Formation of Apollo 16 impactites and the composition of late accreted material: Constraints from Os isotopes, highly siderophile elements and sulfur abundances
Formation of Apollo 16 impactites and the composition of late accreted material: Constraints from Os isotopes, highly siderophile elements and sulfur abundances
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DOI：
10.1016/j.gca.2016.12.017
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发表时间：
2017-03-01
影响因子：
5
通讯作者：
Becker, Harry
中科院分区：
文献类型：
--
作者：
Gleissner, Philipp;Becker, Harry
Apollo 16号撞击熔体岩石中Fe-Ni金属-硅铝榴石-硫铁矿共生以及新的高度亲铁元素和S丰度数据表明，冲击熔体中冲击器衍生的金属熔滴冷却过程中的毫米尺度闭合体系分异结晶过程是造成Apollo 16号撞击熔体多组分中HSE丰度和比值差异较大的主要原因。因此，从这种数据的线性回归获得的元素比率容易出错，但加权平均考虑了样本中的全部元素预算，从而更准确地估计了它们对影响因素的贡献。Fe-Ni-S-P系中固态金属-液态金属分配的模拟和不同着陆点撞击岩中HSE模式的模拟表明，古月球撞击岩的主体成分应代表撞击熔体的组成，撞击熔体片中固态金属或硫化物液体的原位分凝很可能不会发生HSE的大规模分馏。月球撞击岩的成分记录表明，在早期喷发沉积的重熔过程中，不同数量的球粒陨石和非球粒陨石物质积累，并将这些成分混合在一起。非软骨体成分主要出现在一些Apollo 16撞击岩中，其特征是超软骨体的HSE/Ir比值从难熔的HSE增加到中等挥发的HSE，表现出Ru相对于Pt的特征富集性。高P/S的富硫富磷金属熔体在小行星核或胚核中的大规模分离结晶是目前最有可能产生这些成分的过程。类似的材料或工艺可能促成了块状硅酸盐地球(BSE)的HSE签名。(C)2016爱思唯尔有限公司。保留所有权利。
Fe-Ni metal-schreibersite-troilite intergrowths in Apollo 16 impact melt rocks and new highly siderophile element (HSE) and S abundance data indicate that millimeter-scale closed-system fractional crystallization processes during cooling of impactor-derived metal melt droplets in impact-melts are the main reason for compositional variations and strong differences in abundances and ratios of HSE in multiple aliquots from Apollo 16 impact melt rocks. Element ratios obtained from linear regression of such data are therefore prone to error, but weighted averages take into account full element budgets in the samples and thus represent a more accurate estimate of their impactor contributions. Modeling of solid metal-liquid metal partitioning in the Fe-Ni-S-P system and HSE patterns in impactites from different landing sites suggest that bulk compositions of ancient lunar impactites should be representative of impact melt compositions and that large-scale fractionation of the HSE by in situ segregation of solid metal or sulfide liquid in impact melt sheets most likely did not occur. The compositional record of lunar impactites indicates accretion of variable amounts of chondritic and non-chondritic impactor material and the mixing of these components during remelting of earlier ejecta deposits. The non-chondritic composition appears most prominently in some Apollo 16 impactites and is characterized by suprachondritic HSE/Ir ratios which increase from refractory to moderately volatile HSE and exhibit a characteristic enrichment of Ru relative to Pt. Large-scale fractional crystallization of solid metal from sulfur and phosphorous rich metallic melt with high P/S in planetesimal or embryo cores is currently the most likely process that may have produced these compositions. Similar materials or processes may have contributed to the HSE signature of the bulk silicate Earth (BSE). (C) 2016 Elsevier Ltd. All rights reserved.