An Intercomparison Study of the Germanium Isotope Composition of Geological Reference Materials

Recent analytical developments in germanium stable isotope determination by multicollector ICP-MS have provided new perspectives for the use of Ge isotopes as geochemical tracers. Here, we report the germanium isotope composition of the NIST SRM 3120a elemental reference solution that has been calibrated relative to internal isotopic standard solutions used in the previous studies. We also intercalibrate several geological reference materials as well as geological and meteoritic samples using different techniques, including online hydride generation and a spray chamber for sample introduction to MC-ICP-MS, and different approaches for mass bias corrections such as samplecalibrator bracketing, external mass bias correction using Ga isotopes and double-spike normalisation. All methods yielded relatively similar precisions at around 0.1 parts per thousand (2s) for d74/70Ge values. Using igneous and mantle-derived rocks, the bulk silicate Earth (BSE) d74/70Ge value was re-evaluated to be 0.59 +/- 0.18 parts per thousand (2s) relative to NIST SRM 3120a. Several sulfide samples were also analysed and yielded very negative values, down to -4.3 parts per thousand, consistent with recent theoretical study of Ge isotope fractionation. The strong heavy isotope depletion in ore deposits also contrasts with the generally positive Ge isotope values found in many modern and ancient marine sediments.