Worsham Emily A, Bermingham Katherine R, Walker Richard J
Department of Geology, University of Maryland, College Park, MD 20742, USA.
Geochim Cosmochim Acta. 2016 Aug 1;188:261-283. doi: 10.1016/j.gca.2016.05.019. Epub 2016 May 21.
Siderophile trace element abundances and the Re-Os isotopic systematics of the metal phases of 58 IAB complex iron meteorites were determined in order to investigate formation processes and how meteorites within chemical subgroups may be related. Close adherence of Re-Os isotopic data of most IAB iron meteorites to a primordial isochron indicates that the siderophile elements of most members of the complex remained closed to elemental disturbance soon after formation. Minor, presumably late-stage open-system behavior, however, is observed in some members of the sLM, sLH, sHL, and sHH subgroups. The new siderophile element abundance data are consistent with the findings of prior studies suggesting that the IAB subgroups cannot be related to one another by any known crystallization process. Equilibrium crystallization, coupled with crystal segregation, solid-liquid mixing, and subsequent fractional crystallization can account for the siderophile element variations among meteorites within the IAB main group (MG). The data for the sLM subgroup are consistent with equilibrium crystallization, combined with crystal segregation and mixing. By contrast, the limited fractionation of siderophile elements within the sLL subgroup is consistent with metal extraction from a chondritic source with little subsequent processing. The limited data for the other subgroups were insufficient to draw robust conclusions about crystallization processes involved in their formation. Collectively, multiple formational processes are represented in the IAB complex, and modeling results suggest that fractional crystallization within the MG may have been a more significant process than has been previously recognized.
测定了58颗IAB复合铁陨石金属相的亲铁微量元素丰度和铼-锇同位素体系，以研究其形成过程以及化学亚群内陨石之间的关系。大多数IAB铁陨石的铼-锇同位素数据紧密遵循原始等时线，这表明该复合体大多数成员的亲铁元素在形成后不久就保持封闭，未受元素干扰。然而，在sLM、sLH、sHL和sHH亚群的一些成员中观察到了轻微的、可能是后期的开放体系行为。新的亲铁微量元素丰度数据与先前研究结果一致，表明IAB亚群之间无法通过任何已知的结晶过程相互关联。平衡结晶，再加上晶体分离、固液混合以及随后的分步结晶，可以解释IAB主群（MG）内陨石之间亲铁元素的变化。sLM亚群的数据与平衡结晶、晶体分离和混合的情况相符。相比之下，sLL亚群内亲铁元素的有限分馏与从球粒陨石源提取金属且后续加工很少的情况一致。其他亚群的有限数据不足以就其形成过程中涉及的结晶过程得出可靠结论。总体而言，IAB复合体代表了多种形成过程，建模结果表明MG内的分步结晶可能是一个比之前认识到的更为重要的过程。
