月球物质的Re-Os同位素组成研究:对月球后期增生历史的指示

Re-Os isotopic compositions of lunar materials:Implication for late accretion history of the Moon

Re-Os同位素体系是理解月球强亲铁元素的分布规律和示踪月球的后期增生历史的重要手段。目前人们对月球物质Re-Os同位素成分的了解还是十分有限的,已有的Re-Os同位素数据显示一些能代表月幔成分特征的月海玄武岩具有很低的Re和Os的浓度,以及类似于球粒陨石的187Os/188Os成分特征,而月球火山玻璃和月壤等表现出相对高的Re、Os丰度和相对富放射成因Re-Os同位素成分。一般认为月球月幔的Re、Os和其他强亲铁元素相对球粒陨石是非常亏损的,而地球地幔则具有相对较高的强亲铁元素丰度(0.008倍CI球粒陨石的丰度)。新的Re-Os同位素结果证明月幔确实是亏损的,但是月球和地球在太阳系演化的较晚时期都有外来的球粒陨石物质的大量加入,即后期增生(late accretion)过程,导致了月球和地球上部物质(如月球火山玻璃、月壤等)相对地富集Os同位素和强亲铁元素,这些外来物质的后期增生可能是长期和持续的,增生过程主要发生在3.9～4.4Ga。但目前仍不清楚后期增生的陨石物质是被逐渐加入的,还是在一个相对较短的时期大量加入的,因此尚需对更多的月球物质做进一步的Re-Os同位素和强亲铁元素成分的研究。

Re-Os isotopic system is a particularly valuable tool to understand distributions of highly siderophile elements （HSE） within lunar mantle, and trace late accretion history of the Moon. It is still unclear for Re-Os isotopic compositions of the lunar materials. Published Re-Os isotopic data show that the mare basalts have very low Re and Os concentrations as well as long-term, chondritic 187Os/lSSOs ratios, meanwhile some lunar volcanic glasses and soils have relatively high Re and Os abundances as well as radiogenic Re-Os isotopic composition. It is generally accepted that HSE of the lunar mantle were depleted relative to chondrites, but those of Earth＇s mantle has sufficiently high abundances of the HSE （ca. 0. 008 times CI chondrite abundances）. New Re-Os isotopic results of the lunar materials comfirm that the lunar mantle is indeed depleted, but a significant portion of the lunar and terrestrial late veneer may have been added relatively late in the evolution of the solar system by addition of meteoritic contaminant, i.e., late accretion, resulting in enrichment of Os isotope and HSE on upper materials of the Earth and Moon. This late accretion process was long and prolonged. The late accreted materials were mainly added to the Earth-Moon system during 3.9 - 4.4 Ga. However, it is remain unknown if this late-accreted material was progressively added, or largely added during a relatively short time interval, which requires further Re-Os isotopic and HSE investigation for more lunar materials.