An oxygen isotope study of Wark–Lovering rims on type A CAIs in primitive carbonaceous chondrites

Jean-David Bodénan,Natalie A Starkey,Sara S Russell,Ian P Wright,Ian A Franchi

doi:10.1016/j.epsl.2014.05.035

Jean-David Bodénan, Natalie A Starkey + Show 3 more

Open Access

https://doi.org/10.1016/j.epsl.2014.05.035

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Abstract

Calcium–aluminium-rich Inclusions (CAIs) and the thin Wark–Lovering (WL) rims of minerals surrounding them offer a record of the nature of changing conditions during the earliest stages of Solar System formation. Considerable heterogeneity in the gas composition in the immediate vicinity of the proto-Sun had previously been inferred from oxygen isotopic variations in the WL rim of a CAI from Allende (Simon et al., 2011). However, high precision and high spatial resolution oxygen isotope measurements presented in this study show that WL rim and pristine core minerals of individual CAIs from meteorites that had experienced only low degrees of alteration or low grade metamorphism (one from Léoville (reduced CV3), two in QUE 99177 (CR3.0) and two in ALHA 77307 (CO3.0)) are uniformly 16O-rich. This indicates that the previously observed variations are the result of secondary processes, most likely on the asteroid parent body, and that there were no temporal or spatial variations in oxygen isotopic composition during CAI and WL rim formation. Such homogeneity across three groups of carbonaceous chondrites lends further support for a common origin for the CAIs in all chondrites. 16O-poor oxygen reservoirs such as those associated with chondrule formation, were probably generated by UV photo-dissociation involving self-shielding mechanisms and must have occurred elsewhere in outer regions of the solar accretion disk.

Highlights

Understanding Solar System formation can be guided by an appreciation of the various reservoirs present in its early history, and their subsequent interactions and evolution
Our study shows that calcium– aluminium-rich inclusions (CAIs) in unaltered meteorites do not show variations in O isotope composition from the core through to the and are given for reference
High-precision O isotopic spot analyses in the core and WL rims of 5 CAIs from unaltered meteorites belonging to three different groups of carbonaceous chondrites (CO, CV, CR) show that these CAIs and their WL rims formed in a single O isotopic reservoir

Summary

Introduction

Understanding Solar System formation can be guided by an appreciation of the various reservoirs present in its early history, and their subsequent interactions and evolution. In light of its ubiquity (in gas, ice and dust), and involvement in many processes, oxygen cannot be ignored in models for Solar System formation In this regard, the evidence recorded by the oxygen-bearing constituents of primitive meteorites is of critical importance. Of particular interest are calcium– aluminium-rich inclusions (CAIs); the oldest known Solar System objects (4567.2±0.6 Myr; Amelin et al, 2002; 4567.30±0.16 Myr, Connelly et al, 2012) that were formed in a short time interval (Larsen et al, 2011; MacPherson et al, 2012). They are often surrounded by thin mono- or bi-mineralic rims known as Wark–

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