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

作者：Ugiura Naiji^*; Krot Alexander N

来源：Meteoritics & Planetary Science, 2007, 42(7-8): 1183-1195.

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 Mg-26 excesses corresponding to initial Al-26/Al-27 ratios between similar to 5 x 10(-5) and similar to 7 x 10(-5), suggesting that formation of the Acfer 094 CAIs may have lasted for similar to 300,000 years. Four CAIs show no evidence for radiogenic Mg-26; three of these inclusions (a corundum-rich, a grossite-rich, and a pyroxene-hibonite spherule CAI) are very refractory objects and show deficits in Mg-26, suggesting that they probably never contained Al-26. The fourth object without evidence for radiogenic Mg-26 is an anorthite-rich, igneous (type C) CAI that could have experienced late-stage melting that reset its Al-Mg systematics. Significant excesses in Mg-26 were observed in two chondrules. The inferred Al-26/Al-27 ratios in these two chondrules are (10.3 +/- 7.4) x 10(-6) and (6.0 +/- 3.8) x 10(-6) (errors are 2 sigma), suggesting formation 1.6(-0.6)(+1.2) and 2.2(-0.3)(+0.4) Myr after CAIs with the canonical Al-26/Al-27 ratio of 5 x 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.

• 出版日期2007-8