Other
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...v32a10h&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #V32A-10
Other
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 3630 Experimental Mineralogy And Petrology, 8147 Planetary Interiors (5430, 5724)
Scientific paper
Germanium is a compatible, moderately siderophile and volatile element. The approximately constant Ge/Si ratio in various types of terrestrial rocks reflects the diadochic relationship between Ge and Si as Ge4+ substitutes for Si4+ in silicates. The crustal Ge abundance is about 40% higher than the Ge content of the primitive upper mantle. This is the same crust/mantle ratio as for SiO2. The stable Ge species at the conditions of core formation is unclear. Schmitt et al. (GCA, 53, 173-185, 1989) obtained divalent Ge cations dissolved in a silicate liquid, while Capobianco et al. (GCA, 63, 2667-2677, 1999) provided evidence for Ge4+ as dominant Ge species in liquid silicate. We therefore decided to experimentally reinvestigate the valence of Ge in silicate melts by equilibrating pure Ge metal with silicate melts at various oxygen fugacities and constant temperature. All experiments were conducted in a vertical tube furnace at one atmosphere and controlled oxygen fugacities. In all experiments powdered silicates of anorthite-diopside eutectic composition were inserted on top of pure Ge metal lumps into small Al2O3 crucibles with ratios of approximately 1:2.5 (by weight). Crucibles were equilibrated at different fugacities, isothermal conditions (1329+/-3° C), and run durations of about 2 hours. Samples were quenched to glass by withdrawing them from the hot zone to the top of the furnace. Glass samples were separated by crushing the glass-containing crucible and handpicking individual glass fragments. Fragments adhering to the crucible were avoided. Germanium concentrations in glasses were analyzed by instrumental neutron activation analysis (Koeln, Germany), electron microprobe (Muenster, Germany) and proton microprobe (Heidelberg, Germany). The preliminary results show a decrease of the Ge solubility in the silicate with decreasing oxygen fugacity. The decrease is compatible with Ge4+ at oxidized conditions, Ge2+ at oxygen fugacities slightly above the Fe-FeO (IW) buffer and Ge0 at more reduced conditions. Further experiments to clarify the situation are in progress.
Holzheid Astrid
Palme Herbert
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