Chabot Nancy L, Zhang Bidong
Johns Hopkins University Applied Physics Laboratory 11100 Johns Hopkins Rd Laurel Maryland 20723 USA.
Department of Earth, Planetary, and Space Sciences University of California, Los Angeles Los Angeles California 90095-1567 USA.
Meteorit Planet Sci. 2022 Feb;57(2):200-227. doi: 10.1111/maps.13740. Epub 2021 Oct 18.
As the largest magmatic iron meteorite group, the IIIAB group is often used to investigate the process of core crystallization in asteroid-sized bodies. However, previous IIIAB crystallization models have not succeeded in both explaining the scatter among IIIAB irons around the main crystallization trends and using elemental partitioning behavior consistent with experimental determinations. This study outlines a revised approach for modeling the crystallization of irons that uses experimentally determined partition coefficients and can reproduce the IIIAB trends and their associated scatter for 12 siderophile elements simultaneously. A key advancement of this revised trapped melt model is the inclusion of an effect on the resulting solid metal composition due to the formation of troilite. The revised trapped melt model supports the previous conclusion that trapped melt played an important role in the genesis of IIIAB irons and matches the trace element fractionation trends observed in the Cape York suite as due to different amounts of trapped melt. Applying the revised trapped melt model to 16 elements as well as S and Fe, the bulk composition of the IIIAB core is found to have a composition consistent with that expected from a chondritic precursor for refractory siderophile elements but with evidence for depletions of more volatile elements. The bulk S composition of the IIIAB core is estimated as 9 ± 1 wt%, implying that a substantial amount of S-rich material from the IIIAB core is underrepresented in our meteorite collections. Future applications of the revised trapped melt model to other magmatic iron meteorite groups can enable comparisons between the core compositions and crystallization processes across the early solar system.
作为最大的岩浆铁陨石群，IIIAB群常被用于研究小行星大小天体的核心结晶过程。然而，先前的IIIAB结晶模型在解释IIIAB铁陨石围绕主要结晶趋势的离散情况以及使用与实验测定一致的元素分配行为方面均未成功。本研究概述了一种用于模拟铁陨石结晶的修正方法，该方法使用实验测定的分配系数，能够同时再现12种亲铁元素的IIIAB趋势及其相关离散情况。这种修正的捕获熔体模型的一个关键进展是考虑了陨硫铁的形成对最终固体金属成分的影响。修正的捕获熔体模型支持了先前的结论，即捕获熔体在IIIAB铁陨石的成因中起重要作用，并且与约克角陨石群中观察到的微量元素分馏趋势相匹配，这是由于捕获熔体的量不同所致。将修正的捕获熔体模型应用于16种元素以及硫和铁，发现IIIAB核心的整体成分与难熔亲铁元素的球粒陨石前驱体预期的成分一致，但有证据表明挥发性更强的元素存在亏损。IIIAB核心的整体硫成分估计为9±1 wt%，这意味着来自IIIAB核心的大量富硫物质在我们的陨石收藏中未得到充分体现。修正的捕获熔体模型未来应用于其他岩浆铁陨石群，能够对早期太阳系中核心成分和结晶过程进行比较。
