4:15 PM - 4:30 PM
[SCG14-09] C-O-H volatiles, redox variation and hydrogen isotope fractionation in silicate-COH systems in subduction zones
Keywords:melt structure, COH volatiles, spectroscopy, isotope fractionation, high-pressure experiments
Under oxidizing conditions, melt and fluid comprise CO2, CO3-groups, HCO3-groups, H2O, and silicate components. The abundance ratios, CO3/CO2 and CO3/HCO3, decreases with increasing temperature and pressure with ∆H-values for the exchange equilibria between -15 and -25 kJ/mol. The abundance ratio, CO3/silicate, also decreases with increasing temperature and pressure. Hydrogen isotope exchange within coexisting fluids and melts yields ∆H-values near 14 and 34 kJ/mol, respectively, which results in ∆H=-25 kJ/mol for D/H exchange between coexisting fluid and melt. Lack of spectroscopic resolution precluded determination of D/H behavior in the bicarbonate (HCO3) species.
Under reducing conditions (near that to the iron-wustite oxygen fugacity buffer), melt and fluid comprise molecular CH4, CH3-groups, H2, OH-groups, and H2O. Hydrogen isotope exchange within fluids and melts yields ∆H near -5 and -1 kJ/mol, respectively with a ∆H-value for D/H exchange between coexisting fluid and melt of -4kJ/mol. The D/H exchange between CH4 and CD4 species results in ∆H near 40 kJ/mol, whereas the ∆H-value is near -4 kJ/mol under oxidizing conditions where ?H-values average near -6 kJ/mol for D/H exchange between hydrous melt and silicate-saturated aqueous fluid.
The redox-dependent D/H-behavior reflects species-dependent bond strengths. Therefore, D/H fractionation between C-O-H volatiles, released during subduction, and both residual subducting materials, and overlying mantle wedge likely will vary with depth. Somewhat analogous pressure-dependence can be seen in olivine/melt transition metal partition coefficients, which may vary by as much as 100% simply by transforming carbon-speciation from methane-rich to carbon dioxide-rich and, therefore-likely depend on depth in subduction zones. This variation depth dependence partly is because subduction zone redox conditions vary with depth and partly because the solubility of C-O-H volatiles in silicate melts and of silicate in coexisting fluid are pressure-dependent.