Barium stable isotopic composition of chondrites and its implication for the Earth

Abstract

Barium is a refractory and fluid mobile element that is highly incompatible in basaltic systems. The elemental and isotopic compositions of Ba are used to trace fluid processes or sediment recycling during subduction. However, the Ba isotopic composition of primitive meteorites (chondrites), which represents the possible building blocks of terrestrial planets, is presently poorly known. Here, we report high precision Ba isotopic compositions of major types of chondrites using a double-spike method. Chondritic meteorites without clear terrestrial weathering show relatively homogeneous Ba isotopic distributions and have an average δ137/134Ba (the permil deviation of the 137Ba/134Ba ratio from the SRM3104a standard) value of 0.09 ± 0.09‰ (2SD, N = 14). Given the refractory and lithophile behavior of Ba and the limited variations of Ba isotope composition between various chondrite groups, this value can represent the Ba isotopic composition of the bulk Earth and the Bulk Silicate Earth (BSE). It is actually consistent with the present estimate of BSE (0.05 ± 0.06‰, Li et al., 2020; Nan et al., 2022) based on the measurements of mid-ocean ridge basalts and carbonatites. Small Ba isotope variations are observed between carbonaceous chondrite groups where CO and CV show slightly heavier (by 0.09‰) Ba isotopic compositions than CI and CM. Significant light stable Ba isotopes enrichment has previously been documented in calcium, aluminum-rich inclusions (CAIs) in the Allende CV chondrite, and the Ba isotopic differences between CO and CV might result from the various proportions of CAIs based on a mass balance calculation. However, the parent body of CI and CM might have experienced aqueous alteration and secondary carbonates are most likely responsible for their light Ba isotopic signatures.