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

Journal Article · Mon Aug 01 00:00:00 EDT 2016 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2016.04.025· OSTI ID:1326453

Simon, Justin I. ; Matzel, Jennifer E. P.; Simon, Steven B.; Hutcheon, Ian D.; Ross, D. Kent; Weber, Peter K.; Grossman, Lawrence

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. In addition, 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.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC52-07NA27344
• OSTI ID: 1326453
• Alternate ID(s): OSTI ID: 1324514
• Report Number(s): LLNL-JRNL-696173; S0016703716301843; PII: S0016703716301843
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Vol. 186 Journal Issue: C; ISSN 0016-7037
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
58 GEOSCIENCES
37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
oxygen isotopes
CAIs
Wark-Lovering rims
NanoSIMS
protoplanetary disk