Oxygen and magnesium isotopic compositions of amoeboid olivine aggregates from the Semarkona LL3.0 chondrite

Abstract

Amoeboid olivine aggregates (AOAs) in the LL3.0 Semarkona chondrite have been studied by secondary ion mass spectrometry. The AOAs mainly consist of aggregates of olivine grains with interstitial Al-Ti-rich diopside and anorthite. Oxygen-isotopic compositions of all phases are consistently enriched in 16O, with delta-17,18O = ~-50 ppm. The initial 26Al/27Al ratios are calculated to be 5.6 +/- 0.9 (2-sigma) x 10^(-5). These values are equivalent to those of AOAs and fine-grained calciumaluminum- rich inclusions (FGIs) from pristine carbonaceous chondrites. This suggests that AOAs in ordinary chondrites formed in the same 16O-rich calcium-aluminum-rich inclusion (CAI)-forming region of the solar nebula as AOAs and FGIs in carbonaceous chondrites, and subsequently moved to the accretion region of the ordinary chondrite parent body in the solar nebula.