Oxygen isotopic compositions of individual minerals in Antarctic micrometeorites: further links to carbonaceous chondrites

We report in situ measurements of oxygen isotopic abundances in individual silicate and oxide minerals from 16 Antarctic micrometeorites (AMMs). The oxygen isotopic compositions of 10 olivine and 11 pyroxene grains are enriched in ¹⁶O relative to terrestrial minerals, and on an oxygen three-isotope diagram they plot on the low δ¹⁸O side of the ¹⁶O mixing line defined by calcium-aluminum-rich inclusions (CAI) from chondritic meteorites. AMM olivine and pyroxene δ¹⁸O values range from -9.9‰ to +8.0‰ and δ¹⁷O ranges from -11.3‰ to +5.5‰, similar to values measured in individual olivine grains and whole chondrules from carbonaceous chondrites. These data indicate that the mineral grains preserve their pre-terrestrial oxygen isotopic compositions, and provide another link between AMMs and carbonaceous chondrites. However, no clear relationship with one single subgroup of carbonaceous chondrite can be established. Based on their textures, crystal chemistries, and oxygen isotopes, some coarse-grained crystalline AMMs could originate from chondrule fragmentation. Whether the remaining mineral grains were formed by igneous or condensation processes is unclear. No clear correlation is observed between isotopic compositions and mineral compositions of AMM olivine grains, suggesting that the FeO- and ¹⁶O-enrichment processes are not coupled in a simple way. Nor are any relatively large ¹⁶O enrichments measured in any of the olivine grains, however two Mg-Al spinels and a melilite grain are ¹⁶O enriched at the level of δ¹⁸O ∼ δ¹⁷O ∼ -40‰. The discovery of an ¹⁶O-enriched melilite grain in AMMs supports the hypothesis that refractory minerals throughout the solar nebula formed from a relatively uniformly ¹⁶O-enriched reservoir. This unique ¹⁶O-rich signature of refractory minerals in primitive solar system materials suggests that they either formed from a widespread ¹⁶O-rich reservoir in the solar nebula, or that an efficient mechanism (such as bipolar outflows) was acting to spread them from a highly localized ¹⁶O-rich region over the early solar nebula.