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Metasomatized mantle xenoliths containing hydrous minerals, such as amphiboles, serpentine, and phlogopite, likely represent the potential mineralogical compositions of the metasomatized upper mantle, where low seismic velocities are commonly observed. This study presents the first experimentally determined single‐crystal elasticity model of an Fe‐free near Ca, Mg‐endmember amphibole tremolite at high pressure and/or temperature conditions (maximum pressure 7.3(1) GPa, maximum temperature 700 K) using Brillouin spectroscopy. We found that sound velocities of amphiboles strongly depend on the Fe content. We then calculated the sound velocities of 441 hydrous‐mineral‐bearing mantle xenoliths collected around the globe, and quantitatively evaluated the roles that amphiboles, phlogopite and serpentine played in producing the low velocity anomalies in the metasomatized upper mantle. Plain Language Summary: Amphiboles are the most widely distributed hydrous minerals resulting from metasomatism in the upper most mantle. We measured sound velocities of tremolite (Ca, Mg endmember of the amphibole series) at high pressures and high temperatures by Brillouin spectroscopy. Based on global hydrous‐mineral‐bearing mantle xenoliths record, we quantitively evaluated the contributions of amphiboles, serpentine, and phlogopite to low velocity anomalies and water storage in the upper most mantle. We found the existence of hydrous minerals (amphiboles, serpentine, and phlogopite) remains a viable explanation for the low velocity anomalies in the upper most mantle (e.g., mid‐lithosphere discontinuity). Compared to serpentine and phlogopite, although the amount of velocity reduction caused by amphibolization is moderate, the formation of amphiboles does not require K, Al, Si‐rich environments like phlogopite, or exceedingly water‐rich environments like serpentine. Key Points: The single‐crystal elasticity of tremolite is determined by Brillouin spectroscopy up to 7.3 GPa and 700 KSound velocities of uppermost mantle amphiboles mainly depend on Fe contentHydrous minerals (amphiboles, serpentine, phlogopite) are plausible causes of the low velocity anomalies in the uppermost mantle [ABSTRACT FROM AUTHOR]

Abstract Over the past few decades, hydrous minerals were thought to be absent in the lower mantle, due to their instability at high-pressure conditions. Recently, however, hydrous phases including phase H (MgSiO $$_4$$ 4 H $$_2$$ 2 ), pyrite-type FeOOH, and $$\delta$$ δ -AlOOH have been discovered to be thermodynamically stable at lower mantle pressures. Investigations using ab initio calculations methods play a key role in identifying these novel phases and determining their geophysical properties (i.e., compressibility, elasticity, and sound velocities). These calculations suggest that the hydrous minerals which are stable at lower mantle pressure conditions (i.e., phase H, FeOOH, and AlOOH), have symmetric hydrogen bonds at these pressures. This indicates that hydrogen bond strength is closely connected to the stability and physical properties of hydrous minerals at extreme pressures. In this review, we summarize the theoretical and experimental studies of hydrous minerals stable at the high-pressure conditions of the Earth’s lower mantle in light of the role of hydrogen bonding.

The electrical conductivity and elasticity of deep hydrous phases are essential to constraining water distribution, as well as deciphering the origins of conductivity anomalies in the lower mantle. To uncover the impact of iron-bearing δ-AlOOH on the geophysical properties of the lower mantle, we carried out synchrotron X‑ray diffraction and electrical conductivity measurements on δ-(Al0.52Fe0.48)OOH and (Al0.95Fe0.05)OOH in diamond-anvil cells at pressures up to 75 GPa at room temperature. A sharp volume reduction of ~6.5% was observed in δ-(Al0.52Fe0.48)OOH across the spin transition at 40.8–43.3 GPa, where its electrical conductivity increases steadily without abrupt changes. The electrical conductiv‑ ity of δ-(Al0.52Fe0.48)OOH is greater than that of pure δ-AlOOH at high pressure, suggesting that both small polaron and proton conduction mechanisms dominate in iron-bearing δ-AlOOH. Furthermore, the high-pressure electrical conductivity profiles are comparable between δ-(Al0.95Fe0.05)OOH and δ-(Al0.52Fe0.48)OOH, indicating that high-iron content only marginally influences the conductivity of iron-bearing δ-AlOOH. Notably, the electrical conductivity of iron-bearing δ-AlOOH along the North Philippine geotherm is greater than the average 1D electrical conductivity profile in the mantle (Ohta et al. 2010a). This result suggests that δ-(Al,Fe)OOH is a promising candidate to account for high conductivity in some subducting slabs. [ABSTRACT FROM AUTHOR]

Over the past few decades, hydrous minerals were thought to be absent in the lower mantle, due to their instability at high-pressure conditions. Recently, however, hydrous phases including phase H (MgSiO 4 H 2 ), pyrite-type FeOOH, and δ -AlOOH have been discovered to be thermodynamically stable at lower mantle pressures. Investigations using ab initio calculations methods play a key role in identifying these novel phases and determining their geophysical properties (i.e., compressibility, elasticity, and sound velocities). These calculations suggest that the hydrous minerals which are stable at lower mantle pressure conditions (i.e., phase H, FeOOH, and AlOOH), have symmetric hydrogen bonds at these pressures. This indicates that hydrogen bond strength is closely connected to the stability and physical properties of hydrous minerals at extreme pressures. In this review, we summarize the theoretical and experimental studies of hydrous minerals stable at the high-pressure conditions of the Earth's lower mantle in light of the role of hydrogen bonding. [ABSTRACT FROM AUTHOR]

We report the synthesis, at 7 GPa and 923 K, and the thermoelastic characterization, up to 16 GPa and 850 K, of a single crystal of Mg-sursassite, Mg5Al5Si6O21(OH)7. In situ high-pressure and high-temperature single-crystal diffraction allowed the study of structural variation at non-ambient conditions and the determination of bulk elastic properties. The refined parameters of a second-order Birch-Murnaghan equation of state (BM-II EoS) are V0 = 446.02(1) Å3 and KT0 = 135.6(7) GPa. The thermal expansion coefficients of a Berman-type EoS are α0 = 3.14 (5) × 10−5 K−1, α1 = 2.50(16) × 10−8 K−2, and V0 = 445.94(3). For comparison, the P-V EoS is determined for a natural sursassite sample, ideally Mn4Al6Si6O22(OH)6. The refined parameters of BM-II EoS [V0 = 470.2(3) Å3, KT0 = 128(4) GPa] indicate that composition has a minimal effect on elastic properties. The similarity of density and bulk properties of Mg-sursassite if compared to olivine and other anhydrous mantle minerals suggests that this phase could be overseen by geophysical methods. [ABSTRACT FROM AUTHOR]

Dense hydrous magnesium silicates (DHMSs) are considered important water carriers in the deep Earth. Due to the significant effect of Fe on the stability of DHMSs, Fe-bearing Phase D (PhD) deserves much attention. However, few experiments have been conducted to determine the stability of PhD in different bulk compositions. In this study, we provide experimental constraints for the stability of PhD in the AlOOH-FeOOH-Mg1.11Si1.89O6H2.22 system between 18 and 25 GPa at 1000–1600 °C, corresponding to the P-T conditions of the mantle transition zone and uppermost lower mantle. Fe3+-bearing PhD was synthesized from the FeOOH-Mg1.11Si1.89O6H2.22 binary system with two different Fe3+ contents. The resultant Al,Fe3+-bearing compositions are close to analog specimens of the fully oxidized mid-ocean ridge basalt (MORB) and pyrolite in the AlOOH-FeOOH-Mg1.11Si1.89O6H2.22 ternary system. The substitution mechanism of Fe is shown to be dependent on pressure, and Fe3+ occupies both Mg and Si sites in PhD at pressures below 21 GPa. In contrast, Fe3+ only occupies Si site at pressures exceeding 21 GPa. The presence of Fe3+ results in a slight reduction in the thermal stability field of PhD in the FeOOH-Mg1.11Si1.89O6H2.22 system in comparison to Mg-bearing, Fe-free PhD. In contrast, Al,Fe3+-bearing PhD is more stable than Mg-bearing PhD in both MORB and pyrolite compositions. In this regard, Al,Fe3+-bearing PhD could act as a long-term water reservoir during subduction processes to the deep mantle. [ABSTRACT FROM AUTHOR]

The sound velocities of polycrystalline Mg-phase D were investigated at the pressure up to 13.6 GPa and room temperature, using ultrasonic interferometry in a multi-anvil apparatus. The P (VP) and S (VS) wave velocities obtained at ambient conditions produced the results of VP = 9.26(3) km/s and VS = 5.46(1) km/s. The results are in accordance with the values for Mg-phase D by Brillouin scattering (VP = 9.25 km/s and VS = 5.51km/s). The results, i.e. Ks0 = 153.8(13) GPa and G0 = 99.8(5) GPa obtained through two-dimensional linear fitting analysis are self-consistent with Ks0 = 153.3(16) GPa and G0 = 99.5(10) GPa derived by third-order Eulerian finite strain equations. We also find that Vp of phase D is 1.8–2.2% lower than Fe-ringwoodite, but Vs is higher than Fe-ringwoodite at the bottom of the mantle transition zone by up to 1.3%. [ABSTRACT FROM AUTHOR]

• Efficient two-step phase transition extracted valuable metals from gypsum. • Path-0.5 outpaced path-0, achieving 99.9 % Cr and 98.8 % Cd extraction. • Atom rearrangement in two-step transition triggered atom remove form the lattice. • Extraction efficiencies for trace Pb, Zn, Hg, and As were > 98.5 %, and 95.6 % for Sr. Retrieving valuable metals (VMs) from industrial waste is an attractive approach to reduce the continuous consumption of metallic resources and prevent the environmental contamination by heavy metals. However, metal extraction efficiencies are extremely low due to their entrapment in crystal lattices. Herein, a universal strategy for extracting VMs from gypsum (CaSO 4 ·2H 2 O), a common industrial waste, was reported using a two-step phase transition of dehydration–rehydration; metal extraction was achieved by designing an appropriate phase transition route and controlling the phase transformation kinetics. As the phase transition route to CaSO 4 ·0.5H 2 O (path-0.5) proceeded faster than that to CaSO 4 (path-0), higher extraction efficiencies were realized for Cr (99.9%) and Cd (98.8%) from gypsum. Mechanistic investigations indicated that during the selected path-0.5 dehydration, water molecule loss from gypsum initiated a direct solid-state transformation, triggering the first rearrangement of local atoms and excluding partially incorporated metals from the CaSO 4 ·0.5H 2 O lattice. The subsequent rehydration from CaSO 4 ·0.5H 2 O to CaSO 4 ·2H 2 O, with a phase transition rate faster than that of path-0, triggered the secondary atom rearrangement. The synergism between the two atomic rearrangements finally led to the complete release of trapped metals. Additionally, this strategy was used for retrieving other metals (e.g., Pb, Zn, Hg, As, and Sr) from gypsum, thereby potentially offering a new approach for designing an appropriate phase transition route for extracting VMs from other hydrous minerals. [ABSTRACT FROM AUTHOR]

Abstract Interpretation of electrical conductivity anomalies observed in magnetotelluric models provides an important opportunity to understand the nature of the lithospheric mantle and its dynamics. Over the course of the last two decades, a great number of experimental petrology studies have been carried out which can be utilized to construct electrical conductivity distribution models for a given composition and geotherm. We have developed an open‐source software (MATE, Mantle Analysis Tool for Electromagnetics) with an easy‐to‐use graphical interface that creates such theoretical models. The program is developed in such a way that additional effects and models can be added very easily. To investigate the conductivity distribution of the cratonic mantle, a series of experiments was made. Results indicate that it is of utmost importance to analyze the magnetotelluric models using accurate compositions, water distributions, and geometric models. Hence, using only olivine conductivity models can lead to erroneous interpretations of both conductivity and estimated water content. Analysis of the potential causes for conductive anomalies shows that the upper and lower lithospheric mantle can be interpreted separately with the transition between them at 75–125 km. Conductive anomalies in the upper lithospheric mantle (

Hydrogen transport from the surface to the deep interior and distribution in the mantle are important in the evolution and dynamics of the Earth. An aluminum oxy-hydroxide, δ-AlOOH, might influence hydrogen transport in the deep mantle because of its high stability extending to lower mantle conditions. The compressional behavior and spin states of δ-(Al,Fe3+)OOH phases were investigated with synchrotron X‑ray diffraction and Mössbauer spectroscopy under high pressure and room temperature. Pressure-volume (P-V) profiles of the δ-(Al0.908(9)57Fe0.045(1))OOH1.14(3) [Fe/(Al+Fe) = 0.047(10), δ-Fe5] and the δ-(Al0.832(5)57Fe0.117(1))OOH1.15(3) [Fe/(Al+Fe) = 0.123(2), δ-Fe12] show that these hydrous phases undergo two distinct structural transitions involving changes in hydrogen bonding environments and a high- to low-spin crossover in Fe3+. A change of axial compressibility accompanied by a transition from an ordered (P21nm) to disordered hydrogen bond (Pnnm) occurs near 10 GPa for both δ-Fe5 and δ-Fe12 samples. Through this transition, the crystallographic a and b axes become stiffer, whereas the c axis does not show such a change, as observed in pure δ-AlOOH. A volume collapse due to a transition from high- to low-spin states in the Fe3+ ions is complete below 32–40 GPa in δ-Fe5 and δ-Fe12, which is ~10 GPa lower than that reported for pure e-FeOOH. Evaluation of the Mössbauer spectra of δ-(Al0.824(10)57Fe0.126(4))OOH1.15(4) [Fe/(Al+Fe) = 0.133(3), δ-Fe13] also indicate a spin transition between 32–45 GPa. Phases in the δ-(Al,Fe)OOH solid solution with similar iron concentrations as those studied here could cause an anomalously high r/vF ratio (bulk sound velocity, defined as K / ρ) $\sqrt{\left. K/\rho \right)}$ at depths corresponding to the spin crossover region (~900 to ~1000 km depth), whereas outside the spin crossover region a low r/vF anomaly would be expected. These results suggest that the δ-(Al,Fe)OOH solid solution may play an important role in understanding the heterogeneous structure of the deep Earth. [ABSTRACT FROM AUTHOR]

Zoisite is a typical accessory mineral of eclogite; understanding its compressional behavior is important for the knowledge of the properties and processes within subduction zones. In this study, the compressional behavior of a natural eclogitic zoisite Ca1.99(Al2.87Fe0.11)Si3.00O12OH was investigated at ambient temperature and high pressure to 34 GPa, using a diamond anvil cell (DAC) combined with synchrotron-based single-crystal X-ray diffraction (XRD) method. Our results indicate that zoisite is stable over the experimental pressure range. The pressure–volume (P–V) data were fitted to a third-order Birch–Murnaghan equation of state (BM3 EoS), and the equation of state coefficients including zero-pressure unit-cell volume (V0), isothermal bulk modulus (KT0), and its pressure derivative ( K T 0 ′ ) were obtained as: V0 = 904.77(8) Å3, KT0 = 118(1) GPa, and K T 0 ′ = 6.3(2), respectively. The axial compressibilities (β) for a-, b-, and c-axes were also obtained using a parameterized form of the BM3 EoS, and the results show βa0 < βb0 < βc0 with βa0:βb0:βc0 = 1:1.28:1.50. In addition, the bulk modulus of this study is very consistent with previously studied zoisite with similar Fe content. However, the axial compressibility is significantly different with the previous study and the compression of zoisite in this study is more isotropic, which may result from the difference in the pressure-transmitting medium. [ABSTRACT FROM AUTHOR]

Highlights • A screening methodology for thermochemical materials based on a mineral classification is firstly introduced. • Potential reaction systems are experimentally identified, which have yet not been considered. • Two reaction systems are identified to match the defined screening criteria. • The hysteresis at a defined water vapor partial pressure is determined. • Cycling stability of the system CaZn 2 (PO 4) 2 ·2H 2 O / CaZn 2 (PO 4) 2 ·H 2 O is foremost proven. Abstract Thermochemical energy storage and conversion is yet considered as key technology enhancing the efficiency of energy production and utilization from renewables. Recently, several search algorithms and screening approaches for new materials for thermochemical energy storage and conversion have been published. Since respective reaction systems have to match numerous requirements, the demand for suitable materials in the low, middle and high temperature range is quite significant. However, recent works on reversible de- and rehydrations for application in the middle temperature range from 100 °C–400 °C are predominantly restricted to simple, binary compounds containing water of crystallization. Within the scope of the present work, a new search methodology based on a mineral classification system also including ternary or higher compounds is firstly subjected and experimentally validated. The experimental elaboration started from a small number of 29 compounds, from which already two suitable reaction systems are identified. For one of these reaction systems, orthorhombic calcium dizinc-bisphosphate dihydrate to calcium dizinc-bisphosphate monohydrate, the cycling stability is firstly proven on a mineral sample. [ABSTRACT FROM AUTHOR]

It is demonstrated that quasielastic neutron scattering is a novel and effective method to analyse atomic scale hydrogen transport processes occurring within a mineral crystal lattice. The method was previously characterized as sensitive for analysing the transport frequency and distance of highly diffusive hydrogen atoms or water molecules in condensed matter. Here are shown the results of its application to analyse the transport of much slower hydrogen atoms which are bonded into a crystal lattice as hydroxyls. Two types of hydrogen transport process were observed in brucite, Mg(OH)2: a jump within a single two‐dimensional layer of the hydrogen lattice and a jump into the next nearest layer of it. These transport processes observed within the prototypical structure of brucite have direct implications for hydrogen transport phenomena occurring within various types of oxides and minerals having layered structures. Quasielastic neutron scattering is applied to analyse the atomic scale hydrogen transport processes occurring within the crystal lattices of hydrous minerals. Two types of transport processes were observed in Mg(OH)2, which has a prototypical layered hydrogen lattice structure. [ABSTRACT FROM AUTHOR]

Abstract: Amazonian‐aged Lyot crater is the best‐preserved and deepest peak‐ring impact crater (diameter, D = 220 km) in the northern lowlands of Mars. Morphological features including scouring channels emanating from its ejecta and small channels within the crater have been examined previously to understand hydrological activity associated with the crater. In this study, we analyze images acquired by the Compact Reconnaissance Imaging Spectrometer for Mars on board the Mars Reconnaissance Orbiter to investigate the mineralogical record in Lyot and its surroundings, which are presently enriched in ground ice, to understand the associated aqueous processes, their relative timing, and a possible role for ground ice in hydrous mineral formation. We find diverse hydrous minerals, including Fe/Mg phyllosilicates, chlorite, illite/muscovite, and prehnite in Lyot crater walls, central peak, and ejecta, as well as in two craters to the west of Lyot. The exposure and distribution of the hydrous minerals suggest that they are related to the impact process, either exposed by the excavation of hydrothermally altered rocks or formed through syn‐depositional hydrothermal alteration immediately after impacts. The Lyot impact induced channel formation to the north, but no mineralogical evidence of aqueous alteration associated with the channels is observed. The sinuous channels within Lyot, diverted by bedrock units with hydrous mineral detections, did not cause mineralization but likely represent the last stage of water activity in Lyot crater. The separate episodes of water activity indicate flow of liquid water on Mars' surface during the Amazonian but limited to no aqueous alteration to generate hydrous minerals. [ABSTRACT FROM AUTHOR]

Quantitative analysis of hydrated minerals from hyperspectral remote sensing data is fundamental for understanding Martian geologic process. Because of the difficulties for selecting endmembers from hyperspectral images, a sparse unmixing algorithm has been proposed to be applied to CRISM data on Mars. However, it's challenge when the endmember library increases dramatically. Here, we proposed a new methodology termed Target Transformation Constrained Sparse Unmixing (TTCSU) to accurately detect hydrous minerals on Mars. A new version of target transformation technique proposed in our recent work was used to obtain the potential detections from CRISM data. Sparse unmixing constrained with these detections as prior information was applied to CRISM single-scattering albedo images, which were calculated using a Hapke radiative transfer model. This methodology increases success rate of the automatic endmember selection of sparse unmixing and could get more accurate abundances. CRISM images with well analyzed in Southwest Melas Chasma was used to validate our methodology in this study. The sulfates jarosite was detected from Southwest Melas Chasma, the distribution is consistent with previous work and the abundance is comparable. More validations will be done in our future work. [ABSTRACT FROM AUTHOR]

The hydrogen and oxygen stable isotope composition (δ 2 H and δ 18 O values) of mineral hydration waters can give information on the environment of mineral formation. Here we present and validate an approach for the stable isotope analysis of mineral hydration waters based on coupling a thermogravimetric analyzer with a laser-based isotope ratio infrared spectroscopy instrument (Picarro L-2130i), which we abbreviate as TGA-IRIS. TGA-IRIS generates δ 2 H and δ 18 O values of liquid water samples with precision for δ 2 H of ± 1.2‰, and for δ 18 O of ± 0.17‰. For hydration waters in goethite, precision for δ 2 H ranges from ± 0.3‰ to 1.6‰, and for δ 18 O ranges from ± 0.17‰ to 0.27‰. The ability of TGA-IRIS to generate detailed water yield data and δ 2 H and δ 18 O values of water at varying temperatures allows for the differentiation of water in varying states of binding on mineral surfaces and within the mineral matrix. TGA-IRIS analyses of hydrogen isotopes in goethite yields δ 2 H values that reflect the hydrogen of the OH − phase in the mineral and are comparable to that made by IRMS and found in the literature. In contrast, δ 18 O values on goethite reflect the oxygen in OH − groups bound to Fe (Fe-OH group), and not the oxygen bound only to Fe (Fe-O group) in the mineral crystal lattice, and may not be comparable to literature δ 18 O values made by IRMS that reflect the total O in the mineral. TGA-IRIS presents the possibility to isotopically differentiate the various oxygen reservoirs in goethite, which may allow the mineral to be used as a single mineral geothermometer. TGA-IRIS measurements of hydration waters are likely to open new avenues and possibilities for research on hydrated minerals. [ABSTRACT FROM AUTHOR]

The advent of continuous-flow isotope-ratio mass spectrometry (CF-IRMS) coupled with a high temperature conversion (HTC) system enabled faster, more cost effective, and more precise δ 2 H analysis of hydrogen-bearing solids. Accurate hydrogen isotopic analysis by on-line or off-line techniques requires appropriate isotopic reference materials (RMs). A strategy of two-point calibrations spanning δ 2 H range of the unknowns using two RMs is recommended. Unfortunately, the supply of the previously widely used isotopic RM, NBS 30 biotite, is exhausted. In addition, recent measurements have shown that the determination of δ 2 H values of NBS 30 biotite on the VSMOW-SLAP isotope-delta scale by on-line HTC systems with CF-IRMS may be unreliable because hydrogen in this biotite may not be converted quantitatively to molecular hydrogen. The δ 2 H VSMOW-SLAP values of NBS 30 biotite analyzed by on-line HTC systems can be as much as 21 mUr (or ‰) too positive compared to the accepted value of − 65.7 mUr, determined by only a few conventional off-line measurements. To ensure accurate and traceable on-line hydrogen isotope-ratio determinations in mineral samples, we here propose two isotopically homogeneous, hydrous mineral RMs with well-characterized isotope-ratio values, which are urgently needed. The U.S. Geological Survey (USGS) has prepared two such RMs, USGS57 biotite and USGS58 muscovite. The δ 2 H values were determined by both glassy carbon-based on-line conversion and chromium-based on-line conversion, and results were confirmed by off-line conversion. The quantitative conversion of hydrogen from the two RMs using the on-line HTC method was carefully evaluated in this study. The isotopic compositions of these new RMs with 1-σ uncertainties and mass fractions of hydrogen are: USGS57 (biotite) δ 2 H VSMOW-SLAP = − 91.5 ± 2.4 mUr ( n = 24) Mass fraction hydrogen = 0.416 ± 0.002% ( n = 4) Mass fraction water = 3.74 ± 0.02% ( n = 4) USGS58 (muscovite) δ 2 H VSMOW-SLAP = − 28.4 ± 1.6 mUr ( n = 24) Mass fraction hydrogen = 0.448 ± 0.002% ( n = 4) Mass fraction water = 4.03 ± 0.02% ( n = 4). These δ 2 H VSMOW-SLAP values encompass typical ranges for solid unknowns of crustal and mantle origin and are available to users for recommended two-point calibration. [ABSTRACT FROM AUTHOR]

The hydrous minerals on Mars preserve records of potential past aqueous activity. Quantitative information regarding mineralogical composition would enable a better understanding of the formation processes of these hydrous minerals, and provide unique insights into ancient habitable environments and the geological evolution of Mars. The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) has the advantage of both a high spatial and spectral resolution, which makes it suitable for the quantitative analysis of minerals on Mars. However, few studies have attempted to quantitatively retrieve the mineralogical composition of hydrous minerals on Mars using visible-infrared (VISIR) hyperspectral data due to their distribution characteristics (relatively low concentrations, located primarily in Noachian terrain, and unclear or unknown background minerals) and limitations of the spectral unmixing algorithms. In this study, we developed a modified sparse unmixing (MSU) method, combining the Hapke model with sparse unmixing. The MSU method considers the nonlinear mixed effects of minerals and avoids the difficulty of determining the spectra and number of endmembers from the image. The proposed method was tested successfully using laboratory mixture spectra and an Airborne Visible Infrared Imaging Spectrometer (AVIRIS) image of the Cuprite site (Nevada, USA). Then it was applied to CRISM hyperspectral images over Gale crater. Areas of hydrous mineral distribution were first identified by spectral features of water and hydroxyl absorption. The MSU method was performed on these areas, and the abundances were retrieved. The results indicated that the hydrous minerals consisted mostly of hydrous silicates, with abundances of up to 35%, as well as hydrous sulfates, with abundances ≤10%. Several main subclasses of hydrous minerals (e.g., Fe/Mg phyllosilicate, prehnite, and kieserite) were retrieved. Among these, Fe/Mg- phyllosilicate was the most abundant, with abundances ranging up to almost 30%, followed by prehnite and kieserite, with abundances lower than 15%. Our results are consistent with related research and in situ analyses of data from the rover Curiosity ; thus, our method has the potential to be widely used for quantitative mineralogical mapping at the global scale of the surface of Mars. [ABSTRACT FROM AUTHOR]

The newly described hydrous phase H, MgSiO 4 H 2 , is stable to the pressures and temperatures of the Earth's lower mantle, suggesting that the Earth's lower mantle may participate in the cycling of water. We present the results of ab initio calculations on the stability of this phase within the phase H– δ AlOOH–SiO 2 system between 20 and 60 GPa, exploring the wide pressure- and temperature stability field of this mineral. We find that hydrogen in phase H is bonded in a single well at all pressures, and only the pure phase results in pressure-induced symmetrization, while the solid solution maintains asymmetric bonding to high pressure. Substitutions on octahedral sites are locally charge-balanced by H. Furthermore, small amounts of Al + H in SiO 2 stabilize the CaCl 2 -structure of SiO 2 to pressures as low as 20 GPa while phase H and δ AlOOH form an ideal mixture at all pressures considered. The resulting structure is stable relative to the anhydrous assemblage of the Earth's lower mantle by 16–19 kJ/mol. In alumina-rich phase H this increases the stability of the mineral by ∼800 K relative to the pure substance, making it potentially stable under lower mantle temperatures along typical geotherms. [ABSTRACT FROM AUTHOR]

In low-temperature, high-pressure hydrothermal environments coesite transforms into hydrous forms of stishovite. We studied hydrous stishovite produced from hydrothermal treatment of silica glass as initial SiO2 source at temperatures of 350-550 °C and pressures around 10 GPa. The P-T quenched samples were analyzed by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermal analysis, and IR and magic-angle spinning (MAS) NMR spectroscopy. The presence of significant amounts of H2O (ranging from 0.5 to 3 wt%) is shown from thermogravimetric measurements. PXRD reveals that at temperatures below 400 °C, hydrous stishovite is obtained as two distinct phases that may relate to the solid ice-VII environment present at prevailing P-T conditions. Initially formed hydrous stishovite is metastable and dehydrates over time in the low-temperature, high-pressure hydrothermal environment. The primary mechanism of H incorporation in stishovite is a direct substitution of 4H+ for Si4+ yielding unique octahedral hydrogarnet defects. In IR spectra this defect manifests itself by two broad but distinct bands at 2650 and 2900 cm-1, indicating strong hydrogen bonding. These bands are shifted in the deuteride to 2029 and 2163 cm-1, respectively. Protons of the octahedral hydrogarnet defect produce 1H MAS NMR signals in the 9-12 ppm region. The presence of multiple resonances suggests that the octahedral defect is associated with various proton arrangements. At elevated temperatures, the NMR signals narrow considerably because of proton dynamics. [ABSTRACT FROM AUTHOR]

Hydrous minerals are important water carriers in the crust and the mantle, especially in the subduction zone. With the recent development of the experimental technique, studies of the electrical conductivity of hydrous silicate minerals under controlled temperature, pressure and oxygen fugacity, have helped to constrain the water distribution in the Earth's interior. This paper introduces high pressure and temperature experimental study of electrical conductivity measurement of hydrous minerals such as serpentine, talc, brucite, phase A, super hydrous phase B and phase D, and assesses the data quality of the above minerals. The dehydration effect and the pressure effect on the bulk conductivity of the hydrous minerals are specifically emphasized. The conduction mechanism of hydrous minerals and the electrical structure of the subduction zone are discussed based on the available conductivity data. Finally, the potential research fields of the electrical conductivity of hydrous minerals is presented. [ABSTRACT FROM AUTHOR]

The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low (≤5°C/km), lawsonite may carry water into great depths of ≤300 km. In the hot subduction zone where the geothermal gradient is high (>25°C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of <80 km to form felsic melts, into which water is highly dissolved. In this case, only a minor amount of water can be transported into great depths. A number of intermediate modes are present between these two end-member dehydration modes, making subduction-zone dehydration various. Low- T/low- P hydrous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of ≤60-80 km to release large amounts of water. In contrast, the low- T/low- P hydrous minerals are replaced by low- T/high- P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80-160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultrahigh-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately. [ABSTRACT FROM AUTHOR]

The presence of water in the Earth's deep mantle is an issue of increasing interest in the field of highpressure mineralogy. An important task for further advancing research in the field is to create homogeneous single crystals of candidate deep-mantle water-bearing minerals of 1 mm or larger in size, which is required for applying them for the time-of-flight (TOF) single-crystal Laue diffraction method with a third-generation neutron instrument. In this study, we perform several experiments to demonstrate an improved methodology for growing hydrous crystals of such large sizes at relevant transition zone and lower-mantle conditions via very slow cooling over a maximum period of 1 day. Successfully synthesized crystals using this methodology include dense hydrous magnesium silicate (DHMS) phase E, hydrous wadsleyite, hydrous ringwoodite, and bridgmanite (silicate perovskite). It is also demonstrated that these hydrous crystals can be grown from deuterium enriched starting materials in addition to those having a natural hydrogen isotope ratio. Magnitudes of chemical and crystallographic heterogeneities of the product crystals were characterized by comprehensive analysis of X-ray precession photography, single-crystal X-ray diffraction (SCXRD), field-emission scanning electron microscope (FE-SEM), electron probe microanalyzer (EPMA), secondary ion mass spectroscopy (SIMS), powder X-ray diffraction (PXRD), and TOF neutron powder diffraction (TOF-NPD). The product crystals were confirmed to be inclusion free and crystallographically homogeneous. Compositional and isotopic differences of major elements and hydrogen isotope abundances were lower than 1 and 3%, respectively, among intracrystals and intercrystals within each recovered sample capsule. Phase E crystals up to 600 µm in the largest dimension were grown at a constant temperature of 1100 °C kept for 3 h. Using a lattice parameter-to-temperature relation of phase E, the thermal gradient in the sample capsules for the phase E synthesis has been evaluated to be 20 °C/mm. Hydrous wadsleyite crystals up to 1100 µm in the largest dimension were grown at 1390 °C with a temperature reduction of 70 °C during heating for 10 h. Hydrous ringwoodite crystals up to 1000 µm in the largest dimension were grown at around 1400 °C with a temperature reduction of 110 °C during heating for 12 h. Bridgmanite crystals up to 600 µm in the largest dimension were grown at 1700 °C with a temperature reduction of 30 °C during heating for 12 h. A TOF single-crystal diffraction instrument has been successfully used for analyzing one of the hydrous wadsleyite crystals, which demonstrated that single crystals appropriate for their expected usage are created using the method proposed in the present study. [ABSTRACT FROM AUTHOR]

The hypocentres of intermediate-depth earthquakes have been shown to overlap with the regions in subducting slabs that contain high abundances of hydrous minerals. This relationship was initially revealed using geophysical and numerical modelling and until recently has lacked corroboration from direct field-based research. We evaluated the relationship of the coincidence of intermediate-depth earthquakes with hydrous minerals in the slab by undertaking detailed geochemical analyses of blueschist to lawsonite to eclogite facies pseudotachylytes and their hostrocks located within an exhumed ophiolite, the Eocene Schistes Lustres Complex in Corsica. These units comprise incompletely metamorphosed metagabbro and peridotite. The wallrocks of the pseudotachylytes contain variable amounts of hydrous minerals: tremolite, Mg-hornblende, glaucophane in the metagabbro, and serpentine, tremolite and chlorite in the peridotite. Back-scatter-electron images show that the hydrous minerals entrained in the melt undergo fusion rather than dehydration. Vesicular and H 2 O-rich melt veins are observed cross-cutting partially molten pseudotachylyte fault veins and show evidence of H 2 O exsolution during melt solidification. The crystallisation products of these melts indicate formation under high temperature, high pressure conditions (1400–1700 °C; 1.5 GPa). The peridotite-hosted pseudotachylytes crystallised olivine, orthopyroxene and diopside, which are surrounded by interstitial Al- and H 2 O-rich glass. The metagabbro pseudotachylyte is dominated by Al-rich omphacite, ilmenite and high-Fe anorthite. XRF bulk analyses of the wallrock of the pseudotachylyte and electron microprobe analyses of the pseudotachylyte matrix, entrained survivor clasts and the crystallisation products show that near-total disequilibrium melting took place. The peridotite-hosted pseudotachylyte composition is skewed strongly towards chlorite; however, the preservation of delicate dendritic diopside and olivine hopper crystals suggests that the pseudotachylyte is unaltered, indicating that preferential fusion of chlorite took place. The metagabbro-hosted pseudotachylyte matrix composition is very similar to the bulk wallrock composition but slightly skewed by the preferential melting of Mg-hornblende and tremolite. Not all the pseudotachylytes are hydrous as the H 2 O content of the melts is highly variable; the metagabbro–pseudotachylyte ranges from 0 to 4 wt.% and the peridotite–pseudotachylyte ranges from 0 to 14 wt.%. The range in H 2 O content of the pseudotachylytes has lead us to conclude that the localised dehydration of hydrous minerals may be a second order factor in initiating intermediate-depth seismicity. However, we have observed that the pseudotachylytes with the most chaotic vein networks, thickest fault veins and most comminuted material have the highest abundances of hydrous wallrock minerals, possibly owing to repeated fluid ingress in between pseudotachylyte-generating events. This implies that free fluids enhance pseudotachylyte generation and possibly seismicity, but are not a first order requirement. Microtextural and geochemical results from this study suggest that the presence of abundant H 2 O-rich minerals in the slab exerts a strong rheological control during high strain-rate deformation, facilitating thermally-triggered localising shear instabilities. These field-based observations allow us to explore the assumption of the causal link between slab hydration and earthquake nucleation, and offer fresh insight into the debate of how intermediate-depth earthquakes take place. [ABSTRACT FROM AUTHOR]

[Display omitted] • Talc is one of the most elastically anisotropic minerals in subduction zone settings. • Talc-bearing lithology can readily explain ultra-slow velocity, high V P / V S ratio, and large delay time. • Pressure dependence of the components of the elastic constant tensor exhibits anomalous behavior. Talc is a layered hydrous silicate mineral that plays a vital role in transporting water into Earth's interior and is crucial for explaining geophysical observations in subduction zone settings. In this study, we explored the structure, equation of state, and elasticity of both triclinic and monoclinic talc under high pressures up to 18 GPa using first principles simulations based on density functional theory corrected for dispersive forces. Our results indicate that principal components of the full elastic constant tensor C 11 and C 22 , shear components C 66 , and several off-diagonal components show anomalous pressure dependence. This non-monotonic pressure dependence of elastic constant components is likely related to the structural changes and is often manifested in a polytypic transition from a low-pressure polytype talc-I to a high-pressure polytype talc-II. The polytypic transition of talc occurs at pressures within its thermodynamic stability. However, the bulk and shear elastic moduli show no anomalous softening. Our study also shows that talc has low velocity, extremely high anisotropy, and anomalously high V P / V S ratio, thus making it a potential candidate mineral phase that could readily explain unusually high V P / V S ratio and large shear wave splitting delays as observed from seismological studies in many subduction systems. [ABSTRACT FROM AUTHOR]

Nominally hydrous high-pressure silicate phases such as the superhydrous phase B are of considerable importance for the understanding of the water-cycle between the surface and the interior of the Earth. This study tackles the controversial issue of hydrogen positions in superhydrous phase B, a phase believed to be potentially stable in cold subducting ultramafic slabs. To investigate the nature of hydrogen incorporation into the structure of superhydrous phase B, neutron powder diffraction experiments have been performed. A structural model based on Pnn2 symmetry has been used for the analysis of the data, which is consistent with earlier spectroscopic studies. Application of Fourier synthesis with subsequent analyses of difference nuclear density maps and Rietveld fits reveal two distinct positions for deuterium, at 4c (0.194, 0.052, 0.596) and at 4c (0.186, 0.119, 0.388). This unambiguously shows that deuterium lies within large channels, which are formed between the edge-shared octahedra and vertex-linked tetrahedra along the b-axis of the structure. These results contrast with recent polarized single-crystal infrared spectroscopy studies where the position of one of two H atoms was estimated to lie close to the octahedral edge of an MgO6 octahedron, thereby leaving the large structural channel empty. [ABSTRACT FROM AUTHOR]

The exhumation mechanism of high-pressure ( HP) and ultrahigh-pressure ( UHP) eclogites formed by the subduction of oceanic crust (hereafter referred to as oceanic eclogites) is one of the primary uncertainties associated with the subduction factory. The phase relations and densities of eclogites with MORB compositions are modelled using thermodynamic calculations over a P-T range of 1-4 GPa and 400-800 °C, respectively, in the NCKFMASHTO ( Na2O- CaO- K2O- FeO- MgO- Al2O3- SiO2- H2O- TiO2- Fe2O3) system. Our modelling suggests that the mineral assemblages, mineral proportions and density of oceanic crust subducted along a cold P-T path are quite different from those of crust subducted along a warm P-T path, and that the density of oceanic eclogites is largely controlled by the stability of low-density hydrous minerals, such as lawsonite, chlorite, glaucophane and talc. Along a cold subduction P-T path with a geotherm of ~6 °C km−1, lawsonite is always present at 1.1 to >4.0 GPa, and chlorite, glaucophane and talc can be stable at pressures of up to 2.3, 2.6 and 3.6 GPa respectively. Along such a P-T path, the density of subducted oceanic crust is always lower than that of the surrounding mantle at depths shallower than 110-120 km (< 3.3-3.6 GPa). However, along a warm subduction P-T path with a geotherm of ~10 °C km−1, the P-T path is outside the stability field of lawsonite, and the hydrous minerals of chlorite, epidote and amphibole break down completely into dry dense minerals at relatively lower pressures of 1.5, 1.85 and 1.9 GPa respectively. Along such a warm subduction P-T path, the subducted oceanic crust becomes denser than the surrounding mantle at depths >60 km (>1.8 GPa). Oceanic eclogites with high H2O content, oxygen fugacity, bulk-rock X Mg [ = MgO/( MgO + FeO)], X Al [ = Al2O3/( Al2O3 + MgO + FeO)] and low X Ca [ = CaO/( CaO + MgO + FeO + Na2O)] are likely suitable for exhumation, which is consistent with the bulk-rock compositions of the natural oceanic eclogites on the Earth's surface. On the basis of natural observations and our calculations, it is suggested that beyond depths around 110-120 km oceanic eclogites are not light enough and/or there are no blueschists to compensate the negative buoyancy of the oceanic crust, therefore explaining the lack of oceanic eclogites returned from ultradeep mantle (>120 km) to the Earth's surface. The exhumed light-cold-hydrous oceanic eclogites may have decoupled from the top part of the sinking slab at shallow depths in the forearc region and are exhumed inside the serpentinized subduction channel, whereas the dense-hot-dry eclogites may be retained in the sinking slab and recycled into deeper mantle. [ABSTRACT FROM AUTHOR]

Abstract: Teleseismic converted phases are used to probe the composition of the downgoing oceanic crust as a function of depth along the Cocos slab in central and southern Mexico. Previously, modeling of the receiver function (RF) conversion amplitude of the flat Cocos slab beneath central Mexico at 45km depth revealed a thin low-velocity upper oceanic crust of a thickness of 4±1km, which has much lower seismic velocities ( reduction in shear wave velocities) than (normal) lower crust. High Vp/Vs ratio () also suggested a large concentration of hydrous minerals such as talc in combination of high pore-fluid pressure in the horizontal segment. We extend this previous effort to examine seismic properties of both the steeply subducting Cocos oceanic crust beneath the Trans-Mexican Volcanic Belt (TMVB) in central Mexico and the shallowly dipping crust beneath southern Mexico. Inverted seismic velocities using the converted amplitudes at the top and bottom of the dipping oceanic crust are compared with experimentally constrained seismic velocities of candidate mineral phases in a range of likely pressures and temperatures. The composition of the oceanic crust downdip in the steep part of slab beneath the TMVB includes the minerals such as lawsonite and zoisite at 60–100km depth, and the eclogitization occurs around 100km depth. This is related to arc volcanism in the TMVB directly above the slab as well as the slab rollback. In contrast, the dominant mineral phase in the upper oceanic crust of southern Mexico beneath the Isthmus of Tehauntepec is amphibole on top of unaltered gabbroic oceanic crust. The difference in mineral assemblages of the subducted oceanic crust may help explain the difference in slab geometries between central and southern Mexico. [Copyright &y& Elsevier]

The low-temperature heat capacity behavior of synthetic katoite, Ca3Al2H12O12, was investigated for the first time using microcalorimetry. The sample was synthesized hydrothermally in Au capsules at 250 °C and 3 kb water pressure. X-ray powder measurements show that about 98-99% katoite was obtained. Heat capacities were measured with a commercially designed relaxation calorimeter between 5 and 300 K on a milligram-sized sample and around ambient temperatures with a differential scanning calorimeter. The heat capacity data are well behaved at T < 300 K and show a monotonic decrease in magnitude with decreasing temperature. There is no evidence for any phase transition. A standard third-law entropy value of S° = 421.7 ± 1.6 J/(mol⋅K) was calculated. Published experimentally based S° values for katoite are slightly lower than this value. Estimations of S° based on empirical corresponding state schemes give S° values that are much too low. This is ultimately attributed to an inability to account for the vibration behavior of the OH groups in katoite that have very weak or no H-bonding. Using this new calorimetric-based S° value and published standard enthalpy of formation data for katoite, a calorimetric-based Gibbs free energy of formation at 298 K can be obtained as Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed = -5021.2 ± 16.5 kJ/mol. [ABSTRACT FROM AUTHOR]

A high-pressure single-crystal X-ray diffraction (XRD) study has been carried out on two natural zoisite samples Ca2Al3-xFexSi3O12OH, one Fe-free (x = 0) and one Fe-rich (x = 0.12). The unit-cell parameters were determined for the Fe-free sample at 18 different pressures up to 7.76 GPa and for the Fe-rich sample at 13 different pressures up to 7.63 GPa. The P(V) data for both of the samples were fitted by a third-order Birch-Murnaghan equation of state (BM3 EoS). The equation of state coefficients are: V0 = 903.39(5) Å, KT0 = 122.1(7) GPa, and Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed = 6.8(2) for the Fe-free sample and V0 = 906.95(5) ų, KT0 = 119.1(7) GPa, and Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed = 7.3(2) for the Fe-rich sample. This shows that the addition of Fe in to the crystal structure of zoisite leads to a slight softening of the structure. Both compositions exhibit axial compressibilities &beta:c > βa >> βb, with the compressibilities of the a and b axes of the two samples being indistinguishable. The softening of the bulk modulus of zoisite with Fe content follows from softening of the c-axis of the structure. A high-pressure structural study of the Fe-free sample showed that the main compression mechanisms in the structure are the compression of soft inter-octahedral distance along [001] and soft intra-octahedral distances along [010] directions, while along [100] the main compression occurs because of the compression of stiff intra-octahedral distances. The substitution of Fe on to the M3 octahedral site of the structure leads to an increase of the intra-octahedral distance of the M3 that triggers the rotation of M12 and therefore leads to the softening of the M12 inter-octahedral distances that accounts for the softening of the c-axis of the structure. [ABSTRACT FROM AUTHOR]

Abstract: Isotopic fractionation between two phases can be calculated if the vibrational properties of isotopic end-members are fully characterized. We assessed a theoretical approach based on first-principles density-functional density (DFT) prediction of vibrational frequencies by comparing with spectroscopic data on isotopically substituted brucite, a model mineral for which detailed experimental data on isotopic effects and brucite–water D/H partitioning exist. The deviation from experimental values averages to less than 1% for lattice modes, and 4% for OH stretching modes. The reduced partition function ratio (RPFR) for D/H substitution in brucite was combined with experimental RPFR for water to calculate D/H fractionation factor between water and brucite. Results within the harmonic approximation systematically deviate from experimental data by +20–25‰. RPFR is very sensitive to the frequency and isotopic shift of high frequency OH stretching modes, for which anharmonic effects are important and usually not explicitly taken introduced in calculations for minerals. The asymmetry of the O–H potential was calculated by DFT and accounts for spectroscopic measurements of the anharmonic shifts of OH stretching mode overtones and isotopic frequency ratio. Calculations of D/H fractionation introducing anharmonic corrections to the RPFR of brucite are fitted with the expression 1000lnα D/H(brucite/H2O)=−23.3 103 / T +2.55 106 / T 2 −1.51 109 / T 3, that yields an improved agreement with experiments at high temperature, but the deviation at 25 °C is −30‰. Uncertainties in calculated fractionation factors can arise from dispersion effects or from DFT errors. For instance, a 1% change of the stretching mode frequencies or a 3% change of lattice mode frequencies could account for the discrepancy between model and experimental fractionation factors. The pressure dependence of brucite RPRF was calculated, and is given by 1000lnГ Pbrucite = P (−1.005 103 / T +2.18 106 / T 2 −0.213 109 / T 3), with P in GPa. The large calculated temperature dependence of the fractionation factor suggests that a single experimental fractionation curve can be fitted to a partial consistent set of experimental data as 1000lnαD/H(brucite/H2O) =−27.9(30) 103 / T +8.8(29) 106 / T 2 −2.24(63) 109 / T 3. These equations allow calculating the temperature and pressure dependence of the D/H fractionation factor between brucite and water in the 300–900 K and 0.1–100 MPa range when combined with the pressure correction for water RPFR [Polyakov, V.B., Horita, J. and Cole, D.R., 2006. Pressure effects on the reduced partition function ratio for hydrogen isotopes in water. Geochimica Cosmochimica Acta, 70: 1904–1913.] for geochemical applications. The DFT model used here for estimating RPFR of the model mineral brucite can be extended to other hydrous minerals using vibrational spectroscopy as a test for the accuracy of the DFT prediction, and the brucite–water equations proposed here as a reference for determining mineral–water fractionation factors and geochemical applications. [Copyright &y& Elsevier]

[Display omitted] • Partial dehydration of single-crystal brucite at low temperatures through electrical conductivity measurement. • Water activity reduces the temperature stability of hydrous minerals. • Partial dehydration likely induces the breakdown of hydrous minerals. Hydrous minerals within the subducting oceanic slab are important hosts for water. Clarification of the stability field of hydrous minerals helps to understand transport and distribution of water from the surface to the Earth's interior. We investigated the stability of brucite, a prototype of hydrous minerals, by means of electrical conductivity measurements in both open and closed systems at 3 GPa and temperatures up to 1300 K. Dramatic increase of conductivity in association with characteristic impedance spectra suggests that partial dehydration of single-crystal brucite in the open system with a low water fugacity occurs at 950 K, which is about 300 K lower than those previously defined by phase equilibrium experiments in the closed system. By contrast, brucite completely dehydrates at 1300 K in the closed system, consistent with previous studies. Partial dehydration may generate a highly defective structure but does not lead to the breakdown of brucite to periclase and water immediately. Water activity plays a key role in the stability of hydrous minerals. Low water activity (a H 2 O) caused by the high wetting behavior of the subducted oceanic slab at the transition zone depth may cause the partial dehydration of the dense hydrous magnesium silicates (DHMSs), which significantly reduces the temperature stability of DHMS (this mechanism has been confirmed by previous study on super hydrous phase B). As a result, the transition zone may serve as a 'dead zone' for DHMSs, and most water will be stored in wadsleyite and ringwoodite in the transition zone. [ABSTRACT FROM AUTHOR]

Pseudosections for two sediments and one basalt calculated in the system K2O–Na2O–CaO–MgO–FeO–Fe2O3–Al2O3–TiO2–SiO2–H2O for the P–T range 10 to 35 kbar, 300 to 900°C give useful insights into the amount of H2O released from oceanic crust in subduction zones. In cold subduction zones (20 kbar–300°C to 35 kbar–500°C) hydrous minerals storing 3 to 4 wt% H2O are still present in metasediments at depths of 120 km. In the same environment, metabasite releases 1 wt% H2O in the depth range 100 to 120 km, but 4.5 wt% H2O is transported to greater depths. In hot subduction zones (300°C hotter than the cold subduction zone at 100 km depth), dehydration events of metasediments in the depth range 50 to 80 km correspond to the breakdown of chlorite and paragonite. In the calculations no further water is released at greater depths because the modal content of phengite, the only hydrous mineral phase at these depths, remains almost constant. For the same P–T path, metabasite shows continuous dehydration between 40 and 80 km releasing almost 3 wt% H2O. At 120 km depth less than 0.4 wt% of H2O remains. In an average modern subduction zone (∼6°C/km) most dehydration of sediments occurs at depths of 70 to 100 km and that of basalts at depths of 80 to 120 km. Only 1.3 wt% H2O in metasediments and 1.6 wt% H2O in metabasalt has the potential to be subducted to depths greater than 120 km. The dehydration behavior of sediments concurs with the generally held idea that subduction zone fluids are most effectively transported to great depths by cold subduction. In hot subduction zones, such as those characteristic of early Earth, most H2O carried by oceanic crust is liberated at depths less than 120 km and, thus, would not contribute to island-arc magmatism. [ABSTRACT FROM AUTHOR]

Abstract: We investigate a weak, but persistent low S-velocity anomaly in the upper mantle beneath the US East Coast. This anomaly sits above the high-velocity lower-mantle S-velocity structure generally interpreted as relatively cool, subducted Farallon lithosphere. We argue that the most likely explanation for the lowered S-velocities is elevated water content, with hydrogen incorporated in defects and oxygen at regular lattice sites in the crystal structure of major mantle minerals olivine, wadsleyite, and ringwoodite. The subducted Farallon lithosphere is a potential source for this water while the lithosphere at the Atlantic North American margin is a likely recipient of the water. This water may be the vital element needed to allow the margin lithosphere to break and initiate subduction of the Atlantic lithosphere. In a broader geodynamic context we propose that a deep water cycle may be responsible for the longevity of plate tectonics in general and the Wilson Cycle in particular. [Copyright &y& Elsevier]

Abstract: Using high-resolution, triplicate shear-waveforms recorded by broadband seismic arrays, we show that a corresponding anomaly of high S-wave speed (V S ) is absent where an anomaly of high P-wave speed (V P ) was recently recognized in the transition zone of the mantle (TZ) beneath central Tibet. A likely cause of the discrepancy between anomalies in VP and V S is a minor amount of water in nominally anhydrous polymorphs of olivine. Prior to thickening by continent-continent collision, the Tibetan mantle was part of a mantle wedge which has been hydrated during past episodes of subduction. Hydration of the sub-continental lithospheric mantle (SCLM) provides a natural mechanism to facilitate ductile deformation, so rapid thickening of the SCLM, which would have been hindered by advection of cold materials, can take place. Convective instability would then lead to removal and sinking of thickened, cold SCLM, leaving a remnant in the TZ detectable only in V P . This interpretation not only is consistent with previous reconstruction of the SCLM in Tibet based on several types of independent data, but also provides a new pathway for water to enter and be stored in the TZ. [Copyright &y& Elsevier]

Abstract: We report here the electrical resistivity measurements on two natural zeolites–natrolite and scolecite (from the Killari borehole, Maharashtra, India) as a function of pressure up to 8 GPa at room temperature. High-pressure electrical resistivity studies on hydrous alumino-silicate minerals are very helpful in understanding the role of water in deep crustal conductivities obtained from geophysical models. The results obtained by magneto-telluric (MT) soundings and direct current resistivity surveys, along with the laboratory data on the electrical resistivity of minerals and rocks at high-pressure–temperature are used to determine the electrical conductivity distribution in continental lithosphere. The electrical resistivity of natural natrolite decreases continuously from 2.9×109 Ω cm at ambient condition to 7.64×102 Ω cm at 8 GPa, at room temperature. There is no pressure-induced first order structural phase transitions in natrolite, when it is compressed in non-penetrating pressure transmitting medium up to 8 GPa. On the other hand scolecite exhibits a pressure-induced transition, with a discontinuous decrease of the electrical resistivity from 2.6×106 to 4.79×105 Ω cm at 4.2 to 4.3 GPa. The observed phase transition in scolecite is found to be irreversible. Vibrational spectroscopic and X-ray diffraction studies confirm the amorphous nature of the high-pressure phase. The results of the present high-pressure studies on scolecite are in good agreement with the high-pressure Raman spectroscopic data on scolecite. The thermo gravimetric studies on the pressure-quenched samples show that the samples underwent a pressure-induced partial dehydration. Such a pressure-induced partial dehydration, which has been observed in natural scolecite could explain the presence of high conductive layers in the earth''s deep-crust. [Copyright &y& Elsevier]

OH−-bearing (hydrous) ringwoodite compressed non-hydrostatically in a diamond anvil cell supports a differential stress that increases from 2.9 to 4.5 GPa over the pressure range of 6.7–13.2 GPa at room temperature. This result suggests a significant water weakening effect when compared with results from similar experiments on the anhydrous counterpart [Kaver and Duffy, Geophys. Res. Lett. 28 (2001) 2691–2694]. The elastic anisotropy (=2C44/(C11−C12) of hydrous ringwoodite is measured to be 0.87(7) throughout this pressure range, similar to measured values for anhydrous ringwoodite [Kaver and Duffy, Geophys. Res. Lett. 28 (2001) 2691–2694]. This lattice anisotropy cannot be explained by anelastic effects such as faulting and twinning within the structure. These results suggest that hydrous minerals in the upper mantle and transition zone may have higher ductile strain rates for a fixed shear stress at high temperature, resulting in stronger preferred lattice orientation. This, in turn, may be seismically detectable, which opens the possibility of using seismic anisotropy as a marker for local volatile-containing areas within the upper mantle and transition zone. [Copyright &y& Elsevier]

Fluids released from subducting slabs induce partial melting of the mantle wedge above the slab, which in turn is responsible for arc volcanism at the Earth’s surface. The partially hydrated basaltic layer of the slab is a potential source of these fluids and a major reservoir for H2O at depth. Constraining the stability domains of hydrous phases and the position of the dehydration reactions in this system in pressure–temperature (P–T) space is essential to describe and quantify the fluid release from subducting oceanic crust into the overlying mantle wedge. Experiments were conducted in the ranges of 2.2–3.4 GPa and 625–750°C to determine phase equilibria in an H2O-saturated natural basalt at conditions relevant to subduction zones. The experimental duration was typically 1 month, although some experiments were replicated with a shorter run duration (1–2 weeks) in order to identify potentially metastable phases. A mixture of a natural mid-ocean ridge basalt (MORB) glass and mineral seeds was used as the starting material. Oxygen fugacity was buffered within ±1.3 log units of nickel-bunsenite (NiNiO). The results obtained show that a calcic amphibole (barroisite) is stable from 2.2 to about 2.4 GPa. At 2.6 GPa, it is replaced by a sodic amphibole (near end-member glaucophane), which is stable up to 3 GPa at 625°C. This high-pressure assemblage constitutes a true analog of a natural amphibole-bearing eclogite and the first synthesis of glaucophane from a rock of basaltic composition. As opposed to the results of previous studies on basaltic compositions [A.R. Pawley, J.R. Holloway, Science 260 (1993) 664–667; S. Poli, Am. J. Sci. 293 (1993) 1061–1107; S. Poli, M.W. Schmidt, J. Geophys. Res. 100 (1995) 22299–22314; M.W. Schmidt, S. Poli, Earth Planet. Sci. Lett. 163 (1998) 361–379], chloritoid is present only as a metastable phase in the pressure–temperature range investigated here. Metastability of chloritoid in earlier experiments, due to short run duration, is the most likely explanation for this difference, and suggests that chloritoid does not play an important role in the overall dehydration process of the basaltic layer in subduction zones. At pressures above the stability field of amphibole, zoisite/clinozoisite becomes the stable hydrous phase at temperatures above 645°C, whereas lawsonite is stable at lower temperatures. The positions of the zoisite-out and lawsonite-out reactions determined in this study indicate that, for an intermediate temperature subduction zone, the basaltic layer of the slab would be completely dehydrated between 90 and 110 km depth. [Copyright &y& Elsevier]

We have measured the elastic wave velocities and single-crystal elastic moduli of lawsonite (CaAl2(Si2O7)(OH)2·H2O) by Brillouin spectroscopy at high temperatures to 450 °C and at room pressure. Lawsonite is generally characterized by high longitudinal (vP) and shear (vS) elastic wave velocities. A number of the elastic properties increase with increasing temperature, including the shear modulus, G (∂G/∂T=+2.01 GPa/100 K), and the aggregate velocities vP and vS. These anomalous properties are likely related to a displacive phase transition at ∼0 °C. This is supported by the small value of the elastic modulus c66 (18(1) GPa at room temperature), and the fact that c66 softens as temperature is decreased approaching the ∼0 °C phase transition. The anomalous behavior in c66 leads to a decrease in vP/vS and Poisson’s ratio with increasing temperature at 1 atm pressure. Our results show that the aggregate bulk modulus (KS=125(2) GPa at 21 °C) decreases monotonically with increasing temperature (∂KS/∂T=−1.8 GPa/100 K), in contrast to the results of Daneil et al. (1999) who observed a minimum in the bulk modulus at ∼230 °C. In general, the aggregate elastic wave velocities of lawsonite under high P–T conditions of a subducted slab are expected to remain comparable to those of anhydrous mafic silicates. This observation reinforces the notion that other hydrous minerals persist in subducted oceanic crust to considerable depth (∼250 km), in order to explain the low velocity layers observed in some subduction zones (e.g. Helffrich, 1996; Abers, 2000). [Copyright &y& Elsevier]

The Izu-Bonin Trench (IBT) has characteristics different from other trenches in many aspects. A chain of serpentine seamounts is exposed at the forearc slope 40–50 km west of the Izu-Bonin Trench axis. In this area, few large earthquakes (>M7.0) have occurred at shallow depths (0–100 km) and many large earthquakes have occurred at greater depths (>400 km). To reveal the mechanism of serpentine diapiring and the relation between heterogeneity in earthquake occurrence and plate subduction, seismic refraction/reflection studies at the forearc slope, with two lines perpendicular and parallel to the Izu-Bonin Trench axis, were carried out in 1998.Detailed seismic velocities were obtained for both lines by seismic tomography using refracted first arrivals and reflected arrivals. On the E–W line, the subducting plate interface and the Moho of the subducting oceanic plate were recognized. The velocity of the mantle wedge is lower than that of the average velocity of oceanic mantle. Along the subducting plate, a layer between the two plates (Plate Boundary Layer (PBL)) with a low-velocity appears just beneath the serpentine diapir and forms a sill shape, which becomes thicker with increasing velocity from the diapir toward the west, and connects to the mantle wedge. The low-velocity materials in the PBL are interpreted as chrysotile, which is a low-temperature phase of serpentine produced by the hydration of peridotite by water supplied by the subducting slab. The chrysotile in the PBL might act as a lubricant and decrease seismic activity along the subduction zone, and this can explain the characteristics of seismicity in the Izu-Bonin subduction zone (IBSZ). In contrast to the E–W line, the crustal structure along the N–S line is rather homogeneous. [Copyright &y& Elsevier]