Harvey R P, McSween H Y
Department of Geological Sciences, Case Western Reserve University, Cleveland, Ohio 44106-7216, USA.
Nature. 1996 Jul 4;382(6586):49-51. doi: 10.1038/382049a0.
The meteorite Allan Hills (ALH) 84001, commonly accepted to be of martian origin, is unique among known martian meteorites in containing abundant, zoned, pre-terrestrial carbonate minerals. Previous studies of the oxygen isotope compositions of these minerals have suggested that they precipitated from a low-temperature (0-80 degrees C) aqueous fluid in the martian crust--perhaps in a near-surface hydrothermal system. Here we report analyses of the major-element compositions of the carbonates, which provide an independent constraint on the composition and temperature of the fluid from which they formed. We argue that the most likely explanation for the observed compositions, and for the absence of co-existing hydrons minerals, is that the carbonates were formed by reactions between hot (> 650 degrees C), CO2-rich fluids and the ultramatic host rock during an impact event. Impact processes on the martian surface can produce both the hot, CO2-rich fluid (by volatilization of surface carbonates or other CO2 sources) and--by brecciation--the condults through which it flowed. Impact metasomatism is also consistent with the observed oxygen isotope disequillbrium, sequence of mineral formation, and carbonate mineral zoning, reflecting carbonate formation during rapid cooling from high temperatures rather than prolonged exposure to low-temperature fluids.
阿伦丘陵（ALH）84001陨石通常被认为源自火星，在已知的火星陨石中独一无二，因为它含有丰富的、呈带状分布的地球形成前的碳酸盐矿物。此前对这些矿物氧同位素组成的研究表明，它们是从火星地壳中低温（0 - 80摄氏度）的水流体中沉淀形成的——可能是在近地表热液系统中。在此，我们报告了对碳酸盐主要元素组成的分析，这为其形成时流体的组成和温度提供了独立的限制条件。我们认为，对于观察到的组成以及不存在共生氢氧化物矿物的最可能解释是，碳酸盐是在撞击事件期间由高温（>650摄氏度）、富含二氧化碳的流体与超镁铁母岩之间的反应形成的。火星表面的撞击过程既能产生高温、富含二氧化碳的流体（通过表面碳酸盐或其他二氧化碳源的挥发作用），又能通过角砾化作用形成其流动的通道。撞击交代作用也与观察到的氧同位素不平衡、矿物形成顺序以及碳酸盐矿物分带现象相一致，反映出碳酸盐是在从高温快速冷却过程中形成的，而非长时间暴露于低温流体中。
