Abstract
Nucleosynthetic isotope anomalies show that the first few million years of solar system history were characterized by two distinct cosmochemical reservoirs, CC (carbonaceous chondrites and related differentiated meteorites) and NC (the terrestrial planets and all other groups of chondrites and differentiated meteorites), widely interpreted to correspond to the outer and inner solar system, respectively. At some point, however, bulk CC and NC materials became mixed, and several dynamical models offer explanations for how and when this occurred. We use xenoliths of CC materials in polymict ureilite (NC) breccias to test the applicability of such models. Polymict ureilites represent regolith on ureilitic asteroids but contain carbonaceous chondrite-like xenoliths. We present the first 54Cr isotope data for such clasts, which, combined with oxygen and hydrogen isotopes, show that they are unique CC materials that became mixed with NC materials in these breccias. It has been suggested that such xenoliths were implanted into ureilites by outer solar system bodies migrating into the inner solar system during the gaseous disk phase ~3-5 Myr after CAI, as in the "Grand Tack" model. However, combined textural, petrologic, and spectroscopic observations suggest that they were added to ureilitic regolith at ~50-60 Myr after CAI, along with ordinary, enstatite, and Rumuruti-type chondrites, as a result of breakup of multiple parent bodies in the asteroid belt at this time. This is consistent with models for an early instability of the giant planets. The C-type asteroids from which the xenoliths were derived were already present in inner solar system orbits.
Highlights
Focused ion beam/transmission electron microscopy (FIB/TEM) analyses of this “phase” showed Carich, poorly crystalline material containing flakes of a layered phase with a basal spacing of 0.95–0.96 nm, which would be correct for completely dehydrated smectites
Oxygen, and hydrogen isotope analyses, combined with petrologic observations, for several carbonaceous chondrite-like xenoliths in polymict ureilites show that they are derived from the CC isotopic reservoir and became intimately mixed with a variety of NC reservoir materials in ureilitic asteroidal regolith
These xenoliths appear to represent several unique lithologies that have not been sampled as whole meteorites on Earth
Summary
Introduction and BackgroundNucleosynthetic isotope anomalies in a number of elements (including Cr, Ti, Mo, W, Ru, and Ni) show that solar system materials are starkly divided into two suites, referred to as CC and NC (Trinquier et al 2007, 2009; Yin et al 2009; Burkhardt et al 2011; Warren 2011; Budde et al 2016; Kruijer et al 2017; Poole et al 2017; Nanne et al 2019; Sanborn et al 2019; Worsham et al 2019; Williams et al 2020). The NC suite includes terrestrial, lunar, and Martian samples, as well as ordinary, enstatite, and Rumuruti-type chondrites (OC, EC, and RC), some iron meteorite groups, all major groups of achondrites, and some ungrouped meteorites (e.g., Figure 1). This large-scale division suggests that the earliest solar system was characterized by two distinct cosmochemical reservoirs, which remained isolated from one another for a least a few million years (Warren 2011; Budde et al 2016; Kruijer et al 2017; Worsham et al 2019).
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