Application of Ge/Si ratios to ultramafic alkaline rocks using a novel LA-ICP- MS/MS method

Abstract

Germanium has, until recently, found limited interest in igneous and metamorphic rock studies due to analytical limitations, specifically low concentrations and many interferences. However, Ge and Ge/Si ratios have shown potential to distinguish between magma sources based on preferential substitution of Ge for Si in hydrous minerals such as amphibole, mica, and garnet. Here, we present a novel LA-ICP-MS/MS method that employs N2O as a reaction gas to analyse Ge as an oxide. We compare this method to previously used techniques (LA-ICPMS and "H2" mode LA-ICP-MS), and find improvements in sensitivity and a reduction of interferences when measuring Ge as GeO. Application of this novel method to the Bilibin and Inagli alkaline intrusive complexes (Aldan Shield, Siberia) shows that Ge/Si ratios are lower in amphibole and mica, and higher in clinopyroxene in these alkaline rocks compared to similar minerals in basaltic and intermediate compositions. The differences are attributed to the mineral chemistry and further show the potential for the use of Ge/Si as a tracer.