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The Sheahan Diamond Literature Reference Compilation - Scientific and Media Articles based on Major Keyword - Olivine
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.
Olivine is a magnesium iron silicate that is a primary constituent of peridotite, the dominant material in the mantle. oOlivines have a greenish color linked to the presence of nickel which inserts itself into the olivine mineral's crystal lattice which can account for a nickel grade as high as 0.25% that has zero economic value because of the very high energy cost of liberating the nickel from its crystal prison. Olivines are relevant to diamonds because they occur in kimberlites.
American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME) Preprint, Annual Meeting held Phoenix Arizona Feb. 24-27th. 1992, Preprint No. 92-102, 4p
Documenting the chemical, physical and thermodynamic changes associated with all possible geochemical reactions in rocks using Gale vector space:Jericho
Geological Association of Canada, Gac-Mac Yellowknife 2007, May 23-25, Volume 32, 1 pg. abstract p.78-79.
De Assis Janasi, V., Andrade, S., Svisero, D.P.,Vieira de Almeida, V.
Inferencias sobre a evolucao petrologica do manto no sudeste brasileiro a partir de microanalises de elementos traco em piroxenios e olivin a de xenolitos de espinelio peridotitos.
5th Brasilian Symposium on Diamond Geology, Nov. 6-12, abstract p. 85.
Journal of African Earth Sciences, Vol. 111, pp. 26-40.
Africa, Cameroon
Lherzolite, Harzburgite, Olivine websterite
Abstract: Ultramafic xenoliths (lherzolite, harzburgite and olivine websterite) have been discovered in basanites close to Ngaoundéré in Adamawa plateau. Xenoliths exhibit protogranular texture (lherzolite and olivine websterite) or porphyroclastic texture (harzburgite). They are composed of olivine Fo89-90, orthopyroxene, clinopyroxene and spinel. According to geothermometers, lherzolites have been equilibrated at 880-1060 °C; equilibrium temperatures of harzburgite are rather higher (880-1160 °C), while those of olivine websterite are bracketed between 820 and 1010 °C. The corresponding pressures are 1.8-1.9 GPa, 0.8-1.0 GPa and 1.9-2.5 GPa, respectively, which suggests that xenoliths have been sampled respectively at depths of 59-63 km, 26-33 km and 63-83 km. Texture and chemical compositional variations of xenoliths with temperature, pressure and depth on regional scale may be ascribed to the complex history undergone by the sub-continental mantle beneath the Adamawa plateau during its evolution. This may involve a limited asthenosphere uprise, concomitantly with plastic deformation and partial melting due to adiabatic decompression processes. Chemical compositional heterogeneities are also proposed in the sub-continental lithospheric mantle under the Adamawa plateau, as previously suggested for the whole Cameroon Volcanic Line.
Abstract: Most kimberlites contain abundant dunitic nodules. These are centimetre-sized, rounded and multi-grained assemblages of xenocrystic olivine with a wide range of compositions (Fo83 to Fo94). The absence of orthopyroxene and other mantle minerals and the range of olivine compositions have been attributed to reaction between mantle peridotite and (proto)kimberlitic fluid or melt, but the timing of the reaction is a subject of debate. In a kimberlite from the Kangamiut region of Greenland, nodule cores are surrounded by fine-grained outer margins with near-constant Fo contents (~Fo88) but highly variable minor element contents (e.g. 500-2500 ppm Ni). These margins crystallized from the kimberlite melt and we show that their compositions can be explained by crystallization of olivine alone, if a high partition coefficient for Ni between melt and olivine (DNi > 20) is assumed. Orthopyroxene assimilation is not required, removing the constraint that its dissolution occurred during ascent of the kimberlite magma. Within some nodules, in addition to the usual core-to-margin gradients, we observe asymmetric compositional changes (variable Fo but near-constant minor element contents) across mobile grain boundaries. These changes document fluid percolation at the grain scale that occurred during dynamic recrystallization in the deforming lithospheric mantle. We note that chemical gradients associated with mobile grain boundaries are observed in olivines that cover the entire compositional range of the nodules, and propose that fluid-assisted dynamic recrystallization took place in dunite that was already compositionally heterogeneous. Reaction between peridotite and protokimberlitic melt or fluid and dissolution of orthopyroxene thus occurred within the lithospheric mantle, immediately (a few days) prior to the ascent of the kimberlite melt and the entrainment of the dunite nodules. We propose that the grain boundary zones probably mimic, at a fine scale, the fluid-peridotite interaction that caused, at a larger scale, orthopyroxene dissolution and formation of compositionally diverse olivine in kimberlites.
Journal of Geophysical Research,, Vol. 120, 12, pp. 8259-8280.
Mantle
Ringwoodite
Abstract: Review of recent mineral physics literature shows consistent trends for the influence of Fe and H2O on the bulk modulus (K0) of wadsleyite and ringwoodite, the major phases of Earth's mantle transition zone (410-660?km). However, there is little consensus on the first pressure derivative, K0??=?(dK/dP)P=0, which ranges from about 4 to >5 across experimental studies and compositions. Here we demonstrate the importance of K0? in evaluating the bulk sound velocity of the transition zone in terms of water content and provide new constraints on the effect of H2O on K0? for wadsleyite and ringwoodite by conducting a comparative compressibility study. In the experiment, multiple crystals of hydrous Fo90 wadsleyite containing 2.0 and 0.25?wt?% H2O were loaded into the same diamond anvil cell, along with hydrous ringwoodite containing 1.4?wt?% H2O. By measuring their pressure-volume evolution simultaneously up to 32?GPa, we constrain the difference in K0? independent of the pressure scale, finding that H2O has no effect on K0?, whereas the effect of H2O on K0 is significant. The fitted K0? values of hydrous wadsleyite (0.25 and 2.0?wt?% H2O) and hydrous ringwoodite (1.4?wt?% H2O) examined in this study were found to be identical within uncertainty, with K0? ~3.7(2). New secondary-ion mass spectrometry measurements of the H2O content of these and previously investigated wadsleyite samples shows the bulk modulus of wadsleyite is reduced by 7.0(5)?GPa/wt?% H2O, independent of Fe content for upper mantle compositions. Because K0? is unaffected by H2O, the reduction of bulk sound velocity in very hydrous regions of transition zone is expected to be on the order of 1.6%, which is potentially detectible in high-resolution, regional seismology studies.
Abstract: Chromium valence ratios in igneous olivine may hold a wealth of redox information about the melts from which they crystallized. It has been experimentally shown that the Cr2+/?Cr of olivine varies systematically with fO2, therefore measurements of Cr valence in olivine could be employed as a quantitative oxybarometer. In situ synchrotron ?-XANES analyses of Cr valence ratios of individual olivine phenocrysts in thin section have the potential to unlock this stored magmatic redox information on a fine spatial scale. However, there are still obstacles to obtaining accurate XANES measurements of cation valence in crystalline materials, as the results from these measurements can be compromised by anisotropic absorption effects related to the crystallographic orientation of the sample. Improving the accuracy of XANES measurements of Cr valence ratios in olivine by calibrating an anisotropy correction is a vital step in developing Cr valence measurements in olivine as a rigorous oxybarometer. To accomplish this goal, we have used an integrated approach that combined experiments, electron backscatter diffraction analysis, and XANES measurements in olivine to systematically examine how orientation affects the resultant Cr K-edge XANES spectra and the Cr valence ratios that are calculated from them. The data set generated in this work was used to construct a model that mitigates the effects of anisotropy of the calculated Cr2+/?Cr values. The application of this correction procedure as a part of spectral processing improves the overall accuracy of the resultant Cr2+/?Cr values by nearly a factor of five. The increased accuracy of the XANES measured Cr valence ratios afforded by the anisotropy correction reduces the error on calculated fO2 values from approximately ±1.2 to ±0.25
Earth and Planetary Science Letters, Vol. 483, 1, pp. 90-104.
Europe, Iceland
olivines
Abstract: Trace element variability in oceanic basalts is commonly used to constrain the physics of mantle melting and the chemistry of Earth's deep interior. However, the geochemical properties of mantle melts are often overprinted by mixing and crystallisation processes during ascent and storage. Studying primitive melt inclusions offers one solution to this problem, but the fidelity of the melt-inclusion archive to bulk magma chemistry has been repeatedly questioned. To provide a novel check of the melt inclusion record, we present new major and trace element analyses from olivine macrocrysts in the products of two geographically proximal, yet compositionally distinct, primitive eruptions from the Reykjanes Peninsula of Iceland. By combining these macrocryst analyses with new and published melt inclusion analyses we demonstrate that olivines have similar patterns of incompatible trace element (ITE) variability to the inclusions they host, capturing chemical systematics on intra- and inter-eruption scales. ITE variability (element concentrations, ratios, variances and variance ratios) in olivines from the ITE-enriched Stapafell eruption is best accounted for by olivine-dominated fractional crystallisation. In contrast, ITE variability in olivines and inclusions from the ITE-depleted Háleyjabunga eruption cannot be explained by crystallisation alone, and must have originated in the mantle. Compatible trace element (CTE) variability is best described by crystallisation processes in both eruptions. Modest correlations between host and inclusion ITE contents in samples from Háleyjabunga suggest that melt inclusions can be faithful archives of melting and magmatic processes. It also indicates that degrees of ITE enrichment can be estimated from olivines directly when melt inclusion and matrix glass records of geochemical variability are poor or absent. Inter-eruption differences in olivine ITE systematics between Stapafell and Háleyjabunga mirror differences in melt inclusion suites, and confirm that the Stapafell eruption was fed by lower degree melts from greater depths within the melting region than the Háleyjabunga eruption. Although olivine macrocrysts from Stapafell are slightly richer in Ni than those from Háleyjabunga, their overall CTE systematics (e.g., Ni/(Mg/Fe), Fe/Mn and Zn/Fe) are inconsistent with being derived from olivine-free pyroxenites. However, the major element systematics of Icelandic basalts require lithological heterogeneity in their mantle source in the form of Fe-rich and hence fusible domains. We thus conclude that enriched heterogeneities in the Icelandic mantle are composed of modally enriched, yet nonetheless olivine-bearing, lithologies and that olivine CTE contents provide an incomplete record of lithological heterogeneity in the mantle. Modally enriched peridotites may therefore play a more important role in oceanic magma genesis than previously inferred.
Physics and Chemistry of Minerals, Vol. 45, 2, pp. 95-131.
Mantle
olivines
Abstract: Elasticity is a key property of materials, not only for predicting volumes and densities of minerals at the pressures and temperatures in the interior of the Earth, but also because it is a major factor in the energetics of structural phase transitions, surface energies, and defects within minerals. Over the 40 years of publication of Physics and Chemistry of Minerals, great progress has been made in the accuracy and precision of the measurements of both volumes and elastic tensors of minerals and in the pressures and temperatures at which the measurements are made. As an illustration of the state of the art, all available single-crystal data that constrain the elastic properties and pressure–volume–temperature equation of state (EoS) of mantle-composition olivine are reviewed. Single-crystal elasticity measurements clearly distinguish the Reuss and Voigt bulk moduli of olivine at all conditions. The consistency of volume and bulk modulus data is tested by fitting them simultaneously. Data collected at ambient pressure and data collected at ambient temperature up to 15 GPa are consistent with a Mie–Grünesien–Debye thermal-pressure EoS in combination with a third-order Birch–Murnaghan (BM) compressional EoS, the parameter V 0 = 43.89 cm3 mol?1, isothermal Reuss bulk modulus KTR,0=126.3(2) GPa, K?TR,0=4.54(6), a Debye temperature ?D=644(9)K, and a Grüneisen parameter ? 0 = 1.044(4), whose volume dependence is described by q = 1.9(2). High-pressure softening of the bulk modulus at room temperature, relative to this EoS, can be fit with a fourth-order BM EoS. However, recent high-P, T Brillouin measurements are incompatible with these EoS and the intrinsic physics implied by it, especially that (?K?TR?T)P>0. We introduce a new parameterisation for isothermal-type EoS that scales both the Reuss isothermal bulk modulus and its pressure derivative at temperature by the volume, KTR(T,P=0)=KTR,0[V0V(T)]?T and K?TR(T,P=0)=K?TR,0[V(T)V0]??, to ensure thermodynamic correctness at low temperatures. This allows the elastic softening implied by the high-P, T Brillouin data for mantle olivine to be fit simultaneously and consistently with the same bulk moduli and pressure derivatives (at room temperature) as the MGD EoS, and with the additional parameters of ? V0 = 2.666(9) × 10?5 K?1, ?E=484(6), ?T = 5.77(8), and ?? = ?3.5(1.1). The effects of the differences between the two EoS on the calculated density, volume, and elastic properties of olivine at mantle conditions and on the calculation of entrapment conditions of olivine inclusions in diamonds are discussed, and approaches to resolve the current uncertainties are proposed.-
Physics and Chemistry of Minerals, Vol. 45, 2, pp. 139-172.
Mantle
olivines
Abstract: Interfaces in rocks, especially grain boundaries in olivine dominated rocks, have been subject to about 40 years of studies. The grain boundary structure to property relation is fundamental to understand the diverging properties of polycrystalline samples compared to those of single crystals. The number of direct structural observations is small, i.e. in range of 100 micrographs, and the number of measurements of properties directly linked to structural observations is even smaller. Bulk aggregate properties, such as seismic attenuation, rheology and electrical conductivity, are sensitive to grain size, and seem to show influences by grain boundary character distributions. In this context we review previous studies on grain boundary structure and composition and plausible relations to bulk properties. The grain boundary geometry is described using five independent parameters; generally, their structural width ranges between 0.4-1.2 nm and the commonly used 1 nm seems a good approximation. This region of enhanced disorder is often enriched in elements that are incompatible in the perfect crystal lattice. The chemical composition of grain boundaries depends on the bulk rock composition. We determined the 5 parameter grain boundary character distribution (GBCD) for polycrystaline Fo90 and studied structure and chemistry at the nm-scale to extend previous measurements. We find that grain boundary planes close to perpendicular to the crystallographic c-direction dominate the grain boundary network. We conclude that linking grain boundary structure in its full geometric parameter space to variations of bulk rock properties is now possible by GBCD determination using EBSD mapping and statistical analyses.
Abstract: The mantle transition-zone discontinuities are usually attributed to isochemical phase transformations of olivine and its high-pressure polymorphs. However, recent seismic observations have shown complexities in these discontinuities that cannot be explained by conventional models of thermal variations. Here we analyze SS precursor stacking results to investigate global mantle transition-zone properties. The precursor waveforms provide information on the seismic velocity and density profiles across and near the major mantle discontinuities. A sporadic low-velocity layer immediately above the 410- km discontinuity is observed worldwide, including East Asia, western North America, eastern South America, and 33-50% of the resolved Pacific Ocean [1]. The 520-km discontinuity exhibits significant variations in its sharpness and depth, and occasionally appears to be split. A sub-discontinuity beneath 660 km is detected in some regions, possibly suggesting enrichment of garnet. All of these lateral variations show no geographical correlation with discontinuity topography or tomographic models of seismic velocity, suggesting that they are not caused by regional thermal anomalies. Alternatively, our observations can be explained by compositional heterogeneities in the mid-mantle, including major minerals and volatile content, which may result in additional phase transformations and partial melting. In addition, flat slabs that extend horizontally for at least a few thousand kilometers are imaged beneath several subduction zones at various depths in the lower mantle. This suggests a variety of slab sinking at different stages. A slab morphology in the lower mantle seems to be controlled by its individual evolution rather than specific changes in mantle rheology.
Journal of Geophysical Research: Solid Earth, Vol. 123, 10, pp. 8459-8473.
Mantle
olivine
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 [100]?slip to [001]?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.
Geochimica et Cosmochimica Acta, Vol. 243, pp. 133-148.
Mantle
olivine
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 ?26Mg composition of the fluid relative to olivine during the initial carbonation and serpentinization reactions. This was followed by a decrease in ?26Mg under higher pH conditions associated with the formation of brucite. Our experimental and modeling results therefore demonstrate that the ?26Mg 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 ?26Mg 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 ?26Mg may be a useful tool for tracing reactions that are critical to geological CO2 sequestration.
Abstract: The ductile behavior of olivine-rich rocks is critical to constrain thermal convection in the Earth's upper mantle. Classical olivine flow laws for dislocation or diffusion creep fail to explain the fast post-seismic surface displacements observed by GPS, which requires a much weaker lithosphere than predicted by classical laws. Here we compare the plasticity of olivine aggregates deformed experimentally at mantle pressures and temperatures to that of single crystals and demonstrate that, depending on conditions of stress and temperature, strain accommodated through grain-to-grain interactions - here called intergranular strain - can be orders of magnitude larger than intracrystalline strain, which significantly weakens olivine strength. This result, extrapolated along mantle geotherms, suggests that intergranular plasticity could be dominant in most of the upper mantle. Consequently, the strength of olivine-rich aggregates in the upper mantle may be significantly lower than predicted by flow laws based on intracrystalline plasticity models.
Physics of the Earth and Planetary Interiors, doi.org/10.10106/j.pepi.2019.03.001
Mantle
wadsleyite, ringwoodite
Abstract: In a polymorphic change in which the phases differ only by a reversible difference in specific volume, kinematics requires a unit mass to suffer deviatoric strain in the instant it is transformed. Unlike the Eshelby stress-free strain, this strain is a property of the motion. Its existence must be considered when formulating the constitutive relation for the product of an incoherent transformation. To show this, two models are compared: in both, the (Nabarro) condition of vanishing shear stress is imposed at the incoherent interface; they differ only in the treatment of the deviatoric strain at issue. In the existing model, deviatoric stress within a unit mass of product is determined by total deviatoric strain from its initial state as parent phase. In the new model, lattice reconstruction is assumed to erase all memory within the unit mass of deviatoric strain suffered before, or during, its transformation. The existing model is not consistent with experiments on the olivine spinel-phase change in single crystals. It predicts that when the pressure applied exceeds a critical value, samples should transform completely at almost constant rate; instead, growth is seen to slow, and may even cease. The new model predicts this. Without adjustable constants, fair agreement is obtained with experiments on samples having 75-200?ppmw of water. Because elastic deformation by itself can explain those observations, the very thin rims seen on even drier samples suggest that water may be essential to lattice reconstruction in this phase change.
Earth and Planetary Science Letters, Vol. 519, pp. 1-11.
Mantle
olivine
Abstract: Velocity and density jumps across the 410-km seismic discontinuity generally indicate olivine contents of ?30 to 50 vol.% on the basis of the elastic properties of anhydrous olivine and wadsleyite, which is considerably less than the ?60% olivine in the widely accepted pyrolite model for the upper mantle. A possible explanation for this discrepancy is that water dissolved in olivine and wadsleyite affects their elastic properties in ways that can reconcile the pyrolitic model with seismic observations. In order to more fully constrain the olivine content of the upper mantle near the 410-km discontinuity, and to place constraints on the mantle water content at this depth, we determined the full elasticity of hydrous wadsleyite at the P-T conditions of the discontinuity based on density functional theory calculations. Together with previous determinations for the effect of water on olivine elasticity, we simultaneously modeled the density and seismic velocity jumps (??, , ) across the olivine-wadsleyite transition. Our models allow for several scenarios that can well reproduce the density and seismic velocity jumps across the 410-km discontinuity when compared to globally averaged seismic models. When the water content of olivine and wadsleyite is assumed to be equal as in a simple binary system, our modeling indicates a best fit for low water contents (<0.1 wt.%) with an olivine proportion of ?50%, suggesting a relatively dry, non-pyrolitic mantle at depths of the 410-km discontinuity. However, our modeling can be reconciled with a pyrolitic mantle if the water content in wadsleyite is ?0.9 wt.% and that in olivine is at its storage capacity of ?500-1500 ppm. The result would be consistent with a hydrous melt phase produced at depths just above the phase transition.
Geostandards and Geoanalytical Research, in press available, 21p.
Asia, Mongolia
olivine
Abstract: A new olivine reference material - MongOL Sh11?2 - for in situ analysis has been prepared from the central portion of a large (20 × 20 × 10 cm) mantle peridotite xenolith from a ~ 0.5 My old basaltic breccia at Shavaryn?Tsaram, Tariat region, central Mongolia. The xenolith is a fertile mantle lherzolite with minimal signs of alteration. Approximately 10 g of 0.5-2 mm gem quality olivine fragments were separated under binocular microscope and analysed by EPMA, LA?ICP?MS, SIMS and bulk analytical methods (ID?ICP?MS for Mg and Fe, XRF, ICP?MS) for major, minor and trace elements at six institutions world?wide. The results show that the olivine fragments are sufficiently homogeneous with respect to major (Mg, Fe, Si), minor and trace elements. Significant inhomogeneity was revealed only for phosphorus (homogeneity index of 12.4), whereas Li, Na, Al, Sc, Ti and Cr show minor inhomogeneity (homogeneity index of 1-2). The presence of some mineral and fluid?melt micro?inclusions may be responsible for the inconsistency in mass fractions obtained by in situ and bulk analytical methods for Al, Cu, Sr, Zr, Ga, Dy and Ho. Here we report reference and information values for twenty?seven major, minor and trace elements.
Abstract: The strength of subducted slabs in the mantle transition zone influences the style of mantle convection. Intense deformation is observed particularly in relatively old subducted slabs in the deep mantle transition zone. Understanding the cause of this regional and depth variation in slab deformation requires constraint of the rheological properties of deep mantle materials. Here, we report results of in situ deformation experiments during the olivine to ringwoodite phase transformation, from which we infer the deformation process under the conditions of cold slabs deep in the mantle transition zone. We find that newly transformed fine-grained ringwoodite deforms by diffusion creep and that its strength is substantially smaller than that of coarser-grained minerals but increases with time. Scaling analysis, based on a model of transformation kinetics and grain-size evolution during a phase transformation, suggests that a cold slab will be made of a mixture of weak, fine-grained and strong, coarse-grained materials in the deep transition zone, whereas a warm slab remains strong because of its large grain size. We propose that this temperature dependence of grain size may explain extensive deformation of cold slabs in the deep transition zone but limited deformation of relatively warm slabs.
Contributions to Mineralogy and Petrology, Vol. 175, 9p. Pdf
Africa, Lesotho
olivine
Abstract: This paper highlights published and new field and petrographic observations for late-stage (crustal level) deformation associated with the emplacement of kimberlites and other mantle-derived magmas. Thus, radial and tangential joint sets in the competent 183 Ma Karoo basalt wall rocks to the 5 ha. Lemphane kimberlite blow in northern Lesotho have been ascribed to stresses linked to eruption of the kimberlite magma. Further examples of emplacement-related stresses in kimberlites are brittle fractures and close-spaced parallel shears which disrupt olivine macrocrysts. In each of these examples, there is no evidence of post-kimberlite regional tectonism which might explain these features, indicating that they reflect auto-deformation in the kimberlite during or immediately post-emplacement. On a microscopic scale, these inferred late-stage stresses are reflected by fractures and domains of undulose extinction which traverse core and margins of some euhedral and anhedral olivines in kimberlites and olivine melilitites. Undulose extinction and kink bands have also been documented in olivines in cumulates from layered igneous intrusions. Our observations thus indicate that these deformation features can form at shallow levels (crustal pressures), which is supported by experimental evidence. Undulose extinction and kink bands have previously been presented as conclusive evidence for a mantle provenance of the olivines—i.e. that they are xenocrysts. The observation that these deformation textures can form in both mantle and crustal environments implies that they do not provide reliable constraints on the provenance of the olivines. An understanding of the processes responsible for crustal deformation of kimberlites could potentially refine our understanding of kimberlite emplacement processes.
Contributions to Mineralogy and Petrology, Vol. 175, 15p.
Canada
olivine
Abstract: Interactions between carbonated ultramafic silicate magmas and the continental lithospheric mantle results in the formation of dunite—a ubiquitous xenolith type in kimberlites and aillikites. However, whether this process dominantly occurs in the mantle source region or by subsequent interactions between lithospheric mantle fragments and transporting silica-undersaturated magmas during ascent remains debated. Aillikite magmas, which are derived from the fusion of carbonate-phlogopite metasomes under diamond-stability field upper mantle conditions, have a mineralogically more complex source than kimberlites, providing an opportunity to more fully constrain the origin of dunite xenoliths in such deeply sourced carbonated silicate magmas. Here we present a major and trace element study of olivine occurring in xenoliths and as phenocrysts in an aillikite dike located on the southern Superior Craton. We show that olivine within the dunite microxenoliths exhibits extreme enrichment in Al, Cr, Na, and V when compared to equivalent xenoliths carried by kimberlites. We interpret these results as evidence for the presence of carbonate-phlogopite metasomes left residual in the cratonic mantle source during aillikite magma formation. Our results are inconsistent with models of dunite formation through orthopyroxene dissolution upon kimberlite/aillikite magma ascent, supporting an origin for such dunites that is more closely linked to primary melt generation at the base of relatively thick continental lithosphere. Our work demonstrates that it is possible to constrain the precursor composition of cratonic mantle dunite at depth, thereby facilitating the further exploration of how carbonated silicate magmas modify and weaken continental lithospheric roots.
Earth and Planetary Science Letters, Vol. 547, 10p. Pdf
Mantle
Wadsleyite, ringwoodite, majorite garnet
Abstract: The dynamics of the Earth's mantle is still poorly constrained due to the lack of understanding the transfer of matter between the upper and the lower mantle and their convective vigor. The transition zone (TZ) might play a crucial role as the interface connecting the upper to the lower mantle. Here, we examine the rheology of the main TZ minerals, wadsleyite, ringwoodite and majorite garnet based on a mineral physics approach. Using the results of lattice friction modeling and dislocation glide mobilities together with the available data on self-diffusion in the TZ minerals, we quantify their plastic deformation by diffusion and dislocation creep from theoretical plasticity models. We show that pure climb creep is expected to contribute to the plasticity of the TZ without the need of significant diffusion-related hydrolytic weakening, matching well the geophysical observations. Our model results predict that crystallographic preferred orientations (CPO) might only develop along with stress concentrations as present around cold subducting slabs which can be locally weaker than the surrounding TZ despite their lower temperatures.
Abstract: Nickel is a strongly compatible element in olivine, and thus fractional crystallization of olivine typically results in a concave-up trend on a Fo-Ni diagram. "Ni-enriched" olivine compositions are considered those that fall above such a crystallization trend. To explain Ni-enriched olivine crystals, we develop a set of theoretical and computational models to describe how primitive olivine phenocrysts from a parent (high-Mg, high-Ni) basalt re-equilibrate with an evolved (low-Mg, low-Ni) melt through diffusion. These models describe the progressive loss of Fo and Ni in olivine cores during protracted diffusion for various crystal shapes and different relative diffusivities for Ni and Fe-Mg. In the case when the diffusivity of Ni is lower than that for Fe-Mg interdiffusion, then olivine phenocrysts affected by protracted diffusion form a concave-down trend that contrasts with the concave-up crystallization trend. Models for different simple geometries show that the concavity of the diffusion trend does not depend on the size of the crystals and only weakly depends on their shape. We also find that the effect of diffusion anisotropy on trend concavity is in the same magnitude as the effect of crystal shape. Thus, both diffusion anisotropy and crystal shape do not significantly change the concave-down diffusion trend. Three-dimensional numerical diffusion models using a range of more complex, realistic olivine morphologies with anisotropy corroborate this conclusion. Thus, the curvature of the concave-down diffusion trend is mainly determined by the ratio of Ni and Fe-Mg diffusion coefficients. The initial and final points of the diffusion trend are in turn determined by the compositional contrast between mafic and more evolved melts that have mixed to cause disequilibrium between olivine cores and surrounding melt. We present several examples of measurements on olivine from arc basalts from Kamchatka, and several published olivine datasets from mafic magmas from non-subduction settings (lamproites and kimberlites) that are consistent with diffusion-controlled Fo-Ni behaviour. In each case the ratio of Ni and Fe-Mg diffusion coefficients is indicated to be?
Geochemistry, Geophysics, Geosystems, 10.1029/ 2020GC009159 22p. Pdf
Mantle
olivine
Abstract: The uppermost layer of Earth's mantle, the asthenosphere, experiences large deformations due to a variety of tectonic processes. During deformation, grains of olivine, the main rock?forming mineral in the asthenosphere, rotate into a preferred direction parallel to the deformation, developing a texture that can affect the response of the asthenosphere to tectonic stresses. Laboratory measurements show that the deformation rate depends on the orientation of the shear stress relative to the olivine texture. We use numerical models to apply the findings of the laboratory measurements to geodynamic situations that are difficult to simulate in a laboratory. These models track the development of olivine texture and its directional response to shear stress, which are highly coupled. Our results suggest that anisotropic viscosity in the asthenosphere can significantly affect the motions of tectonic plates, as plate motion in a continuous direction should become faster, while abrupt changes in the direction of plate motion should meet high resistance in the underlying asthenosphere. We suggest that olivine textures in the asthenosphere play a critical role in upper mantle dynamics.
Abstract: A dense magnesium iron silicate polymorph with a structure intermediate between olivine, ringwoodite, and wadsleyite was theoretically predicted about four decades ago. As this group of minerals constitute the major component of shocked meteorites, constraining their transitional forms and behaviour is of potential importance for understanding impact events on their parent bodies. Here we use high-resolution transmission electron microscopy techniques and single-crystal X-ray diffraction analyses to identify naturally occurring examples of this mineral - recently named poirierite - in shocked chondritic meteorites. We observe nanoscale lamellar poirierite topotactically intergrown within wadsleyite, and additionally within ringwoodite as recently reported. Our results confirm the intermediate structure of poirierite and suggest it might be a relay point in the shear transformations between its polymorphs. We propose that poirierite formed during rapid decompression at relatively low temperature in retrograde shock metamorphism of the meteorites.
Contribution to Mineralogy and Petrology, Vol. 176, 99 16p. Pdf
Mantle
olivine
Abstract: We report experimental measurements of volume and grain boundary diffusion of 26Mg in Mg2SiO4 bi-crystals at asthenosphere temperatures as a ground reference for olivine. By analysis of literature and combination with previous data, we provide Arrhenius laws D = D0 exp(- E/RT) at ambient pressure for volume diffusion of Mg in Mg2SiO4 in the intrinsic regime along the three crystallographic axes as well as grain boundary diffusion.
Nature Communications, 10.10.1038/s4167-021-266668-z 14p. Pdf
Mantle
olivine
Abstract: Oxygen isotope ratios in mantle-derived magmas that differ from typical mantle values are generally attributed to crustal contamination, deeply subducted crustal material in the mantle source or primordial heterogeneities. Here we provide an alternative view for the origin of light oxygen-isotope signatures in mantle-derived magmas using kimberlites, carbonate-rich magmas that assimilate mantle debris during ascent. Olivine grains in kimberlites are commonly zoned between a mantle-derived core and a magmatic rim, thus constraining the compositions of both mantle wall-rocks and melt phase. Secondary ion mass spectrometry (SIMS) analyses of olivine in worldwide kimberlites show a remarkable correlation between mean oxygen-isotope compositions of cores and rims from mantle-like 18O/16O to lower ‘crustal’ values. This observation indicates that kimberlites entraining low-18O/16O olivine xenocrysts are modified by assimilation of low-18O/16O sub-continental lithospheric mantle material. Interaction with geochemically-enriched domains of the sub-continental lithospheric mantle can therefore be an important source of apparently ‘crustal’ signatures in mantle-derived magmas.
Earth and planetary Science Letters, Vol. 579, 117359, 11p. Pdf
Mantle
ringwoodite
Abstract: The 520 km discontinuity (the 520) and the 660 km discontinuity (the 660) are primarily caused by the wadsleyite to ringwoodite and ringwoodite to bridgmanite + ferropericlase phase transitions, respectively. Global seismic studies show significant regional variations of the 520, which are likely due to chemical and thermal heterogeneities in the Mantle Transition Zone (MTZ). However, the effects of chemical composition and temperature on the detectability of the 520 are unclear. Additionally, it remains unknown whether the possibly existing metastable ringwoodite in the core of the cold and fast subducting slabs could create a detectable seismic signature near the top of the lower mantle. Our understanding of both issues is hindered by the lack of single-crystal elasticity measurements of ringwoodite at simultaneous high pressure-temperature (P-T) conditions. In this study, we measured the single-crystal elasticity of an anhydrous Fe-bearing ringwoodite up to 32 GPa and 700 K by Brillouin spectroscopy, and then modeled the composition-dependent elastic properties of ringwoodite to calculate the compositional effects on the velocity jumps at the 520. We found that opposite to the effect of Fe, water enhances the Vp (P-wave velocity) jump, yet decreases the Vs (S-wave velocity) jump of the 520 across the wadsleyite to ringwoodite transition. Higher temperature increases both Vp and Vs contrasts across the 520. At depths between 660-700 km in the lower mantle, the existence of metastable ringwoodite may only result in ?1-2% low velocity anomaly, which is seismically difficult to resolve. The low velocity anomaly caused by metastable ringwoodite increases to 5-7% at 750 km depth due to the weak pressure dependence of Vs in ringwoodite at lower mantle conditions, but whether it is seismically detectable depends on the extension of the regions in subducted slabs that are sufficiently cold to host metastable ringwoodite.
Abstract: The cold, rigid, and melt-depleted mantle underlying Archean cratons plays an important role in the preservation of the overlying continental crust and is one of the main sources of diamonds. However, with the possible exception of rare earth elements (REE) and platinum group-elements (PGE), the concentrations and host mineral phases for many other critical trace elements within lithospheric mantle remain very poorly understood. Here we address that knowledge gap, presenting new electron microprobe and laser-ablation inductively-coupled-plasma mass-spectrometry results for a suite of mantle xenoliths (n = 12) and olivine xenocrysts (n = 376) from the Jericho, Muskox, and Voyageur kimberlites (northern Slave craton, Canada). Low-temperature (<1000 °C) harzburgite xenoliths and olivine xenocrysts suggest that the shallowest portions of the garnet-bearing mantle (?160 km) underlying the northern Slave craton is chemically depleted and becomes increasing re-fertilized from 160 to 200 km. High-temperature (>1000 °C) garnet and clinopyroxene crystals with Ti/Eu ratios > > 1000, and olivine xenocrysts suggest that interaction with ultramafic silicate melts is the most likely mechanism to re-fertilize melt-depleted peridotite with incompatible elements toward the base of the lithosphere (~200 km). In contrast, lower temperature garnet and clinopyroxene with Ti/Eu ratios <1000 are more likely related to metasomatism by carbonatitic melts and/or fluids. Carbonatitic metasomatism is also interpreted as the preferred explanation for the trend of Nb (4 ppm)- and Ta (185 ppb)-rich concentrations of olivine xenocrysts sampled from mid-lithosphere depths (~140 km). With the exception of a few elements that substitute into the olivine crystal structure during sub-solidus re-equilibration (e.g., Ca, Cr, Cu, Na, Sc, V, Zn), most other olivine-hosted trace elements do not systematically vary with depth. Instead, we interpret olivine-hosted trace element concentrations that are significantly above the analytical detection and/or quantification limits to reflect trapped fluid (e.g., As, Mo, Sb, Sn), base-metal sulphide (e.g., Ag, Au, Bi, Pd, Pt, Se, Te), and other mineral inclusions (e.g., U, Th) rather than enrichments of these elements due to substitution reactions or analytical artefacts. We interpret that these inclusions occur in olivine throughout the garnet stability field, but are relatively rare. As a result, these trapped carbonatitic, proto-kimberlite, and/or other ultramafic silicate melts do not represent a significant source for the suite of trace elements that become enriched to economic levels in the crust.
