Amoeboid olivine aggregates (AOAs) in the Efremovka, Leoville and Vigarano (CV3) chondrites: A record of condensate evolution in the solar nebula

Abstract

Amoeboid olivine aggregates (AOAs) in the Efremovka, Leoville, and Vigarano CV3 chondrites were studied using petrographic and microanalytical techniques to evaluate the origins of these inclusions and their relationships to other chondrite components. Our data support the idea that the inclusions formed by dust growth in the solar nebula as condensates and that they preserve a record of mineralogical and chemical evolution. Spinel-rich nodules and ribbons in AOAs often show Group II trace element patterns and formed either by fractional condensation or by condensation following partial vaporization. The dominant olivine component in AOAs can be explained as a condensate produced in the same way, but with fractionation occurring at a lower temperature. Other spinel-rich material in AOAs appears to be a vaporization residue. Ca-rich pyroxene+anorthite patches in AOAs show chemical signatures similar to those observed for spinel- or melilite-bearing regions but with lower refractory element and higher Mg and Si contents, supporting the idea that the patches formed by reaction between CAI-like material and Mg–Si-rich gas. Different trace element patterns (Group II and non-Group II refractory) are sometimes found for Al-rich regions (Ca-pyroxene+anorthite, or melilite-bearing) in a given AOA, suggesting that AOAs agglomerated from materials that formed under different conditions and that they did not subsequently homogenize. AOAs appear to have originated in a system with near-canonical solar composition and a low pressure gas (∼10−6–10−4bar) over a range of temperatures (mainly ∼1200–1384K) under somewhat non-equilibrium conditions. Relative to predicted equilibrium condensates, most AOAs show an apparent paucity of metal, possibly reflecting differential, density-dependent sorting between olivine and metal grains during aggregate formation. Trace element data are consistent with a simple model involving melting and igneous fractionation of AOA-like olivine to produce Type I chondrule olivine, but other data suggest that open-system melting of chondrules occurred in a different gaseous environment than that which produced AOAs.