Oxygen Isotope Measurements of a Rare Murchison Type a Cai and Its Rim

Introduction: Ca-, Al-rich inclusions (CAIs) from CV chondrites commonly show oxygen isotope heterogeneity among different mineral phases within individual inclusions reflecting the complex history of CAIs in both the solar nebula and/or parent bodies [1]. The degree of isotopic exchange is typically mineralspecific, yielding O-rich spinel, hibonite and pyroxene and O-depleted melilite and anorthite. Recent work [1] demonstrated large and systematic variations in oxygen isotope composition within the margin and Wark-Lovering rim of an Allende Type A CAI. These variations suggest that some CV CAIs formed from several oxygen reservoirs and may reflect transport between distinct regions of the solar nebula or varying gas composition near the proto-Sun. Oxygen isotope compositions of CAIs from other, less-altered chondrites show less intra-CAI variability and O-rich compositions [2-3]. The record of intraCAI oxygen isotope variability in CM chondrites, which commonly show evidence for low-temperature aqueous alteration, is less clear, in part because the most common CAIs found in CM chondrites are mineralogically simple (hibonite ± spinel or spinel ± pyroxene) and are composed of minerals less susceptible to O-isotopic exchange. No measurements of the oxygen isotope compositions of rims on CAIs in CM chondrites have been reported. Here, we present oxygen isotope data from a rare, Type A CAI from the Murchison meteorite, MUM-1. The data were collected from melilite, hibonite, perovskite and spinel in a traverse into the interior of the CAI and from pyroxene, melilite, anorthite, and spinel in the Wark-Lovering rim. Our objectives were to (1) document any evidence for intra-CAI oxygen isotope variability; (2) determine the isotopic composition of the rim minerals and compare their composition(s) to the CAI interior; and (3) compare the MUM-1 data to oxygen isotope zoning profiles measured from CAIs in other chondrites. MUM-1 Petrography: The petrography of MUM1 (MUrchison Melilite) was previously described by MacPherson et al. [4], who reported that it is one of three small angular fragments so similar in texture and composition, they may be pieces of a single broken inclusion. MUM-1 is similar in mineralogy and composition to Allende compact Type A CAIs. The interior of MUM-1, away from the layered rim, consists primarily of blocky melilite crystals (~150 μm in length) that enclose euhedral spinel crystals (~20 μm), euhedral hibonite blades (~30-40 μm in length) and irregularlyshaped perovskite grains (~1-10 μm) (Fig. 1). Spinel tends to be slightly rounded and, in places, forms long chains. Hibonite and perovskite are sparse in the interior and are closely associated with spinel.