Olivine/silicate melt partitioning of germanium: an example of a nearly constant partition coefficient

Abstract

The partitioning of germanium between forsterite (Fo) and liquids in the diopside-anorthiteforsterite join was investigated by electron microprobe analysis of Ge-doped samples equilibrated at 1300°–1450°C. Germanium is somewhat incompatible in Fo relative to the haplobasaltic melts, with a grand mean for all simple partition coefficients ( D Fo- l Ge ) of 0.68 ± 0.06. For the melt composition range studied, D Fo- l Ge is virtually constant in isothermal series of experiments, and shows only minor overall temperature dependence. The exchange reaction partition coefficient K D = ( Mg 2GeO 4) Fo(SiO 2) l (Mg 2SiO 4) Fo(GeO 2) l ] is near unity in all cases, with a grand mean of 0.93 ± 0.11. One exploratory run at 20 kbar yielded a distinctly lower partition coefficient ( D Fo- l Ge = 0.54 ± 0.04), which confirms the negative pressure dependence predicted by the thermodynamics of Ge ai Si exchange. These new data indicate that absolute Ge enrichment must occur in terrestrial magmas undergoing olivine fractionation, while Ge Si remains nearly constant.