The indium isotopic composition of the bulk silicate Earth

Indium (In) behaves as a moderately volatile metal during nebular and planetary processes, and its volatility depends strongly on oxygen fugacity. The In isotopic composition of the bulk silicate Earth (BSE) could provide a critical constraint on the nature of Earth's building blocks and mechanisms that lead to its volatile depletion. However, accurately and precisely determining the isotopic composition of In of the silicate Earth is challenging due to its low abundance in igneous rocks and the presence of significant isobaric interferences on its isotopes (e. g., 113Cd+ on 113In+ and 115Sn+ on 115In+). Here, we present a purification procedure for In from rock matrices and report the first dataset of In isotopic compositions of 30 terrestrial igneous rocks, one biotite geostandard, and one carbonaceous chondrite (Allende) measured on a Nu Sapphire collision-cell equipped multi-collector inductively-coupled-plasma mass-spectrometer (CC-MC-ICP-MS) with an external reproducibility of 0.11 parts per thousand (2SD). At this level of precision, we find no statistically significant difference in the In isotopic compositions of mid-ocean-ridge basalts (MORB), oceanic island basalts (OIB), and continental flood basalts (CFB). Furthermore, Canary Islands, Iceland and Afar lavas display no analytically resolvable In isotopic variations from basalts to rhyolites. Therefore, In isotope fractionation during igneous processes is smaller than our analytical uncertainty and the In isotopic compositions of basalts are likely to be representative samples of their mantle sources. The twenty-one terrestrial basalts from diverse geological settings have an average delta 115In of 0.35 +/- 0.07 parts per thousand (2SD). This value represents the current best estimate of the In isotopic composition of the mantle as well as of the bulk silicate Earth, assuming limited In isotope fractionation during mantle partial melting, and due to the small contribution of the continental crust to the In budget (<5%). This isotopic composition provides a baseline with which to compare with chondrites and differentiated planetary bodies in future studies.

Automated summary

Indium (In) behaves as a moderately volatile metal during nebular and planetary processes and its volatility depends on oxygen fugacity. Accurately determining the isotopic composition of In in silicate Earth is challenging due to its low abundance in rocks and significant isobaric interferences. This study presents a purification procedure for In and reports the first dataset of its isotopic compositions in terrestrial igneous rocks, showing no significant variation between different types of basalts. The average δ115In of 0.35‰ (2SD) in basalts represents the best estimate of the In isotopic composition of the mantle and the bulk silicate Earth.