Experimentally-determined carbon isotope fractionation in and between methane-bearing melt and fluid to upper mantle temperatures and pressures

- Vibrational spectroscopy to determine stable isotope fractionation.
- Determine 13C/12C of fluid and melt while at temperature and pressure.
- Fractionation factors vary with temperature at magmatic temperatures.
- Heavy isotope depletion/enrichment of magmatic systems depends on temperature.

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+H2)-saturated aluminosilicate melts and (CH4+H2O+H2)-fluids. The samples were characterized with vibrational spectroscopy while at temperatures and pressures from 475° to 850 °C and 92 to 1158 MPa, respectively.The solution equilibrium is 2CH4+Qn=2CH3−+H2O+Qn+1 where the superscript, n, in the Qn-notation describes silicate species where n denotes the number of bridging oxygen. The solution equilibrium affects the carbon isotope fractionation factor between melt and fluid, αmelt/fluid. Moreover, it is significantly temperature-dependent. The αmelt/fluid<1 with temperatures less than about 1050 °C, and is greater than 1 at higher temperature.Methane-bearing melts can exist in the upper mantle at fO2≤fO2(MW) (Mysen et al., 2011). Reduced (CH)-species in present-day upper mantle magma, therefore, are likely. During melting and crystallization in this environment, the δ13C of melts increases with temperature at a rate of ∼0.6‰/°C. From the simple-system data presented here, at T≤1050°C, melt in equilibrium with a peridotite-(CH4+H2O+H2)-bearing mantle source will be isotopically lighter than fluid. At higher temperatures, melts will be isotopically heavier. Degassing at T≤1050°C will shift δ13C of degassed magma to more positive values, whereas degassing at T≥1050°C, will reduce the δ13C of the degassed magma.