 4 and 11 oxygen atoms. The crystal structures have been refined by single-crystal X-ray diffraction. Orthorhombic unitcell parameters for CaAl2O4 are a = 8.8569(10) Å; b = 2.8561(4) Å; c = 10.2521(11) Å; V = 259.34(5) Å3; Z = 8 (space group Pnma). The Ca2Al6O11 phase was obtained for the first time. It crystallizes with a space group P42/mnm and has lattice parameters a = b = 11.1675(4) Å; c = 2.83180(10) Å; V = 353.16(2) Å3; Z = 2. A Raman spectrum was obtained for a new phase for the first time. Our results suggest that both studied phases are stable under the condition of the transition zone and can be considered as potential aluminum concentrators in the Earth's deep geospheres.
Contribution to Mineralogy and Petrology, Vol. 177, 19 , 18p. Pdf
Global
garnet
Abstract: Garnet is a common U-bearing mineral in various magmatic and metamorphic rocks with a high U-Pb closure temperature (>?850 °C), rendering it a potentially valuable U-Pb geochronometer. However, a high U (>?10 ppm) garnet reference material that suits both quadrupole and/or multi-collector inductively coupled plasma mass spectrometry (ICP-MS) is yet to be established. This study evaluates a potential reference material for in situ garnet U-Pb analysis with anomalously high U content from the Prairie Lake alkaline complex, Canada. The PL57 garnet, occurring in a calcite ijolite, has high TiO2 (6.5-15.0 wt%, average 12.7 wt%) and Fe2O3 (17.1-21.3 wt%) contents and is a member of the andradite (26-66 mol.%)-morimotoite (18-41 mol.%)-schorlomite (16-35 mol.%) solid solution series. Four samples were dated by U-Pb ID-TIMS to assess reproducibility. Twelve TIMS analyses produced concordant, equivalent results. Garnet PL57 yielded a concordant age of 1156.2?±?1.2 Ma (2?, n?=?10, MSWD?=?1.0), based on ten analyses with two results discarded due to possible mineral inclusions (if included, the concordia age is 1156.6?±?1.8 Ma; n?=?12, MSWD?=?2.0). PL57 had 27-76 ppm (average 41 ppm) U with Th/U of 0.51-0.68 (average 0.63). The total common Pb content ranged from 0.4 to 3.9 pg (average 1.1 pg). Laser ablation coupled with ICP-MS and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging provide direct evidence that U is incorporated and homogeneously distributed within the garnet lattice rather than as defects or pore spaces. Published garnet samples and standards were then tested by calibrating the Willsboro, Mali, Qicun, and Tonglvshan garnet against PL57, which gave accurate ages within the recommended values. Case studies of garnet from the Archean Musselwhite orogenic gold deposit in Canada and the Cenozoic Changanchong and Habo skarn deposits in China yield reliable ages. This suggests that PL57 is a robust U-Pb isotope reference material. The limited variations of U and Pb isotopic ratios, together with the high U concentration and extremely low initial common Pb, make PL57 an ideal calibration and monitor reference material for in situ measurements.
