Deep Water Cycle Research Articles

Stability and physical properties of brucite at high pressures and temperatures: Implication for Earth’s deep water cycle

Brucite is a common hydrous mineral on Earth and may contribute to the deep water cycle of the Earth, but its stability and structure under mantle conditions remain uncertain. In this study, we investigated the stability of brucite up to 60 GPa at 800 K and 45 GPa at 1850 K. Within the experiment P-T conditions, no theoretically predicted new phase was observed, and brucite remained in the P3¯m1 structure. With the determined thermal EoS of brucite and the elastic parameters of mantle minerals, we modeled the velocity and density profile of subducted hydrous harzburgite in the top lower mantle, assuming that the water was stored in brucite and phase D. Based on the modelling, 1 wt% water will reduce the velocity and density of harzburgite by ∼ 5 % and ∼ 2 %, respectively, yet whether the water is stored in brucite or phase D has weak influence on both density and velocity. With a water content up to 2.4 wt%, the density of hydrous harzburgite could be reduced to 2.2(2)%–2.8(2)% lower than the surrounding mantle, while the VP and VS of hydrous harzburgite are still 0.3(1)%–0.7(1)% and 0.7(2)%–1.8(2)% higher than that of the normal mantle. Thus, the low-density hydrous harzburgite may slow down the subducting of slab, despite being a high-velocity body in seismic observations.

(De)hydration Front Propagation Into Zero‐Permeability Rock

AbstractHydration and dehydration reactions play pivotal roles in plate tectonics and the deep water cycle, yet many facets of (de)hydration reactions remain unclear. Here, we study (de)hydration reactions where associated solid density changes are predominantly balanced by porosity changes, with solid rock deformation playing a minor role. We propose a hypothesis for three scenarios of (de)hydration front propagation and test it using one‐dimensional hydro‐mechanical‐chemical models. Our models couple porous fluid flow, solid rock volumetric deformation, and (de)hydration reactions described by equilibrium thermodynamics. We couple our transport model with reactions through fluid pressure: the fluid pressure gradient governs porous flow and the fluid pressure magnitude controls the reaction boundary. Our model validates the hypothesized scenarios and shows that the change in solid density across the reaction boundary, from lower to higher pressure, dictates whether hydration or dehydration fronts propagate: decreasing solid density causes dehydration front propagation in the direction opposite to fluid flow while increasing solid density enables both hydration and dehydration front propagation in the same direction as fluid flow. Our models demonstrate that reactions can drive the propagation of (de)hydration fronts, characterized by sharp porosity fronts, into a viscous medium with zero porosity and permeability; such propagation is impossible without reactions, as porosity fronts become trapped. We apply our model to serpentinite dehydration reactions with positive and negative Clapeyron slopes and granulite hydration (eclogitization). We use the results of systematic numerical simulations to derive a new equation that allows estimating the transient, reaction‐induced permeability of natural (de)hydration zones.

Water Evolution and Inventories of Super-Earths Orbiting Late M Dwarfs

Super-Earths orbiting M dwarf stars may be the most common habitable planets in the Universe. However, their habitability is threatened by intense irradiation from their host stars, which drives the escape of water to space and can lead to surface desiccation. We present simulation results of a box model incorporating deep-water cycling between interior and atmosphere and water loss to space for terrestrial planets of mass 1–8 M ⊕ orbiting in the habitable zone of a late M dwarf. Energy-limited loss decreases with planetary mass, while diffusion-limited loss increases with mass. Depending on where it orbits in the habitable zone, a 1 M ⊕ planet that starts with 3–8 Earth Oceans can end up with an Earthlike surface of oceans and exposed continents; for an 8 M ⊕ super-Earth, that range is 3–12 Earth Oceans. Planets initialized with more water end up as waterworlds with no exposed continents, while planets that start with less water have desiccated surfaces by 5 Gyr. Since the mantles of terrestrial planets can hold much more water than is currently present in Earth’s atmosphere, none of our simulations result in Dune planets—such planets may be less common than previously thought. Further, more water becomes sequestered within the mantle for larger planets. A super-Earth at the inner edge of the habitable zone tends to end up as either a waterworld or with a desiccated surface; only a narrow range of initial water inventory yields an Earthlike surface.

Estimation of antigorite wave velocities in subduction conditions based on first-principles thermoelasticity

The most abundant serpentine mineral in subduction settings, antigorite has one of the highest water storage capacities and is involved in seismicity. Seismic wave velocities of antigorite are important for detecting and quantifying serpentinization within the mantle wedge and the subducting oceanic plate. At present, the elastic properties of antigorite at high pressures and temperatures are unclear. In this study, we have investigated pressure-volume-temperature (P-V-T) data and thermodynamic properties of antigorite using first-principles molecular dynamics (FPMD) simulations. Using these simulations results, we computed the relevant thermoelastic parameters and estimated compressional and shear wave velocities (vP and vS) of antigorite in subduction conditions. A simplified velocity model of antigorite with its coexisting mantle anhydrous phases was introduced to help us understand the potential effect of serpentinization on the seismic velocity of mantle rocks. Combined with seismic observations, we re-evaluated some velocity anomalies within forearc mantle wedges and established reliable serpentinization budgets. These results can provide preliminary evaluations and reliable constraints on serpentinization and water content in mantle rocks, which has important implications for understanding global plate dynamics and the deep water cycle.

The Subduction of Hydrogen: Deep Water Cycling, Induced Seismicity, and Plate Tectonics

The dynamic equilibrium between mantle degassing and water recycling in subduction zones controls the variation of sea level in deep geologic time, as well as the size of Earth’s interior hydrogen reservoir. While the principles of water transport and water release by common hydrous minerals in the subducted crust are relatively well understood, the importance of deep serpentinization of the slab, the contribution of nominally anhydrous minerals and dense hydrous magnesium silicates to water transport, and the mechanisms of water subduction into the lower mantle are still subjects of active research. A quantitative understanding of these processes is required to constrain the evolution of Earth’s deep water cycle through geologic time and the role of water in stabilizing plate tectonics.

The Geological History of Water: From Earth’s Accretion to the Modern Deep Water Cycle

The abundance of water on Earth and its distribution between surficial and deep reservoirs are the outcome of 4.6 billion years of geological history involving various mechanisms of water in and outgassing. Here, we use the metaphor of a pipeline connecting Earth’s deep and surface water reservoirs. The net flux through this pipeline has changed over time due to contrasting Hadean, Archean, and modern geodynamic regimes. Most water was dissolved in the primordial magma ocean, entrapped in the solidifying mantle, and massively released by volcanism during the Hadean and Archaean. As Earth cooled, plate tectonics enabled water ingassing into the mantle, which appears to exceed outgassing under the modern tectonic regime, implying that Earth’s surface has been drying out and will continue to do so.

Probing the state of hydrogen in δ−AlOOH at mantle conditions with machine learning potential

Hydrous and nominally anhydrous minerals are a fundamental class of solids of enormous significance to geophysics. They are the water carriers in the deep geological water cycle and impact structural, elastic, plastic, and thermodynamic properties and phase relations in Earth's forming aggregates (rocks). They play a critical role in the geochemical and geophysical processes that shape the planet. Their complexity has prevented predictive calculations of their properties, but progress in materials simulations ushered by machine-learning potentials is transforming this state of affairs. Here, we adopt a hybrid approach that combines deep learning potentials (DPs) with the strongly constrained and appropriately normed meta-generalized gradient approximation functional to simulate a prototypical hydrous system. We illustrate the success of this approach to simulate δ−AlOOH (δ), a phase capable of transporting water down to near the core-mantle boundary of the Earth (∼2900km depth and ∼135GPa) in subducting slabs. A high-throughput sampling of phase space using molecular dynamics simulations with DPs sheds light on the hydrogen-bond behavior and proton diffusion at geophysical conditions. These simulations provide a pathway for a deeper understanding of these crucial components that shape Earth's internal state. Published by the American Physical Society 2024

Water in Omphacite and Garnet From Pristine Xenolithic Eclogite: T‐X‐fO2 Controls, Retentivity, and Implications for Electrical Conductivity and Deep H2O Recycling

AbstractKimberlite‐borne eclogite xenoliths having Precambrian oceanic crustal protoliths and entrained from ≥100 km depth can retain pristine geochemical features despite extended residence in the cratonic lithospheric mantle, making them valuable archives of deep chemical cycling including that of water. We determined, by Fourier Transform Infrared Spectroscopy, structural OH contents in clinopyroxene and garnet from 15 unmetasomatized eclogite xenoliths. Calculated total c(H2O) is 100–510 wt.ppm for clinopyroxene and below detection (∼2 wt.ppm) to 200 wt.ppm for garnet, while garnet δ18O, determined by Secondary Ion Mass Spectrometry, ranges from +5.0‰ to +7.3‰, (similar to high‐ and low‐temperature seawater‐altered oceanic crust). Estimated electrical conductivity in pristine eclogites increases with temperature (i.e., depth for conductive geotherms), while clinopyroxene‐garnet H2O partition coefficients decrease with increasing temperature and garnet grossular component (i.e., Ca#), similar to other incompatible components. Various considerations suggest the retention of primary H2O in the samples, likely occurring in km‐sized pods of coarse‐grained eclogite. High Al2O3 in clinopyroxene as omphacite component, stabilized during high‐pressure metamorphism, facilitates H2O uptake. Therefore, the high bulk c(H2O) estimated for samples with plagioclase‐rich, deep crustal protoliths (median 290 wt.ppm) may indicate an interaction with fluids expelled at depth from serpentinites. The c(H2O) of ancient and modern subducted bulk oceanic crust (∼220–240 wt.ppm) are similar, suggesting constant mantle ingassing since at least 3 Ga ago. This places constraints on factors, such as mantle temperatures, that determine the efficiency of deep water cycling.

The role of magma oceans in maintaining surface water on rocky planets orbiting M-dwarfs

ABSTRACT Earth-like planets orbiting M-dwarf stars, M-Earths, are currently the best targets to search for signatures of life. Life as we know it requires water. The habitability of M-Earths is jeopardized by water loss to space: high flux from young M-dwarf stars can drive the loss of 3–20 Earth oceans from otherwise habitable planets. We develop a 0-D box model for Earth-mass terrestrial exoplanets, orbiting within the habitable zone, which tracks water loss to space and exchange between reservoirs during an early surface magma ocean phase and the longer deep-water cycling phase. A key feature is the duration of the surface magma ocean, assumed concurrent with the runaway greenhouse. This time-scale can discriminate between desiccated planets, planets with desiccated mantles but substantial surface water, and planets with significant water sequestered in the mantle. A longer-lived surface magma ocean helps M-Earths retain water: dissolution of water in the magma provides a barrier against significant loss to space during the earliest, most active stage of the host M-dwarf, depending on the water saturation limit of the magma. Although a short-lived basal magma ocean can be beneficial to surface habitability, a long-lived basal magma ocean may sequester significant water in the mantle at the detriment of surface habitability. We find that magma oceans and deep-water cycling can maintain or recover habitable surface conditions on Earth-like planets at the inner edge of the habitable zone around late M-dwarf stars – these planets would otherwise be desiccated if they form with less than ∼10 terrestrial oceans of water.

Water contents and hydrogen isotope compositions of amphibole in aillikites from the Tarim large igneous province, NW China: Insight into Earth’s deep water cycle

Water is known to play a crucial role in the generation of many large igneous provinces (LIP) worldwide, but the amount and origin of the water in their sources is still under debate. To address this question, this paper presents in situ major-, trace-element, and Sr isotope data coupled with bulk-mineral O-H-He isotope analyses of amphibole in a suite of aillikites from the Tarim LIP (NW China). The cores of zoned macrocrysts and microcrysts display partially overlapping compositions ranging between edenite and pargasite (75−83 versus 75−80 Mg#), which suggest a common origin from an evolving magma. The rims (Mg# = 75−80) of both macrocrysts and microcrysts are very similar to their cores for many elements, except for higher Sr and Ba contents. All the amphibole zones show similar primitive mantle−normalized trace element patterns, suggesting that they crystallized at different stages during magmatic evolution. This interpretation is confirmed by the homogenous Sr isotope compositions (87Sr/86Sr(i) = 0.70298−0.70394) of these amphiboles, which overlap with those of magmatic apatites and perovskites in these aillikites. The hydrogen isotope compositions (δD = −120‰ to −140‰) of the amphiboles are significantly lower than average mantle values. Given the low water contents (<0.66 wt%) of these minerals, the low H isotope signatures of the amphiboles might be caused by variable H2O loss during magma ascent. However, open-system Rayleigh fractionation modeling suggests that the hydrogen isotope compositions of these amphibole phenocrysts cannot be fully reproduced by crystallization following magmatic degassing. These low δD values require incorporation of recycled altered oceanic crust containing hydrous components in the plume source of these aillikites, which is consistent with the previously published moderately radiogenic He isotope ratios of olivine separates and bulk-rock Os and Pb isotope data for these same samples. We conclude that water in these magmas was derived from a plume source containing recycled water-bearing oceanic crust.

Large-Scale Cretaceous Adakitic Magmatism Induced by Water-Fluxed Melting of Continental Crust during the North China Craton Destruction

ABSTRACT The mechanism behind the destabilization of the North China Craton (NCC) remains a contentious topic among researchers. Large-scale Cretaceous adakitic magmatism in the NCC offers insights into the decratonization process. This study focuses on the Huashan and Laoniushan plutons located in the Lesser Qinling on the southern margin of the NCC and compiles published data for coeval adakitic rocks to investigate the role of water in adakitic rock petrogenesis during the peak destruction of NCC. Both the Huashan and Laoniushan plutons exhibit adakitic signatures, including high Sr (193–1080 ppm), low Yb (<14.8 ppm) and Y (<1.24 ppm) concentrations, as well as high Sr/Y (18–100) and La/Yb (24–58) ratios. The zircon Hf–O isotope compositions suggest that the primary source for the Huashan and Laoniushan plutons is the mafic lower crust of NCC. Nevertheless, there are significant differences in trace element characteristics between the two plutons. Specifically, the Huashan pluton displays higher Na2O/K2O ratios, lower levels of Rb, Rb/Sr, Nb, Ta content, and a weak Eu anomaly in comparison to the Laoniushan pluton. These variations in geochemical attributes cannot be accounted for by mechanisms like mantle-derived magma mixing, crustal contamination, or fractional crystallization processes. Instead, these disparities are attributed to distinct modes of crustal anatexis, involving both water-fluxed and dehydration melting. Subsequently, we conducted thermodynamic simulations of the melting process of mafic lower crust under different pressure (0.5–1.5 GPa) and water content conditions (1–3 wt.%). The simulation results suggest that the Huashan pluton is most likely formed through water-fluxed melting in a scenario with normal crustal thickness (1 GPa). On the other hand, the Laoniushan pluton might have originated from dehydration melting under normal crustal thickness and pressure conditions. Notably, high pressure (>1.5 GPa) is not necessary for the formation of intracontinental adakitic rocks. The release of water from metasomatized lithospheric mantle and subsequent hydration of the lower continental crust triggers extensive adakitic magmatism in the NCC. These findings emphasize the significance of deep water cycling in understanding large-scale magmatic events and illuminate the decratonization mechanism.

OH incorporation and retention in eclogite-facies garnets from the Zermatt–Saas area (Switzerland) and their contribution to the deep water cycle

Abstract. The incorporation mechanisms of OH groups in garnet were investigated in a suite of high-pressure rocks from the Zermatt–Saas area (Switzerland) using a combination of Fourier transform infrared spectroscopy (FTIR) and electron probe micro-analysis (EPMA). Investigated garnet specimens include grossular–andradite–uvarovite solid solutions in serpentinite and rodingite and almandine–grossular–pyrope–spessartine solid solutions in eclogite, mafic fels and meta-sediment. All rocks experienced the same peak metamorphic conditions corresponding to a burial depth of ∼ 80 km (∼ 540 ∘C, 2.3 GPa), allowing determination of the OH content in garnet as a function of rock type. The capacity for OH incorporation into garnet strongly depends on its composition. Andradite-rich (400–5000 µg g−1 H2O) and grossular-rich garnet (200–1800 µg g−1 H2O) contain at least 1 order of magnitude more H2O than almandine-rich garnet (< 120 µg g−1 H2O). Microscale analyses using FTIR and EPMA profiles and maps reveal the preservation of OH zoning throughout the metamorphic history of the samples. The OH content correlates strongly with Mn, Ca and Ti zoning and produces distinct absorption bands that are characteristic of multiple nano-scale OH environments. The use of 2D diffusion modelling suggests that H diffusion rates in these rocks is as low as log(D[m2 s−1]) = −24.5 at 540 ∘C. Data were collected for the main garnet-bearing rock types of the Zermatt–Saas area allowing a mass balance model of H2O to be calculated. The result shows that ∼ 3360 kg H2O km−1 (section of oceanic crust) yr−1 could be transported by garnet in the subducting slab beyond 80 km depth and contributed to the deep-Earth water cycle during the Eocene subduction of the Piemonte–Liguria Ocean.

Multistage hydration during oceanic serpentinisation revealed by in situ oxygen isotope and trace element analyses

Serpentinisation of mantle peridotites below the seafloor is the most important hydration reaction in the Earth's deep water cycle. This critical step in water–rock interaction occurs over multiple serpentinisation stages and at variable temperatures and fluid compositions. We present the first study using spatially coupled in-situ analysis of oxygen isotopes (secondary ionization mass spectrometry) and trace elements (laser ablation inductively coupled plasma mass spectrometry) to unravel the multistage hydration history of oceanic serpentinites. We study samples from the Newfoundland-Iberia extended passive margins, which represents a magma-poor ocean-continent transition zone (Ocean Drilling Program cores, Leg 173 Site 1070 from Iberia, Leg 210 Site 1277 from Newfoundland). The concentrations of the fluid mobile elements chlorine and boron in serpentine are used as a proxy for the salinity of the serpentinising fluid. The correlation of Cl/B with δ18Oserpentine compositions provides new insights to disentangle temperature from fluid composition effects. The transition metal composition (V, Co, Sc, Mn, Zn, Ni, Cr) of dominantly lizardite in mesh after olivine and in bastite after orthopyroxene shows a chemical redistribution between textural sites in the Newfoundland samples, indicating the simultaneous serpentinisation of olivine and orthopyroxene. This feature is not observed in the Iberian samples, for which we propose sequential reactions. Lizardite in samples from both localities varies considerably in oxygen isotope composition at the scale of tens of micrometres depending on texture, with a range in δ18O of 3.3–13.5‰ for Iberia samples and a more restricted range of 5.7–9.3‰ for Newfoundland samples. Temperatures calculated from the δ18Oserpentine corresponding to the lowest Cl/B ratio (interpreted as closest to seawater composition) indicate sequential serpentinisation with decreasing temperature from ∼190 to ∼60 °C in the Iberia setting. The Newfoundland samples were serpentinised at a lower temperature (100–130 °C), possibly at a shallower depth. Serpentinisation in the Newfoundland samples probably produced a concentrated amount of H2 in a shorter period of time, whereas in the Iberian samples a small but prolonged amount of H2 was produced, possibly creating more favourable conditions for microbial activity.

Mantle mineralogy limits to rocky planet water inventories

ABSTRACT Nominally anhydrous minerals in rocky planet mantles can sequester multiple Earth-oceans’ worth of water. Mantle water storage capacities therefore provide an important constraint on planet water inventories. Here we predict silicate mantle water capacities from the thermodynamically-limited solubility of water in their constituent minerals. We report the variability of upper mantle and bulk mantle water capacities due to (i) host star refractory element abundances that set mantle mineralogy, (ii) realistic mantle temperature scenarios, and (iii) planet mass. We find that transition zone minerals almost unfailingly dominate the water capacity of the mantle for planets of up to ∼1.5 Earth masses, possibly creating a bottleneck to deep water transport, although the transition zone water capacity discontinuity is less pronounced at lower Mg/Si. The pressure of the ringwoodite-perovskite phase boundary defining the lower mantle is roughly constant, so the contribution of the upper mantle reservoir becomes less important for larger planets. If perovskite and postperovskite are relatively dry, then increasingly massive rocky planets would have increasingly smaller fractional interior water capacities. In practice, our results represent initial water concentration profiles in planetary mantles where their primordial magma oceans are water-saturated. This work is a step towards understanding planetary deep water cycling, thermal evolution as mediated by rheology and melting, and the frequency of ocean planets.

Hydrogen Concentrations and He Isotopes in Olivine From Ultramafic Lamprophyres Provide New Constraints on a Wet Tarim Plume and Earth's Deep Water Cycle

AbstractWater enters Earth's mantle via subduction of hydrated oceanic slab and largely returns to the ocean‐atmosphere system through arc volcanism. However, the extent to which H2O is transferred into the deep mantle is poorly constrained. Here, we address this question by combining mineral chemistry and bulk‐rock geochemistry data for aillikites related to the deep mantle plume which generated the Permian Tarim large igneous province (NW China). The water contents of olivine phenocrysts are 75–168 ppm H2O and positively correlated with Ti contents. These results, combined with infrared hydroxyl peaks at 3,572 and 3,525 cm−1, suggest that H is mainly present in the form of Ti‐clinohumite‐like point defects. Hydrogen concentration profiles across olivine reveal that H loss during decompression was limited to the outermost rims, and yield dehydration durations of 15–417 min. Based on the water contents of the highest‐Fo olivine cores, the water contents of the primitive aillikite melts and their mantle source are estimated as 1.6–4.7 wt% and 150–1,200 ppm H2O, respectively. 3He/4He ratios (5.31–5.84 Ra) of olivine phenocrysts are slightly lower than MORBs and suggest involvement of recycled slab containing U (and hence radiogenic 4He) in the plume source. This interpretation is consistent with Pb isotope compositions of the aillikites which are intermediate between PREMA (Prevalent Mantle) and EM (Enriched Mantle) compositions. These lines of evidence combined with the depleted Sr‐Nd isotopes and moderately radiogenic Os isotopes of the aillikites suggest that water in these rocks derived from a plume source marginally contaminated by deeply subducted hydrated material.

Buffering of mantle conditions through water cycling and thermal feedbacks maintains magmatism over geologic time

The Earth has remained magmatically and volcanically active over its full geologic history despite continued planetary cooling and a lack of thermal equilibrium in the mantle. Here we investigate this conundrum using data-constrained numerical models of deep-water cycling and thermal history. We find that the homologous temperature - the ratio of upper mantle to melting temperatures - initially declined but has been buffered at a nearly constant value since 2.5-2.0 billion years ago. Melt buffering is a result of the dependence of melting temperature and mantle viscosity on both mantle temperature and water content. We show that thermal and water cycling feedbacks lead to a self-regulated mantle evolution, characterised by a near-constant mantle viscosity. This occurs even though the mantle remains far from thermal equilibrium. The added feedback from water-dependent melting allows magmatism to be co-buffered over geological time. Thus, we propose that coupled thermal and water cycling feedbacks have maintained melting on Earth and associated volcanic/magmatic activity.

Elasticity of Phase H Under the Mantle Temperatures and Pressures: Implications for Discontinuities and Water Transport in the Mid‐Mantle

AbstractPhase H (MgSiO4H2), one of the lower mantle's dense hydrous magnesium silicates (DHMSs), may form and exist in cold slabs and is crucial in carrying water into the deep mantle. Its sound velocities and density are crucial for inferring the mid‐mantle water cycling via seismic approaches. Here we obtain the elastic and thermodynamic properties of phase H under lower‐mantle conditions using first‐principles calculations and discuss the effect of the Mg‐Si disorder on elasticity. The density of phase H is ∼15% and ∼6% lower than that of bridgmanite and periclase, respectively. The dehydration reaction from phase H to bridgmanite, which may occur at the depth of ∼1,300–1,700 km in cold slabs, will cause an increase of 1.0%, 2.7%, and 15% at 1,500 km on VP, VS, and density, respectively. The dehydration of phase H in subduction zones could produce a seismic VS impedance contrast of ∼17% in the mid‐mantle, which can provide an explanation for some seismic discontinuities detected by previous studies. Meanwhile, phase H has remarkable anisotropies and this may help explain the observed seismic anisotropy within subduction zones. Collectively, our results suggest that some seismic observations in mid‐mantle slabs may be related to the presence of phase H formed via the deep water cycle, further constraining the potential water content in local regions of the subducted slabs.

Dense hydrous silica could carry water to Earth’s mantle

The deep water cycle may alter mantle properties and regulate surface reservoirs.

Elastic Anisotropy of Lizardite at Subduction Zone Conditions

AbstractSubduction zones transport water into Earth's deep interior through slab subduction. Serpentine minerals, the primary hydration product of ultramafic peridotite, are abundant in most subduction zones. Characterization of their high‐temperature elasticity, particularly their anisotropy, will help us better estimate the extent of mantle serpentinization and the Earth's deep water cycle. Lizardite, the low‐temperature polymorph of serpentine, is stable under the P‐T conditions of cold subduction slabs (<260°C at 2 GPa), and its high‐temperature elasticity remains unknown. Here we report ab initio elasticity and acoustic wave velocities of lizardite at P‐T conditions of subduction zones. Our static results agree with previous studies. Its high‐temperature velocities are much higher than previous experimental‐based lizardite estimates with chrysotile but closer to antigorite velocities. The elastic anisotropy of lizardite is much larger than that of antigorite and could better account for the observed large shear‐wave splitting in some cold slabs such as Tonga.

Intramolecular hydrogen isotope exchange inside silicate melts – The effect of deuterium concentration

Tracing the deep geological water cycle requires knowledge of the hydrogen isotope systematics between and within hydrous materials. For quenched hydrous alkali-silicate melts, hydrogen NMR reveals a distinct heterogeneity in the distribution of stable hydrogen isotopes (D, H) within the silicate tetrahedral network, where deuterons concentrate strongly in network regions that are associated with alkali cations. Previous hydrogen NMR studies performed in the sodium tetrasilicate system (Na2O x 4SiO2, NS4) with a 1:1 D2O/H2O ratio showed on average 1300 ‰ deuterium enrichment in the alkali-associated network, but the effect on varying bulk D2O/H2O ratios on this intramolecular isotope effect remained unconstrained. Experiments in the hydrous sodium tetrasilicate system with 8 wt% bulk water and varying bulk D2O/H2O ratios were performed at 1400 °C and 1.5 GPa. It is found that both hydrogen isotopes preferably partition into the silicate network that is associated with alkali ions. The partitioning is always stronger for the deuterated than for the protonated hydrous species. The relative enrichment of deuterium over protium in the alkali-associated network, i.e., the intramolecular isotope effect, correlates positively with the D2O/H2O bulk ratio of the hydrous NS4 system. Modeled for natural deuterium abundance (D/H near 1.56 × 10−4), a 1.4-fold (c. 340 ‰) deuterium enrichment in the alkali-associated silicate network is predicted. The partitioning model further predicts a positive correlation between the bulk water content of the silicate melt and the intramolecular deuterium partitioning into the alkali-associated silicate network. Such heterogeneities may explain the magnitude and direction of hydrogen isotope fractionation in hydrous silicate melts coexisting with silicate-saturated fluids. As such, this intramolecular isotope effect appears to be an effective mechanism for deuterium separation, particularly in hydrous magmatic settings, such as subduction zones.