Osmium isotope and highly siderophile element systematics of lunar impact melt breccias: Implications for the late accretion history of the Moon and Earth
Osmium isotope and highly siderophile element systematics of lunar impact melt breccias: Implications for the late accretion history of the Moon and Earth
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DOI：
10.1016/j.gca.2008.04.006
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发表时间：
2008-06
影响因子：
5
通讯作者：
I. Puchtel;R. Walker;O. James;D. Kring
中科院分区：
文献类型：
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作者：
I. Puchtel;R. Walker;O. James;D. Kring
为了表征地球-月球系统中约4.5和3.8Ga之间的物质的组成，我们已经确定了Os同位素组成和一些高度亲铁元素（HSE：Re，Os，Ir，Ru，Pt和Pd）的丰度在48个子样品的6个月球角砾岩。这些是：阿波罗17号嵌晶熔融角砾岩72395和76215;阿波罗17号隐晶质熔融角砾岩73215和73255;阿波罗14号多晶熔融角砾岩14321;以及月球陨石NWA 482，一种结晶的撞击熔融物。Ir与其他HSE的图定义了良好的线性相关性，表明所有数据集可能主要代表低HSE目标（推测为内源性组分）和高HSE（推测为外源性组分）的双组分混合物。这些趋势的线性回归产生的截距是从零统计上无法区分的所有HSE，除了钌和钯在两个样品。线性回归的斜率对观测到的Ru和Pd的目标岩石贡献不敏感;因此，趋势线斜率近似于影响这些岩石的撞击物成分中存在的元素比率。阿波罗17号隐晶质熔融角砾岩和月球陨石的187 Os/188 Os和回归导出的元素比值表明，这些样品中的撞击物成分与南极陨石有密切的关系。然而，阿波罗17号隐晶质熔融角砾岩中的HSE可能部分或全部反映了在其创建时被纳入冲击熔体中的富含HSE的麻粒角砾岩碎屑的HSE特征。在这种情况下，这些岩石的HSE特征可能反映了导致熔融角砾岩形成的撞击事件之前的撞击物的特征。阿波罗17号嵌晶熔融角砾岩和阿波罗14号角砾岩中的撞击物成分具有较高的187 Os/188 Os、Pt/Ir和Ru/Ir，而Os/Ir比大多数斜长岩低。这些成分表明，它们所代表的撞击物在化学上与已知的球粒陨石类型不同，并且可能代表一种目前未作为陨石运送到地球的原始材料。
To characterize the compositions of materials accreted to the Earth–Moon system between about 4.5 and 3.8Ga, we have determined Os isotopic compositions and some highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundances in 48 subsamples of six lunar breccias. These are: Apollo 17 poikilitic melt breccias 72395 and 76215; Apollo 17 aphanitic melt breccias 73215 and 73255; Apollo 14 polymict breccia 14321; and lunar meteorite NWA482, a crystallized impact melt. Plots of Ir versus other HSE define excellent linear correlations, indicating that all data sets likely represent dominantly two-component mixtures of a low-HSE target, presumably endogenous component, and a high-HSE, presumably exogenous component. Linear regressions of these trends yield intercepts that are statistically indistinguishable from zero for all HSE, except for Ru and Pd in two samples. The slopes of the linear regressions are insensitive to target rock contributions of Ru and Pd of the magnitude observed; thus, the trendline slopes approximate the elemental ratios present in the impactor components contributed to these rocks. The187Os/188Os and regression-derived elemental ratios for the Apollo 17 aphanitic melt breccias and the lunar meteorite indicate that the impactor components in these samples have close affinities to chondritic meteorites. The HSE in the Apollo 17 aphanitic melt breccias, however, might partially or entirely reflect the HSE characteristics of HSE-rich granulitic breccia clasts that were incorporated in the impact melt at the time of its creation. In this case, the HSE characteristics of these rocks may reflect those of an impactor that predated the impact event that led to the creation of the melt breccias. The impactor components in the Apollo 17 poikilitic melt breccias and in the Apollo 14 breccia have higher187Os/188Os, Pt/Ir, and Ru/Ir and lower Os/Ir than most chondrites. These compositions suggest that the impactors they represent were chemically distinct from known chondrite types, and possibly represent a type of primitive material not currently delivered to Earth as meteorites.