| Summary: | Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum-rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum-rich chondrules and two plagioclase fragments measured for ²â¶Al-²â¶Mg systematics, only one aluminum-rich chondrule contains excess ²â¶Mg from the in situ decay of ²â¶Al; the inferred initial ratio (²â¶Al/²â·Al)â‚’ = (6.8 ± 2.4) × 10â»â¶ is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites. The oxygen isotopic compositions of five aluminum-rich chondrules and one plagioclase fragment define a line of slope Ëœ0.6 ± 0.1 on a three-oxygen-isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more ¹â¶O-rich compositions with a range in δ¹â¸O of about Ëœ12‰. Based on their oxygen isotopic compositions, aluminum-rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium-aluminum-rich inclusions (CAIs). Relative to their counterparts from unequilibrated ordinary chondrites, aluminum-rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess ²â¶Mg from the in situ decay of ²â¶Al, probably resulting from higher degrees of equilibration and isotopic exchange during post-crystallization metamorphism. However, the presence of ²â¶Al-bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that ²â¶Al was at least approximately homogeneously distributed across the chondrite-forming region. The manuscript was improved by reviews from S. Mostefaoui, M. Wadhwa, and Associate Editor E. Zinner. We thank the Antarctic Meteorite Working Group and the meteorite curation facility at the NASA Johnson Space Center for providing many of ...
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