Redox-controlled mechanisms of C and H isotope fractionation between silicate melt and COH fluid in the Earth's interior

The behavior of COH fluids, their isotopes (hydrogen and carbon), and their interaction with magmatic liquids are at the core of understanding formation and evolution of the Earth. Experimental data are needed to aid our understanding of how COH volatiles affect rock-forming processes in the Earth's interior. Here, I present a review of experimental data on structure of fluids and melts and an assessment of how structural factors govern hydrogen and carbon isotope partitioning within and between melts and fluids as a function of redox conditions, temperature, and pressure. The solubility of individual COH components in silicate melts can differ by several orders of magnitude and ranges from several hundred ppm to several wt%. Silicate solubility in fluid can reach several molecular at mantle temperatures and pressures. Different solubility of oxidized and reduced C-bearing species in melts reflects different solution equilibria. These equilibria are 2CH(4) + Q(n) = 2CH(3)(-) + H2O + Q(n+1) and 2CO(3)(2-) + H2O + 2Q(n+1) = HCO3- + 2Q(n), under reducing and oxidizing conditions, respectively. In the Q(n)-notations, the superscript, n, denotes the number of bridging oxygen in the silicate species (Q-species). The structural changes of carbon and silicate in magmatic systems (melts and fluids) with variable redox conditions result in hydrogen and carbon isotope fractionation factors between melt, fluid, and crystalline materials that depend on redox conditions and can differ significantly from 1 even at magmatic temperatures. The Delta H of D/H fractionation between aqueous fluid and magma in silicate-COH systems is between -5 and 25 kJ/mol depending on redox conditions. The Delta H values for C-13/C-12 fractionation factors are near - 3.2 and 1 kJ/mol under oxidizing and reducing conditions, respectively. These differences are because energetics of O-D, O-H, O-C-13, and O-C-12 bonding environments are governed by different solution mechanisms in melts and fluids. From the above data, it is suggested that (COH)-saturated partial melts in the upper mantle can have delta D values 100%, or more, lighter than coexisting silicate-saturated fluid. This effect is greater under oxidizing than under reducing conditions. Analogous relationships exist for C-13/C-12. At magmatic temperatures in the Earth's upper mantle, C-13/C-12 of melt in equilibrium with COH-bearing mantle in the - 7 to - 30% range increases with temperature from about 40 to > 100% and 80-120% under oxidizing and reducing conditions, respectively.