Small-scale Cr-Ti isotopic heterogeneity in Ryugu samples induced by aqueous alteration

Abstract

The Ryugu samples collected by Hayabusa2 show kinship with CI chondrites in terms of mineralogy, chemistry, and isotopic composition. However, for small samples (＜ 25 mg), the epsilon-⁵⁴Cr values vary beyond the range of literature values for CI, whereas the epsilon-⁵⁰Ti values were relatively uniform. We performed Cr and Ti isotope analysis on eight Ryugu particles (1.6-4.3 mg) provided by JAXA's Announcement of Opportunity (AO2). The epsilon-⁵⁴Cr values of the Ryugu samples ranged from +0.89 (+/-) 0.11 to +2.24 (+/-) 0.13, and the variation was larger than in previous studies. However, in contrast to previous studies, the epsilon-⁵⁰Ti values of the Ryugu samples also varied widely, from +0.75 (+/-) 0.20 to 2.18 (+/-) 0.16. These results suggest that the Ryugu sample contains microscale heterogeneity in Cr and Ti isotopic compositions due to the heterogeneous distribution of ⁵⁴Cr- and ⁵⁰Ti-rich presolar grains. This heterogeneous distribution is possibly due to the selective destruction of chemically labile phases with low epsilon-⁵⁴Cr and epsilon-⁵⁰Ti values by aqueous alteration of the parent body and the removal of low epsilon-⁵⁴Cr and epsilon-⁵⁰Ti components by fluids.