Mg-isotopes in terrestrial and extraterrestrial olivines.

O-isotope studies of meteoritic materials have shown thatthe solar nebula was heterogeneous in composition, but alsothat some material as CAI and Ureilites have a non-massdependant fractionation regime. Recent advances in MC-ICPMS allow the investigation of other stable isotopic systems,and of particular interest are the 3-isotope systems of Fe andMg (e.g. Zhu et al., 2001; Galy et al., 2000). Zhu et al. (2000)have shown that in a 56Fe/54Fe-57Fe/54Fe space terrestrial andextraterrestrial (chondrite and achondrite) materials plot on asingle mass fractionation line, thus suggesting that the solarnebula was homogeneous in term of Fe-isotopes.In this study Mg isotopes have been measured using MCICP-MS in terrestrial and extraterrestrial olivines, withoutchemical separation. Olivine has high Mg content, lowminor/trace element contents, and is a rather ubiquitous phase.Analysis of Fe(Ni)-doped SRM-980 shows that there is noeffect on the Mg isotopic composition. Furthermore, analysisof a number of mantle olivines (n=12; δ25Mg=1.63±0.06;δ26Mg=3.07±0.08, relative to SRM-980) shows a perfectagreement with the terrestrial mass fractionation line. Pallasiteolivines have δ25Mg ranging from 1.05 to 1.35 and show amass dependant fractionation within error of the TerrestrialFractionation Line (TFL). Ureilites show a wider range of Mgfractionation (-0.30≤δ25Mg≤1.54), however, they still define amass dependant fractionation line, with a slopeindistinguishable to that of the TFL.Mg isotopes depict a contrasting view of the solar nebulaheterogeneity to that shown by O isotopes. No difference isobserved between the pallasites (Vesta line in O- space) andthe Earth. Ureilites show a mass dependant fractionation linein Mg isotopes which strongly contrasts with their slope (ca.1) in O-space, yet is in agreement with the CAI Mg-isotopiccomposition (Galy et al., 2000, Young et al., 2002). Thesedata suggest that the process/es responsible for O isotopefractionation did not affect Mg isotopes, or did not affect themin the same manner.