Ge/Si ratios in hydrous minerals and their role in igneous processes

Germanium/Silicon (Ge/Si) ratios provide an insight into magma differentiation, early Earth crustal development and potential sources of economically important mineralisation. Ge/Si ratios, whilst important, are understudied, partly due to undeveloped analytical methods. This study improves the methodology for measuring Ge and was specifically designed to reduce interferences whilst improving precision, and sensitivity to Ge/Si ratios. Measuring oxide shifted ⁷⁴Ge (mass 90) with the use of Laser Ablation-Inductively Coupled Plasma-Triple Quadrupole-Mass Spectrometry (LA-ICP-QQQ-MS) using N₂O as a reaction gas is shown here to improve sensitivity, precision and accuracy compared to the research-standard LA-ICP-MS method. The new method was utilised to measure Ge concentrations and Ge/Si ratios of natural samples from the Bilibin and Inagli Massifs: ultramafic alkaline ring structures in the Aldan Shield, Siberia. This study shows higher Ge/Si ratios in phlogopites and lower ratios in clinopyroxenes from Inagli compared to the Bilibin Massif. When compared to a collated global dataset of available Ge/Si values, both the Bilibin and Inagli samples show lower values for mica and amphiboles, but higher values for clinopyroxene. This complicates the predictions from previous studies, showing that further work is needed to fully understand the role of Ge/Si ratios as a potential tracer for magma sources.