Lithium- and oxygen-isotope compositions of chondrule constituents in the Allende meteorite
Lithium- and oxygen-isotope compositions of chondrule constituents in the Allende meteorite
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DOI：
10.1016/j.gca.2019.02.038
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发表时间：
2019-05
影响因子：
5
通讯作者：
T. Kunihiro;T. Ota;E. Nakamura
中科院分区：
文献类型：
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作者：
T. Kunihiro;T. Ota;E. Nakamura
我们报告原位离子探针分析的锂和O同位素组成的橄榄石，低钙辉石，高钙辉石，和球粒中/斜长石在9个球粒从阿连德CV 3球粒陨石。根据它们的矿物学和O-同位素组成，我们推断球粒中稳态/斜长石和铁橄榄石轮辋广泛修改或形成的交代蚀变和变质作用的阿连德母小行星。我们排除这些矿物，以确定锂和O和橄榄石和低钙辉石的球粒和他们的火成岩轮辋的化学成分之间的相关性。根据橄榄石的O同位素组成，将9个球粒分为3组。第1组和第2组球粒中橄榄石（Fo>65）的平均Δ 17 O分别为−5.3 ± 0.4和−6.2 ± 0.4‰。第3组球粒的特征是富含16 O的残留颗粒，其橄榄石的Δ 17 O介于−23.7至−6.2‰之间。在第1组橄榄石中，随着Fa含量的增加，δ 7 Li的变化变小，δ 7 Li接近全岩值（2.4‰; Seitz等人，2012年），这表明几乎完全的锂同位素平衡。在第2组和第3组橄榄石中，δ 7 Li的变化有限，即使具有显著的Fa含量范围。我们的结论是，1组球粒中橄榄石的锂同位素组成的修改不是由小行星的过程，但由火成环的形成过程，因此球粒橄榄石保留了锂同位素组成的原太阳星云收购。在第3族球粒PO-8的橄榄石中，我们观察到O和Li同位素之间的相关性：在残留的富含16 O的橄榄石颗粒中，Δ 17 O为−25至−20‰，δ 7 Li的范围为−23至−3‰;在Δ 17 O> −20‰的橄榄石颗粒中，δ 7 Li几乎恒定（−8 ± 4‰）。根据低钙辉石的锂同位素组成，推测存在δ 7 Li <$-11‰的气体储集层，这些辉石是在主球粒和岩浆边缘结晶过程中由熔体形成的。
We report in situ ion-microprobe analyses of Li- and O-isotope compositions for olivine, low-Ca pyroxene, high-Ca pyroxene, and chondrule mesostasis/plagioclase in nine chondrules from the Allende CV3 chondrite. Based on their mineralogy and O-isotope compositions, we infer that the chondrule mesostasis/plagioclase and ferroan olivine rims were extensively modified or formed during metasomatic alteration and metamorphism on the Allende parent asteroid. We excluded these minerals in order to determine the correlations between Li and both O and the chemical compositions of olivines and low-Ca pyroxenes in the chondrules and their igneous rims. Based on the O-isotope composition of the olivines, nine chondrules were divided into three groups. Average Δ17O of olivines (Fo>65) in group 1 and 2 chondrules are −5.3 ± 0.4 and −6.2 ± 0.4‰, respectively. Group 3 chondrules are characterized by the presence of16O-rich relict grains and the Δ17O of their olivines range from −23.7 to −6.2‰. In group 1 olivines, as Fa content increases, variation of δ7Li becomes smaller and δ7Li approaches the whole-rock value (2.4‰; Seitz et al., 2012), suggesting nearly complete Li-isotope equilibration. In group 2 and 3 olivines, variation of δ7Li is limited even with a significant range of Fa content. We conclude that Li-isotope compositions of olivine in group 1 chondrules were modified not by an asteroidal process but by an igneous-rim formation process, thus chondrule olivines retained Li-isotope compositions acquired in the protosolar nebula. In olivines of the group 3 chondrule PO-8, we observed a correlation between O and Li isotopes: In relict16O-rich olivine grains with Δ17O of ∼−25 to −20‰, δ7Li ranges from −23 to −3‰; in olivine grains with Δ17O > −20‰, δ7Li is nearly constant (−8 ± 4‰). Based on the Li-isotope composition of low-Ca pyroxenes, which formed from melt during the crystallization of host chondrules and igneous rims, the existence of a gaseous reservoir with a δ7Li ∼ −11‰ is inferred.