Precise measurements of highly siderophile elements in ancient lunar impact rocks

地球地幔中高亲铁元素（HSE, Ir, Rh, Ru, Pd, Pt, Os, Re, Au）的过量丰度和比例通常可以用地核形成后大型星子的晚期吸积来解释。来自地幔样品的新的HSE丰度数据显示，这些元素的独特比例与已知的原始陨石中发现的不同。可能的解释包括：出现了成分有所不同的古代原始陨石，存在源自岩心形成的残余HSE分馏物，或者人们对地幔中HSE分馏物的过程知之甚少。与地球不同的是，在地球上，3.8亿年前被陨石污染的地壳被重新注入地幔，而月球地壳自45亿年前形成以来，仍然保持着晚期的增生记录。阿波罗17号和14号撞击熔石的精确丰度数据确实显示，月球撞击物的HSE成分与地球地幔中的成分非常相似。由于此类岩石缺乏完整的HSE数据集，因此有必要更好地约束具有不同HSE丰度模式的撞击物在月球上的分布。我们建议在阿波罗14号和阿波罗16号撞击熔融岩和粒状角砾岩上获得完整的HSE丰度模式和锇同位素数据。这些结果将对解释地幔的HSE成分产生重大影响。

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.