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 δ17,18O = ∼−50‰. The initial ²⁶Al/²⁷Al ratios are calculated to be 5.6 ± 0.9 (2σ) × 10⁻⁵. These values are equivalent to those of AOAs and fine‐grained calcium‐aluminum‐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.