Murchison陨石中草莓状磁铁矿的成因及意义

The Origin and Significance of Framboidal Magnetite in the Murchison Meteorite

碳质小行星富含水和有机物,被认为是地球水与生命的重要潜在来源,是当前深空探测的重要目标。通过研究碳质球粒陨石中蚀变矿物的特征,可有效揭示其母体小行星早期的水活动历史。以Murchison碳质球粒陨石(CM2)中的草莓状磁铁矿为研究对象,利用扫描电镜和透射电镜对其微观结构和元素组成进行分析。结果显示,Murchison陨石中的草莓状磁铁矿具有交代黄铁矿的五角十二面体晶型假象,其颗粒边缘具有次生环带结构以及颗粒间隙填充有蛇纹石等次生层状硅酸盐矿物。研究结果表明,受^(26)Al等放射性元素衰变生热影响,Murchison陨石母体小行星早期曾发育有上限为150~200℃的水热蚀变作用,随着流体由酸性向碱性演变以及溶解氧的形成,早期形成的草莓状黄铁矿被后期流体交代而形成草莓状磁铁矿。

Carbonaceous asteroids are considered to be important potential sources of water and life on the Earth as they are relatively rich in water and organic matters.Therefore,they have become important targets for the deep space exploration.By studying the characteristics of altered minerals in carbonaceous chondrites,the early water activity history of the parent asteroid can be effectively revealed.In this study,framboidal magnetite in the Murchison carbonaceous chondrite(CM2)was selected as the main research object to carry out detailed study on its microstructure and elemental composition using SEM and TEM.The results show that framboidal magnetite particles in the Murchison meteorite were characterized with pentagonal dodecahedron pseudomorphs of replaced pyrite crystals,with the secondary alteration amorphous thin rims.In addition,there are lots of secondary phyllosilicates minerals such as serpentine filling in interval pores among framboidal magnetite particles.This study suggests that the parent body of the Murchison meteorite could have been hydrothermally altered with an upper limit of 150-200℃in its early evolution stage due to the influence of heat produced by the decay of radioactive elements including 26Al.With the transformation of acidic fluid to alkaline one and formation of dissolved oxygen,the early formed framboidal pyrite was replaced by framboidal magnetite via the late stage hydrothermal metasomatism.