Xenoliths in ordinary chondrites and ureilites: Implications for early solar system dynamics

Abstract

Foreign clasts (xenoliths) in meteoritic breccias are a serendipitous source of information about the impact environment in which their hosts formed, including impactor flux and cosmochemical types. These parameters may be related to timing and/or heliocentric distance of xenolith origin and implantation, and thus can be used to test or inform models of early solar system dynamics. We use xenoliths in ordinary chondrites (OCs) and ureilites to do this. We first conducted a petrologic and oxygen isotope study of a new, cm‐sized igneous‐textured clast in L3.7 Northwest Africa (NWA) 092, which highlighted some of the difficulties of identifying xenoliths in meteorites. Results indicate that this clast is not a xenolith but an impact melt of non‐local OC material. We add this result to a literature survey of more than 3000 OCs and find that the fraction of OCs that contain xenoliths is <<1%, and, even in these, the abundance and the diversity of xenoliths are very low. This contrasts markedly with the ureilites, ˜5% of which contain ˜1–10 vol% xenoliths from every major meteorite class, including multiple groups and petrologic types. To investigate reasons for this difference, we compare the histories of OC and ureilite parent bodies. The OC and ureilitic parent bodies accreted in the inner solar system within ˜1 AU of one another. The OC bodies accreted ˜2–3 Myr after calcium‐aluminum‐rich inclusion (CAI) formation and were heated slowly, experiencing thermal metamorphism over ˜50–60 Myr. The ureilite parent body (UPB) accreted <1 Myr after CAIs and was heated rapidly, experiencing partial melting over ˜4 Myr. Both OC parent bodies and the UPB were catastrophically disrupted and reassembled into rubble piles. For ureilites, this occurred ˜5.0–5.4 Ma after CAIs, while for OCs, it did not occur until 50–60 Myr after CAIs. Xenoliths in OC and ureilitic breccias were acquired as fragments of impactors on the rubble piles. The presence in polymict ureilites of xenoliths of all OC groups (H, L, LL) and petrologic types (3–6), and the intimate scale on which these and myriad other xenolith types are mixed, indicate that most xenoliths were acquired within a short time period around ˜50–60 Myr after CAIs when OC (likely also Rumuruti chondrite and enstatite chondrite) parent bodies were disrupted. This timing is consistent with the early instability dynamical model for a period of excitation in the asteroid belt. Outer solar system (CC) xenoliths were also acquired during this period, but were derived indirectly from C‐type bodies that had already been emplaced in orbits in the asteroid belt. The large discrepancy in xenolith abundance between ureilites and OC may be due to different physical properties of their regoliths at 50–60 Myr after CAIs. CC‐like xenoliths in OC may represent a different, more recently acquired, population than those in polymict ureilites.

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

“…Our best estimates of the 40 Ar/ 39 Ar ages of the chondritic clasts (Table 4) in AhS are 4535 AE 10 Ma (L4), ~4540 Ma (H5), and 4513 AE 17 Ma (EL6). Goodrich, Sanborn, et al (2021), andGoodrich, Kring, et al (2021) present evidence that 2008 TC 3 acquired carbonaceous chondrite and other xenolithic clasts under mild thermal conditions. If the xenolithic material in AhS remained essentially unperturbed on and after ejection from their parent bodies, then the 40 been happened at any time after 4513 Ma.…”

“…Polymict ureilites subsequently formed as regolith that developed on the surfaces of these daughter bodies, among them being the progenitor from which 2008 TC 3 /AhS broke off (Downes et al, 2008;Goodrich et al, 2004;Goodrich, Hartmann, et al, 2015). Based on numerous lines of evidence, Goodrich, Sanborn, et al (2021) and Goodrich, Kring, et al (2021) deduce that most of the xenoliths in polymict ureilites were implanted into ureilitic regolith 40-60 Myr after CAI, or even later.…”

“…In more recent work, Bischoff et al (2022) specify this time interval, stating that 2008 TC 3 was "a polymict C1 object that may have formed by late accretion at least 50-100 Ma after calcium-aluminum-rich inclusions." Goodrich, Sanborn, et al (2021) and Goodrich, Kring, et al (2021) argue that UDBs, including the progenitor(s) of both typical polymict ureilites and 2008 TC 3 , formed ~5 Ma after CAI. The ancient 40 Ar/ 39 Ar age of the ureilitic crustal clast MS-MU-011 (4549 AE 10 Ma) is consistent (1-σ) with this result.…”

Analytical database of Martian minerals (ADaMM): Project synopsis and Raman data overview

“…However, Martian olivine mostly differs from Terrestrial ones by their higher concentration of Fe, thus having a chemical composition close to fayalite. This information was extrapolated from OMEGA orbital data [38] and confirmed by laboratory analysis of Martian meteorites [39–41]. The ADaMM sample list provides access to several olivine samples within the forsterite‐fayalite solid solutions (Table 1).…”

Unique evidence of fluid alteration in the Kakowa (L6) ordinary chondrite

“…Shock parameters can be inferred from several lines of evidence, including brecciation, deformation in minerals, and the presence and textural features of melt veins (MVs) that often contain high-pressure (HP) minerals 12 – 20 . A notable group of meteorites known as polymict breccias contain fragments of multiple objects, presumably derived from both impactor and target of one or more collisions and reassembled as rubble piles 21 . Although such breccias are not unusual, they typically represent low-velocity collisions; polymict breccias from impacts fast enough to form HP minerals are uncommon 22 , 23 .…”