Experimental Germanium Isotopic Fractionation Under HT, fO2-controlled Conditions of Core Formation and Accretion
Résumé
Terrestrial planets and the Moon underwent metal-silicatedifferentiation in their earliest history under reducingconditions. This process partly explained the depletion insiderophile (iron-loving) and volatile elements with respect tothe solar composition (i.e. CI chondrites) in their silicatereservoir. Metal-silicate experiments and models of pressure,temperature, and increasingly oxidized conditions pertinent tothe end of accretion fail to reproduce the Earth’s mantlegermanium (Ge) concentration, a moderately siderophile andvolatile element, unless unrealistically high amounts ofchondritic “late veneer” is added to the silicate reservoir [1].In order to understand the specific behaviour ofgermanium and its isotopes under the T, fO2 conditions ofcore formation and accretion processes, we have undertaken aseries of metal-silicate experiments. The silicate phase of 1baranorthite - diopside eutectic doped with ~4000 ppm of Ge-Aldrich standard is placed in pure Ni capsules at 1 atm in avertical drop quench furnace, at T= 1355°C for 2 to 60 hoursover a range of ƒO2 from 4 log units below, to 2.5 log unitsabove, the IW buffer [2]. Ge isotope data on metal phase weregiven in [2]. New Ge isotopic analyses of the final low-Gesilicate have been performed using hydride generator systemcoupled to the NeptunePlus MC-ICPMS (CRPG-Nancy) [3].At very low ƒO2, Ge diffusion in the metal was observed(δ74/70Gemetal slightly lower that Ge-CMAS starting material).Under increasingly oxidizing conditions, competition wasseen between diffusivity and volatility (strong increase inδ74/70Ge in metal and silicate associated to a decrease in Gecontents). With time, it is shown an inversion of Δ74/70Gemetalsilicate, from negative to positive. These results are consistentwith the sense of Ge isotopic fractionation as seen in metaland silicate phases of pallasites and chondrites [4, 5], andbetween Fe-meteorites and the silicate Earth [3].[1] Siebert et al. (2011). GCA 75, 1451. [2] Luais et al.(2007). Eos Trans. AGU88(5), V51E-0833. [3] Luais et al.(2012) Chem. Geol. 334, 295. [4] Luais et al. (2017) Goldsch.Abst. 2474. [5] Florin et al. (2018) MetSoc LPI Contrib. 2067.