Effect of terrestrial weathering on stable Sr and Ba isotope compositions of eucrites

Abstract

Stable strontium and barium isotopes are potential tracers for understanding planetary differentiation and the nature of the building blocks of terrestrial planets. Strontium and barium are fluid-mobile elements, but it remains unclear how terrestrial weathering affects the Sr-Ba isotopes compositions in achondrites, thus hampering the utility of Sr-Ba isotopes in cosmochemistry. In this study, we conducted acetic acid leaching on three eucrites with varying weathering degrees (fall: Qiquanhu, hot desert find: Northwest Africa (NWA) 13583, and Antarctic find: Grove Mountains (GRV) 13001). Combined with detailed petrography observations and major and trace element analyses, we investigated the variations in Sr-Ba isotopes during terrestrial weathering. The degree of weathering follows an order of: NWA 13583 > GRV 13001 > Qiquanhu, evaluated based on several alteration signs, including: the presence of secondary carbonate, the enrichment of large ion lithophile elements (e.g., Sr, Ba, and U), and the Ce and Eu anomalies. The concentrations of Sr and Ba in the leachates of NWA 13583 show a good correlation with Ca, suggesting that the soluble Sr and Ba are derived from secondary carbonate. Differently, the concentrations of Sr and Ba in the leachates of Qiquanhu correlate with Al and Na, suggesting that the soluble Sr and Ba in Qiquanhu are derived from primary plagioclases. This also indicates that silicates dissolution may be inevitable in an acid leaching experiment for achondrites, even when using weak acetic acid. GRV 13001 shows no variation in Sr and Ba isotopes during leaching experiments. The δ138/134Ba in the leachate (0.26 ± 0.02 ‰) of Qiquanhu is higher than that of the residue (0.04 ± 0.03 ‰), reflecting that aqueous fluids preferentially uptake heavy Ba isotopes during plagioclase dissolution. Conversely, the leachate of NWA 13583 shows lower δ138/134Ba (-0.19 ± 0.05 ‰) than that of residue (-0.10 ± 0.03 ‰), reflecting the lighter Ba isotope composition in carbonate. Notably, the residue of NWA 13583 has δ138/134Ba ∼ 0.1 ‰ lower than those of Qiquanhu and GRV 13001. This discrepancy may reflect plagioclase dissolution during hot-desert weathering rather than magmatism on the parent body. Different from Ba isotopes, the δ88/86Sr of Qiquanhu shows tiny variation in the leaching experiment, suggesting that the dissolution of plagioclase may cause negligible Sr isotope fractionation. For NWA 13583, the δ88/86Sr of leachate is slightly heavier than that of leaching residue and bulk rock, reflecting the high δ88/86Sr in terrestrial fluids. Our results suggest that Ba and Sr isotopes of eucrites show different behaviors during terrestrial weathering. Sr isotopes show a smaller fractionation scale and may have greater resistance for terrestrial weathering than Ba isotopes.