The generation of oxidized CO 2-bearing basaltic melts from reduced CH 4-bearing upper mantle sources
Redox states of asthenospheric basaltic melts suggest that the asthenosphere is more oxidized than the lithosphere. Theoretical considerations, on the other hand, require the opposite, i.e., that the asthenosphere is more reduced. The implication is that the ƒ O 2 range of basaltic melts at the Earth's surface cannot reflect the ƒ O 2 range of their mantle sources at depth. We present evidence to show that the asthenosphere is significantly more reduced than the lithosphere. Mantle whose oxygen fugacity is buffered predominantly by ferric-ferrous iron equilibria will experience reduction with increasing depth, due to (1) the negative contributions to free energy from molar volume changes.(2) effects of phase transitions on the chemical potentials of the ferric iron components in solid solution. The extent of reduction per GPa unit pressure increase is modeled to be on the order of 0.6-0.8 log 10-bar units in ƒ O 2 relative to the FMQ buffer. Basaltic melts experience oxidation during melt segregation and decompression relative to their mantle sources. The redox state of a basaltic melt at the Earth's surface is roughly proportional to the depth of first mantle melting.