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The key features of the interaction between peridotites of the continental lithospheric mantle and reduced hydrocarbon-rich fluids have been studied in experiments conducted at 5.5 GPa and 1200 °C. Under this interaction, the original harzburgite undergoes recrystallization while the composition of the fluid changes from CH4-H2O to H2O-rich with a small amount of CO2. The oxygen fugacity in the experiments varied from the iron-wustite (IW) to enstatite-magnesite-olivine-graphite/diamond (EMOG) buffers. Olivines recrystallized in the interaction between harzburgite and a fluid generated by the decomposition of stearic acid contain inclusions composed of graphite and methane with traces of ethane and hydrogen. The water content of such olivines slightly exceeds that of the original harzburgite. Redox metasomatism, which involves the oxidation of hydrocarbons in the fluid by reaction with magnesite-bearing peridotite, leads to the appearance of additional OH absorption bands in the infrared spectra of olivines. The water content of olivine in this case increases by approximately two times, reaching 160–180 wt. ppm. When hydrocarbons are oxidized by interaction with hematite-bearing peridotite, olivine captures Ca-Mg-Fe carbonates, which are products of carbonate melt quenching. This oxidative metasomatism is characterized by the appearance of specific OH absorption bands and a significant increase in the total water content in olivine of up to 500–600 wt. ppm. These findings contribute to the development of criteria for reconstructing metasomatic transformations in mantle rocks based on the infrared spectra and water content of olivines. [ABSTRACT FROM AUTHOR]

This study aims to experimentally constrain the water storage capacities of olivine and wadsleyite at a depth near 410 km (12–14 GPa) under water-saturated conditions, as a function of temperature, oxygen fugacity, and the presence of carbon (molar H / C of 2). Experiments have been conducted in the multi-anvil press, with sealed double capsules to preserve fluids, at 1200 to 1400 ∘ C and three different oxygen fugacities fixed at the rhenium–rhenium oxide buffer (RRO), nickel–nickel oxide buffer (NNO), and iron-wüstite (IW) for oxidizing, intermediate, and reducing conditions, respectively. The water contents of minerals were measured by Raman spectroscopy that allows a very small beam size to be used and were cross-checked on a few samples with NanoSIMS analyses. We observe an effect, although slight, of fO2 on the water storage capacity of both wadsleyite and olivine and also on their solidus temperatures. At 1200 ∘ C, the storage capacity of the nominally anhydrous minerals (NAMS) increases with increasing oxygen fugacity (from the IW to the RRO buffer) from 1 wt % to 1.5 wt % H 2 O in wadsleyite and from 0.1 wt % to 0.2 wt % in olivine, owing to the increase in H 2 O / H 2 speciation in the fluid, whereas at 1400 ∘ C the storage capacity decreases from 1 wt % to 0.75 wt % H 2 O in wadsleyite and down to 0.03 wt % for olivine. At high temperature, the water storage capacity is lowered due to melting, and the more oxidized the conditions are the more the solidus is depressed. Still, at 1400 ∘ C and IW, wadsleyite can store substantial amounts of water: 0.8 wt % to 1 wt % H 2 O. The effect of carbon is to decrease water storage capacity in both wadsleyite and olivine by an average factor 2 at 1300–1400 ∘ C. The trends in water storage as a function of fO2 and C presence are confirmed by NanoSIMS measurements. The solidus at IW without C is located between 1300 and 1400 ∘ C in the wadsleyite stability field and drops to temperatures below 1300 ∘ C in the olivine stability field. With the addition of C, the solidus is found between 1200 and 1300 ∘ C in both olivine and wadsleyite stability fields. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The diffusion coefficient associated with the 3,612 cm−1 infrared absorbance band in H‐bearing olivine, attributed to a defect involving a vacant Si site balanced by four H+ (i.e., (4H)Si× ${(4\mathrm{H})}_{\mathrm{S}\mathrm{i}}^{\times }$), was determined in H in‐diffusion experiments at 1200°C–1400°C and 1.5–3.0 GPa in piston‐cylinder and multi‐anvil apparatuses. The retrieved values are in good agreement with experimentally determined diffusion coefficients ascribed to the same process from previous experiments, despite those being conducted at atmospheric pressure and representing out‐diffusion rather than in‐diffusion. Overall, the diffusivity of the Si vacancy is several orders of magnitude lower than that of the M‐site vacancy (∼4 orders of magnitude at 1000°C), with a higher activation energy (∼450–500 vs. 200–250 kJ mol−1), when both are charge‐compensated solely by H+. From these data, the diffusion coefficient of Si in H‐bearing olivine can be estimated using the relationship between vacancy concentration, vacancy diffusivity, and diffusivity of an ion moving by a vacancy mechanism. However, these diffusivities are inconsistent with published results for Si diffusivity, with a considerable discrepancy of around two orders of magnitude. This suggests that simple relationships between diffusion coefficients of vacancies and species moving by vacancy diffusion mechanisms may be inappropriate, highlighting again the complexity of H incorporation and diffusion mechanisms in olivine. Key Points: Hydrogen diffusion associated with Si vacancies in olivine is confirmed to be slowH‐bearing defects form by complex processes involving inter‐site reactionsSimple relationships between vacancy diffusivity and vacancy concentration cannot explain Si diffusion in H‐bearing olivine [ABSTRACT FROM AUTHOR]

Trace concentrations of H2O in olivine strongly affect diverse mantle and magmatic processes. H2O in olivine has been difficult to accurately quantify due to challenges in sample preparation and measurement, as well as significant uncertainties in standard calibrations. Here we directly compare secondary-ion mass spectrometry (SIMS) measurements of the olivine standards of Bell et al. (2003, hereafter Bell03) and Withers et al. (2012, hereafter Withers12) upon which most SIMS and Fourier transform infrared (FTIR) spectroscopy analyses are based. In the same SIMS session, we find that the olivine standards from the two studies are offset by ~50%, forming lines of different slope when comparing SIMS measurements to the independent nuclear reaction analysis (NRA) in Bell03 and elastic recoil detection analysis (ERDA) in Withers12. This offset is similar to the ~40% offset that exists in the FTIR absorption coefficients determined by those two studies, and points to the NRAERDA data as the cause for the offset more than different IR absorption characteristics of the different olivines. We find that the Withers12 olivine standards form the most precise calibration line, and that the measured Bell03 olivine standards have issues of reproducibility and accuracy due to the presence of hydrous inclusions (as documented previously by Mosenfelder et al. 2011). Owing to the limited availability of the Withers12 olivine standards, however, we recommend using orthopyroxene standards (Kumamoto et al. 2017) to calibrate H2O in olivine by SIMS due to similar calibration slopes. We revise the reference values of current orthopyroxene standards to account for uncertainties in the Bell et al. (1995) manometry data. With these revised values, the orthopyroxene calibration line is within 12% of the Withers12 olivine line, which is within the long-term uncertainty of the SIMS olivine measurements. We apply our SIMS calibration protocol to revise estimates of the partition coefficients for H2O between olivine and melt, resulting in a value of 0.0009 ± 0.0003 at pressures ~0.2–2 GPa. This brings into closer agreement between the partition coefficients determined from experimental studies and those based on natural studies of olivine-hosted melt inclusions. [ABSTRACT FROM AUTHOR]

Hydroxyl defects in nominally anhydrous minerals (NAMs) were potential carriers of water in the early Solar System and might have contributed to the accretion of terrestrial water. To better understand this, we have conducted a nanoscale secondary ion mass spectrometry survey of water contents in olivine and orthopyroxene from a set of equilibrated ordinary chondrites of the L and LL groups (Baszkówka, Bensour, Kheneg Ljouâd, and Tuxtuac) and several ultramafic achondrites (Zakłodzie, Dhofar 125, Northwest Africa [NWA] 4969, NWA 6693, and NWA 7317). For calibration, we used terrestrial olivine and orthopyroxene with H2O contents determined by Fourier transform infrared. Our 99.7% (~3SD) detection limits are 3.6–5.4 ppmw H2O for olivine and 7.7–10.9 ppmw H2O for orthopyroxene. None of the meteoritic samples studied consistently shows water contents above the detection limits. A few exceptions slightly above the detection limits are suspected of terrestrial contamination by ferric oxyhydroxides. If the meteorite samples investigated accreted in the presence of small amounts of water ice, the upper limits of water contents provided by our survey suggest that the retention of hydrogen during thermal metamorphism and differentiation was ineffective. We suggest that loss occurred through combinations of low internal pressures, high permeability along grain boundaries, and speciation of hydrogen into reduced compounds such as H2 and methane, which are less soluble in NAMs than in water. [ABSTRACT FROM AUTHOR]

Whilst the diffusivity of hydrogen (H) in forsterite (Mg2SiO4) has been extensively studied, there still remain some puzzling observations. Firstly, experiments measuring ostensibly the same process have provided different results in terms of diffusion coefficients. Secondly, despite H diffusion in forsterite generally being associated with diffusion of M-vacancies charge compensated by 2H+, a plethora of H-bearing point defects have been observed, including those associated with Si vacancies, trivalent cations and tetravalent cations in the form of so-called 'clinohumite-type' point defects. This has been tentatively associated with some form of inter-site reaction, such as one in which a M-site vacancy associated with 2H+ reacts with tetrahedrally coordinated Ti4+. Equivalent reactions can be constructed to form all point defects mentioned above. Here, we present a series of numerical models in which these processes are simulated. In the models, the mobility of H is described using a diffusion coefficient (D*) for the hydrogenated M-site vacancies and an equilibrium constant (K) for the relevant inter-site reaction(s). Reevaluation of published data shows that the extracted D* and K values are consistent between some different datasets, even in situations where the phenomenological (chemical) diffusion coefficient D ~ , extracted simply using solutions of Fick's second law, did not agree. The 'true' mobility (D*) of the M-site vacancy associated with 2H+ must be between 1 and 2 orders of magnitude greater than previously determined D ~ in order to form measurable profiles of all point defects observed by vibrational spectroscopy. Density functional theory calculations of the K of each of the inter-site reactions implemented in our model show good agreement (within an order of magnitude) with those determined experimentally for the reactions forming 'clinohumite-type' defects, but considerable disagreement (~ 3 orders of magnitude) for the defects involving trivalent cations, potentially due to assumptions related to binding of different components within individual defects. Overall, the first-order implication is that H diffusion profiles that we observe in natural and experimental samples are unlikely to be formed by simple diffusion alone. These models provide a new methodological framework for further understanding of complex ionic diffusion mechanisms in olivine and the other nominally anhydrous minerals. [ABSTRACT FROM AUTHOR]

Infrared spectroscopy (IR) is the most widely used analytical tool to quantify trace water in silicate and silica minerals. A prerequisite for highly accurate IR measurements of trace water is a good understanding of the effect of the second Si–O vibrational overtones/combination bands (2nd Si–O VOCBs) on the water peaks. Silicate and silica minerals can be divided as isolated (Q0), paired (Q1), ring (Q2), chain (Q1 or Q2), sheet (Q3) and framework (Q4) structures according to the polymerization of their SiO4 tetrahedral units, and the 2nd Si–O VOCBs of these different structural types attain different vibrational features which are expected to affect the water peaks to different extents. Here, we selected olivine (Q0) and α-quartz (Q4) as two endmember-like structural examples, performed extensive IR measurements on both pristine and heat-treated thin sections prepared for these two minerals, and explored the vibrational features of the 2nd Si–O VOCBs. We have found that the 2nd Si–O VOCBs are well separated from the water peaks in olivine, but severely overlap with the water peaks in α-quartz, confirming the different roles that the 2nd Si–O VOCBs play in quantifying trace water in silicate and silica minerals with different structural polymerizations. To remove the influence of the 2nd Si–O VOCBs (or any other species rather than water), an experimental protocol has been successfully developed, as approved by some fundamental equations and verified by the data of α-quartz in the literature. This development should lead to significant accuracy improvement in quantifying trace amounts of water in Earth and planetary materials. [ABSTRACT FROM AUTHOR]

Nominally anhydrous minerals (NAM) such as olivine are important reservoirs for water in the Earth's upper mantle, although the association of water with crystallographic defects is poorly understood. Here, hydrated regions at the atomic scale in fayalite (Fe2SiO4) are directly resolved using atom probe tomography. Site‐specific analyses reveal regions that are either compositionally homogenous or contain nanometer‐long channels that are consistent with hydrated defects. Na as a minor element is also enriched within the interlayers, indicating the formation environment is associated with Na geochemically. The ability to directly visualize and characterize hydrated regions at the nanometer scale can provide critical insight into the material properties of NAM and their impact on planetary‐scale processes, such as the influence of water‐bearing minerals on mantle rheology and geodynamics. Plain Language Summary: Anhydrous minerals such as olivine, containing seemingly small amounts of water per unit volume, are significant reservoirs of water on Earth and play a critical role in planetary geodynamics. Our knowledge of how water incorporates into these minerals is however lacking because it is challenging to actually see where the water resides in the crystal structure at the atomic scale. Here, we utilize the novel chemical imaging technique atom probe tomography to map water within minerals in three dimensions atom‐by‐atom. We find signatures for water within nanometer‐long structurally defective channels in the crystal. Sodium, thought to be a constituent of the formation environment, is also found to coincide with these defects. While the mode of water incorporation in minerals has been speculated for decades, our observations are important because it provides the first direct evidence for this occurrence. The approach developed here can be further applied to understand mechanisms for water incorporation in minerals in various environments. By understanding the fundamental properties of minerals at the atomic level, we can gain important insight into processes at the planetary scale such as the influence of water‐bearing minerals on mantle rheology. Key Points: At the planetary scale nominally anhydrous minerals are significant water reservoirs, but the mode of incorporation is poorly understoodHydrated interlayers and minor element partitioning in fayalite are visualized at the near atomic level using atom probe tomographyThis approach provides unique insight into the distribution of water in geomaterials, enabling a deeper understanding of their properties [ABSTRACT FROM AUTHOR]

We investigate the coupling between H, minor, trace, and ultra-trace element incorporations in 17 olivines from ten different locations covering various petrological origins: magmatic, hydrothermal, and mantle-derived context. Concentrations in major element are determined by micro X-ray fluorescence. Minor, trace, and ultra-trace elements are determined by laser ablation inductively coupled plasma mass spectrometry. Hydrogen concentrations are quantified using unpolarized and polarized Fourier transform infrared spectroscopy (FTIR). Forsterite contents (83.2–94.1%) reflect the petrogenetic diversity. Hydrogen concentrations range from 0 to 54 ppm H2O wt. Minor element concentrations (Ni + Mn) range from 3072 to 4333 ppm, and impurities are dominated by Ni, Mn, Ca or B. Total trace element concentrations range from 8.2 to 1473 ppm. Total rare Earth and extended ultra-trace elements concentrations are very low (< 0.5 ppm). Magmatic and hydrothermal olivines show the most and least amount of impurities, respectively, and mantle-derived olivines have concentrations between these two extremes. Combined with minor, trace, and ultra-trace element concentrations, the hydrogen concentrations, and FTIR OH bands reflect the point defect diversity imposed by different geological settings. Hydrogen concentrations are inversely correlated with divalent impurities, indicating their competition for vacancies. However, a broad positive correlation is also found between OH bands at 3575 and 3525 cm−1 and Ti, confirming the existence of Ti-clinohumite-like point defect in mantle olivines. Nonetheless, Ti does not exclusively control hydrogen incorporation in olivine due to the co-existence with other mechanisms, and its effect appears diluted. Our results confirm that hydrogen behaves as a peculiar incompatible element, and furthermore as an opportunistic impurity in olivine. [ABSTRACT FROM AUTHOR]

The study of nominally anhydrous minerals with vibrational spectroscopy, despite its sensitivity, tends to produce large uncertainties (in absorbance or intensity) if the observed dispersion of the values arising from the anisotropy of interaction with light in non-cubic minerals is not assessed. In this study, we focused on Raman spectroscopy, which allows the measurement of crystals down to a few micrometers in size in backscattered geometry, and with any water content, down to 200 ppm by weight of water. Using synthetic hydrous single-crystals of olivine and wadsleyite, we demonstrate that under ideal conditions of measurement and sampling, the data dispersion reaches ±30% of the average (at 1σ) for olivine and ±32% for wadsleyite, mostly because of their natural anisotropy. As this anisotropy is linked to physical properties of the mineral, it should not be completely considered as an error without treatment. By simulating a large number of measurements with a 3D model of the OH/Si spectral intensity ratio for olivine and wadsleyite as a function of orientation, we observe that although dispersion increases when increasing the number of measured points in the sample, analytical error decreases, and the contribution of anisotropy to this error decreases. With a sufficient number of points (five to ten, depending on the measurement method), the greatest contribution to the error on the measured intensities is related to the instrument's biases and reaches 12 to 15% in ideal cases, indicating that laser and power drift corrections have to be carefully performed. We finally applied this knowledge on error sources (to translate data dispersion into analytical error) on olivine and wadsleyite standards with known water contents to build calibration lines for each mineral to convert the intensity ratio of the water bands over the structural bands (OH/Si) to water content. The conversion factors from OH/Si to parts per million by weight of water (H2O) are 93 108 ± 24 005 for olivine, 250 868 ± 53 827 for iron-bearing wadsleyite, and 57 862 ± 12 487 for iron-free wadsleyite, showing the strong effect of iron on the spectral intensities. [ABSTRACT FROM AUTHOR]

We investigated the electrical conductivity of Ti‐H‐doped synthetic olivine aggregates at 4 GPa, 873–1273 K, and controlled oxygen fugacities. Under a given pressure and temperature, electrical conductivity depends on both hydrogen and Ti content, but these samples show different conductivity behavior from that observed in Ti‐poor sample such as San Carlos olivine. We found that when Ti content is comparable to or larger than hydrogen content, Ti has notable effects on electrical conductivity, but the effects of Ti is different between the H‐rich and the H‐poor regimes. In the H‐rich regime, electrical conductivity of olivine is weakly dependent on Ti content but has different sensitivity to water content than a Ti‐poor olivine. In contrast, in the H‐poor regime, electrical conductivity of Ti‐rich olivine is substantially higher than the conductivity of Ti‐poor olivine. As a consequence, the effect of hydrogen for the Ti‐rich synthetic olivine on electrical conductivity is smaller than for the Ti‐poor (natural) olivine for the modest H content expected in the asthenosphere, whereas in the H‐poor lithosphere Ti will enhance the electrical conductivity substantially. Possible models to explain these observations are proposed including the interaction of Ti‐related defects and H‐related defects as well as the charge transfer caused by the hopping conduction due to Ti3+ ⇔ Ti4+ under the H‐poor conditions. We conclude that the addition of Ti to olivine affects the behavior of H‐related defects, and therefore the applications of results from Ti‐rich olivine samples to the Ti‐poor real Earth need to be made with great care. Key Points: Ti has a strong effect on electrical conductivity of sample; however, its influence is completely different in H‐rich and H‐poor regimesDefect models to explain these observations are proposed by interaction of Ti‐related and H‐related defects and hopping of small polaronIt is suggested that the applications of results from Ti‐rich olivine samples to the Ti‐poor real Earth need to be made with great care [ABSTRACT FROM AUTHOR]

In Part 1 of this study (Yabe et al., 2020, https://doi.org/10.1029/2020JB019415), we established an olivine grain‐boundary diffusion creep law that incorporates the chemically induced enhancement of creep rates, which becomes significant with increasing temperature. The effect was attributed to grain‐boundary disordering that starts at ~0.92 × solidus (bulk eutectic temperature). In this study, we estimated solidus temperatures of the samples used in the previous diffusion creep experiments. These temperatures were used to compare previously reported diffusion creep rates for olivine with our established diffusion creep law. We found that the law explains a difference of up to 2 orders of magnitude in olivine creep rates at the same temperatures, stresses, grain sizes, and water contents in the previous studies. The geotherm normalized by the mantle solidus was calculated for the upper mantle with water contents ranging from 0 to 300 μg/g, which predicts depths where grain‐boundary diffusion creep is enhanced due to grain‐boundary disordering. Constructed viscosity‐depth profiles reveal a very thin mantle lithosphere beneath mid‐ocean ridges, with development of the lithosphere away from the ridge, leaving a low‐viscosity region below. Given a grain size of 1 mm and depending on the water content, a viscosity of 2–5 × 1019 Pa·s is predicted for the low‐viscosity mantle beneath 50‐million‐year‐old seafloor. Key Points: Solidus‐dependent grain‐boundary diffusion creep explains differences among previous olivine diffusion creep resultsEnhanced grain‐boundary diffusion creep at >0.92 × solidus results in weakening of the upper mantleDepth profiles of viscosity based on grain‐boundary diffusion creep in the oceanic upper mantle are presented [ABSTRACT FROM AUTHOR]

High‐temperature ionic conductivity in olivine single crystals has been measured in the [100], [010], and [001] crystallographic orientations as a function of pressure from 2 to 10 GPa, temperature from 1450 to 2180 K, and H2O content from 20 to 580 wt. ppm using multianvil presses with in situ impedance analyses. The experimental results yield an activation energy, activation volume, and H2O content exponent of 250–405 kJ/mol, 3.2–5.3 cm3/mol, and 1.3 ± 0.2, respectively, for the high‐temperature ionic conduction regime. Olivine ionic conductivity has negative pressure and positive temperature dependences and is significantly enhanced by H2O incorporation. The [001] direction is more conductive than the [100] and [010] directions. The H2O‐enhanced ionic conductivity may contribute significantly to the electrical conductivity profile in the asthenosphere, especially in the regions under relatively high‐temperature and low‐pressure conditions. Key Points: Pressure, temperature, and water content dependences of olivine ionic conductivity are obtainedOlivine ionic conductivity is dramatically enhanced by water incorporationOlivine ionic conductivity may contribute significantly to the bulk conductivity of olivine in the asthenosphere [ABSTRACT FROM AUTHOR]

Fingerprinting hydrogen storage sites in olivine at high temperature and high pressure is fundamental to understand water distribution and its impact on the upper mantle. We carried out in situ high-temperature and high-pressure IR spectroscopic investigations on hydrogen storage sites in the natural olivine and synthetic Fe-free forsterite. Based on in situ observations of hydrogen in both the natural olivine and synthetic Fe-free forsterite at high temperatures and high pressures, we find that hydrogen does not transfer between storage sites with increasing temperature, but displays disordering at temperatures over 600 °C. In contrast, pressure can induce re-configuration of hydrogen storage sites corresponding to the 3610 and 3579 cm–1 bands. Hydrogen storage sites also exhibit disordering at high pressure. In addition, the dehydrogenation experiments of the natural olivine indicate interacts of hydrogen storage sites. Protons released from titanium-clinohumite defects move to pure Si vacancies, and also to Mg vacancies coupling with trivalent cations. This study is the first attempt to fingerprint hydrogen storage sites in olivine at high temperature and high pressure using in situ IR spectroscopy. The implications of the temperature- and pressure-induced disordering and re-configuration of hydrogen storage sites are discussed. The disordering and re-configuration of hydrogen storage sites at high temperature and high pressure favor better understanding of the water effects on physical properties of olivine. The interactions of hydrogen storage sites during dehydrogenation warn that some hydrogen if observed in dehydrated mantle-derived samples may not be original and also make hydrogen diffusivity complex. [ABSTRACT FROM AUTHOR]

Abstract: The pressure and temperature sensitivity of the two most important point hydrous defects in mantle olivine involving Si vacancies (associated to trace amounts of titanium [TiChu‐PD] or exclusively to Si vacancies [Si]) was investigated at subsolidus conditions in a fluid‐saturated natural peridotite from 0.5 to 6 GPa (approximately 20–200 km depth) at 750 to 1050°C. Water contents in olivine were monitored in sandwich experiments with a fertile serpentine layer in the middle and olivine and pyroxene sensor layers at the border. Textures and mineral compositions provide evidence that olivine completely recrystallized during the weeklong experiments, whereas pyroxenes displayed only partial equilibration. A site‐specific water solubility law for olivine has been formulated based on the experiments reconciling previous contradictory results from low‐ and high‐pressure experiments. The site‐specific solubility laws permit to constrain water incorporation into olivine in the subducting slab and the mantle wedge, as these are rare locations on Earth where fluid‐present conditions exist. Chlorite dehydration in the hydrated slab is roughly parallel to the isopleth of 50 ± 20 ppm wt H2O in olivine, a value which is independent of the pressure and temperature trajectory followed by the slab. Hydrous defects are dominated by [Si] under the relevant conditions for the mantle wedge affected by fluids coming from the slab dehydration (slab‐adjacent low viscosity/seismic low‐velocity channel, P > 3 GPa). In cold subduction zones at 5.5 km from the slab surface the storage capacity of the mantle wedge at depths of 100–250 km ranges from 400 to 2,000 ppm wt H2O. [ABSTRACT FROM AUTHOR]

Hydrogen distribution between nominally anhydrous minerals (NAMs) of a garnet-lherzolite under subsolidus conditions has been investigated. Separated NAMs from a garnet-peridotite from Patagonia (Chile) are annealed together (olivine, orthopyroxene, clinopyroxene, and garnet) using a piston-cylinder at 3 GPa and 1100 °C using talc-pyrex cell assembly for 10, 25, and 100 h. The talc-pyrex assembly provides enough hydrogen in the system to re-equilibrate the hydrogen concentrations at high pressure. The three coexisting nominally anhydrous minerals (NAMs, i.e., olivine, orthopyroxene, and clinopyroxene) were successfully analyzed using FTIR. The resulting hydrogen concentrations exceed significantly the initial hydrogen concentration by a factor of 13 for olivine and a factor of 3 for both pyroxenes. Once mineral-specific infrared calibrations are applied, the average concentrations in NAMs are 115 ± 12 ppm wt H2O for olivine, 635 ± 75 ppm wt H2O for orthopyroxene, and 1214 ± 137 ppm wt H2O for clinopyroxene, garnet grains are dry. Since local equilibrium seems achieved over time (for 100 h), the calculated concentration ratios are interpreted as mineral-tomineral hydrogen partition coefficients (i.e., Nernst's law) for a garnet-peridotite assemblage. It yields, based on mineral-specific infrared calibrations, DOpx/Ol = 5 ± 1, DCpx/Ol = 10 ± 2, and DCpx/Opx = 1.9 ± 0.4. While DCpx/Opx is in agreement (within error) with previous results from experimental studies and concentration ratios observed in mantle-derived peridotites, the DPx/Ol from this study are significantly lower than the values reported from mantle-derived xenoliths and also at odd with several previous experimental studies where melt and/or hydrous minerals co-exists with NAMs. The results confirm the sensitivity of hydrogen incorporation in olivine regarding the amount of water-derived species (H) in the system and/or the amount of water in the coexisting silicate melt. The results are in agreement with an important but incomplete dehydration of mantle-derived olivine occurring at depth, during transport by the host magma or during slow lava flow cooling at the surface. The rapid concentration modification in mantle pyroxenes also points out that pyroxenes might not be a hydrogen recorder as reliable as previously thought. [ABSTRACT FROM AUTHOR]

The water solubility in olivine $$ \left({C}_{{\mathrm{H}}_2\mathrm{O}}\right) $$ has been investigated at 1050 °C and 3 GPa as a function of water activity $$ \left({a}_{{\mathrm{H}}_2\mathrm{O}}\right) $$ at subsolidus conditions in the piston-cylinder apparatus, with $$ {a}_{{\mathrm{H}}_2\mathrm{O}} $$ varied using HO-NaCl fluids. Four sets of experiments were conducted to constrain the effect of $$ {a}_{{\mathrm{H}}_2\mathrm{O}} $$ on the four main substitution mechanisms. The experiments were designed to grow olivine in situ and thus achieve global equilibrium (G-type), as opposed to hydroxylating olivine with a pre-existing point-defect structure and impurity content (M-type). Olivine grains from the experiments were analysed with polarised and unpolarised FTIR spectroscopy, and where necessary, the spectra have been deconvoluted to quantify the contribution of each substitution mechanism. Olivine buffered with magnesiowüstite produced absorbance bands at high wavenumbers ranging from 3566 to 3612 cm. About 50% of the total absorbance was found parallel to the a-axis, 30% parallel to the b-axis and 20% parallel to the c-axis. The total absorbance and hence water concentration in olivine follows the relationship of $$ {C}_{{\mathrm{H}}_2\mathrm{O}}\propto {a_{{\mathrm{H}}_2\mathrm{O}}}^2 $$ , indicating that the investigated defect must involve four H atoms substituting for one Si atom (labelled as [Si]). Forsterite buffered with enstatite produced an absorbance band exclusively aligned parallel the c-axis at 3160 cm. The band position, polarisation and observed $$ {C}_{{\mathrm{H}}_2\mathrm{O}}\propto {a}_{{\mathrm{H}}_2\mathrm{O}} $$ are consistent with two H substituting for one Mg (labelled as [Mg]). Ti-doped, enstatite-buffered olivine displays absorption bands, and polarisation typical of Ti-clinohumite point defects where two H on the Si-site are charge-balanced by one Ti on a Mg-site (labelled as [Ti]). This is further supported by $$ {C}_{{\mathrm{H}}_2\mathrm{O}}\propto {a}_{{\mathrm{H}}_2\mathrm{O}} $$ and a 1:1 relationship of molar HO and TiO in these experiments. Sc-doped, enstatite-buffered experiments display a main absorption band at 3355 cm with $$ {C}_{{\mathrm{H}}_2\mathrm{O}}\propto {a_{{\mathrm{H}}_2\mathrm{O}}}^{0.5} $$ and a positive correlation of Sc and H, indicating the coupled substitution of a trivalent cation plus a H for two Mg (labelled as [triv]). Our data demonstrate that extreme care has to be taken when inferences from experiments conducted at $$ {a}_{{\mathrm{H}}_2\mathrm{O}}=1 $$ are applied to the mantle, where in most cases, a low $$ {a}_{{\mathrm{H}}_2\mathrm{O}} $$ persists. In particular, the higher exponent of the [Si] substitution mechanism means that the contribution of this hydrous defect to total water content will decrease more rapidly with decreasing $$ {a}_{{\mathrm{H}}_2\mathrm{O}} $$ than the contributions of the other substitution mechanisms. The experiments confirm previous results that the [Mg] mechanism holds an almost negligible amount of water under nearly all T-P-fO-fHO conditions that may be anticipated in nature. However, the small amounts of HO we find in substituting by this mechanism are similar in the experiments on forsterite doped with either Sc or Ti to those in the undoped forsterite at equivalent $$ {a}_{{\mathrm{H}}_2\mathrm{O}} $$ (all buffered by enstatite), confirming the assumption that, thermodynamically, $$ {C}_{{\mathrm{H}}_2\mathrm{O}} $$ substituting by each mechanism does not depend on the water concentration that substitutes by other mechanisms. [ABSTRACT FROM AUTHOR]

Hydrogen incorporation in olivine involves many OH defects, which will control the hydrogen solubility at mantle conditions. Several of these OH defects are identified from the investigation of forsterite (the olivine Mg end-member). We study here the effect of Fe2+, Fe3+, Al3+, and Cr3+ on OH defects to improve our understanding of the hydrogen speciation in natural olivine. Low-temperature infrared spectra (-194 °C) are collected on synthetic and natural olivines. These spectra are then interpreted in the light of the theoretical determination of the structural, vibrational, and infrared spectroscopic properties of Fe-related OH defects, using first-principles calculations based on density functional theory. The presence of Fe2+ changes the cationic environment around the fully protonated vacancies in forsterite, leading to a slight modification of their infrared signatures. In particular, the presence of Fe2+ in an octahedral site adjacent to a hydrogarnet-type defect is likely responsible for the additional bands observed at 3599 cm-1 and around 3520-3550 cm-1 in Fe-doped olivines. Results show that the OH bands between 3310 and 3380 cm-1 are associated with the presence of trivalent cations. Specifically, two bands at 3323 and 3358 cm-1, commonly observed in natural olivine, are associated with the substitution of Mg2+ by Cr3+ while two similar bands at 3328 and 3353 cm-1 are associated with the substitution of Mg2+ by Fe3+. The presence of these defects and the "titanoclinohumite" defect in natural olivine clearly underlines the prominent role of trace elements on the hydrogen incorporation in lithospheric olivine. [ABSTRACT FROM AUTHOR]

Raman spectroscopic data were obtained for (Mg,Fe)2SiO4 samples during compression to 57 GPa. Single crystals of San Carlos olivine compressed hydrostatically above 41 GPa showed appearance of a new "defect" peak in the 820–840 cm–1 region associated with SiOSi linkages appearing between adjacent SiO44– tetrahedra to result in five- or sixfold-coordinated silicate species. Appearance of this feature is accompanied by a broad amorphous background. The changes occur at lower pressure than metastable crystalline transitions of end-member Mg2SiO4 forsterite (Fo-I) into Fo-II and Fo-III phases described recently. We complemented our experimental study using density functional theory (DFT) calculations and anisotropic ion molecular dynamics (AIMD) simulations to study the Raman spectra and vibrational density of states (VDOS) of metastably compressed Mg2SiO4 olivine, Fo-II and Fo-III, and quenched melts at high and low pressures. By 54 GPa all sharp crystalline peaks disappeared from observed Raman spectra indicating complete pressure-induced amorphization (PIA). The amorphous (Mg,Fe)2SiO4 spectrum contains Si-O stretching bands at lower wavenumber than expected for SiO44– indicating high coordination of the silicate units. The amorphous spectrum persisted on decompression to ambient conditions but with evidence for reappearance of tetrahedrally coordinated units. Nonhydrostatic compression of polycrystalline olivine samples showed similar appearance of the defect feature and broad amorphous features between 43–44 GPa. Both increased in intensity as the sample was left at pressure overnight but they disappeared during decompression below 17 GPa with recovery of the starting olivine Raman signature. A hydrated San Carlos olivine sample containing 75–150 ppm OH was also studied. Significant broadening of the SiO44– stretching peaks was observed above 43 GPa but without immediate appearance of the defect or broad amorphous features. However, both of these characteristics emerged after leaving the sample at 47 GPa overnight followed by complete amorphization that occurred upon subsequent pressurization to 54 GPa. During decompression the high-density amorphous spectrum was retained to 3 GPa but on final pressure release a spectrum similar to thermally quenched low-pressure olivine glass containing isolated SiO44– groups was obtained. Leaving this sample overnight resulted in recrystallization of olivine. Our experimental data provide new insights into the metastable structural transformations and relaxation behavior of olivine samples including material recovered from meteorites and laboratory shock experiments. [ABSTRACT FROM AUTHOR]

Experimental and theoretical evidence for the coupled substitution of B and H in synthetic forsterite and a natural olivine is presented. The intensities of OH bands at 3704 cm-1 (//z), 3598 cm-1 (//x,y), and 3525 cm-1 (//x) in a heterogeneous B-doped synthetic forsterite crystal matches the zoning of B concentration measured by ion probe. The two anti-symmetric stretching vibrations of BO3 groups occur at 1301 cm-1 (//x) and 1207 cm-1 (//z) for the 10B and at 1256 and 1168 cm-1 for the 11B isotope. A microscopic model of the mixed (B,H) defect that accounts for experimental observations is obtained from quantum mechanical calculations. The BO3 group lies on the (O3-O1-O3) face of the vacant Si site and the H atom is bonded to the O2 atom at the remaining apex. The occurrence of the same OH bands associated with v3 BO3 vibrations in a natural olivine sample from Pakistan confirms the occurrence of this defect in nature. The three diagnostic OH bands can be used as a signature of H associated with boron substitution in olivine and forsterite, leading to a quantitative analysis of their contribution to H-defects. [ABSTRACT FROM AUTHOR]

We have measured the water contents in forsterites and olivines synthesized in the multi-anvil press using confocal Raman spectroscopy. These samples were previously characterized for water contents by polarized FTIR and contain from 75 to 1300 ppm wt H2O. We find that both forsterite and olivine follow the same trend in water content vs. integrated Raman OH/Si intensity. In addition three synthetic enstatites also display a linear trend in water vs. OH/Si integrated Raman intensity but with a different slope than for olivine, indicating that the calibration for measuring water by Raman is matrix dependent. Three glasses of different compositions (two rhyolites and one basalt) and different water contents were also analyzed. Comparison with the forsterites and olivines shows that the Raman cross-section of these glasses is very different and their intensities must be corrected by different factors. Therefore, to be able to use glasses as external calibrants, prior knowledge of their behavior compared to well-characterized NAM standards is necessary. [ABSTRACT FROM AUTHOR]

There is an understandable desire to use simple unpolarized infrared analysis of unoriented anisotropic samples to extract quantitative information, rather than using more demanding polarized techniques. Owing to the fact that unpolarized infrared absorbance in birefringent media deviates from the Beer-Lambert law, previous studies have either warned against using unpolarized spectroscopy for quantitative purposes, or have used flawed error analysis to justify using simple averages of integrated absorbance of multiple absorbance bands as a proxy for total integrated polarized absorbance in the principal spectra. It is shown here that unpolarized infrared absorbance is correctly calculated by averaging in the transmission domain. The errors in estimates of principal absorbance by averaging of unpolarized absorbance spectra are evaluated using correct theory of unpolarized infrared transmission. Correction schemes for integrated absorbance based on linear-absorbance error calculations are shown to be inappropriate. A theory is developed that allows the sum of the polarized principal absorbance spectra to be estimated from multiple unpolarized measurements of randomly oriented samples. The systematic errors that arise when averaging in the absorbance domain are avoided by use of exact theory rather than an approximation. Numerical simulation shows that applying the new procedure to 10 unpolarized measurements of OH stretching bands in olivine results in convergence of the estimated total integrated principal polarized absorbance to within 10% of the true value for a sample size of 10 measurements, but the technique is limited to spectral regions that do not contain absorption bands that are simultaneously intensely absorbing and strongly anisotropic. [ABSTRACT FROM AUTHOR]

The infrared spectroscopic properties of selected defects in orthoenstatite are investigated by first-principles calculations. The considered defects include doubly protonated Mg vacancies at M1 and M2 sites, fully protonated SiA and SiB vacancies (hydrogarnet defects), and doubly protonated SiA and SiB vacancies associated with interstitial Mg cations. The bands observed at 3,070 and 3,360 cm in the spectrum of synthetic enstatite samples are ascribed to O2A-H and O2B-H groups, respectively, associated with M2 vacancies. The theoretical models suggest that bands observed at 3,590 and 3,690 cm in the spectrum of enstatite samples synthesized under low silica-activity conditions correspond to O2H and O1H groups associated with SiB vacancies partially compensated by interstitial Mg cations in fivefold coordination. The theoretical relation between the integrated absorption coefficient of OH-defects and vibrational frequencies is consistent with previous observations indicating that the absorption coefficients of OH-defects are comparatively stronger in enstatite than in the olivine polymorphs. [ABSTRACT FROM AUTHOR]

The onset of hydrous partial melting in the mantle above the transition zone is dictated by the HO storage capacity of peridotite, which is defined as the maximum concentration that the solid assemblage can store at P and T without stabilizing a hydrous fluid or melt. HO storage capacities of minerals in simple systems do not adequately constrain the peridotite water storage capacity because simpler systems do not account for enhanced hydrous melt stability and reduced HO activity facilitated by the additional components of multiply saturated peridotite. In this study, we determine peridotite-saturated olivine and pyroxene water storage capacities at 10-13 GPa and 1,350-1,450°C by employing layered experiments, in which the bottom ~2/3 of the capsule consists of hydrated KLB-1 oxide analog peridotite and the top ~1/3 of the capsule is a nearly monomineralic layer of hydrated Mg# 89.6 olivine. This method facilitates the growth of ~200-μm olivine crystals, as well as accessory low-Ca pyroxenes up to ~50 μm in diameter. The presence of small amounts of hydrous melt ensures that crystalline phases have maximal HO contents possible, while in equilibrium with the full peridotite assemblage (melt + ol + pyx + gt). At 12 GPa, olivine and pyroxene water storage capacities decrease from ~1,000 to 650 ppm, and ~1,400 to 1,100 ppm, respectively, as temperature increases from 1,350 to 1,450°C. Combining our results with those from a companion study at 5-8 GPa (Ardia et al., in prep.) at 1,450°C, the olivine water storage capacity increases linearly with increasing pressure and is defined by the relation $$ C_{{{\text{H}}_{2} {\text{O}}}}^{\text{olivine}} \left( {\text{ppm}} \right) = 57.6\left( { \pm 16} \right) \times P\left( {\text{GPa}} \right) - 169\left( { \pm 18} \right). $$ Adjustment of this trend for small increases in temperature along the mantle geotherm, combined with experimental determinations of $$ D_{{{\text{H}}_{2} {\text{O}}}}^{\text{pyx/olivine}} $$from this study and estimates of $$ D_{{{\text{H}}_{2} {\text{O}}}}^{{{\text{gt}}/{\text{olivine}}}} $$, allows for estimation of peridotite HO storage capacity, which is 440 ± 200 ppm at 400 km. This suggests that MORB source upper mantle, which contains 50-200 ppm bulk HO, is not wet enough to incite a global melt layer above the 410-km discontinuity. However, OIB source mantle and residues of subducted slabs, which contain 300-1,000 ppm bulk HO, can exceed the peridotite HO storage capacity and incite localized hydrous partial melting in the deep upper mantle. Experimentally determined values of $$ D_{{{\text{H}}_{2} {\text{O}}}}^{{{\text{pyx}}/{\text{olivine}}}} $$ at 10-13 GPa have a narrow range of 1.35 ± 0.13, meaning that olivine is probably the most important host of HO in the deep upper mantle. The increase in hydration of olivine with depth in the upper mantle may have significant influence on viscosity and other transport properties. [ABSTRACT FROM AUTHOR]

To investigate the influence of chemical composition on the behavior of H2O in Fe-rich nominally anhydrous minerals, and to determine the difference between H2O behavior in the martian and terrestrial mantles, we conducted high-pressure H2O storage capacity experiments employing a wide range of olivine compositions. Experiments were conducted with bulk compositions in the system FeO-MgO-SiO2-H2O with Mg no. [Mg no. = 100 x molar Mg/(Mg+Fe)] ranging between 50 and 100 at 3 GPa in a piston-cylinder and at 6 GPa in a multi-anvil apparatus. Experiments at 3 GPa were conducted at 1200 °C, with fO2 buffered by the coexistence of Fe and FeO, and at 1300-1500 °C in unbuffered assemblies. Experiments at 6 GPa were conducted at 1200 °C without buffers. Experiments at 1200 °C produced olivine+orthopyroxene+hydrous liquid (liq), and higher T experiments produced olivine+liq. Additionally, we synthesized a suite of 7 olivine standards (Mg no. = 90) for low blank secondary ion mass spectrometry (SIMS) analysis of H in multi-anvil experiments at 3-10 GPa and 1250 °C, resulting in large (200-400 µm) homogeneous crystals with 0.037 to 0.30 wt% H2O. Polarized Fourier transform infrared (FTIR) measurements on randomly oriented grains from the synthesis experiments were used to determine principal axis spectra through least-squares regression, and H contents were calculated from the total absorbance in the OH stretching region. Using these olivines as calibrants for SIMS analyses, the H contents of olivines and pyroxenes from the variable Mg no. experiments were measured by counting 16OH ions. Ignoring any matrix effects owing to variation in Mg no., H contents of olivine and pyroxene increase linearly with decreasing Mg no. At 6 GPa and 1200 °C, olivine H contents increase from 0.05 to 0.13 wt% H2O (8360 to 23 900 H/106 Si) as olivine Mg no. decreases from 100 to 68, and at 3 GPa and 1200 °C olivine H contents 6 increase from 0.017 to 0.054 wt% (278 to 10 000 H/106 Si) as Mg no. decreases from 100 to 55. The partition coefficient for H between pyroxene and olivine, Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed., decreases from 1.05 at 3 GPa and 1200 °C to 0.61 at 6 GPa and 1200 °C. The storage capacity of Fe-rich olivines with compositions expected in the martian mantle is ~1.5 times greater than those in the terrestrial mantle, suggesting that the geochemical behavior of H2O in the mantles of the two planets are quite similar. If 50% of the K2O on Mars remains in its mantle (Taylor et al. 2006), then a similar or greater proportion of the H2O is also in the mantle. Given accretionary models of the total martian H2O budget (Lunine et al. 2003), this suggests concentrations of 100-500 ppm H2O in the martian mantle and 0.1-1.9 wt% H2O in primary martian basalts. [ABSTRACT FROM AUTHOR]

We conducted a series of hydroxylation experiments using mm-sized cuboids cut from six different crystals of San Carlos olivine with a range of trace-element concentrations. The cuboids were pre-annealed and then hydroxylated under identical conditions, ensuring that variation in the amounts of H incorporated depended only on the compositional variables. The pre-anneal was at 1400 °C, atmospheric pressure and an oxygen fugacity equivalent to Δlog FMQ + 1, with the subsequent hydroxylation at 800 °C and 1.5 GPa, for 3 days. Hydrogen was incorporated into all six crystals by the four main substitution mechanisms [Si], [Mg], [Ti] and [triv], with homogeneous H contents in the cores of the crystals, indicating H diffusion rates faster than 10− 11 m2/s. Total H as H2O in the homogeneous cores calculated by summing all the infrared absorbance bands ranges from 13 to 27 wt. ppm. The total H2O in the six pre-annealed crystals is poorly correlated with any measured compositional variable. However, when the H2O associated with individual infrared bands is compared, clear trends emerge. The intensity of absorption bands at 3572 and 3525 cm− 1 are strongly correlated with Ti concentrations, whose range in the six crystals exceeds an order of magnitude. Bands between 3400 and 3300 cm− 1, correlate negatively with Na+, but are positively correlated with the difference between molar Cr3+ and Na+. This highlights a previously unrecognised role for Na in suppressing H incorporation in natural olivines. The results confirm the important role that the trace constituents of olivine play in H incorporation. Two of these trace elements, Na and Ti, tend to be similarly enriched or depleted by partial melting or metasomatism of the mantle, but have opposite effects on H incorporation, with Ti enhancing it but Na suppressing it. Models estimating the effect of H in olivine on mantle rheology must, therefore, consider carefully the availability of these trace elements. [ABSTRACT FROM AUTHOR]