EVOLUTION OF EARTHS REDOX STATE DURING UPWELLING OF CARBON-BEARING MANTLE

The oxygen fugacity (f(O-2)) values recorded by diamondiferous peridot ite and eclogite xenoliths from Siberia indicate that the redox state of the ancient lithosphere is heterogeneous on a scale of at least fou r log units, mainly in the range between the wustite-magnetite (WM) an d iron-wustite (IW) oxygen buffers. Highly reduced peridotites can be interpreted as relict from earlier lower f(O-2). The f(O-2) values rec orded by 'fertile' and less modified spinel peridotites from Mongolia, Baikal and Tien-Shan show that the redox state of the lithosphere ben eath central Asia and Tien-Shan is heterogeneous on a scale of 2-3 log units, mainly in the range between the WM and IW + 1 oxygen buffers. These data provide evidence for the presence of a lower-f(O-2) regime of carbon-bearing mantle beneath the Baikal rift zone and Tien-Shan, a nd the oxidation of diapirs ascending from the asthenosphere. The 'dry ' xenoliths from Mongolia primarily reflect closed system behavior in the upper mantle, the f(O-2) of which is buffered by ferric-ferrous re dox equilibrium. The observed evolution of f(O-2) values is closely li nked to the distribution of volatile species in the mantle. H2O and CO 2 are the dominant volatiles for the more depleted and oxidized part o f peridotites, and CH4 for the more reduced and less modified part. It is proposed that the upper mantle was originally more reduced and has become progressively more oxidized, resulting perhaps largely from th e preferential loss of hydrogen and carbon during melting. The oxygen budget of the upper mantle results from the opposing contributions of crustal recycling and transfer of carbon-bearing material from the dee p mantle. (C) 1997 Elsevier Science B.V.