Natural Olivine Research Articles

Re-evaluating the diffusivity of phosphorus in olivine: Implications of low diffusive mobility for thermochronology

Heterogeneities in the phosphorus (P) content of olivine are relatively resistant to diffusive homogenization when compared with other compositional heterogeneities. Thus, heterogeneities in the spatial distribution of P can preserve petrological information about olivine crystals from the earliest stages of crystallization which have been otherwise eliminated. However, compared to independent determinations of protracted cooling timescales in slowly cooled rocks, P enrichments are so sharp as to suggest a rate of diffusive mobility that is many orders of magnitude slower than previously suggested. Here, we heat single natural olivine crystals with sharply defined P heterogeneities (0.02–0.15 wt% P2O5 over 0.5 μm spatial scale) in a 1-atmosphere gas-mixing furnace for durations of 10–20 days at 1400°C, thus exposing them to conditions that should be sufficient for complete diffusive relaxation according to published P diffusivity values. However, high-precision chemical analysis of the same interior section before and after heating reveals no discernable difference in the sharpness of phosphorus concentration patterns. Therefore, diffusion chronometry applied to skeletal P enrichments in olivine currently provides erroneously short diffusive timescales. We discuss several possible causes for these discrepancies and the implications for diffusion chronometry as applied to phosphorus in olivine.

Quantification of grain boundary mobilities in natural olivine by annealing experiments and full-field modelling

We investigate olivine grain boundary (GB) migration in natural peridotites experimentally annealed at high pressure and high temperature, and couple the experimental observations to full-field grain growth models to provide the distribution of GB mobilities in natural olivine polycrystals. A stack of four slices of natural mylonitic peridotite (Oman ophiolite) was annealed at 1473 K for 5 h under a confining pressure of 300 MPa of argon in a Paterson press. The three sintered interfaces of the stack are characterized using scanning electron microscopy and electron backscatter diffraction (EBSD) to extract a distribution of apparent (2D) GB displacements. Full-field simulations of numerically sintered interfaces are then used to infer the GB mean mobility and distribution in olivine polycrystal allowing forward modelling exercise to reproduce the experimental GB displacement distribution.This yields widely dispersed mobilities, which can be approximated by a log-normal distribution of 10−15.85±2.58m4.J−1.s−1. Both the average and the dispersion of GB mobilities can be explained by silicon grain boundary diffusion. Finally, we demonstrate that the high variability of GB mobilities in olivine implies a decrease of the mean growth rate with time. This elucidates the difficulties of extrapolating experimental grain growth rates to geological timescales and observed microstructures in peridotites.

Prediction of feasibility of Li and Fe doped natural olivine Mg2SiO4 for Li ion battery applications using Density Functional Theory

Li and Fe doped olivine Mg2SiO4 was studied using density functional theory. It was found that Li atom prefers to occupy M1 site to M2 site of Pbnm space group symmetry. Also, probability of Fe occupying M1 site over M2 site was also investigated and was observed, like Li, Fe also preferably occupies M1 site to M2 site. Two samples like Li2RLi2iMg6Si4O16 (R stands for replacement and i stands for interstitial site) and Li2RLi2iFe1Mg5Si4O16 were formulated by inserting Li at replacement site and interstitial site to maintain charge neutrality while Fe atom was inserted at replacement site. The probability of oxygen evolution during charging was minimized by incorporating redox active Fe+2 ion in Li doped Mg2SiO4. It was found that extraction of all the Li ions from Li2RLi2iMg6Si4O16 and Li2RLi2iFe1Mg5Si4O16 could be done at 3.68 V and 3.29 V with specific capacities of 198 mAh/g and 187 mAh/g respectively.

New Olivine Reference Materials for Secondary Ion Mass Spectrometry Oxygen Isotope Measurements

To accurately analyze the oxygen isotope of olivine using secondary ion mass spectrometry (SIMS), appropriate standard materials are required to calibrate for matrix effects caused by chemical composition differences between the samples and the standard materials. In this study, we investigated the homogeneity of oxygen isotopes in two natural olivine minerals using a secondary ion mass spectrometer to evaluate their potential as standard materials. The two minerals, JAY03-3 and JAY02-4, with forsterite contents of 99.3% and 99.6%, respectively, were evaluated for homogeneity in oxygen isotope composition. The recommended oxygen isotope values were characterized using CO2 laser fluorination, and the homogeneity was tested with in situ SIMS oxygen isotope measurements. Our results show that the δ18O value determined via CO2 laser fluorination for JAY03-3 is 16.37 ± 0.22‰ (2 s) and for JAY02-4 is 18.29 ± 0.28‰ (2 s). The precision of SIMS oxygen isotope measurements is 0.57‰ (2 s) for JAY03-3 and 0.70‰ (2 s) for JAY02-4. These two minerals have the potential to be used as standard materials for calibrating the oxygen isotope value of end-member forsterite during in situ analysis of SIMS oxygen isotope.

High precision measurement of trace F and Cl in olivine by electron probe microanalysis

Fluorine (F) and chlorine (Cl) are important volatiles in olivine and its high-pressure polymorphs, which would significantly affect olivine phase transition, melting temperature, and physical property of the mantle. F and Cl concentrations in olivine can be detected by electron probe microanalysis (EPMA). However, the analytical accuracy and precision can be impeded by severe peak overlaps, low peak intensities of traditional analytical crystals, and secondary fluorescence effects. In this study, we constructed an optimized analytical method with high accuracy and precision to analyze trace F and Cl in olivine. Key parameters of analytical crystals, beam conditions, peak overlaps, and secondary fluorescence effects were discussed. Variations in the levels of the analyzed trace elements fall within ± 10%. The detection limits (3σ) for F and Cl are lowered to 30 ppm and 5 ppm, respectively. This method can provide precise F and Cl analysis for natural olivine samples and help to provide significant information on its formation process.

Intracrystalline deformation microstructures in natural olivine with implications for stress estimation

Constraining the stress related to lithospheric deformation in natural rocks is key to develop and test a geodynamic model. However, the cautions of extrapolating piezometers that are established on experimental samples to natural rocks are less addressed. In this study, we investigated the microstructures of a natural harzburgite sample using the electron backscatter diffraction (EBSD) technique. Subgrain boundary (SGB) geometries suggest large percentages of (010)[100] and {0kl}[100] dislocation slip systems in olivines. More importantly, multiple low-angle misorientation boundaries (LAMBs) variants are recognized for the first time in olivine based on their distinctive characteristics with the change of EBSD mapping step size. The LAMBs that exist at a small step size (≤ 1 μm) are mostly equivalent to real SGBs, while other LAMBs that appear only when the step size is larger (> 1 μm) are artificial SGBs. Besides, the former develop mainly in the high LAMB density grains, whereas the latter are mostly found in the low LAMB density grains. This result reinforces the previous knowledge that the stress calculated using subgrain-related piezometers is meaningful only when real SGBs are captured at sufficiently small step size. Furthermore, we provide a proof of concept that SGB density and kernel average misorientation (KAM) are two viable metrics to estimate stress. These two alternative piezometers, which still need calibrations using the experimentally deformed samples, are anticipated to have wide applications in natural rocks.

CO2 transport and carbonate precipitation in the coupled diffusion-reaction process during CO2 storage

During geological CO2 storage and utilization, CO2 plume migration forms widespread static retention regions including the dead-end fractures and pores in the host rock, where the mass transfer is dominated by diffusion. A series of packed column experiments with natural olivine were conducted to investigate how the coupled CO2 diffusion–reaction process influences the CO2 transport and the temporal-spatial evolution of carbonate precipitation in diffusion-limited zones. The carbonate precipitations were mainly magnesite, and some dolomite due to the combined effect of carbonate precipitation kinetics and the low Ca content of the solution with Mg enrichment. Compared with the pure diffusion in porous media, the geochemical gradients in the pore solution resulting from the coupled diffusion–reaction interaction promoted mass transfer and broad migration, which led to an increase in effective diffusion coefficients of CO2 by approximately-one order of magnitude. The free water layer and pressure mode could affect the distribution of carbonate precipitation and the overall sequestration capacity. In particular, the coupled CO2 diffusion–reaction interaction under pressure decay mode could accelerate the carbonation in the deep region in the water-saturated column. This study advances the fundamental understanding on the transport and reaction of injected CO2 for mafic and ultramafic formations.

Rates of dehydration in hydrous, high-Fo, magmatic olivines

Natural olivine from the Etna Fall Stratified eruption (3930 BP) was dehydrated to determine hydrogen diffusivity. Dehydration was carried out in several time-steps, from 0.5 to 40 cumulative hours, in a 1 atm. furnace at 810 and 1000 °C. This starting material has high H2O concentration (∼50 ppm), high forsterite (Fo ∼ 90 %), and high fO2 (∼NNO + 1.2), reflecting characteristics of its host magma. H2O concentration was measured using Fourier transform infrared spectroscopy after each heating step. Spectral peaks were fit with Gaussian curves to determine the contribution from each H+ defect. Zonation profiles along each crystallographic direction were modelled using the ‘whole-block’ method to determine bulk and defect-specific H+ diffusivity.Our modelling reveals a common apparent diffusivity for all defects, which is faster along ([100]) than b ([010]) or c ([001]) by more than an order of magnitude. A common defect-specific diffusivity has not been found in experiments on pure forsterite, so it appears that the presence of Fe enables H+ from all defects to diffuse via the proton-polaron mechanism (supported by the observed anisotropy). However, the rates that we observe are slower than the proton-polaron mechanism, which supports the recent proposal by Ferriss et al. (2018) that the apparent diffusivity is rate-limited by reactions between H+ bound in defect sites and Fe2+. A coupled reaction and diffusion process may explain the observed decrease in diffusivity along a over time, as well as an apparent non-zero H2O concentration at the crystal edge. Both of these effects need to be considered when modelling H+ diffusion profiles to determine decompression rate – fixing the edge concentration to zero will underestimate the diffusion timescale.Arrhenius laws describing diffusion parallel to a (at the start and end of the experiments), b, and c are developed from the diffusivity at 810 and 1000 °C: Dastart=10-2.94exp-182000RT, Daend=10-4.63exp-154000RT, Db=10-2.35exp-243000RT,Dc=10-4.01exp-187000RT, where R is the gas constant 8.314 (J/mol K); T is the temperature (K), the units of the diffusivities (D) are in m2/s. A comparison of experimental dehydration studies on natural olivines reveals that the diffusivity during dehydration likely depends on how the olivine was experimentally hydrated. To avoid this experimental effect and understand the diffusivity operating during ascent in a volcanic conduit, crystals which are naturally hydrous (such as the Etna olivines in this study) are particularly useful. Despite some ambiguity in the literature owing to different experimental approaches, our results suggest that for the studied range of pressure and temperature, forsterite content is the major control on H+ diffusivity, while H2O concentration and the fO2 at which the crystal last equilibrated have a smaller effect within the ranges studied for magmatic olivines.

Iron and silicon isotope fractionation in silicate melts using first-principles molecular dynamics

The direct determination of silicate melts iron and silicon isotopes signature remains a major challenge of high-temperature isotope geochemistry. For this reason, melts are often approximated by silicate glasses. Calculation of precise equilibrium Si and Fe isotopes fractionation factors between minerals and melt would indeed allow us to distinguish equilibrium fractionation from diffusion-driven kinetic fractionation involved in the iron and silicon isotopes signatures of Earth and other planets. In this study, we use for the first time, first-principles molecular dynamics based on density functional theory to determine iron and silicon isotope compositions of different silicate melts, namely: iron-rich basalt, iron-depleted basalt, basanite, trachyte and phonolite. The 57Fe/54Fe reduced partition function ratios (β-factors) of the different melts span over a 1.1 ‰ range at 1000 Kelvin (K) while 30Si/28Si β-factors are much less influenced by the melt composition with a 0.5 ‰ fractionation range at the same temperature. The main parameter controlling iron isotope fractionation in silicate melts having similar iron oxidation state is, after temperature, the average Fe-O bond length. The chemical environment around iron (e.g. Fe-Fe distances) is suggested to contribute to Fe isotope fractionation as well. Silicon isotopes fractionation seems also affected, but to a lesser extent, by its local chemical composition with decreasing Si-Fe distances leading to slightly higher Si β-factor in the melt. From these melts Fe and Si β-factors, a new set of equilibrium fractionation factors between a variety of minerals and melts has been calculated. These new Δ57Femin-melt and Δ30Simin-melt sets allow us to discuss whether processes such as fractional crystallization, partial melting and diffusion could be responsible for the documented Fe and Si isotopes variations in igneous rocks. Our results suggest that: 1) fractional crystallization may explain at least part of the Fe and Si isotopic evolution during magmatic differentiation, for values up to δ57Fe = 0.65 ‰ and δ30Si = -0.1 ‰, respectively, 2) partial melting of the upper mantle can produce the Mid-Ocean Ridge Basalts (MORB) iron isotopes signature. Finally, we calculated that olivine at equilibrium with a basaltic melt could display an iron isotope composition down to −0.1 ‰ for δ57Fe. Hence, the lower isotopic compositions (δ57Fe < -0.1 ‰) observed in natural olivines are most likely due to diffusion-driven kinetic fractionation.

Scandium diffusion in forsterite: concentration dependence, inter-site reactions and the effect of trivalent cations on Fe diffusion

Diffusion of scandium (Sc) in synthetic forsterite (Mg2SiO4) was studied at 1400 °C at atmospheric pressure, in air. The Sc source was a powder of forsterite and protoenstatite (Mg2Si2O6) in order to buffer the silica activity, doped with various Sc contents, from ∼9 wt. ppm Sc to ∼8.5 wt% Sc2O3. The powder with the highest Sc content also contained thortveitite (Sc2Si2O7), which additionally buffers the activity of Sc2O3. All experiments were conducted for the same duration (550 h), but the length of the Sc diffusion profile (i.e. the distance over which the concentration decreased from the rim to core/background value) varied from ∼200 to 800 μm, with higher Sc concentrations associated with longer diffusion profiles. The geometry of Sc diffusion profiles also changes systematically with the Sc concentration – profiles with low Sc (<10 wt. ppm at the interface) have stepped shapes, those with medium Sc contents (80–400 wt. ppm at the interface) have near-linear profiles, while experiments with high Sc at the interface (>500 wt. ppm) show profiles with a shape that is approximately, but not exactly, describable by error function forms. Small but variable amounts of Fe contamination resulted in Fe diffusion profiles with interface concentrations of 80 to 1100 wt. ppm. The diffusivity of Fe, at these trace concentrations, appears to be a function of the concentration of Sc, rather than its own concentration, with the increased Sc concentration associated with an increase in Fe diffusivity of at least an order of magnitude. Similar behaviour is observed for other contaminants, Ni, Co, Mn and Li, at trace concentrations. Additionally, Al, present in the original forsterite crystal at the level of a few 10s of wt. ppm developed concentration profiles with a stepped form, broadly showing out-diffusion. The point along the profiles at which the Al concentration changes dramatically, coincides approximately with the end of the Sc profiles. Models are tested in which Sc diffusion is assumed to be concentration-dependent. In these models, the concentration-dependence is defined using the energy of binding between Sc3+ and M-site vacancy, and the diffusivity of the cation-vacancy pair. Such models are generally successful, but cannot fully describe the behaviour observed in these experiments. Rather, the diffusive behaviour of Sc and Al is better described by a model in which Sc3+ substitutes for Mg2+, charge balanced by vacancies (the Sc4/3[vac]2/3SiO4 substitution), and diffuses relatively rapidly in a concentration-dependent regime. Interaction of Sc with Al (present in the starting material) is proposed to lead to the formation of an MgScAlO4 component, upon which the diffusivity of Sc is drastically reduced. The model is capable of describing the majority of Sc and Al profiles observed. One experiment was hydroxylated following the diffusion anneal, revealing a multitude of defects supporting the suggestion that this system is more complex than simple concentration-dependent diffusion. Whilst these experiments were conducted in a simple system with lower Fe concentrations and higher fO2 than those typical of natural olivine, an example from a previous study (Cr diffusion in San Carlos olivine) shows that such complex behaviour may also be found in systems closer to natural conditions. The addition of Cr (∼2000 wt. ppm at the diffusion interface) appears to cause a ∼ 1 order of magnitude increase in the diffusivity of Fe-Mg, Ca and Ni relative to the published values conventionally used for diffusion chronometry.

Seismological Evidence for Girdled Olivine Lattice‐Preferred Orientation in Oceanic Lithosphere and Implications for Mantle Deformation Processes During Seafloor Spreading

AbstractSeismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid‐ocean ridge (MOR) dynamics. Yet, interpreting anisotropy in the context of grain‐scale deformation processes and strain observed in laboratory experiments and natural olivine samples has proven challenging due to incomplete seismological constraints and length scale differences spanning orders of magnitude. To bridge this observational gap, we estimate an in situ elastic tensor for oceanic lithosphere using co‐located compressional‐ and shear‐wavespeed anisotropy observations at the NoMelt experiment located on ∼70 Ma seafloor. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil‐spreading direction, consistent with corner‐flow deformation fabric. We compare this model with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250%–400% in the shallow mantle. We find evidence for D‐type olivine lattice‐preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes perpendicular to shear. D‐type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during MOR spreading; we hypothesize that grain‐boundary sliding during dislocation creep relaxes strain compatibility, allowing D‐type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation‐accommodated grain‐boundary sliding (disGBS) has implications for in situ stress and grain size during MOR spreading and implies grain‐size dependent deformation, in contrast to pure dislocation creep.

Viscosity of Crystal‐Mushes and Implications for Compaction‐Driven Fluid Flow

AbstractCentrifuge experiments on olivine, chromite, and plagioclase aggregates saturated in basaltic liquid show evidence of viscous compaction by grain‐boundary diffusion‐controlled creep. The experiments confirm that the exponential dependence of shear viscosity on melt fraction, observed at low porosities in earlier shear deformation experiments, extends to sedimentary porosities. The compaction profiles are inconsistent with the porosity‐dependence commonly ascribed to viscosity in macroscopic compaction models, which underestimate the effect of matrix disaggregation and consequently overestimate the viscosities of crystal mushes by 1–2 orders of magnitude. The time to halve the porosity of natural olivine igneous sediments by compaction is estimated from the centrifuge experiments to be O(103)y. Half‐times for plagioclase and chromite layers are O(104–105)y, suggesting that such layers compact on magmatic time scales only if they are loaded by additional sedimentation. At conditions relevant to melt flow in asthenospheric settings and trans‐crustal magmatic systems, the bulk and shear viscosities inferred for olivine and plagioclase are O(1017)Pa s, 4 orders of magnitude less than inferred from earlier experimental studies of the diffusion creep rheology. The reduced viscosities imply time‐ and length‐scales for compaction processes that are substantially shorter than previously anticipated. Our analysis serendipitously reveals that the oft‐neglected solidity term of the Carman‐Kozeny porosity‐permeability relation is essential to prevent non‐physical behavior in models of cumulate compaction.

Carbon mineralization with concurrent critical metal recovery from olivine

Carbon dioxide utilization for enhanced metal recovery (EMR) during mineralization has been recently developed as part of CCUS (carbon capture, utilization, and storage). This paper describes fundamental studies on integrating CO2 mineralization and concurrent selective metal extraction from natural olivine. Nearly 90% of nickel and cobalt extraction and mineral carbonation efficiency are achieved in a highly selective, single-step process. Direct aqueous mineral carbonation releases Ni2+ and Co2+ into aqueous solution for subsequent recovery, while Mg2+ and Fe2+ simultaneously convert to stable mineral carbonates for permanent CO2 storage. This integrated process can be completed in neutral aqueous solution. Introduction of a metal-complexing ligand during mineral carbonation aids the highly selective extraction of Ni and Co over Fe and Mg. The ligand must have higher stability for Ni-/Co- complex ions compared with the Fe(II)-/Mg- complex ions and divalent metal carbonates. This single-step process with a suitable metal-complexing ligand is robust and utilizes carbonation processes under various kinetic regimes. This fundamental study provides a framework for further development and successful application of direct aqueous mineral carbonation with concurrent EMR. The enhanced metal extraction and CO2 mineralization process may have implications for the clean energy transition, CO2 storage and utilization, and development of new critical metal resources.

Natural Forsterite Strongly Enriched in Boron: Crystal Structure and Spectroscopy

Boron is a typical crustal element and largely incompatible in olivine. Most natural olivine samples have very low concentrations of boron. Recently, forsterite with high boron content (up to 60.53 wt% MgO and 1795.91 ppm B) has been discovered in the Jian forsterite jade in the Jian area of northeast China. In this study, B-rich forsterite was examined by electron microprobes, Laser Ablation-Inductively Coupled Plasma Mass Spectrometry, Single crystal X-ray diffraction, Raman spectroscopy, and infrared spectroscopy. The B-rich forsterite is orthorhombic, existing in space group Pnma, and its unit-cell parameters are: a = 10.1918(7) Å, b = 5.9689(4) Å, c = 4.7484(3) Å, α = 90°, β = 90°, γ = 90°, and V = 288.86(3) Å3. The results of single crystal X-ray diffraction analysis indicate that the unit-cell parameters (a, b, and c) and unit-cell volume of forsterite in Jian forsterite jade are much smaller than those of known olivine. An equivalent set of Raman and infrared spectra were measured for the natural B-rich forsterite and compared to the results for mantle forsterite with a Fo value of ~91. The Raman spectrum of B-rich forsterite is similar to that of mantle olivine. We conclude that the systematic peak position shifts towards higher Raman shift with increasing Fo content. The infrared spectrum of B-rich forsterite crystals is characterized by strong absorption bands at 761, 1168, 1259, and 1303 cm−1, which are assigned to stretching vibrations of BO3 groups. Our data further confirm the existence of the B(F, OH)Si–1O–1 coupled substitution in natural B-rich forsterite.

Diffusion rates of hydrogen defect species associated with site-specific infrared spectral bands in natural olivine

We investigated hydrogen transport in naturally occurring, iron-bearing samples of San Carlos olivine that were hydrogenated at confining pressures of 200 or 300 MPa and 1173 to 1303 K or dehydrogenated at room pressure and 1191 to 1358 K. Chemical diffusion coefficients were determined from diffusion profiles for individual O-H-stretching bands from series of infrared spectra in orthogonal directions across each sample. Within experimental uncertainty, the diffusivities associated with all the individual bands are in good agreement with one another in both the hydrogenation and the dehydrogenation experiments. Hydrogenation proceeds by two diffusion mechanisms, as reported previously. The faster process involves interstitial diffusion of protons coupled with a counter-flux of polarons, with proton diffusion rate-limiting hydrogenation. For this mechanism, diffusion is faster along the olivine [100] direction than along [010] and [001], consistent with the anisotropy reported for proton diffusion and conductivity in olivine. The slower process involves interstitial proton diffusion coupled with a parallel flux of metal vacancies, with vacancy diffusion rate-limiting hydrogenation. For this mechanism, diffusion is faster along [001] than along [100] and [010], consistent with the anisotropy previously reported for the diffusion of metal cations in olivine. Diffusivities from our new dehydrogenation experiments are identical in both magnitude and anisotropy to those determined in our earlier hydrogenation experiments. This agreement demonstrates the validity of studies that used the results of our hydrogenation experiments to analyze dehydrogenation profiles in olivine xenocrysts and olivine in mantle xenoliths to determine rates of magma ascent from the source regions in Earth's interior.

Small effect of water incorporation on dislocation mobility in olivine: Negligible creep enhancement and water-induced fabric transition in the asthenosphere

To constrain the effect of water on upper mantle dynamics, we measured the annihilation rate coefficients (k) of [100](010) and [001](100) dislocations in olivine, referred to as a-dislocations and c-dislocations, respectively, as a function of water content. Natural olivine single crystals were doped with 5–800 wt. ppm water and sheared in the [100] or [001] directions along the (010) and (100) planes to produce a- and c-dislocations, respectively, and then annealed under quasi-hydrostatic conditions at a constant pressure and temperature of 5 GPa and 1473 K. The obtained annihilation rate coefficients are fitted to a power-law equation, yielding astonishingly small water-content exponents of 0.0±0.1 and 0.2±0.2 for the a- and c-dislocations, respectively. The overall effect of water on dislocation mobility is therefore small because these two slip systems are considered to be the least and most sensitive to water, respectively. These results imply that water incorporation does not effectively increase the dislocation-creep rate and that a water-induced fabric transition is unlikely. The effects of water on asthenospheric dynamics may thus be limited, and the lateral seismic anisotropy changes observed in the asthenosphere may solely reflect changes of mantle flow geometry.

Where did the largest diamonds grow? The experiments on percolation of Fe-Ni melt through olivine matrix in the presence of hydrocarbons

Recently it was found that large natural diamonds can grow from a metal liquid. One of the principal issues of the proposed hypothesis is the formation of so-called “pockets” filled with Fe-Ni melt and hydrocarbons in the Earth's mantle. The existing models of Fe migration imply percolation of liquid melt through interconnected interstices between silicate minerals, although these models face several fundamental problems in explaining the process of penetration of Fe melt between solid crystalline phases like silicate and oxide minerals. The aim of the present study is to contribute to the mechanism of Fe-Ni melt migration, and to elucidate the evolution of the “pockets” in the presence of hydrocarbons. The experiments were performed using a high-pressure apparatus “BARS” at pressures 3 and 5 GPa, and temperature 1600 °C. A silicate matrix consisting of natural olivine grains was used. The interstices in olivine were filled with anthracene that decomposes under high P-T into a complex hydrocarbon fluid. Percolation of Fe-Ni (64/36 wt%) melt through the interstices was demonstrated which occurred at relatively high rates. The basis of the proposed mechanism is “solubility-enhanced infiltration”: Fe-Ni occupies the space filled with light elements or substances that are soluble in the melt. It is suggested that the following simple, but efficient mechanism supports the growth of large diamonds as well as their resorption and storage within silicate mantle of the Earth for a long time.

The effect of excimer laser irradiation on mid-IR spectra of mineral mixtures for remote sensing

We examined the influence of micrometeorite bombardment simulated with an excimer laser, which irradiated mixtures of minerals with an energy density between 1.9 J/cm2 and 3.3 J/cm2. As starting material for the mixtures, we selected a natural olivine (Fo91) and a natural pyroxene (En87). Olivine and pyroxene are important rock-forming minerals of Earth's upper mantle and also occur on other terrestrial planetary bodies. The minerals were used to prepare a compositional range of mixtures that were pressed into pellets and examined especially by diffuse reflectance mid-infrared spectroscopy to identify changes caused by micrometeorite impacts as one tracer of space weathering on airless bodies such as Mercury or Moon. Spectral mid-IR measurements were carried out on untreated and treated samples in high vacuum. For comparison, the untreated samples are investigated in low vacuum under various phase angles. Especially the spectral variation between 7 μm–14 μm is of interest as the most important mid-IR features of rock-forming minerals as the Christiansen feature, Reststrahlen bands, and Transparency feature are located in this spectral region.As a result, we found that the mid-IR spectra of the powder pellets show signs of agglutination after irradiation. Here, olivine demonstrates a larger effect on irradiation than pyroxene, whereby pyroxene in mixtures can mask the presence of olivine in the mid-IR spectra after micrometeorite bombardment. We also observed a shift of the CF to longer wavelengths (shorter wavenumbers) after irradiation, and from high to low vacuum, a modification more prominent in olivine than in pyroxene. Therefore, this study shows that the identification and quantification of minerals in mixtures solely on the base of the CF location of the respective material can lead to an overestimation of modal abundances, which is important for the interpretation of data coming back from space missions, e.g., from the MERTIS experiment onboard BepiColombo.

The Principal Hugoniot of Iron‐Bearing Olivine to 1465 GPa

AbstractShock compression experiments on natural compositions are imperative to accurately model planetary accretion and the interior dynamics of planets. Combining shock compression experiments from the Sandia Z Machine and the OMEGA EP laser facility with density functional theory‐based molecular dynamics calculations, we report the first pressure‐density‐temperature (P‐ρ‐T) relationship of natural iron (Fe)‐bearing olivine ((Mg0.91Fe0.09)2SiO4) on the principal Hugoniot between 166 and 1,465 GPa. Additionally, we report the first reflectivities of natural olivine liquid in this pressure range. Compared to the magnesium‐endmember forsterite (Mg2SiO4), the presence of Fe in typical mantle abundance (∼9 wt% FeO) alters the US‐uP relation of olivine. On the other hand, the shock temperature and reflectivity of olivine are indistinguishable from forsterite where experimental conditions overlap. Both forsterite and olivine increase in reflectivity (and hence optical conductivity) with increasing temperature, with a maximum reflectivity of ∼31% at shock velocities greater than 22 km/s (∼800 GPa).

Ni-in-garnet geothermometry in mantle rocks: a high pressure experimental recalibration between 1100 and 1325\xa0\xb0C

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule left( {{text{lnD}}_{{{text{grt}}/{text{olv}}}}^{{{text{Ni}}}} ; frac{{{text{Ni}}_{{{text{grt}}}} }}{{{text{Ni}}_{{{text{olv}}}} }}} right), the Ca mole fraction in garnet ({mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}}; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet ({mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}}; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as:T left(^circ{rm C} right)=frac{-8254.568}{left(left( {mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}} times 3.023 right)+left({mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}} times 2.307 right)+left({mathrm{lnD}}_{frac{mathrm{grt}}{mathrm{olv}}}^{mathrm{Ni}} - 2.639 right)right)}-273pm 55where {mathrm{D}}_{mathrm{grt}/mathrm{olv}}^{mathrm{Ni}}= frac{{mathrm{Ni}}_{mathrm{grt}}}{{mathrm{Ni}}_{mathrm{olv}}}, Ni is in ppm, {mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}} = Ca/(Ca + Fe + Mg) in garnet, and {mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}}= Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.