Silicate-COH melt and fluid structure, their physicochemical properties, and partitioning of nominally refractory oxides between melts and fluids

Abstract

Structural characterization of silicate melts and aqueous fluids equilibrated at pressures and temperatures corresponding to the Earth's interior requires measurements in-situ while the samples are at the pressure and temperature of interest. To this end, structure and structure–property relations of melts and coexisting fluids in silicate-COH systems have been determined at temperatures up to 1000°C and at pressures to ~2.0GPa.The water component of silicate-H2O systems shows aqueous fluids, supercritical fluids, and hydrous melts to comprise molecular H2O (H2O0) and OH-groups, bonded to Si4+ and likely Al3+. The abundance-ratio, OH/H2O0, is positively correlated with temperature. The extent of hydrogen bonding diminishes with temperature and cannot be detected at above ~450°C and ~0.4GPa. Its ∆H is near 10kJ/mol for water dissolved in hydrous melt as compared with ≥20kJ/mol for pure H2O. Hydrogen bonding cannot, therefore, be the cause of property behavior in hydrous magmatic systems because the temperature in hydrous magmatic systems exceeds 600°C.In SiO2–H2O fluid, silicate solute comprises Q0 and Q1 species with a ∆H of the polymerization reaction of ~15kJ/mol assuming no pressure effect. In the Qn-notations, the value of n indicates the number of bridging oxygen in a silicate or aluminosilicate polymeric species. In chemically more complex alkali aluminosilicate systems, the silicate speciation in melts, in aqueous fluid, and in supercritical fluids comprises the same Q-species, but their abundance and proportions differ with the more polymerized species dominant in melt. Silicate–water interaction in the fluids, melts and supercritical fluids is described with the expression, 12Q3+13H2O⇋2Q2+6Q1+4Q0 with ∆H=400–450kJ/mol.The solubility of geochemically important trace elements such as, for example, HFSE in silicate-saturated aqueous fluid under deep crustal and upper mantle pressure and temperature conditions is orders of magnitude greater than in pure H2O at the same temperature and pressure. For example, the fluid/melt partition coefficients for structural species such as PO4, P2O7 and QnP are in the 0.15–0.7 range and increases rapidly with silica content of the fluid. The fluid/melt partition coefficient of TiOn species in similar Ti-bearing systems increases from ~0.1 to ~0.5 in the 200°–500°C and ~0.4–1.0GPa temperature and pressure range. The Ti abundance in aqueous fluids coexisting with rutile in the same pressure and temperature range is about 2 orders of magnitude lower. These differences reflect solute-dependent structural roles of Ti4+ and P5+ in pure H2O fluid and in silicate-saturated H2O. Other HFSE likely behave similarly.In silicate melt-COH systems, oxygen fugacity (fO2) is an additional variable affecting solubility and solution mechanisms. From haplobasalt to haploandesite melt-COH, the carbon solubility at upper mantle pressures and temperatures decreases from ~2wt.% to ~1wt.% in equilibrium with CO2 gas. Oxidized carbon is dissolved dominantly as CO3 groups. The solubility of reduced carbon in the COH system, on the other hand, is less than 50% of oxidized carbon. Reduced carbon in the COH system is dissolved in melts as a mixture of CH3 groups and CH4 molecules when coexisting fluid is CH4. The isotope fractionation between coexisting COH-saturated silicate melt and silicate-saturated COH fluid is correlated with the CH3/CH4 abundance ratio in the melt.By changing fO2 from oxidized to reduced, resultant changes of C solution mechanisms in melt-COH systems cause NBO/T changes. This change can cause crystal/melt element partition coefficients to vary by several tens of percent within natural abundance ranges of COH in magmatic liquids. Variable redox conditions will also result in variable Q-speciation of melts. This variability, in turn, governs configurational properties of magmatic liquids. As a result, their transport properties such as viscosity and diffusion also vary with redox conditions.