Constraining Volatile Element Loss Processes by Germanium Isotopic Investigations of Iron Meteorites

The concentrations of Ge and other moderately volatile elements (MVE) in iron meteorites vary by orders of magnitude, however, the origin of these variations is poorly understood and nebular as well as planetary processes have been proposed. To better constrain the origin(s) and process(es) of MVE depletion among planetary bodies, we measured the Ge concentrations and mass-dependent Ge isotopic compositions of a set of iron meteorites, using a newly developed Ge double-spike technique.

In total, three IAB, four IC, eight IIAB, five IID, nine IIIAB, and one IIIE iron meteorites were investigated. The Ge concentrations are consistent with reported literature data and range from ∼35 ppm for the IIIAB meteorites up to >300 ppm for the IAB irons. The Ge stable isotopic compositions of the analyzed samples, including the first Ge isotopic data for IC, IID, IIIAB, and IIIE irons, are relatively uniform (δ74/70Ge ≈ 1). There is no resolved within-group variability for most of the investigated groups. Only the IID and IIIAB irons seem to show some internal variability which, however, is neither attributable to fractional crystallization nor to cosmic ray exposure effects. Despite large variations of Ge concentrations, the different iron meteorite groups lack significant Ge isotopic variations among each other, but such variations would be expected for simple evaporative loss of Ge during magma degassing. As such, the new data for magmatic irons do not seem to support the idea that the volatile-depleted nature of their parent bodies is the result of planetary processes.