Oxygen isotopic heterogeneities in refractory inclusions in the ungrouped carbonaceous chondrite Acfer 094

Abstract

Oxygen isotopic compositions of minerals in three Ca-Al-rich inclusions (CAIs), one amoeboid olivine aggregate (AOA) and one Al-rich chondrule (ARC) from the pristine ungrouped carbonaceous chondrite Acfer 094 were analyzed by secondary ion mass spectrometry (SIMS), including conventional spot analyses and O-isotope imaging. Most of the ARC minerals analyzed in this study are ¹⁶O-poor (Δ¹⁷O ≥ −5.4‰), with one outlier in high-Ca pyroxene (Δ¹⁷O = −10.6 ± 2.8‰), indicating that if the ARC precursors formed initially in an ¹⁶O-rich setting, isotopic compositions were mostly reset during chondrule melting in an ¹⁶O-poor environment. The CAIs and AOA analyzed are dominated by ¹⁶O-rich compositions, consistent with previous work, but partial isotopic resetting to ¹⁶O-poor compositions has been identified. Melilite with a moderately ¹⁶O-depleted composition (Δ¹⁷O = −15.7 ± 3.0‰) was identified in an AOA, and ¹⁶O-poor diopside (Δ¹⁷O = −1.9 ± 2.5‰) was identified as the outermost layer of a Wark–Lovering-like rim of an ¹⁶O-rich CAI (Δ¹⁷O ranges from −18 to −22 ± 2.5‰). The diopside layer is bounded by an inner rim of anorthite replacing melilite, which is in turn bounded by the grossite-hibonite-perovskite-spinel-bearing core of the CAI. Isotopic imaging shows that the diopside/anorthite boundary coincides with a steep gradient in O isotopic composition. Based on modeling of O diffusion in the temperature range of 1400–1500 K, thermal events that formed the diopside and anorthite rim layers were limited to durations of no more than approximately 100 days and were probably much shorter. Given the weak metamorphic alteration of Acfer 094, the partial to nearly complete O-isotope resetting of AOA, CAI, and ARC minerals analyzed in this study occurred by short-term thermal events in the solar nebula prior to the formation of the Acfer 094 parent body. Therefore, the isotopic variations identified in this study show that at least some refractory materials were transported from ¹⁶O-rich environments, where initial crystallization took place, to ¹⁶O-poor environments in the solar nebula, where subsequent crystallization and/or isotopic resetting occurred.