Tracing the origin and core formation of the enstatite achondrite parent bodies using Cr isotopes

Enstatite achondrites (including aubrites) are the only differentiated meteorites that have similar isotope compositions to the Earth-Moon system for most of the elements. However, the origin and differentiation of enstatite achondrites and their parent bodies remain poorly understood. Here, we report high-precision mass-independent and mass-dependent Cr isotope data for 10 enstatite achondrites, including eight aubrites, Itqiy and one enstatite-rich clast in Almahatta Sitta, to further constrain the origin and evolution of their parent bodies. The epsilon Cr-54 (per 10,000 deviation of the mass bias corrected Cr-54/Cr-52 ratio from a terrestrial standard) systematics define three groups: main-group aubrites with epsilon Cr-54 = 0.06 +/- 0.12 (2SD, N = 7) that is similar to the enstatite chondrites and the Earth-Moon system, Shallowater aubrite with epsilon Cr-54 = -0.12 +/- 0. 04 and Itqiy-type meteorites with epsilon Cr-54 = -0.26 +/- 0.03 (2SD, N = 2). This shows that there were at least three enstatite achondrite parent bodies in the Solar System. This is confirmed by their distinguished mass-dependent Cr isotope compositions (delta Cr-53 values): 0.24 +/- 0.03%e, 0.10 +/- 0.03%e and -0.03 +/- 0.03%e for main-group, Shallowater and Itqiy parent bodies, respectively. Aubrites are isotopically heavier than chondrites (delta Cr-53 = -0.12 +/- 0.04%e), which likely results from the formation of an isotopically light sulfur-rich core. We also obtained the abundance of the radiogenic Cr-53 (produced by the radioactive decay of Mn-53, T-1/2 = 3.7 million years). The radiogenic epsilon Cr-54 excesses correlate with the Mn-55/Cr-52 ratios for aubrites (except Shallowater and Bustee) and also the Cr stable isotope compositions (delta Cr-53 values). We show that these correlations represent mixing lines that also hold chronological significance since they are controlled by the crystallization of sulfides and silicates, which mostly reflect the main-group aubrite parent body differentiation at 4562.5 +/- 1.1 Ma (i.e., 4.8 +/- 1.1 Ma after Solar System formation). Furthermore, the intercept of these lines with the ordinate axis which represent the initial epsilon(53)Crvalue of main-group aubrites (0.50 +/- 0.16, 20) is much higher than the average epsilon Cr-53 value of enstatite chondrites (0.15 +/- 0 .10, 2SD), suggesting an early sulfur-rich core formation that effectively increased the Mn/Cr ratio of the silicate fraction of the main-group aubrite parent body. (C) 2021 Elsevier Ltd. All rights reserved.