|Title:||Aluminum‐Magnesium and Oxygen Isotope Study of Relict Ca‐Al‐rich Inclusions in Chondrules|
|Authors:||A.N. Krot, K.D. McKeegan, G.R. Huss, K. Liffman, S. Sahijpal, I.D. Hutcheon, G. Srinivasan, A. Bischoff, and K. Keil|
|Publication:||Astrophys. Jour., v. 639, p. 1227‐1237.|
|Publish Date:||Mar 2006|
Relict Ca‐Al‐rich inclusions (CAIs) in chondrules crystallized before their host chondrules and were subsequently partly melted together with chondrule precursors during chondrule formation. Like most CAIs, relict CAIs are 16O enriched (Δ17O < −20‰) compared to their host chondrules (Δ17O > −9‰). Hibonite in a relict CAI from the ungrouped carbonaceous chondrite Adelaide has a large excess of radiogenic 26Mg (26Mg*) from the decay of 26Al, corresponding to an initial 26Al/27Al ratio [(26Al/27Al)I] of (3.7 ± 0.5)×10−5; in contrast, melilite in this CAI and plagioclase in the host chondrule show no evidence for 26Mg* [(26Al/27Al)I of <5×10−6]. Grossite in a relict CAI from the CH carbonaceous chondrite PAT 91546 has little 26Mg*, corresponding to a (26Al/27Al)I of (1.7±1.3)×10−6. Three other relict CAIs and their host chondrules from the ungrouped carbonaceous chondrite Acfer 094, CH chondrite Acfer 182, and H3.4 ordinary chondrite Sharps do not have detectable 26Mg* [(26Al/27Al)I <1×10−5,<(4−6)×10−6, and <1.3×10−5, respectively]. Isotopic data combined with mineralogical observations suggest that relict CAIs formed in an 16O−rich gaseous reservoir before their host chondrules, which originated in an 16O−poor gas. The Adelaide CAI was incorporated into its host chondrule after 26Al had mostly decayed, at least 2 Myr after the CAI formed, and this event reset 26Al−26Mg systematics.