4.4 LIU Yongsheng

Ultra-reduced and strong oxidizing environments caused by deep carbon recycling

Yongsheng Liu, Detao He, Stephen Foley

Carbon has oxidation numbers ranging from −4 to +4, behaving as a cation, as an anion, and as a neutral species in phases. Therefore, the carbon-bearing phase transition during deep carbon recycle can dramatically affect or be affected by the oxygen fugacity. We described a suite of peridotite and carbonatite xenolith from the Dalihu basalt, Inner Mongolia, China. The carbonatitic xenoliths record geochemically and petrologically deep mantle recycling of sedimentary carbonate. A SiC dominated ultra-reduced mineral assemblage, including graphite, diamond, SiC, TiC, native metals (Si, Fe, Ni) and iron silicide, were found in these carbonatite xenoliths. These observation indicate an ultra-reduced environment was locally formed during the deep carbon recycling. SiC in the carbonatite xenoliths has significant 13C-depleted isotopic composition with obvious spatial variation, which can be well-explained by degassing. It is interesting to note that extreme forsterite-rich olivine (Mg# up to 98) and high Fe3+/ΣFe ratio minerals, including magnesioferrite and hematite were observed in some peridotites, suggesting an extremely oxidizing environment (above ΔFMQ +7). Such extremely oxidizing environment could have been related to the recycled sedimentary carbonate as well. Subsolidus oxidation of olivine converts Fe2+ into Fe3+, which cannot fit into the octahedral site and tend to dissolve out and form magnesioferrite and hematite.