The interplay between the chemical composition and the molecular structure of silicate melts was central to the evolution of the Earth's crust, mantle and core. This interplay also affects geochemical records such as the partitioning of isotopes between minerals, melts and fluids in the Earth's interior. For instance, large H-2/H-1 fractionations between silicate melts and aqueous fluids have been observed at high temperature and pressure. Such behaviour may be promoted by the occurrence of H-2/H-1 intramolecular fractionation within the molecular structure of silicate melts. New Raman spectroscopy and H-1 and H-2 Nuclear Magnetic Resonance (NMR) spectroscopy data reveal the source of such H-2/H-1 intramolecular isotopic fractionation, showing that H-1 and H-2 fractionate between the silicate tetrahedral units. Such a process might affect other isotopic systems (e.g., N, C, or S) where the isotopes interact with the melt silicate network.

The behavior of volatiles in silicate-COH melts and fluids and hydrogen isotope fractionation between melt and fluid were determined experimentally to advance our understanding of the role of volatiles in magmatic processes. Experiments were conducted in situ while the samples were at the desired temperature and pressure to 825 degrees C and similar to 1.6 GPa and with variable redox conditions. Under oxidizing conditions, melt and fluid comprised CO2, CO3, HCO3, OH, H2O, and silicate components, whereas under reducing conditions, the species were CH4, H-2, H2O, and silicate components. Temperature-dependent hydrogen isotope exchange among structural entities within coexisting fluids and melts yields Delta H values near 14 and 24 kJ/mol and -5 and -1 kJ/mol under oxidizing and reducing conditions, respectively. This temperature (and probably pressure)-dependent D/H fractionation is because of interaction between D and H and silicate and C-bearing species in silicate-saturated fluids and in COH fluid-saturated melts. The temperature-and pressure-dependent D/H fractionation factors suggest that partial melts in the presence of COH volatiles in the upper mantle can have delta D values 100% or more lighter relative to coexisting silicate-saturated fluid. This effect is greater under oxidizing than under reducing conditions. It is suggested that delta D variations of upper mantle mid-ocean ridge basalt (MORB) sources, inferred from the delta D of MORB magmatic rocks, can be explained by variations in redox conditions during melting. Lower delta D values of the MORB magma reflect more reducing conditions in the mantle source.

The behavior of melts and fluids is at the core of understanding formation and evolution of the Earth. To advance our understanding of their role, high-pressure/-temperature experiments were employed to determine melt and fluid structure together with carbon isotope partitioning within and between (CH4 + H2O + H-2)-saturated aluminosilicate melts and (CH4 + H2O + H-2)-fluids. The samples were characterized with vibrational spectroscopy while at temperatures and pressures from 475 degrees to 850 degrees C and 92 to 1158 MPa, respectively.

Throughout the Earth's history, mass transport involved fluids. In order to address the circumstances under which Zr4+ may have been transported in this manner, its solubility behavior in aqueous fluid with and without NaOH and SiO2 in equilibrium with crystalline ZrO2 was determined from 550 to 950 degrees C and 60 to 1200 MPa. The measurements were carried out in situ while the samples were at the temperatures and pressures of interest. In ZrO2-H2O and ZrO2-SiO2-H2O fluids, the Zr4+ concentration ranges from <= 10 to similar to 70 ppm with increasing temperature and pressure. Addition of SiO2 to the ZrO2-H2O system does not affect these values appreciably. In these two environments, Zr4+ forms simple oxide complexes in the H2O fluid with Delta H similar to 40 kJ/mol for the solution equilibrium, ZrO2(solid) = ZrO2(fluid). The Zr4+ concentration in aqueous fluid increases about an order of magnitude upon addition of 1 M NaOH, which reflects the formation of zirconate complexes. The principal solution mechanism is ZrO2 + 4NaOH = Na4ZrO4 + 2H(2)O with Delta H similar to 200 kJ/mol. Addition of both SiO2 and NaOH to ZrO2-H2O enhances the Zr4+ by an additional factor of about 5 with the formation of partially protonated alkali zircon silicate complexes in the fluid. The principal solution mechanism is 2ZrO(2) + 2NaOH + 2SiO(2) = Na2Zr2Si2O9+ H2O with Delta H similar to 40 kJ/mol. These results, in combination with other published experimental data, imply that fluid released during high-temperature/high-pressure dehydration of hydrous mineral assemblages in the Earth's interior under some circumstances may carry significant concentrations of Zr and probably other high field strength elements (HFSEs). This suggestion is consistent with the occurrence of Zr-rich veins in high-grade metamorphic eclogite and granulite terranes. Moreover, aqueous fluids transported from dehydrating oceanic crust into overlying mantle source rocks of partial melting also may carry high-abundance HFSE of fluids released from dehydrating slabs and transported to the source rock of partial melting in the overlying mantle wedge. These processes may have been operational in the Earth's history within which subduction resembling that observed today was operational.

The bonding and speciation of water dissolved in Na silicate and Na and Ca aluminosilicate melts were inferred from in situ Raman spectroscopy of the samples, in hydrothermal diamond anvil cells, while at crustal temperature and pressure conditions. Raman data were also acquired on Na silicate and Na and Ca aluminosilicate glasses, quenched from hydrous melts equilibrated at high temperature and pressure in a piston cylinder apparatus. In the hydrous melts, temperature strongly influences O-H stretching.(O-H) signals, reflecting its control on the bonding of protons between different molecular complexes. Pressure and melt composition effects are much smaller and difficult to discriminate with the present data. However, the chemical composition of the melt + fluid system influences the differences between the.(O-H) signals from the melts and the fluids and, hence, between their hydrogen partition functions. Quenching modifies the O-H stretching signals: strong hydrogen bonds form in the glasses below the glass transition temperature T-g, and this phenomenon depends on glass composition. Therefore, glasses do not necessarily record the O-H stretching signal shape in melts near Tg. The melt hydrogen partition function thus cannot be assessed with certainty using O-H stretching vibration data from glasses. From the present results, the ratio of the hydrogen partition functions of hydrous silicate melts and aqueous fluids mostly depends on temperature and the bulk melt + fluid system chemical composition. This implies that the fractionation of hydrogen isotopes between magmas and aqueous fluids in water-saturated magmatic systems with differences in temperature and bulk chemical composition will be different.

Understanding what governs the speciation in the C-O-H-N system aids our knowledge of how volatiles affect mass transfer processes in the Earth's interior. Experiments with aluminosilicate melt + C-O-H-N volatiles were, therefore, carried out with Raman and infrared spectroscopy to 800 degrees C and near 700 MPa in situ in hydrothermal diamond anvil cells. The measurements were conducted in situ with the samples at the desired temperatures and pressures in order to avoid possible structural and compositional changes resulting from quenching to ambient conditions prior to analysis. Experiments were conducted without any reducing agent and with volatiles added as H2O, CO2, and N-2 because both carbon and nitrogen can occur in different oxidation states.

The crystal structure of Ba4Ru3O10 has been determined by single-crystal X-ray diffraction at room pressure. From refinements to R = 0.0203 at room temperature and ambient pressure, the material is orthorhombic with space group Cmca (space group No. 64) and has lattice parameters of a = 5.7762(15) Angstrom, b = 13.271(4) Angstrom, and c = 13.083(3) Angstrom. The unit cell thus has a volume of V = 1002.9(8) Angstrom(3) and contains four formula units (Z = 4), Ba4Ru3O10 is therefore of higher symmetry than the previously reported monoclinic structure based on powder X-ray data. It is isostructural with the quaternary oxides Ba-4(Ti,Pt)(3)O-10 and Ba4Ir2AlO10 and the ternary fluorides Cs4M3F10 (M = Mg, Co, Ni, Zn), Kinked chains of RuO6 octahedra run along the c direction, consisting of sets of three face-sharing units joined at the corners of the end units to additional similar sets. The two distinct Pa sites show 10-fold and Ii-fold coordination. Compressibilities and bulk modulus have been determined from lattice parameter variations at pressures up to 5.4 GPa, No phase transition was observed up to this pressure. Compressibility is greatest along the c axis and the bulk modulus obtained from a weighted fit to a Vinet equation of state is 113.3(47) GPa. (C) 2000 Academic Press.

Oxygen fugacity (f(O2)) exerts first-order control on the geochemical evolution of planetary interiors, and the Fe3+/Sigma Fe ratios of silicate glasses provide a useful proxy for f(O2). Fe K-edge micro-X-ray absorption near-edge structure (XANES) spectroscopy allows researchers to micro-analytically determine the Fe3+/Sigma Fe ratios of silicate glasses with high precision. In this study we characterize hydrous and anhydrous basalt glass standards with Mossbauer and XANES spectroscopy and show that synchrotron radiation causes progressive changes to the XANES spectra of hydrous glasses as a function of radiation dose (here defined as total photons delivered per square micrometer), water concentration, and initial Fe3+/Sigma Fe ratio.