Soutenance de thèse de Mme Xueying WANG du Laboratoire de Géologie (LGLTPE) sous la direction de M. Bernard BOURDON
The main aim of this thesis is to investigate the depletion of moderately volatile elements in planets and meteorites. The isotopes of moderately volatile elements have been utilized to distinguish the processes such as partial condensation and partial evaporation in the early Solar System. They can also be used to determine whether the solid phase was in thermodynamic equilibrium with the ambient gas during the thermal events in the protoplanetary disk. In this study, the isotope compositions of the moderately volatile element Sn with a 50% condensation temperature of 704 K was chosen to investigate this issue.
In the first part of this thesis, a new high-precision isotope method for analyzing Sn using the 117Sn-122Sn double-spike technique was developed, which is a great improvement compared with previous studies. A recovery close to 100% was achieved using our new purification protocol of Sn, and a reproducibility better than 0.06‰ (2 s.d.) on δ124Sn for 10 ppb Sn standard solution was found using MC-ICP-MS Neptune Plus equipped with Jet cones. In addition, a best external uncertainty of 0.11‰ (2 s.d.) was obtained for replicate analyses of the USGS standard andesite AGV-1.
In order to compare Sn isotope composition of chondrites with that of the bulk Earth, it is important to investigate the potential effect of core formation and magmatic processes on Sn isotopes in the Earth. Therefore, we analyzed a variety of terrestrial rocks including OIB, MORB, peridotites and granite to understand Sn isotope behavior during partial melting and to estimate the δ124Sn value of the BSE. We found that partial melting did affect Sn isotopes and resulted in the peridotites enriched in lighter Sn isotopes while mantle derived rocks were enriched in heavier Sn isotopes.
This effect is due to the difference in partitioning of Sn(IV) and Sn(II) in the minerals. For instance, Sn(IV) is compatible during partial melting and thus enriched in the silicate melts owing to its compatible behavior in clinopyroxene and incompatible behavior in olivine and orthopyroxene. The situation of Sn(II) is reversed since Sn(II) is compatible in olivine and orthopyroxene, and incompatible in clinopyroxene. Since Sn(IV) is enriched in heavier Sn isotopes relative to Sn(II), this results in a silicate melt enriched in heavier Sn isotopes and a residue enriched in lighter Sn isotopes. On this basis, the δ124Sn of the BSE can be estimated from the most primitive peridotite to a value of -0.08±0.05‰ (2 s.d.) relative to the standard. Then, the effect of core formation was investigated using Sn partition coefficient between metal and silicate. We established a model for calculating the difference of δ124Sn between the BSE and bulk Earth (expressed as Δ124SnBSE-BE) during core formation of the Earth. The maximum Δ124SnBSE-BE (0.09‰) is smaller than the measurement precision (0.11‰, 2 s.d.) and thus the δ124Sn of the BSE can represent that of the bulk Earth, which is -0.08±0.05‰ (2 s.d.).
Amphi BIO - ENS de Lyon