Expérimentation et Géochimie isotopique : la quantification des fractionnements isotopiques du fer lors des ségrégations métal silicate

Résumé: Fractionation of Fe isotopes between a molten silicate and metallic alloys have been quantified experimentally at 1500ºC. The effects of oxygen fugacity and run duration have been investigated such that kinetic fractionation may be distinguished from that at equilibrium. 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 in the form of 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 54Fe along concentration gradients in the alloy, which leads to the formation of a metallic phase which is isotopically light compared to the residual silicate (D56FeM-S= d56Femetal-d56Fesilicate up to ?4.7?). The semi-empirical approach of diffusion-driven isotope fractionation is successfully applied to the data. Equations are parameterized 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 (D56FeM-S=0.4±0.3?). 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 as emphasized here in the case of Iron and Pallasite meteorites. -------------------------------------------------------------------------------- Séminaire de Géochimie --------------------------------------------------------------------------------