Melting boundary of Fe-17%Si up to 5.5 GPa and the timing of core formation

Melting of Fe-17wt%Si has been investigated at temperatures up to 1600 degr ees C and pressures up to 5.5 GPa in a cubic anvil press. The melting tempe rature, T-M,T- was identified with the ty in temperature and pressure depen dence of electricail resistance. Two types of electrode materials, Pt and F e, were used and gave consistent melting results. T-M increases with pressu re from 1200 degrees C at 1 atm to 1430 degrees C at 5.5 GPa. dT(M)/dP decr eases with pressure and reaches a value of zero it P similar to 5 GPa. Comb ined with the thermal profiles of the proto-Earth, the pressure independenc e of the melting boundary of Fe-17%Si at the highest pressures of this stud y suggests core formation began early in Earth accretion with a substantial liquid metal phase fraction between 30 Ma and 42 Ma.