Oxygen isotope systematics of chondrule phenocrysts from the CO3.0 chondrite Yamato 81020: Evidence for two distinct oxygen isotope reservoirs

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• 作者：Travis J. Tenner^a ; ^{tenner@wisc.edu} ; Takayuki Ushikubo^a ; Erika Kurahashi^b ; Noriko T. Kita^a ; Hiroko Nagahara^c
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2013
• 出版时间：1 February, 2013
• 年：2013
• 卷：102
• 期：Complete
• 页码：226-245
• 全文大小：2156 K

文摘

High-precision oxygen three-isotope measurements of olivine and pyroxene were performed on 33 chondrules in the Yamato 81020 CO3.0 chondrite by secondary ion mass spectrometry. In chondrules where oxygen isotopes were measured in both olivine and pyroxene, the majority of grains have similar values, indicating co-magmatic crystallization. However, many chondrules contain relict grains with unique oxygen isotope ratios. A striking feature of Yamato 81020 chondrules is a bimodal distribution of oxygen isotope ratios, as those with Mg# >97 phenocrysts range in ¦¤¹⁷O from ?4.8¡ë to ?6.5¡ë (¡°?5.5¡ë¡± group), and those with Mg# 96-36 phenocrysts have ¦¤¹⁷O values of ?2.1¡ë to ?3.0¡ë (¡°?2.5¡ë¡± group). A single Mg# 99.6 barred olivine chondrule has a ¦¤¹⁷O of ?3.3¡ë. We discuss that ¦¤¹⁷O ¡«?5.5¡ë chondrules are derivative of a reservoir with limited dust enrichment (100¡Á Solar System), which yielded a relatively reduced chondrule-forming environment. In contrast, the ¦¤¹⁷O ¡«?2.5¡ë chondrules may have been influenced by ¹⁶O-poor H₂O ice that sublimed and then homogenized with precursor material. The addition of H₂O, when combined with high dust enrichment (1000¡Á Solar System) and greater bulk Fe content, could have induced an oxidized environment at high temperatures, forming Mg# 96-36 chondrules. Among the 33 chondrules studied, the Al-Mg relative ages of 20 had been obtained previously. Comparing the oxygen isotope ratios and the ²⁶Al ages of these chondrules, it is likely that the ¡°?5.5¡ë¡± and ¡°?2.5¡ë¡± oxygen isotope reservoirs existed contemporaneously. This implies that the snow line was spatially fixed during chondrule formation, and separated the CO chondrite accretion region into two distinct volumes of precursors.