Petrology, trace element abundances and oxygen isotopic compositions of a compound CAI–chondrule object from Allende
Petrology, trace element abundances and oxygen isotopic compositions of a compound CAI–chondrule object from Allende
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DOI：
10.1016/j.gca.2012.10.039
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发表时间：
2013-02
影响因子：
5
通讯作者：
S. Wakaki;S. Itoh;Tsuyoshi Tanaka;H. Yurimoto
中科院分区：
文献类型：
--
作者：
S. Wakaki;S. Itoh;Tsuyoshi Tanaka;H. Yurimoto
本文报道了在阿连德CV 3球粒陨石中发现的复合CAI球粒陨石WI-025的岩石学、微量元素丰度和氧同位素特征。WI-025是一种不规则形状的包裹体，由三个纹理和化学性质不同的部分组成：内部部分、火成岩边缘和位于这两个部分之间的中间区。内部由钙长石、尖晶石、橄榄石和含铝低钙辉石组成。主要元素化学的内部部分对应于富铝球粒，是细粒尖晶石丰富的CAIs和铁镁球粒之间的中间字符。内部富含富16 O尖晶石（Δ 17 O =−14.2 ~ −24.7），稀土元素组成为II类CAI。这些观察结果表明，内部部分包含由分馏冷凝形成的CAI组分。主要和微量元素化学的内部部分表明，CAI随后同化球粒材料通过部分熔融。部分熔化的最高加热温度估计为约1400°C，类似于B型CAI的最高加热温度。内部橄榄石和低钙辉石（Δ 17 O =−6.3）的氧同位素组成表明部分熔融和球粒同化发生在中等贫16 O的星云气体下。火成边缘在结构和化学上类似于铁镁球粒，完全包围着内部。火成岩边缘的橄榄石和低钙辉石的氧同位素组成与内部的橄榄石和含铝低钙辉石的氧同位素组成没有区别。这些观测结果表明，球粒物质，这是熔化在同一星云气体的内部部分，是吸积到内部部分。中间带代表伴随着火成岩边缘形成的反应带。WI-025的形成过程可概括为：（1）初始CAI形成，（2）部分熔融和球粒同化，（3）火成边缘形成，（4）母岩的次生蚀变。
We report the petrology, trace element abundances and oxygen isotopic characteristics of a compound CAI–chondrule object, WI-025, found in the Allende CV3 chondrite. The WI-025 is an irregularly shaped inclusion consisting of three texturally and chemically distinct portions: the interior portion, the igneous rim and the intermediate zone located between these two portions. The interior portion consists of anorthite, spinel, olivine and Al-bearing low-Ca pyroxene. The major element chemistry of the interior portion corresponds to that of Al-rich chondrules and is of intermediate character between fine-grained spinel-rich CAIs and ferromagnesian chondrules. The interior portion has abundant16O-rich spinel (Δ17O=−14.2 to −24.7) and displays a group II CAI-like REE composition. These observations indicate that the interior portion contains a CAI component formed by fractional condensation. The major and trace element chemistry of the interior portion indicate that the CAI had subsequently assimilated chondrule materials through partial melting. The maximum heating temperature of the partial melting is estimated at approximately 1400°C, similar to the maximum heating temperature of Type-B CAIs. The oxygen isotopic compositions of the olivine and low-Ca pyroxene (Δ17O=−6.3) in the interior portion indicate that the partial melting and chondrule assimilation took place under a moderately16O-poor nebular gas. The igneous rim is texturally and chemically similar to ferromagnesian chondrules and entirely surrounds the interior portion. The oxygen isotopic compositions of the olivine and low-Ca pyroxene in the igneous rim are indistinguishable from those of the interior olivine and Al-bearing low-Ca pyroxenes. These observations indicate that a chondrule material, which was melted in the same nebular gas as the interior portion, was accreted to the interior portion. The intermediate zone represents a reaction zone accompanying the igneous rim formation. The formation history of WI-025 can be summarized by the following processes: (1) original CAI formation, (2) partial melting and chondrule assimilation, (3) igneous rim formation and (4) secondary alteration on the parent body.