The gallium isotopic composition of the bulk silicate Earth

• Published in: Chemical geology Vol. 448; pp. 164 - 172
• Main Authors: Kato, Chizu, Moynier, Frédéric, Foriel, Julien, Teng, Fang-Zhen, Puchtel, Igor S.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.2017; Elsevier
• Subjects: Basalt; Bulk Silicate Earth; Earth; Earth Sciences; Estimates; Gallium; Geochemistry; Komatiite; Komatiites; Lava; Mathematical analysis; MC-ICP-MS; MORB; OIB; Sciences of the Universe; Uncertainty; USA; Gallium; MORB; MC-ICP-MS; Basalt; OIB; Komatiite; Bulk Silicate Earth
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2016.11.020

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