The eight iron meteorites currently classified as belonging to the IIC group were characterized with respect to the compositions of 21 siderophile elements. Several of these meteorites were also characterized for mass independent isotopic compositions of Mo, Ru and W. Chemical and isotopic data for one, Wiley, indicate that it is not a IIC iron meteorite and should be reclassified as ungrouped. The remaining seven IIC iron meteorites exhibit broadly similar bulk chemical and isotopic characteristics, consistent with an origin from a common parent body. Variations in highly siderophile element (HSE) abundances among the members of the group can be well accounted for by a fractional crystallization model with all the meteorites crystallizing between ~10 and ~26% of the original melt, assuming initial S and P concentrations of 8 wt.% and 2 wt.%, respectively. Abundances of HSE estimated for the parental melt suggest a composition with chondritic relative abundances of HSE ~6 times higher than in bulk carbonaceous chondrites, consistent with the IIC irons sampling a parent body core comprising ~17% of the mass of the body. Radiogenic 182W abundances of two group IIC irons, corrected for a nucleosynthetic component, indicate a metal-silicate segregation age of 3.2 ± 0.5 Myr subsequent to the formation of Calcium-Aluminum-rich Inclusions (CAI). When this age is coupled with thermal modeling, and assumptions about the Hf/W of precursor materials, a parent body accretion age of 1.4 ± 0.5 Myr (post-CAI) is obtained. The IIC irons and Wiley have 100Ru mass independent "genetic" isotopic compositions that are identical to other irons with so-called carbonaceous chondrite (CC) type genetic affinities, but enrichments in 94,95,97Mo and 183W that indicate greater s-process deficits relative to most known CC iron meteorites. If the IIC irons and Wiley are of the CC type, this indicates variable s-process deficits within the CC reservoir, similar to the s-process variability within the NC reservoir observed for iron meteorites. Nucleosynthetic models indicate that Mo and 183W s-process variability should correlate with Ru isotopic variability, which is not observed. This may indicate the IIC irons and Wiley experienced selective thermal processing of nucleosynthetic carriers, or are genetically distinct from the CC and NC precursor materials.

中文翻译：

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IIC 族铁陨石的遗传学、年龄和结晶历史


目前归类为 IIC 组的 8 颗铁陨石的特征在于 21 种亲铁元素的组成。其中一些陨石还具有与质量无关的 Mo、Ru 和 W 同位素组成的特征。Wiley 陨石的化学和同位素数据表明它不是 IIC 铁陨石，应重新分类为未分类。其余七颗 IIC 铁陨石表现出大致相似的大量化学和同位素特征，与共同母体的起源一致。该组成员中高亲铁元素 (HSE) 丰度的变化可以通过分级结晶模型很好地解释，假设初始 S 和 P 浓度为 8，所有陨石的结晶量在原始熔体的约 10% 到约 26% 之间。分别为 wt.% 和 2 wt.%。对母体熔体估计的 HSE 丰度表明，其球粒状 HSE 相对丰度比块状碳质球粒陨石高约 6 倍，这与对包含约 17% 主体质量的母体核心取样的 IIC 铁一致。两组 IIC 铁的放射性 182W 丰度（针对核合成成分进行校正）表明，在富含钙铝夹杂物 (CAI) 形成后，金属硅酸盐偏析年龄为 3.2 ± 0.5 Myr。当这个年龄与热模型以及关于前体材料的 Hf/W 的假设相结合时，获得了 1.4 ± 0.5 Myr（CAI 后）的母体吸积年龄。 IIC 铁杆和 Wiley 具有 100Ru 质量独立的“遗传”同位素组成，与具有所谓碳质球粒陨石 (CC) 型遗传亲和力的其他铁杆相同，但 94,95,97Mo 和 183W 的富集表明更大的 s 过程缺陷相对于大多数已知的 CC 铁陨石。如果 IIC 铁和 Wiley 属于 CC 类型，则这表明 CC 储层内存在可变的 s 过程缺陷，类似于在铁陨石中观察到的 NC 储层内的 s 过程变异性。核合成模型表明 Mo 和 183W s 过程变异性应与 Ru 同位素变异性相关，但尚未观察到。这可能表明 IIC 铁杆和 Wiley 经历了核合成载体的选择性热处理，或者在遗传上与 CC 和 NC 前体材料不同。