9:30 AM - 9:45 AM
[SCG19-03] Mantle melting and volatiles: Redox-controlled solution behavior in silicate-COH magmatic systems
Keywords:Stable isotopes, melting, structure, fluid, melt, spectroscopy
Hydrogen as H2O is the major component of the silicate Earth. Its solubility in magma can reach >30 wt% under crustal and upper mantle conditions. The CH4 and H2 solubility is in the 0.1-1 wt% range, and CO2-solubility typically 1-5 wt%. These variations reflect bulk chemical and, therefore, structural control of solubility. Water dissolved in magmatic liquids to form predominantly Si-OH and Al-OH bonding together with molecular H2O. The OH/H2O ratio is positively correlated with SiO2 and Al2O3 concentrations in melts. The carbon solution equilibria are, 2CH4+Qn=2CH3+H2O+Qn+1 and, 2CO3+H2O+2Qn+1=HCO3+2Qn, under reducing and oxidizing conditions, respectively. In the Qn-notations, the superscript, n, in the Qn-species denotes number of bridging oxygen in the silicate species (Q-species). The abundance ratios, CH3/CH4 and HCO3/CO3, increase with temperature.
The resulting structural changes result in hydrogen and carbon isotope fractionation factors between melt, fluid, and crystalline materials significantly different from 1 even at magmatic temperatures. The D/H and fractionation between aqueous fluid and magma are also temperature dependent with a small enthalpy change near -5 kJ/mol. In silicate-C-O-H systems, the enthalpy-values for 13C/12C fractionation factors are near -3.2 and 1 kJ/mol under oxidizing and reducing conditions, respectively. These differences reflect different energetics of O-D, O-H, O-13C, and O-12C bonding environments governed by different solution mechanisms in melts of the individual volatile components.
From experimentally-determined temperature- and pressure-dependent D/H fractionation between fluids and melts, it is suggested suggest that (C-O-H)-saturated partial melts in the upper mantle can have dD-values 100%, or more, lighter than coexisting silicate-saturated fluid. This effect is greater under oxidizing than under reducing conditions. Analogous relationships exist for 13C/12C with ∂13C-differences between silicate melts and coexisting CO2+H2O (oxidizing) and CH4+H2O+H2 (reducing) fluid and at magmatic temperatures in the Earth’s upper mantle increases with temperature from about 40 to > 100‰ and 80 – 120 ‰ under oxidizing and reducing conditions, respectively. The use of hydrogen and carbon isotopes of mantle-derived magma to derive isotopic composition of upper mantle source regions, therefore, is discouraged as these are affected by temperature, bulk composition, and redox conditions during melting, crystallization, and degassing events.