Upper-mantle Conditions Research Articles

Synthesis and Stability of Biomolecules in C-H-O-N Fluids under Earth's Upper Mantle Conditions.

How life started on Earth is an unsolved mystery. There are various hypotheses for the location ranging from outer space to the seafloor, subseafloor, or potentially deeper. Here, we applied extensive ab initio molecular dynamics simulations to study chemical reactions between NH3, H2O, H2, and CO at pressures (P) and temperatures (T) approximating the conditions of Earth's upper mantle (i.e., 10-13 GPa, 1000-1400 K). Contrary to the previous assumptions that large organic molecules might readily disintegrate in aqueous solutions at extreme P-T conditions, we found that many organic compounds formed without any catalysts and persisted in C-H-O-N fluids under these extreme conditions, including glycine, ribose, urea, and uracil-like molecules. Particularly, our free-energy calculations showed that the C-N bond is thermodynamically stable at 10 GPa and 1400 K. Moreover, while the pyranose (six-membered ring) form of ribose is more stable than the furanose (five-membered ring) form at ambient conditions, we found that the formation of the five-membered-ring form of ribose is thermodynamically more favored at extreme conditions, which is consistent with the exclusive incorporation of β-d-ribofuranose in RNA. We have uncovered a previously unexplored pathway through which the crucial biomolecules could be abiotically synthesized from geofluids in the deep interior of Earth and other planets, and these formed biomolecules could potentially contribute to the early stage of the emergence of life.

A first-principles molecular dynamics study of molecular hydrogen diffusion in Fe-free olivine

Molecular hydrogen (H2) may be an important form of water in nominally anhydrous minerals in the Earth’s mantle and plays a critical role in mantle water cycle, but the transport properties of H2 remain unclear. Here, the diffusion of H2 in Fe-free olivine lattice is investigated at pressures of 1–13 GPa and temperatures of 1300–1900 K by first-principles molecular dynamics. The activation energy and activation volume for H2 diffusion in Fe-free olivine are determined to be 55 ± 8 kJ/mol and 3.6 ± 0.2 cm3/mol, respectively. H2 diffusion in Fe-free olivine is faster than H+ by 1–4 orders of magnitude and therefore it is more favorable for hydrogen transportation under upper mantle conditions. H2 can be carried to the mantle transition zone by subducting slabs without releasing to the surrounding mantle. The upper mantle may act as a lid, preventing the releasing of H2 produced in the deep mantle to the surface.

The reduced alloy in Earth's upper mantle: Experimental constraints on Fe-Ni-S-C(-O) melt compositions and deep mantle oxygen fugacity (5–16 GPa)

High-pressure experiments conducted at upper mantle conditions reveal increased Fe3+ contents in majoritic garnet with increasing pressure. Consequently, at ∼8 GPa, Fe2+ disproportionation is expected to generate Fe0, leading to a higher metal/sulfur ratio of the initially present monosulfidic melt. This study experimentally investigates equilibrium systematics between the reduced Fe-Ni-S-C(-O) melt and mantle silicates at 5–16 GPa and near-adiabatic temperatures (1420–1590∘C), aiming to constrain the alloy melt composition in equilibrium with bulk silicate Earth-like (BSE) mantle silicates. As analysis of unquenchable liquid alloys requires large melt pools, experimental charges were designed towards a 50:50 ratio of alloy:silicates, with Fe/Ni ratios, sulfur and carbon scaled to correspond to a BSE with 200–250 ppm S and 100–150 ppm C.Observed phase assemblages consist of Fe-Ni-S-C(-O) melt (metal/(S+O): 0.9–3.5, Ni/(Fe+Ni): ∼0.3–0.6 and 0.2–1.6 wt.% oxygen) saturated with graphite/diamond and coexisting with olivine/wadsleyite (XMg: 0.76–0.91), garnet and cpx ± low-Ca pyroxene. Oxygen fugacities (fO2) range from -0.5 to +4.0 relative to the iron-wüstite (IW) buffer. In most runs, Fe0–Fe2+ redox equilibration affected silicate XMg's, resulting in values <0.90. Therefore, an empirical model was developed to predict KDNi-Fealloy-olivine, allowing to recalculate the experimental results for a primitive BSE mantle composition with XMg of 0.90.Modeling BSE, the alloy's metal/(S+O) increases from 1.1 to ∼7 while Ni/(Fe+Ni) decreases from 0.35 to 0.19 at 5–16 GPa. With BSE-like S concentrations, the abundance of the Fe-Ni-S-C(-O) melt increases from 600–750 ppm at 5 GPa to ∼3800 ppm at 16 GPa, 100–150 ppm bulk carbon lead to graphite/diamond saturation to ∼14 GPa. Finally, a method is presented to derive fO2 from modeled liquid alloy compositions, which act as a “redox-sensor”. For a BSE-like mantle with 250 ppm S, fO2 decreases from IW+2.6 at 5 GPa to IW at 8 GPa, further decreasing to IW-1.0 at 12–16 GPa.

Peritectic reaction of olivine in the diamond-forming system carbonate-silicate-(C-O-H) at 6 GPa

Influence of the supercritical C-O-H-fluid (7.5 wt.%) onto melting phase relations of the multicomponent multiphase diamond-forming system olivine-jadeite-diopside-(Mg-Fe-Ca-Na-carbonates)-(C-O-H) in experiments at 6 GPa and 700–1200 °C (the upper mantle conditions) has been studied. The peritectic reaction of olivine and jadeite-bearing melt with garnet formation has been retained as a key mechanism of the ultrabasic-basic evolution of diamond-forming melts. The CO2-fluid and silicate components react forming carbonate phases. The H2O-fluid together with carbonates has essentially lowered temperatures of the liquidus and solidus boundaries. The phase of supercritical water fluid and water-bearing carbonate nesquehonite (Nes) MgCO3·3H2O were identified with the Raman-spectroscopy method after crystallization of the completely mixed silicate-carbonate-(C-O-H-fluid) melt.

Composition effects on equilibrium oxygen isotope fractionation factors between garnets and quartz/calcite/olivine: Implication from vibration frequencies of garnets at high temperatures and high pressures

Garnet-group minerals, with their wide range of compositions, play a significant role in Earth's crust and upper mantle, participating in various petrological and geochemical processes. Oxygen isotope fractionation factors between garnets and other minerals hold crucial implications in these contexts. In this work, vibration frequencies were measured via Raman spectra on five garnet mineral samples in the pyrope-almandine-spessartine ternary system and four synthetic pyrope-grossular solid solutions at temperatures up to 1000 °C and pressures up to 17 GPa. Isobaric (γiP) and isothermal (γiT) mode Grüneisen, as well as anharmonic (ai) parameters, were determined for the observed modes. Our study shows that the vibration bands tend to shift to lower frequencies with increasing temperature and to higher frequencies with increasing pressure. Moreover, the anharmonic correction has been found to contribute positively to equilibrium oxygen isotope fractionation β factors in garnets, typically within 0.5 ‰ above 1000 K. The 103∙lnβ(T) factors calculated from vibration spectra (considering Г points only) align closely with those from theoretical calculations (including all frequencies in the Brillouin zone). Contributions from phonon dispersion (including none-Г points) are typically smaller than the uncertainties propagated from frequency measurements. Additionally, the pressure effect on the oxygen isotope fractionation in garnet, evaluated from the isothermal Grüneisen parameters, is found to be insignificant under crustal and upper mantle conditions, consistent with thermodynamic expectations. Using published oxygen isotope fractionation β factors for quartz and calcite, the equilibrium oxygen isotope fractionation factors (103∙lnα) are calculated between garnets of diverse compositions and any of these phases as functions of temperature, pressure and fraction of garnet endmembers. Considering composition effects helps to reduce the discrepancies among experimental and empirical calibrations of oxygen isotope fractionation factors between garnet and quartz. As an example application, oxygen isotope fractionation factors 103∙lnα(P, T) between garnets and forsterite were calculated under upper mantle conditions. Our findings suggest that these fractionation factors are predominantly dependent on temperature and garnet compositions, with minor influence from pressure. Compared with our calculated fractionation factors between garnet and olivine, the oxygen isotope exchange equilibrium may have been reached in some in kimberlite and mantle xenolith samples between garnet and olivine. However, in other samples, the fractionation factors cannot be explained solely by the composition effect, likely due to metasomatism. Our results underscore the importance of considering garnet compositions when interpreting their oxygen isotope compositions.

Shallow mantle cycle of subducted sedimentary limestone indicated by carbonate xenoliths

In the subduction zone, carbon (C) release from the subducted plate into the mantle wedge takes place by various mechanisms, including metamorphic degassing, carbonate dissolution, and hydrous melting. Recent studies have highlighted the significant role of buoyant limestone diapirs in facilitating C recycling during subduction processes. Here, we report that the Hannuoba carbonate xenoliths of eastern China were introduced into the shallow mantle in the form of sedimentary limestone diapirs. We conducted a comprehensive study of the Hannuoba carbonate xenoliths using petrological analysis, major- and trace-element analysis, in situ zircon dating, and C-O isotopic analysis techniques. The whole-rock major and trace elements, along with high δ18OSMOW values (22.8‰−23.2‰), indicate that the protolith of Hannuoba carbonate xenoliths consists of sedimentary limestones. The spectral age peaks of detrital zircons (400−100 Ma, 500−450 Ma, 2.0−1.6 Ga, and 2.8−2.2 Ga) suggest that the primary source of the protolith is the North China Craton. When sedimentary limestone diapirs enter into the mantle wedge, they undergo partial melting under high-temperature conditions. This process results in the melting of low-melting point pelitic components, while leaving behind resistant minerals such as pure calcite. Due to its low density and viscosity, this pure limestone component will continue to migrate upward and assimilate with the mantle peridotite. In this process, the pure calcites are preserved in the shallow mantle in a solid-state form rather than as carbonatitic melts. The discovery of graphite and spinel in the Hannuoba carbonate xenoliths indicates that the depth of formation of the sedimentary limestone diapirs does not exceed 2.5 GPa (&amp;lt;70 km). The calcite in Hannuoba carbonate xenoliths exhibits extremely low δ13CVPDB (−11.8‰ to −11.0‰) and high δ18OSMOW (22.8‰−23.2‰) contents. This characteristic C-O isotopic signature can be explained by the Rayleigh decarbonation of sedimentary limestones under upper-mantle conditions (&amp;lt;3 GPa). Based on calculations, the Hannuoba carbonate xenoliths are a product of subducted sedimentary limestone experiencing 99% volume decarbonation. Through this metamorphic decarbonation process, limestone diapirs release a significant amount of CO2 into the arc magma system. During subduction processes, sedimentary carbonate rocks can be recycled to the mantle wedge of island arcs as solid limestone diapirs. The C flux released by this mechanism cannot be ignored.

Oxidation state of Cu in silicate melts at upper mantle conditions

Beyond its economic value, copper (Cu) serves as a valuable tracer of deep magmatic processes due to its close relationship with magmatic sulfide evolution and sensitivity to oxygen fugacity (fO2). However, determining Cu’s oxidation state (+ 1 or + 2) in silicate melts, crucial for interpreting its behavior and reconstructing fO2 in the Earth’s interior, has long been a challenge. This study utilizes X-ray Absorption Near Edge Structure spectroscopy to investigate the Cu oxidation state in hydrous mafic silicate melts equilibrated under diverse fO2 (− 1.8 to 3.1 log units relative to the Fayalite–Magnetite–Quartz buffer), temperature (1150–1300 °C), and pressure (1.0–2.5 GPa) conditions. Our results reveal that Cu predominantly exists as Cu+ across all fO2 conditions, with a minor Cu2+ component. This dominance of Cu+ persists even in relatively oxidized melts, highlighting its limited sensitivity to fO2 under upper mantle conditions. This significantly constrains the utility of Cu as an oxybarometer in hydrous silicate melts in the deep Earth. However, our findings suggest that Cu isotopes primarily reflect the interplay of sulfide segregation/accumulation during magmatic differentiation, shedding light on these fundamental processes in Earth’s interior.

Reaction path model of the formation of abiotic immiscible hydrocarbon fluids in subducted carbonated serpentinites, Lanzo Massif (Western Italian Alps)

Fluids generated from subducted slabs participate in the cycling of deep carbon in the crust and upper mantle. In these fluids, aqueous carbon species vary in oxidation state between +IV and -IV depending on whether the fluids are oxidizing or reducing, respectively. Most studies of subduction-zone fluids have focused on oxidized carbon species. However, recent studies of natural samples have demonstrated the occurrence of deep, reducing fluids, generated in both subducted oceanic upper mantle and crustal rocks. CH4-H2-rich fluid inclusions in subducted carbonated serpentinites have demonstrated the existence of abiotic, immiscible, hydrocarbon fluids at upper mantle conditions. To investigate the formation of such immiscible hydrocarbon fluids during the evolution of subducted carbonated serpentinites, we used equilibrium constants from the Deep Earth Water model to carry out predictive chemical mass transfer modeling to simulate the alteration reactions. A novel feature of the models was the inclusion of an immiscible hydrocarbon fluid containing six components (CH4,f,C2H6,f,C3H8,f,isoC4H10,f,CO2,f,H2,f). This feature enabled prediction of the formation of a separate immiscible fluid in equilibrium with aqueous species and minerals.We developed a predictive reaction path model of invasive H2,f reacting with carbonated serpentinites and interstitial aqueous fluids for comparison with the natural samples from the Lanzo Massif, western Italian Alps. Over a range of temperatures and pressures, immiscible hydrocarbon fluids formed in association with altered mineral assemblages. Reaction progress caused the transformation of carbonated serpentinites and the formation of clinopyroxene, brucite, graphite, and hydrocarbon fluids, along with changes of pH, logfO2, and aqueous species. CH4,f was the most abundant hydrocarbon species in all the models. The overall results at 2.0 GPa and 400 to 450 °C were consistent with the natural samples from the Lanzo Massif. Interestingly, large amounts of H2O formed due to oxidation of H2. More hydrocarbons and H2O formed in models with lower fluid/rock mass ratios or with more reactant H2. Models at different pressure and temperature conditions showed similar results with some variation in the relative stabilities of aragonite, graphite and olivine solid solution, and associated differences in mineral sequences, hydrocarbon fluids, values of aqueous species, and the final logfO2 and pH. Our models strongly support the laboratory and field evidence that reduction of carbonated serpentinites by infiltrating H2 fluids can cause the formation of immiscible, abiotic hydrocarbon fluids in subduction zones.

Global Hydrogen Production During High‐Pressure Serpentinization of Subducting Slabs

AbstractSerpentinization is among the most important, and ubiquitous, geological processes in crustal–upper mantle conditions (&lt;6 GPa, &lt;600°C), altering the rheology of rocks and producing H2 that can sustain life. While observations are available to quantify serpentinization in terrestrial and mid‐ocean ridge environments, measurements within subduction zone environments are far more sparse. To overcome this difficulty, we design a methodology to quantify and offer a first‐order estimate of the magnitude of “slab‐serpentinization” that has occurred over the last 5 Ma within the world's subduction zones by coupling four discrete tectonic and geophysical datasets—(a) raster grids of relic abyssal peridotite (peridotite exhumed from slow spreading mid‐ocean ridges but unaffected by pre‐subduction serpentinization) within ocean basins, (b) slab geometry, (c) thermal profiles and a (d) plate‐tectonic model. Averaged per year, our results suggest that 4.2–24 • 107 kg of H2 per annum could be generated from “slab‐serpentinization” within a subduction zone. Our estimate is 1–2 orders of magnitude lower than what is thought to be produced at mid‐ocean ridges, and of a similar magnitude to what could occur when including serpentinization at trench flexure and possible mantle wedge serpentinization. Higher hydrogen production is correlated most strongly with the spreading history of ocean basins, underlaying the importance of the tectonic history of a slab prior to subduction.

Structural Transformations of Minerals in the Earth’s Mantle: the New High Pressure Polymorphous Modifications of Olivine and Wollastonite

There are reviewed important results obtained in the studies of HP-minerals during 2018–2022 years. The emphasis is made on the characterization of HP-polymorphs modifications of olivine (asimovite, poirierite) and wollastonite (breyite and davemaoite) discovered in this period. The structure of poirierite (ε-Mg2SiO4) contains the unique silicate chain Si3O9 with one tetrahedron in its period. Consequently its structural formula is Mg2[SiO3]O, and its intergrowth with wadsleyite/ringwoodite in shocked chondrites support the idea that transformations of olivine into ringwoodite becomes via forming of poirierite. The close ratio of cell parameters in wollastonite (1T) at 8.9 GPa and its synthetic analogue CaGeO3 exhibits the similarity of their structures. The HP/HT data confirm the stability of the wollastonite polymorph (1T, 2M, pseudowollastonite and synthetic analogue of breyite) at the upper mantle conditions. The crystal structures and composition of new НР-minerals discovered as inclusions in diamonds, in meteorites and in the rocks near terrestrial impact craters provide the richer information on the complicate petrology of deep-seated geospheres which cannot be obtained on the base of laboratory experiments.

Oldhamite: a new link in upper mantle for C-O-S-Ca cycles and an indicator for planetary habitability.

In the solar system, oldhamite (CaS) is generally considered to be formed by the condensation of solar nebula gas. Enstatite chondrites, one of the most important repositories of oldhamite, are believed to be representative of the material that formed Earth. Thus, the formation mechanism and the evolution process of oldhamite are of great significance to the deep understanding of the solar nebula, meteorites, the origin of Earth, and the C-O-S-Ca cycles of Earth. Until now, oldhamite has not been reported to occur in mantle rock. However, here we show the formation of oldhamite through the reaction between sulfide-bearing orthopyroxenite and molten CaCO3 at 1.5 GPa/1510 K, 0.5 GPa/1320 K, and 0.3GPa/1273K. Importantly, this reaction occurs at oxygen fugacities within the range of upper-mantle conditions, six orders of magnitude higher than that of the solar nebula mechanism. Oldhamite is easily oxidized to CaSO4 or hydrolysed to produce calcium hydroxide. Low oxygen fugacity of magma, extremely low oxygen content of the atmosphere, and the lack of a large amount of liquid water on the celestial body's surface are necessary for the widespread existence of oldhamite on the surface of a celestial body otherwise, anhydrite or gypsum will exist in large quantities. Oldhamites may exist in the upper mantle beneath mid-ocean ridges. Additionally, oldhamites may have been a contributing factor to the early Earth's atmospheric hypoxia environment, and the transient existence of oldhamites during the interaction between reducing sulfur-bearing magma and carbonate could have had an impact on the changes in atmospheric composition during the Permian-Triassic Boundary.

Sound velocities and single-crystal elasticity of hydrous Fo90 olivine to 12 GPa

Nominally anhydrous minerals (NAMs) may contain significant amounts of water and constitute an important reservoir for mantle hydrogen. The colloquial term ‘water’ in NAMs is related to the presence of hydroxyl-bearing (OH−) point defects in their crystal structure, where hydrogen is bonded to lattice oxygen and is charge-balanced by cation vacancies. This hydrous component may therefore have substantial effects on the thermoelastic parameters of NAMs, comparable to other major crystal-chemical substitutions (e.g., Fe, Al). Assessment of water concentrations in natural minerals from mantle xenoliths indicates that olivine commonly stores ∼0–200 ppm of water. However, the lack of samples originating from depths exceeding ∼250 km coupled with the rapid diffusion of hydrogen in olivine at magmatic temperatures makes the determination of the olivine water content in the upper mantle challenging. On the other hand, numerous experimental data show that, at pressures and temperatures corresponding to deep upper mantle conditions, the water storage capacity of olivine increases to 0.2–0.5 wt%. Therefore, determining the elastic properties of olivine samples with more realistic water contents for deep upper mantle conditions may help in interpreting both seismic velocity anomalies in potentially hydrous regions of Earth's mantle as well as the observed seismic velocity and density contrasts across the 410-km discontinuity.Here, we report simultaneous single-crystal X-ray diffraction and Brillouin scattering experiments at room temperature up to 11.96(2) GPa on hydrous [0.20(3) wt% H2O] Fo90 olivine to assess its full elastic tensor, and complement these results with a careful re-analysis of all the available single-crystal elasticity data from the literature for anhydrous Fo90 olivine. While the bulk (K) and shear (G) moduli of hydrous Fo90 olivine are virtually identical to those of the corresponding anhydrous phase, their pressure derivatives K′ and G′ are slightly larger, although consistent within mutual uncertainties. We then defined linear relations between the water concentration in Fo90 olivine, the elastic moduli and their pressure derivatives, which were then used to compute the sound velocities of Fo90 olivine with higher degrees of hydration. Even for water concentrations as high as 0.5 wt%, the sound wave velocities of hydrous and anhydrous olivines were found to be identical within uncertainties at pressures corresponding to the base of the upper mantle. Contrary to previous claims, our data suggest that water in olivine is not seismically detectable, at least for contents consistent with deep upper mantle conditions. In addition to that, our data reveal that the hydration of olivine is unlikely to be a key factor in reconciling seismic velocity and density contrasts across the 410-km discontinuity with a pyrolitic mantle.

Phase equilibrium modelling of partial melting in the upper mantle: A comparison between different modelling methodologies and experimental results

Recent advances in thermodynamic modelling now allow phase equilibrium studies of partial melting in mafic and ultramafic sources under upper mantle conditions. This potentially offers several advantages over experimental investigations, including the fact that capsule-sample interactions are avoided, there is no uncertainty over the volatile content of the charge, the process is much faster, and open system behaviour can be investigated with relative ease. Two different thermodynamic datasets are in common use for phase equilibrium investigations of mantle rocks, compiled alongside a range of constantly evolving phase solution models. This study compares the results of published experimental studies on partial melting in the upper mantle with phase equilibrium modelling results from the popular pMELTS software (Ghiorso et al., 2002), which uses an embedded dataset from Berman (1988), and with results from the Rcrust software (Mayne et al., 2016), which uses the dataset of Holland & Powell (2011). The aim of this comparative study is to investigate the suitability of each combination of software specific calculation routines, thermodynamic datasets and phase solution models to provide a viable alternative to conventional experimental techniques when studying magma genesis from the upper mantle. The main points of interest are how the phase assemblages, proportions, and compositions predicted by the modelling compare with those produced in the experiments. The set of experimental results used for comparison includes both peridotitic and eclogitic compositions, investigated over a pressure range of 1.0–3.0 GPa and a temperature range of 1165–1950 °C. The results indicate that using the Holland & Powell (2011) dataset and appropriately selected phase solution models provides the best match with experimental results in terms of melt volume, melt composition and the assemblage of coexisting phases produced during partial melting under upper mantle conditions.

Phase Stability and Reactions of Subducting CaCO3 under Upper Mantle Conditions

AbstractCaCO3 is an important component of marine sediments and one of the major deep‐carbon carriers at subduction zones. Some subducted CaCO3 can be dissolved in subduction fluids and recycled back to the surface via arc volcanoes degassing. At the same time, there still remain large amounts of CaCO3 and its reaction products, which could be further transported into Earth's deep interior. These internal processes link atmosphere, hydrosphere and biosphere with the deep solid Earth, modifying the environments of our planet. In this review, we summarize current understanding from high pressure‐temperature experiments and field petrological observations on the physical and chemical properties of CaCO3. In particular, the phase stability and reactions of CaCO3 largely control the migration and reservation of oxidized carbon in subducting slabs. Finally, we present several critical but unsolved questions on CaCO3 subducting in the deep mantle.

Experimental determination of quartz solubility in H2O-CaCl2 solutions at 600–900 °C and 0.6–1.4 GPa

Abstract Fluid-mediated calcium metasomatism is often associated with strong silica mobility and the presence of chlorides in solution. To help quantify mass transfer at lower crustal and upper mantle conditions, we measured quartz solubility in H2O-CaCl2 solutions at 0.6–1.4 GPa, 600–900 °C, and salt concentrations to 50 mol%. Solubility was determined by weight loss of single-crystals using hydrothermal piston-cylinder methods. All experiments were conducted at salinity lower than salt saturation. Quartz solubility declines exponentially with added CaCl2 at all conditions investigated, with no evidence for complexing between silica and Ca. The decline in solubility is similar to that in H2O-CO2 but substantially greater than that in H2O-NaCl at the same pressure and temperature. At each temperature, quartz solubility at low salinity (XCaCl2 &amp;lt; 0.1) depends strongly on pressure, whereas at higher XCaCl2 it is nearly pressure independent. This behavior is consistent with a transition from an aqueous solvent to a molten salt near XCaCl2 ~0.1. The solubility data were used to develop a thermodynamic model of H2O-CaCl2 fluids. Assuming ideal molten-salt behavior and utilizing previous models for polymerization of hydrous silica, we derived values for the activity of H2O (aH2O), and for the CaCl2 dissociation factor (α), which may vary from 0 (fully associated) to 2 (fully dissociated). The model accurately reproduces our data along with those of previous work and implies that, at conditions of this study, CaCl2 is largely associated (&amp;lt;0.2) at H2O density &amp;lt;0.85 g/cm3. Dissociation rises isothermally with increasing density, reaching ~1.4 at 600 °C, 1.4 GPa. The variation in silica molality with aH2O in H2O-CaCl2 is nearly identical to that in H2O-CO2 solutions at 800 °C and 1.0 GPa, consistent with the absence of Ca-silicate complexing. The results suggest that the ionization state of the salt solution is an important determinant of aH2O, and that H2O-CaCl2 fluids exhibit nearly ideal molecular mixing over a wider range of conditions than implied by previous modeling. The new data help interpret natural examples of large-scale Ca-metasomatism in a wide range of lower crustal and upper mantle settings.

Schorl breakdown at upper mantle conditions: Insights from an experimental study at 3.5 GPa

Schorl breakdown at upper mantle conditions: Insights from an experimental study at 3.5 GPa

Diffusional Isotope Effect Based on the Transition-State Theory of Interstitial Mechanism in Solids

It is important to be able to interpret the rapidly growing isotopic diffusion data in mineral grains that are measured using high spatial resolution. Based on previous works, the classical transition-state theory, and quantum statistical mechanics, an atomic theory to evaluate the diffusion coefficient (D) and the kinetic isotope effect (β factor) for the interstitial diffusion mechanism is developed with the unified equations for theoretical calculations. Taking helium diffusion in forsterite as an example, the D and β (4He/3He) were calculated at certain upper mantle conditions (0–20 GPa, 0–1500 °C) based on the developed equations. All parameters needed in our theory were obtained by first-principles calculations. Our calculated results for D at 0 GPa show reasonable agreement with most experimental results, indicating acceptable validity of the developed theory and methods. Meanwhile, possible causes for slight differences between calculations and experimental results at ambient pressure were also explored. We find that the β increases with the increase of temperature, but its pressure dependence is quite limited at least in the case of interstitial helium diffusion in forsterite. Additionally, we confirmed that current widely used simulation protocols with a fixed lattice for diffusion fail to accurately estimate the D values under high pressures due to the neglection of the activation volume. However, we also find that such protocols have almost negligible effects on the β factors for isotope fractionations during diffusion. Finally, potential effects of various approximations and calculation protocols involved in previous studies were carefully evaluated, and we find that some commonly used approximations and techniques are oversimplified and probably lead to significant loss of accuracy, especially at low temperatures (≤∼600 °C). However, it is still quite safe to continue adopting these protocols when dealing with high-temperature problems (e.g., ≥1000 °C).

Fast anisotropic Mg and H diffusion in wet forsterite

Adding hydrogen to forsterite strongly increases the diffusion rate of Mg, but the reason for this is unclear. As Mg diffusion in forsterite can influence its electrical conductivity, understanding this process is important. In this study we use density functional theory to predict the diffusivity of H-bearing Mg vacancies and we find that they are around 1000 times slower than H-free Mg vacancies. H-bearing Mg vacancies are many orders of magnitude more concentrated than H-free Mg vacancies, however, and diffusion is a combination of diffusivity and defect concentration. Overall, the presence of hydrated Mg vacancies is predicted to cause a large (multiple orders of magnitude) increase in both diffusion rate and diffusional anisotropy with a strong preference for diffusion in the [001] direction predicted. In models of experimental data, the effect of water concentration on diffusion is often described by a constant best-fitting exponent. Our results suggest that this exponent will vary between 0.5 and 1.6 across common experimental conditions with pressure decreasing and temperature increasing this exponent. These results suggest that Mg diffusion in forsterite could vary considerably throughout upper mantle conditions in ways that cannot be captured with a simple single-exponent model. Comparisons to measures of hydrogen diffusivity suggest that the diffusion of hydrated Mg vacancies also controls the diffusion of hydrogen in (iron-free) forsterite and that our conclusions above also apply to hydrogen diffusion rates and anisotropy. We also find that cation diffusivity likely cannot explain experimental measurements of the effect of water on electrical conductivity in olivine.

Kimberlitic olivine – a proxy to kimberlite petrogenesis and ascent process

In kimberlite, olivine is an important constituent and it is found as mineral grains of different sizes, which can be of xenocrystic and magmatic origin. To know the processes, that are involved and controls the compositional variations of olivine, can provide distinct understanding into the genesis and evolution of kimberlites. In addition to this the textural features of kimberlitic olivine, which are recorded as textural events during magma assent constrains the ascent of kimberlite and the processes involved.Unambiguous identification of kimberlitic olivine and its textural features require careful petrographic examination combined with mineral compositional analysis and use of high magnification images (BSE-SEM), all this is integrated to know the origin of olivine grain which in turn constrain the process involved in ascent of kimberlite and its petrogenesis. In this study we review the use of kimberlitic olivine in deducing the upper mantle conditions and process for kimberlite petrogenesis.

Zoisite in cratonic eclogite xenoliths - Implications for water in the upper mantle

A significant portion of the Earth's water is found at trace levels in nominally anhydrous rock forming minerals in the crust and mantle. Its distribution has far-reaching implications in melt-generating processes, mantle rheology and the stability of the lithosphere. We report petrographic, major and trace element, Fourier transform infrared spectroscopy and secondary ion mass spectrometry data on four eclogite xenoliths from the Obnazhennaya kimberlite, Siberia that contain hydrous minerals at upper mantle conditions. Al-rich (>9.9 wt% Al2O3) omphacitic clinopyroxene, a major mineral of eclogites, contains crystallographically controlled exsolutions of CaO-rich (up to 23.6 wt%) garnet (coronae, lenses), garnet-zoisite (lamellae) or amphiboles (needle/blade-like exsolutions). The omphacite hosting the exsolved lamellae shows enrichment or depletion in major (Al, Si, Mg) and trace (e.g., HREE) elements compatible with garnet and zoisite, proportional to the abundance of exsolutions.We argue that zoisite likely formed concomitantly with garnet exsolution from a water- and Al-rich precursory omphacite, without any fluid addition. The compositional gradients are consistent with diffusion-controlled partitioning in a closed system, and their preservation indicates the exsolution likely took place shortly prior to eclogite entrapment by the kimberlite. Large, non-exsolved, omphacite grains contain minimum ~870–1500 ppm wt. H2O (determined as OH). We infer that intertwined lamellae of hydrous garnet (600–960 ppm wt. H2O) and minor (<1%) zoisite (2 wt% H2O) formed at mantle conditions due to progressive exsolution, without external fluids. Recalculated whole rock water contents range from ~320 to 970 ppm wt. H2O. These estimates exceed by far those for the surrounding peridotitic mantle and suggest that, although a minor component, eclogites may locally be an important water reservoir in the cratonic mantle.