Experimental Evidence for Iron and Silicon Isotope Fractionation during Earth's Core Formation

Details Experimental Evidence for Iron and Silicon Isotope Fractionation during Earth's Core Formation Experimental Evidence for Iron and Silicon Isotope Fractionation during Earth's Core Formation

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmmr32a..03s&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #MR32A-03

Physics

1025 Composition Of The Mantle, 1041 Stable Isotope Geochemistry (0454, 4870), 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008)

Scientific paper

Several recent studies have shown that comparisons between meteorites and terrestrial stable isotope ratios can be used to elucidate the composition of Earth's core if fractionation factors between silicate and metal are known. In order to understand the fractionation that might occur at core conditions, isotopic experiments need to be conducted at high pressure and temperature. We have developed a method for tracing the evolution towards isotopic equilibrium in piston cylinder experiments based on the so-called three- isotope method. Our implementation of the three-isotope method utilizes mono-isotopic spikes for the elements of interest (e.g., 54Fe, 28Si) in one of the participating phases. We present Fe and Si isotope ratio data from one such set of three-isotope experiments conducted in a piston cylinder apparatus at 2100 K and 1 GPa. In these experiments silicates were spiked with both 54Fe and 28Si. The goal was to determine if core formation could have resulted in isotopic fractionation in both the Fe and Si systems at high pressures and temperatures. Preliminary results suggest that there is a small 57/54Fe fractionation at these conditions between silicate and iron metal of ~ +0.15 ‰ (silicate > metal). These results are inconsistent with some earlier findings (Poitrasson and Roskosz, 2007 and Williams et al. 2008) but are in agreement with estimates based on differences between meteoritical and terrestrial Fe isotope ratios (Georg et al. 2007). We also find a larger silicon isotope fractionation between Si in metal and Si in silicate of ~ 2 ‰ in 30Si/28Si, suggesting that there is approximately 4 wt % Si in the core. These results indicate that isotope fractionation of major elements at high pressure and temperature can be significant, and suggest that experimental calibrations of fractionations between metal and silicate/oxide are essential for interpreting stable isotope data on the planetary scale.

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