Original paper
Determination of the formal Ge-oxide species in silicate melts at oxygen fugacities applicable to terrestrial core formation scenarios
Kegler, Philip; Holzheid, Astrid
European Journal of Mineralogy Volume 23 Number 3 (2011), p. 369 - 378
published: Jul 13, 2011
Abstract
High-temperature, high-pressure experiments were performed at 1350 °C and 0.5 GPa to determine the dependence of germanium partitioning behavior between coexisting solid metal and liquid silicate phases on oxygen fugacity and melt composition. A piston cylinder apparatus was used to perform experiments under effectively closed-system conditions to avoid possible loss of volatile Ge species. The oxygen fugacity of the experiments was adjusted by using silicate melts with different amounts of FeO. This causes a change of the melt composition. Additional experiments with different melt compositions but identical FeO contents were performed to determine the effect of melt composition on the metal-silicate partitioning behavior.The partitioning behavior of Ge depends on both oxygen fugacity (fO2) and silicate melt composition (NBO/T). The dependences can be described by the equation:logDmet-silGe = 3.32(±0.07) - 0.50(0.03)·logfO2 - 0.29 (± 0.04)·NBO/T (R2 = 0.99).The slope of the correlation between Dmet-silGe and oxygen fugacity allows one to determine the valence state of Ge in the silicate melt. Within the fO2 range of the experiments (−0.9 to −2.7 log units relative to the Fe-FeO (IW) buffer), the valence state of Ge is 2.00 (±0.12), i.e. GeO is the stable species in the silicate melt. This is in good agreement with an earlier study performed in 1 atm furnaces at 1300 °C and a comparable fO2 range (−0.5 to −2.7 log units relative to the IW buffer), but at variance with another study that determined tetravalent Ge cations dissolved as oxides in silicate melts in 1 atm experiments performed at more oxidizing conditions (−1.3 to +3.8 log units relative to IW buffer). The data of the present study in combination with these earlier data suggest a change in the dominant formal Ge oxide species in silicate melts at oxygen fugacities slightly below the IW buffer.
Keywords
core formation • germanium • oxygen fugacity • valence state • partition coefficient • experimental petrology