HR: 0800h
AN: V51E-0833    [Abstracts]
TI: Experimental determination of germanium isotopic fractionation during metal-silicate segregation
AU: Luais, B
EM: luais@crpg.cnrs-nancy.fr
AF: Centre de Recherches Pétrographiques et géochimiques, 15, rue Notre-Dame des Pauvres, Vandoeuvre les Nancy, 54501, France
AU: Toplis, M J
EM: toplis@dtp.obs-mip.fr
AF: Laboratoire Dynamique Terrestre et Planétaire, 14 avenue Edouard Belin, Toulouse, 31400, France
AU: * Roskosz, M
EM: mathieu.roskosz@univ-lille1.fr
AF: Laboratoire de Structure et Propriétés de l'Etat Solide, Université des Sciences et Techniques de Lille, Batiment C6, Villeneuve d'Ascq, 59655, France
AU: Tissandier, L
EM: tix@crpg.cnrs-nancy.fr
AF: Centre de Recherches Pétrographiques et géochimiques, 15, rue Notre-Dame des Pauvres, Vandoeuvre les Nancy, 54501, France
AB: Germanium is a moderately siderophile, moderately volatile element, whose concentration has long been used for the classification of iron meteorites. Despite a range in elemental abundance covering four orders of magnitude, it has been recently shown that the isotopic composition of Ge in different classes of magmatic irons is constant within analytical error (Luais, 2007). In order to constrain the processes which have led to these elemental and isotopic characteristics, we have undertaken a series of preliminary experiments to study the isotopic consequences of Ge transfer from an oxidized silicate liquid to a metallic phase. Particular attention has been paid to the role of oxygen fugacity (fO₂), inferred to be an important factor controlling the bulk element chemistry of magmatic irons (e.g. Fe/Ni ratio), and known to affect metal-silicate partition coefficients of Ge. Experiments were performed at 1 atm. in a vertical drop quench furnace. A glass corresponding to the 1bar anorthite - diopside eutectic was synthesized and doped with ~4,000 ppm Ge, added in the form of an ICP Aldrich standard solution. Powders of this glass were placed in pure Ni capsules and heated to 1355°C for 2 to 60 hours over a range of fO₂ from 4 log units below, to 2.5 log units above, the IW buffer. At this temperature, Ni is below its melting point, but the silicate was above its liquidus. Metal and silicate phases were mechanically separated. The metal phase was dissolved in nitric acid, and Ge separated from the Ni matrix using chromatographic methods of Luais (2007). Isotopic measurements were performed on the Isoprobe MC-ICPMS. At the IW buffer, time series experiments show that there is a clear continuous increase in δ⁷⁴Ge in the metal as a function of time, values being isotopically lighter than the reference Aldrich standard after 2 hours (δ⁷⁴Ge ~-0.5‰), reaching a δ⁷⁴Ge of ~+3‰ after 60 hours. For experiments at 24 hours, an increase in δ⁷⁴Ge (from ~-0.3‰ to +10‰) is observed with increasing fO₂. The enrichment of light isotopes in the metallic phase most likely reflects the consequence of Ge diffusion from the silicate to the metal (c.f. case for iron, Roskosz et al., 2006). On the other hand, enrichment in heavy isotopes is indicative of loss of Ge, most probably due to evaporation. More experiments are planned to constrain these processes and to quantify the relative time-scales of Ge transport between silicate, metal and gas reservoirs. Luais B (2007) EPSL In press: Roskosz et al. (2006) EPSL v. 248 p. 851.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 3630 Experimental mineralogy and petrology
DE: 5120 Plasticity, diffusion, and creep
DE: 5139 Transport properties
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting