Petrographic and isotopic investigations of two unusual Ca-Al-rich inclusions from primitive CO3 chondrites

摘要: Abstract We performed high-precision SIMS (secondary ion mass spectrometry) 26Al-26Mg and oxygen isotope analyses of two unique CAIs, “Mesquite” “Y24”, found in the CO3.05 chondrites Northwest Africa 7892 Yamato-81020, respectively. Mesquite is unusually large (∼5 × 3 mm) for a CAI from any CO chondrite exhibits layered texture comprising melilite-rich core surrounded by hibonite- spinel-rich mantle layers semi-continuous spinel-dominated rim. The Y24 stands out because its distinct mineralogy: grossite, hibonite, spinel are accompanied abundant ultra-refractory-element-rich phases such as warkite, kangite, perovskite. Silicates absent Y24. Negatively fractionated δ25Mg values suggest that inclusion whole was never molten and, hence, represents an aggregate condensates. relatively grain sizes melilite (up to ∼300 µm) most likely result solid-state recrystallization coarsening course heating event occurring solar nebula. This event, however, did not disturb Al-Mg systematics Mesquite. Regardless their position within analyzed, spots analyzed plot on single isochron characterized initial 26Al/27Al (4.95 ± 0.08) × 10–5 δ26Mg*0 –0.14 ± 0.05‰. this ratio corresponds formation nebula slightly heterogeneous with respect Mg isotopes. Spinel rim uniform Δ17O (∼–25‰); contrast, hibonite layers, albeit also 16O-rich, show variable ratios (Δ17O ∼ –15‰ –23‰), which would be consistent condensation gas quickly-changing compositions. 16O-poor composition (Δ17O ∼ –1‰ 0‰) could exchange gas, perhaps during caused or fluid parent body. Abundant calcite, phyllosilicates, sodalite witnesses late-stage low-temperature alteration CAI; calcite phyllosilicates terrestrial origin, but have formed Inclusion irregularly-shaped, indicating origin. Completely enclosing other phases, warkite forms matrix Y24, simultaneous growth hibonite. Possibly, replacing grossite both minerals gradually cooling before silicates condensed. indicate occurred 16O-rich when (5.4 ± 1.0) × 10–5. Oxygen body nebular setting compositions kangite.