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Isotope cosmochemistry of gallium and zinc


IPGP - Îlot Cuvier


Soutenances de thèses


Chizu Kato

Géochimie des enveloppes externes (GEE)

Gallium is a moderately volatile element with a 50 % condensation temperature of 968 K. The Earth and the Moon are depleted in volatile elements, including Ga, compared to the primitive meteorites that represent the solar system element abundance. The origin of this depletion is long-standing debate. Furthermore, while Ga behaves as a siderophile element during metal/silicate partitioning and its over-abundance in the Earth’s mantle is puzzling. In order to understand the Ga behavior in the solar system, and more generally moderately volatile elements, we have developed a method to analyze Ga isotope ratio at high precision and we studied terrestrial samples, chondrites and lunar samples for their Ga isotopic composition. We also analyzed the Zn isotopic composition of the lunar samples to further understand the volatile depletion history of the Moon. In this thesis, we first developed a chemical purification method for Ga at a precision of ± 0.05 ‰ (2 s.d.), and samples were measured for their isotopic compositions. To understand the Ga isotopic composition of the Earth, ocean island basalts, mid ocean ridge basalts and a komatiite was analyzed along with crustal granite. Kilauea Iki lava lake samples with a wide range of MgO contents were also analyzed, to investigate the Ga isotopic fractionation during magmatic processes. There was no isotopic fractionation observed for Ga, and the bulk silicate Earth (BSE) value was estimated to be ?71Ga (permil deviation of the 71Ga/69Ga ratio from the Ga IPGP standard) 0.00 ± 0.06 ‰ (2 s.d.). To further understand the Ga isotopic system in the solar system, meteorites and calcium-aluminum-rich inclusions (CAIs) were studied. All chondrites (carbonaceous, ordinary and enstatite) were isotopically lighter compared to the BSE. The Ga isotopic compositions of the carbonaceous chondrites were inversely correlated with the Al/Ga ratio. This correlation between the isotopes and the concentration cannot be created by evaporation, and instead, suggests a two component mixing of a volatile rich isotoically depleted and a volatile poor isotopically enriched sources. The Ga and Zn isotopic composition of CAIs were both highly fractionated compared to the bulk meteorite, and were enriched in both concentrations. This enrichment in the volatile elements and highly fractionated isotopes are due to condensation of a volatile rich gas following a Rayleigh distillation. The lunar mare basalts and the plutonic Mg suite rocks were enriched in the heavy isotopes of both Ga and Zn compared to the BSE, suggesting a global evaporation by the giant impact or the during the lunar magma ocean. The intrusive Mg suite samples were also isotopically heavy although they have not experienced any degassing into vacuum. The ferroan anorthosites on the other hand displayed a large isotopic variation that suggests a secondary distribution of the Ga and Zn isotopes on the surface. The Ga and Zn isotopes can be interpreted, as there were two stages of volatile loss during the formation of the Moon: a first stage of volatile loss following the giant impact and a second stage during the degassing of the magma ocean.