No iron isotope fractionation between molten alloys and silicate melt to 2000 °C and 7.7 GPa: Experimental evidence and implications for planetary differentiation and accretion

摘要

Whether core-mantle differentiation of terrestrial planets fractionates iron isotope is currently a debated issue. Melting experiments corresponding to the conditions inferred for core differentiation in an early silicate magma ocean were performed at 1750 and 2000 °C, and from 1 to 7.7 GPa to address this question. The starting mixtures correspond to a devolatilized CI chondrite composition and oxygen fugacity conditions were ~ 2 log units below the iron-wüstite buffer. Scanning electron microscopy observations, electron microprobe chemical analyses and plasma source mass spectrometric isotope analyses of the experimental charges show that chemical and iron isotope equilibrium was reached at 2000 °C within 100 s. No Fe isotope fractionation was found between the Fe-Ni alloy and the ultramafic silicate melt at this temperature. This result holds within the 2-7.7 GPa pressure range and is likely to remain valid at higher pressures and temperatures. The addition of sulfur to the system does not alter this conclusion. The compilation of all experiments conducted at 2000 °C yields Δ~(57)Fe_(metal-silicate glass) = 0.047 ± 0.063‰. Our results suggest that significant iron isotope fractionation is unlikely during equilibration of molten core-forming materials in a deep magma ocean. This process therefore cannot explain the heavier Fe isotope composition of the Moon relative to the Earth, itself heavier than Mars, Vesta and chondrite parent bodies.