The gallium isotopic composition of the bulk silicate Earth

Abstract

We report a new method for precise analysis of gallium (Ga) isotopic composition in geological samples. The purification of Ga is achieved by a three-step ion exchange chromatography to remove matrix and interfering elements. The 71Ga/69Ga ratios are analyzed by multi-collector inductively-coupled-plasma mass-spectrometer (MC-ICP-MS). The external reproducibility of the measurements (0.05‰, 2 s.d.) was assessed by replicate analyses of the USGS BCR-2 and BHVO-2 standards. This newly developed technique was then used to investigate the extent of Ga isotopic fractionation during igneous processes by analyzing well-characterized samples from the Kilauea Iki lava lake, USA. These samples were formed in a closed system and have MgO contents ranging from 26.9 to 2.4wt.%. We found that igneous processes do not fractionate Ga isotopes within the analytical uncertainty and that the Ga isotopic composition of mafic-ultramafic lavas can be used to estimate the composition of their mantle source. Twelve ocean island basalts, two mid-ocean-ridge basalts, one continental flood basalt and one komatiite have homogeneous and nearly identical Ga isotopic compositions within analytical uncertainties averaging 0.00±0.06‰ (2s.d.). This value represents the best estimate for the Ga isotopic composition of the bulk silicate Earth