THE GENERATION OF OXIDIZED CO2-BEARING BASALTIC MELTS FROM REDUCED CH4-BEARING UPPER-MANTLE SOURCES

Redox states of asthenospheric basaltic melts suggest that the astheno sphere is more oxidized than the lithosphere. Theoretical consideratio ns, on the other hand, require the opposite, i.e., that the asthenosph ere is more reduced. The implication is that the f(O2) range of basalt ic melts at the Earth's surface cannot reflect the f(O2) range of thei r mantle sources at depth. We present evidence to show that the asthen osphere is significantly more reduced than the lithosphere. Mantle who se oxygen fugacity is buffered predominantly by ferric-ferrous iron eq uilibria will experience reduction with increasing depth, due to (1) t he negative contributions to free energy from molar volume changes and (2) effects of phase transitions on the chemical potentials of the fe rric iron components in solid solution. The extent of reduction per GP a unit pressure increase is modeled to be on the order of 0.6-0.8 log1 0-bar units in f(O2) relative to the FMQ buffer. Basaltic melts experi ence oxidation during melt segregation and decompression relative to t heir mantle sources. The redox state of a basaltic melt at the Earth's surface is roughly proportional to the depth of first mantle melting.