Relationship among O, Mg isotopes and the petrography of two spinel-bearing compound chondrules
Relationship among O, Mg isotopes and the petrography of two spinel-bearing compound chondrules
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DOI：
10.1016/s0016-7037(03)00172-8
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发表时间：
2003-10
影响因子：
5
通讯作者：
S. Maruyama;H. Yurimoto
中科院分区：
文献类型：
--
作者：
S. Maruyama;H. Yurimoto
采用扫描电镜和二次离子质谱分析了阿连德CV球粒陨石中2个含尖晶石球粒的岩石学性质和O、Al-Mg同位素组成。条状橄榄石（BO）球粒中的粗尖晶石颗粒在16o中含量较低（Δ17O ~−5‰；Δ17O = Δ17O ~ 0.52 δ18O），而在复合球粒中富含斜长石的球粒中，小尖晶石颗粒极富16o （Δ17O ~−17‰），尖晶石具有强烈的锯齿状特征。富斜长石球粒中尖晶石的岩石学特征和16o富集特征表明，源自粗粒富钙铝包裹体（CAIs）的尖晶石被纳入球粒前体，并在球粒形成事件中幸存下来。各球粒内矿物的16o过量程度与结晶顺序有关。这一证据表明，矿物间O同位素的变化可能是由于富16o的球粒熔体与贫16o的星云气体之间O同位素交换不完全所致。含水蚀变也改变了中间平衡中的o同位素组成。BO球粒中长石介稳态表现出受干扰的Mg-Al等时线，表明BO球粒经历了次生蚀变。复合球粒中富含斜长石的斜长石表现出轻微的26mg过量，对应于（26Al/27Al）0=[4.6±4.0(2σ)] × 10−6，而次生蚀变形成的霞石则没有明显的过量。这些球粒的Al-Mg同位素系统受到Allende母体的水蚀变和热变质作用的干扰。
The petrological properties, and O and Al-Mg isotopic compositions of two spinel-bearing chondrules from the Allende CV chondrite were investigated using scanning electron microscopy and secondary ion mass spectrometry. A coarse spinel grain in a barred-olivine (BO) chondrule is less enriched in16O (Δ17O ∼ −5‰; Δ17O = δ17O – 0.52 δ18O), whereas smaller spinel grains in a plagioclase-rich chondrule member of a compound chondrule are extremely16O-rich (Δ17O ∼ −17‰) and the spinels have a strongly serrated character. The petrological features and16O-enrichments of the spinels in the plagioclase-rich chondrule indicate that spinels originating in coarse-grained Ca-Al-rich Inclusions (CAIs) were incorporated into chondrule precursors and survived the chondrule-forming event. The degree of16O-excesses among minerals within each chondrule is correlated to the crystallization sequences. This evidence suggests that the O isotopic variation among minerals may have resulted from incomplete exchange of O isotopes between16O-rich chondrule melt and16O-poor nebular gas. Aqueous alteration also has changed the O-isotope compositions in the mesostasis. The feldspathic mesostasis in the BO chondrule shows a disturbed Mg-Al isochron indicating that the BO chondrule experienced secondary alteration. While plagioclase in the plagioclase-rich chondrule member of the compound chondrule shows slight26Mg-excesses corresponding to (26Al/27Al)0= [4.6±4.0(2σ)] × 10−6, nepheline formed by secondary alteration shows no detectable excess. The Al-Mg isotopic system of these chondrules was disturbed by aqueous alteration and thermal metamorphism on the Allende parent body.