Petrographic constraints on the formation of silica-rich igneous rims around chondrules in CR chondrites
Petrographic constraints on the formation of silica-rich igneous rims around chondrules in CR chondrites
复制标题
CR 球粒陨石中球粒周围富硅火成岩边缘形成的岩相学限制
DOI：
10.1111/maps.14051
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发表时间：
2023
影响因子：
2.2
通讯作者：
Smith A
中科院分区：
文献类型：
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作者：
Smith A
在CR （Renazzo - like）球粒陨石群中，许多球粒具有连续的火成岩边缘（IR）层，外层含有二氧化硅矿物和/或富二氧化硅玻璃（富二氧化硅火成岩边缘，SIRs）。SIR形成的模型包括：(1)富硅尘埃在固体球粒表面的吸积，随后加热和冷却；(2)部分熔融球粒表面的SiO（气体）冷凝。我们基于对5个南极CR球粒陨石的岩石学研究来评估这些模型，这些陨石经历了最小的次生蚀变。我们利用定量波长色散光谱对矿物和玻璃进行了电子探针分析，并利用拉曼光谱鉴定了二氧化硅多晶。常见的SIRs含有二氧化硅、低钙辉石、富钙辉石、铁、镍金属、±玻璃±斜长石±稀有橄榄石。我们还描述了近单矿物SIRs，其中在球粒的外缘出现一个狭窄的方英石带。所有的硅晶都是方英石，除了一种硅晶硅是由三晶石组成的。一些轮辋含有富二氧化硅玻璃（约80 wt% SiO2），但不含二氧化硅矿物。球粒和SIRs之间的尖锐界面和成分边界等特征表明SIRs是由固体前体形成的。考虑二氧化硅多晶态的稳定性场和计算的液相温度表明，SIRs在有限的时间内加热到bb0 - 1500°C，然后快速冷却，类似于球粒形成的条件。我们推断，在CR球粒形成区，相同的加热机制被重复多次，而星云气体的化学成分演化为高度分馏的富二氧化硅成分。
In the CR (Renazzo‐like) chondrite group, many chondrules have successive igneous rim (IR) layers, with an outer layer that contains a silica mineral and/or silica‐rich glass (silica‐rich igneous rims, SIRs). Models for SIR formation include (1) accretion of Si‐rich dust onto solid chondrule surfaces, followed by heating and cooling and (2) condensation of SiO(gas)onto the surface of partially molten chondrules. We evaluate these models, based on a petrographic study of five Antarctic CR chondrites that have undergone minimal secondary alteration. We obtained electron microprobe analyses of minerals and glass with quantitative wavelength‐dispersive spectroscopy mapping, and identified silica polymorphs with Raman spectroscopy. Common SIRs contain silica, low‐Ca pyroxene, Ca‐rich pyroxene, Fe,Ni metal, ± glass ± plagioclase ± rare olivine. We also describe near‐monomineralic SIRs where a narrow zone of cristobalite occurs at the outer edge of the chondrule. All crystalline silica is cristobalite, except for one SIR that consists of tridymite. Some rims contain silica‐rich glass (>80 wt% SiO2) but no silica mineral. Features such as sharp interfaces and compositional boundaries between chondrules and SIRs indicate that SIRs were formed from solid precursors. Consideration of the stability fields of silica polymorphs and computed liquidus temperatures indicates that SIRs were heated to >1500°C for limited time periods, followed by rapid cooling, similar to conditions for chondrule formation. We infer that in the CR chondrule formation region, the same heating mechanism was repeated multiple times while the chemical composition of the nebular gas evolved to highly fractionated silica‐rich compositions.