⁴⁰Ar/³⁹Arages ofL4,H5,EL6,and feldspathic ureilitic clasts from the Almahata Sitta polymict ureilite (asteroid 2008TC₃)

Abstract

The Almahata Sitta (AhS) meteorite consists of disaggregated clasts from the impact of the polymict asteroid 2008 TC3, including ureilitic (70%–80%) and diverse non‐ureilitic materials. We determined the 40Ar/39Ar release patterns for 16 AhS samples (3–1500 μg) taken from three chondritic clasts, AhS 100 (L4), AhS 25 (H5), and MS‐D (EL6), as well as a clast of ureilitic trachyandesite MS‐MU‐011, also known as ALM‐A, which is probably a sample of the crust of the ureilite parent body (UPB). Based on our analyses, best estimates of the 40Ar/39Ar ages (Ma) of the chondritic clasts are 4535 ± 10 (L4), 4537–4555 with a younger age preferred (H5), and 4513 ± 17 (EL6). The ages for the L4 and the H5 clasts are older than the most published 40Ar/39Ar ages for L4 and H5 meteorites, respectively. The age for the EL6 clast is typical of older EL6 chondrites. These ages indicate times of argon closure ranging up to 50 Ma after the main constituents of the host breccia, that is, the ureilitic components of AhS, reached the >800°C blocking temperatures of pyroxene and olivine thermometers. We suggest that these ages record the times at which the clasts cooled to the Ar closure temperatures on their respective parent bodies. This interpretation is consistent with the recent proposal that the majority of xenolithic materials in polymict ureilites were implanted into regolith 40–60 Ma after calcium–aluminum‐rich inclusion and is consistent with the interpretation that 2008 TC3 was a polymict ureilite. With allowance for its 10‐Ma uncertainty, the 4549‐Ma 40Ar/39Ar age of ALM‐A is consistent with closure within a few Ma of the time recorded by its Pb/Pb age either on the UPB or as part of a rapidly cooling fragment. Plots of age versus cumulative 39Ar release for 10 of 15 samples with ≥5 heating steps indicate minor losses of 40Ar over the last 4.5 Ga. The other five such samples lost some 40Ar at estimated times no earlier than 3800–4500 Ma bp. Clustering of ages in the low‐temperature data for these five samples suggests that an impact caused localized heating of the AhS progenitor ~2.7 Ga ago. In agreement with the published work, 10 estimates of cosmic‐ray exposure ages based on 38Ar concentrations average 17 ± 5 Ma but may include some early irradiation.

Enstatite meteorite clasts in Almahata Sitta and other polymict ureilites: Implications for the formation of asteroid 2008 TC3 and the history of enstatite meteorite parent asteroids

“…The fact that the EC clasts (like the OC) include the entire known range of EC chemical groups and petrologic types implies that the materials that were available to be implanted into ureilitic daughter bodies at ~50-60 Myr after CAI must have been derived from a large range of depths in several (at least five) EC parent bodies, that is, consistent with the interpretation that this was a period of enhanced dynamical activity when many chondritic planetesimals of various types in the asteroid belt were disrupted (Goodrich, Kring, et al, 2021;Goodrich, Sanborn, et al, 2021). A 40 Ar/ 39 Ar age of 4513 AE 16 Ga for an EL6 AhS clast (MS-D), interpreted as recording the time of cooling to the Ar closure temperature on the parent asteroid, is also consistent with this interpretation (Turrin et al, 2023). The excavated EC materials could have directly become impactors onto ureilitic bodies.…”

Nature and timing of a significant reduction event on the L‐chondrite parent asteroid

“…Other “old” L6 ages include MIL 05029 (4517 ± 11 Ma; Weirich et al., 2010); Park, which experienced shock and subsequent annealing (Ruzicka & Hugo, 2018; 4526 ± 5 Ma; Ruzicka, Clay, et al., 2015); and an L4‐chondrite clast recovered from Almahata Sitta (AhS) AhS 100 (4535 ± 10 Ma; see 40 Ar/ 39 Ar age compilation of the L chondrites; tab. S6.1, Turrin et al., 2023).…”

“…The total 38 Ar cos is determined by applying the lever rule to the total measured 36 Ar and 38 Ar, using a value of 5.35 for terrestrial atmospheric and 0.65 for the cosmogenic 36 Ar/ 38 Ar ratio. See Turrin et al (2023) for a more detailed description of the methods.…”