Formation of chondrules in a moderately high dust enriched disk: evidence from oxygen isotopes of chondrules from the Kaba CV3 chondrite

摘要

Oxygen three-isotope analysis by secondary ion mass spectrometry of chondrule olivine and pyroxene in combination with electron microprobe analysis were carried out to investigate 24 FeO-poor (type I) and 2 FeO-rich (type II) chondrules from the Kaba (CV) chondrite. The Mg#’s of olivine and pyroxene in individual chondrules are uniform, which confirms that Kaba is one of the least thermally metamorphosed CV3 chondrites. The majority of chondrules in Kaba contain olivine and pyroxene that show indistinguishable Δ¹⁷O values (= δ¹⁷O − 0.52 × δ¹⁸O) within analytical uncertainties, as revealed by multiple spot analyses of individual chondrules. One third of chondrules contain olivine relict grains that are either ¹⁶O-rich or ¹⁶O-poor relative to other indistinguishable olivine and/or pyroxene analyses in the same chondrules. Excluding those isotopically recognized relicts, the mean oxygen isotope ratios (δ¹⁸O, δ¹⁷O, and Δ¹⁷O) of individual chondrules are calculated, which are interpreted to represent those of the final chondrule melt. Most of these isotope ratios plot on or slightly below the primitive chondrule mineral (PCM) line on the oxygen three-isotope diagram, except for the pyroxene-rich type II chondrule that plots above the PCM and on the terrestrial fractionation line. The Δ¹⁷O values of type I chondrules range from ~ −8‰ to ~ −4‰; the pyroxene-rich type II chondrule yields ~0‰, the olivine-rich type II chondrule ~ −2‰. In contrast to the ungrouped carbonaceous chondrite Acfer 094, the Yamato 81020 CO3, and the Allende CV3 chondrite, type I chondrules in Kaba only possess Δ¹⁷O values below −3‰ and a pronounced bimodal distribution of Δ¹⁷O values, as evident for those other chondrites, was not observed for Kaba.Investigation of the Mg#-Δ¹⁷O relationship revealed that Δ¹⁷O values tend to increase with decreasing Mg#’s, similar to those observed for CR chondrites though data from Kaba cluster at the high Mg# (>98) and the low Δ¹⁷O end (−6‰ and −4‰). A mass balance model involving ¹⁶O-rich anhydrous dust (Δ¹⁷O = −8‰) and ¹⁶O-poor water ice (Δ¹⁷O = +2‰) in the chondrule precursors suggests that type I chondrules in Kaba would have formed in a moderately high dust enriched protoplanetary disk at relatively dry conditions (~50–100× dust enrichment compared to Solar abundance gas and less than 0.6× ice enhancement relative to CI chondritic dust). The olivine-rich type II chondrule probably formed in a disk with higher dust enrichment (~2000× Solar).