A petrographic, chemical, and isotopic study of calcium-aluminum-rich inclusions and aluminum-rich chondrules from the Axtell (CV3) chondrite

Abstract

Petrographic, compositional, and isotopic characteristics were studied for three calcium-aluminum-rich inclusions (CAIs) and four plagioclase-bearing chondrules (three of them Al-rich) from the Axtell (CV3) chondrite. All seven objects have analogues in Allende (CV3) and other primitive chondrites, yet Axtell, like most other chondrites, contains a distinctive suite of CAIs and chondrules. In common with Allende CAIs, CAIs in Axtell exhibit initial ^(26)Al/^(27)Al ratios ((^(26)Al/^(27)Al)0) ranging from ∼5 × 10^(−5) to <1.1 × 10^(−5), and plagioclase-bearing chondrules have (^(26)Al/^(27)Al)0 ratios of ∼3 × 10^(−6) and lower. One type-A CAI has the characteristics of a FUN inclusion. The Al-Mg data imply that the plagioclase-bearing chondrules began to form >2 Ma after the first CAIs. As in other CV3 chondrites, some objects in Axtell show evidence of isotopic disturbance. Axtell has experienced only mild thermal metamorphism (<600 °C), probably not enough to disturb the Al-Mg systematics. Its CAIs and chondrules have suffered extensive metasomatism, probably prior to final accretion. These data indicate that CAIs and chondrules in Axtell (and other meteorites) had an extended history of several million years before their incorporation into the Axtell parent body. These long time periods appear to require a mechanism in the early solar system to prevent CAIs and chondrules from falling into the Sun via gas drag for several million years before final accretion. We also examined the compositional relationships among the four plagioclase-bearing chondrules (two with large anorthite laths and two barred-olivine chondrules) and between the chondrules and CAIs. Three processes were examined: (1) igneous differentiation, (2) assimilation of a CAI by average nebular material, and (3) evaporation of volatile elements from average nebular material. We find no evidence that igneous differentiation played a role in producing the chondrule compositions, although the barred olivine compositions can be related by addition or subtraction of olivine. Methods (2) and (3) could have produced the composition of one chondrule, AXCH-1471, but neither process explains the other compositions. Our study indicates that plagioclase-bearing objects originated through a variety of processes.

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