Oxygen isotopic variations in the outer margins and Wark-Lovering rims of refractory inclusions

1Justin I. Simon, 2Jennifer E.P. Matzel, 3Steven B. Simon, 4D. Kent Ross, 2Peter K. Weber, 3,6Lawrence Grossman
1Center for Isotope Cosmochemistry and Geochronology, Astromaterials Research and Exploration Science Division, Exploration, Integration, and Science Directorate, NASA Johnson Space Center, Houston, TX 77058
2Lawrence Livermore National Laboratory, Livermore, CA 94451
3Department of the Geophysical Sciences, 5734 S. Ellis Ave, The University of Chicago, Chicago, IL 60637
4University of Texas at El Paso / Jacobs Technology, Houston, TX 77058
5Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637

Oxygen isotopic variations across the outer margins and Wark-Lovering (WL) rims of a diverse suite of six coarse-grained Types A and B refractory inclusions from both oxidized and reduced CV3 chondrites suggest that CAIs originated from a 16O-rich protosolar gas reservoir and were later exposed to both relatively 17,18O-rich and 16O-rich reservoirs. The O-isotope profiles of CAIs can be explained by changes in the composition of gas near the protoSun or the migration of CAIs through a heterogeneous nebula. Variability within the inclusion interiors appears to have been set prior to WL rim growth. Modeling the isotopic zoning profiles as diffusion gradients between inclusion interiors and edges establishes a range of permissible time-temperature combinations for their exposure in the nebula. At mean temperatures of 1400 K, models that match the isotope gradients in the inclusions yield timescales ranging from 5×103 to 3×105 years. Assuming CAIs originated with a relatively 16O-rich (protosolar) isotopic composition, differences among the melilite interiors and the isotopic gradients in their margins imply the existence of a number of isotopically distinct reservoirs. Evidence at the edges of some CAIs for subsequent isotopic exchange may relate to the beginning of rim formation. In the WL rim layers surrounding the interiors, spinel is relatively 16O-rich but subtly distinct among different CAIs. Melilite is often relatively 16O-poor, but rare relatively 16O-rich grains also exist. Pyroxene generally exhibits intermediate O-isotope compositions and isotopic zoning. Olivine in both WL and accretionary rims, when present, is isotopically heterogeneous. The extreme isotopic heterogeneity among and within individual WL rim layers and in particular, the observed trends of outward 16O-enrichments, suggest that rims surrounding CAIs contained in CV3 chondrites, like the inclusions themselves, formed from a number of isotopically distinct gas reservoirs. Collectively, these results support numerical protoplanetary disk models in which CAIs were transported between several distinct nebular reservoirs multiple times prior to accretion onto a parent body.

Reference
Simon JI, Matzel JEP, Simon SB, Ross DK, Weber PK, Grossman L (2016) Oxygen isotopic variations in the outer margins and Wark-Lovering rims of refractory inclusions. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.04.025]
Copyright Elsevier

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