Experimental quantification of the fractionation of Fe isotopes during metal segregation from a silicate melt

Fractionation of Fe isotopes between a silicate melt and metallic alloys has been quantified experimentally at 1500 degrees C. The effects of oxygen fugacity and run duration have been investigated to distinguish between kinetic and equilibrium fractionations. A new experimental, setup is presented, in which metallic Fe is produced by reduction of oxidized iron from a silicate melt due to a change of redox conditions. This metallic Fe is physically removed from the silicate and sequestered as a (Pt,Fe) alloy. Bulk analyses of the silicate and metallic fractions using multi-collector ICP-MS methods are coupled with in situ analyses using an ion microprobe. Experimental results indicate that significant isotopic fractionation of Fe occurs during metal segregation. During the early stages of the experiments, we find evidence for kinetic fractionation caused by faster diffusion of 54 Fe along concentration gradients in the alloy. This process leads to the formation of a metallic phase which is isotopically light compared to the residual silicate (metal-silicate fractionation up to -2.35%/a.m.u.). The data are used to quantify the difference in diffusion coefficient of each isotope of iron, providing a means to predict Fe isotope fractionation during Fe diffusion in metallic alloys. On the other hand, this state of affairs is transient in nature, and at superliquidus temperature, the systems rapidly reach a. state of isotopic equilibrium in which the metal is isotopically heavier than the silicate melt by about +0.2 +/- 0.15%/a.m.u. These results are consistent with the range of variation observed in natural samples. These kinetic and equilibrium data provide an experimental framework to understand the observed variability among igneous and meteoritic materials formed at high temperature, particularly for iron and pallasite meteorites. (c) 2006 Elsevier B.V. All rights reserved.