17:15 〜 18:45
[SIT15-P04] The oxidative capacity of metasomatic carbonated melts in the mantle beneath the Cape Verde archipelago.
キーワード：Carbon, mantle, metasomatism, redox conditions, peridotites
Carbonated melt reactive migration is critical to increasing the oxidation state of the mantle. Here, we study a suite of 12 peridotite xenoliths from Sal Island (Cape Verde) and found to sample the oceanic deep lithosphere at pressures ≧ 3 GPa in a temperature range of 827 to 1194°C. Peridotites consist of lherzolites (7 – 10% Cpx) and ultradepleted harzburgites. Rocks record strong chemical gradients and metasomatic refertilisation by carbonated melts, testified by the replacement of Opx by Cpx + Ol and coexisting carbonate (Arg+Dol+Mg-Cal) + silicate glass veins and fluid inclusions. Fluid inclusions contain CO2+CO (XCO up to 0.27) and locally disordered Gr in harzburgites. They are mostly CO2, with locally minor CO in lherzolites. Superimposed on metasomatic refertilisation was an oxidation trend revealed by thermodynamic analysis of C-super-saturated (aC > 1) metastable CO2+CO fluids, along with Ol-Spl fO2 calculations. At 3.4 GPa, harzburgites fO2 increases from -4.00 to -1.10 Dlog FMQ, while lherzolites' fO2 increases from -1.23 to -0.70 Dlog FMQ. Thus, infiltrating metasomatic carbonated melts can oxidise the reduced mantle by three orders of magnitude (i.e., 3 log fO2 units) on reaction.