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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 (aH2O) 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.

&delta, AlOOH has been shown to be stable at the pressure&ndash, temperature conditions of the lower mantle. However, its stability remains uncertain at the conditions expected for the lowermost mantle where temperature is expected to rise quickly with increasing depth. Our laser-heated diamond-anvil cell experiments show that &delta, AlOOH undergoes dehydration at &sim, 2000 K above 90 GPa. This dehydration temperature is lower than geotherm temperatures expected at the bottom &sim, 700 km of the mantle and suggests that &delta, AlOOH in warm slabs would dehydrate in this region. Our experiments also show that the released H 2 O from dehydration of &delta, AlOOH can react with metallic iron, forming iron oxide, iron hydroxide, and possibly iron hydride. Our observations suggest that H 2 O from the dehydration of subducting slabs, if it occurs, could alter the chemical composition of the surrounding mantle and core regions.

Near-Infrared spectroscopy (NIR) is a useful tool for direct and on-site identification of rock mineralogy in spite of the difficulties arising in spectral evaluation, due to limited availability of spectral libraries at the time. Especially in the field, a functional methodology for the identification and evaluation if possible, of the geologic materials, is of interest to many researchers. However, several different parameters (such as grain size, color, mineralogy, texture, water content etc.) can affect the spectroscopic properties of the samples resulting in spectral variability. The subject of the present work focuses in various lithotypes (monzodiorite, diorite, altered diorite, actinolite schist, cataclasite, slate) from Kos Island, Aegean Sea, in Greece, all bearing hydrous minerals in various amounts. The evaluation of the results obtained from NIR spectroscopy offered important qualitative information about the mineralogy of the lithotypes examined. The important asset of the method is that no sample preparation was necessary. From the reflectance spectra, the NIR-active minerals that were identified include chlorite, micas, amphiboles and epidotes. Petrographic and mineralogic analyses were also employed in order to confirm the NIR results and provide more detailed information about the mineralogy of the samples, the grain size and the orientation of the minerals. Correlation of wavelength positions at ~1400 nm with loss on ignition (LOI) values led us to relate the various lithotypes in terms of their petrological affinities. NIR spectroscopy was proved to be a useful tool, especially for the mineralogic identification of rocks underwent low- to medium grade metamorphism, from greenschist to amphibolite facies.

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 δ2H 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 δ2H 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 δ2H 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 δ2HVSMOW-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 δ2H 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) δ2HVSMOW-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) δ2HVSMOW-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 δ2HVSMOW-SLAP values encompass typical ranges for solid unknowns of crustal and mantle origin and are available to users for recommended two-point calibration.

The newly described hydrous phase H, MgSiO4H2, 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 initiocalculations on the stability of this phase within the phase H-delta AlOOH-SiO(2)system between 20 and 60GPa, 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 CaCl2-structure of SiO(2)to pressures as low as 20GPa while phase H and delta 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 16-19kJ/mol. In alumina-rich phase H this increases the stability of the mineral by similar to 800K relative to the pure substance, making it potentially stable under lower mantle temperatures along typical geotherms. (C) 2017 Elsevier B. V. All rights reserved.