Precise measurements of highly siderophile elements in ancient lunar impact rocks

地幔中高亲铁元素（HSE; Ir、Rh、Ru、Pd、Pt、Os、Re、Au）的过量丰度和比例通常被解释为地核形成后大微行星的晚期吸积。来自地球地幔样本的新HSE丰度数据显示，这些元素的独特比例与已知原始陨石中发现的不同。可能的解释包括古代原始陨石的出现与有些不同的组成，存在残留的HSE分馏起源于核心形成，或知之甚少的过程，将压裂HSE在地幔中。与地球相比，地球的再循环使3.8 Ga前的陨石污染地壳返回地幔，月球地壳自45亿年前形成以来仍然保持着晚期吸积的记录。一些阿波罗17号和14号撞击熔融岩石的精确丰度数据确实产生了月球撞击物的HSE成分，这些成分与地球地幔中发现的成分非常相似。由于缺乏关于这些岩石的完整HSE数据集，因此有必要更好地限制月球上具有不同HSE丰度模式的撞击物的分布。我们建议获得完整的HSE丰度模式和锇同位素数据阿波罗14号和阿波罗16号的影响熔融岩石和麻粒角砾岩。这些结果将对解释地球地幔的HSE组成产生重大影响。

Excess abundances and proportions of the highly siderophile ( iron-loving ) elements (HSE; Ir, Rh, Ru, Pd, Pt, Os, Re, Au) in the Earth s mantle are commonly explained by late accretion of large planetesimals after formation of the Earth s core. New HSE abundance data for samples from the Earth s mantle show unique proportions of these elements that are unlike those found in known primitive meteorites. Possible explanations include the occurrence of ancient primitive meteorites with somewhat different compositions, the presence of a residual HSE fractionation originating from core formation, or poorly understood processes that would fractionate the HSE in the mantle. In contrast to Earth where recycling has returned pre-3.8 Ga old meteorite-contaminated crust back into the mantle, the lunar crust still maintains a record of late accretion since its formation 4.5 billion years ago. Precise abundance data on some Apollo 17 and 14 impact melt rocks indeed yielded HSE compositions for lunar impactors that are very similar to compositions found in the Earth s mantle. Because of the scarcity of complete HSE data sets on such rocks, it is necessary to better constrain the distribution of impactors with distinct HSE abundance patterns on the Moon. We propose to obtain complete HSE abundance patterns and Osmium isotopic data on Apollo 14 and Apollo 16 impact melt rocks and on granulitic breccias. The results will have major implications for the interpretation of the HSE composition of the Earth s mantle.