26Al-26Mg systematics of Ca-Al-rich inclusions, amoeboid olivine aggregates, and chondrules from the ungrouped carbonaceous chondrite Acfer 094

Abstract

We report in situ magnesium isotope measurements of 7 porphyritic magnesium-rich (type I) chondrules, 1 aluminum-rich chondrule, and 16 refractory inclusions (14 Ca-Al-rich inclusions [CAIs] and 2 amoeboid olivine aggregates [AOAs]) from the ungrouped carbonaceous chondrite Acfer 094 using a Cameca IMS 6f ion microprobe. Both AOAs and 9 CAIs show radiogenic 26Mg excesses corresponding to initial 26Al/27Al ratios between ~5 x  10^(−5) and ~7 x  10^(−5), suggesting that formation of the Acfer 094 CAIs may have lasted for ~300,000 years. Four CAIs show no evidence for radiogenic 26Mg; three of these inclusions (a corundum-rich, a grossite-rich, and a pyroxene-hibonite spherule CAI) are very refractory objects and show deficits in 26Mg, suggesting that they probably never contained 26Al. The fourth object without evidence for radiogenic 26Mg is an anorthite-rich, igneous (type C) CAI that could have experienced late-stage melting that reset its Al-Mg systematics. Significant excesses in 26Mg were observed in two chondrules. The inferred 26Al/27Al ratios in these two chondrules are (10.3 ± 7.4) × 10^(−6) and (6.0 ± 3.8) × 10^(−6) (errors are 2σ), suggesting formation and Myr after CAIs with the canonical 26Al/27Al ratio of 5 × 10^(−5). These age differences are consistent with the inferred age differences between CAIs and chondrules in primitive ordinary (LL3.0-LL3.1) and carbonaceous (CO3.0) chondrites.