The melting of subducted banded iron formations

摘要

Banded iron formations (BIF) were common shelf and ocean basin sediments 3.5-1.8 Ga ago. To understand the fate of these dense rocks upon subduction, the melting relations of carbonated BIF were determined in Fe-Ca-(Mg)-Si-C-O-2 at 950-1400 degrees C, 6 and 10 GPa, oxidizing (fO(2) = hematite-magnetite, HM) and moderately reducing (fO(2) similar to CO2-graphiteidiamond, CCO) conditions. Solidus temperatures under oxidizing conditions are 950-1025 degrees C with H2O, and 1050-1150 degrees C anhydrous, but 250-175 degrees C higher at graphite saturation (values at 6-10 GPa). The combination of Fe3+ and carbonate leads to a strong melting depression. Solidus curves are steep with 17-20 C/GPa. Near-solidus melts are ferrocarbonatites with similar to 22 wt.% FeOtot, similar to 48 wt% CO2 and 1-5 wt.% SiO2 at fO(2) similar to HM and 49 wt.% FeOtot, similar to 20 wt% CO2 and 19-25 wt.% SiO2 at fO(2) similar to CCO.