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首页> 外文期刊>Space Science Reviews >Alteration assemblages in martian meteorites: Implications for near-surface processes
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Alteration assemblages in martian meteorites: Implications for near-surface processes

机译：火星陨石中的蚀变组合：对近地表过程的影响

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The SNC (Shergotty-Nakhla-Chassigny) meteorites have recorded interactions between martian crustal fluids and the parent igneous rocks. The resultant secondary minerals - which comprise up to similar to1 vol.% of the meteorites - provide information about the timing and nature of hydrous activity and atmospheric processes on Mars. We suggest that the most plausible models for secondary mineral formation involve the evaporation of low temperature (25 - 150 degreesC) brines. This is consistent with the simple mineralogy of these assemblages - Fe-Mg-Ca carbonates, anhydrite, gypsum, halite, clays - and the chemical fractionation of Ca-to Mg-rich carbonate in ALH84001 "rosettes". Longer-lived, and higher temperature, hydrothermal systems would have caused more silicate alteration than is seen and probably more complex mineral assemblages. Experimental and phase equilibria data on carbonate compositions similar to those present in the SNCs imply low temperatures of formation with cooling taking place over a short period of time (e.g. days). The ALH84001 carbonate also probably shows the effects of partial vapourisation and dehydration related to an impact event post-dating the initial precipitation. This shock event may have led to the formation of sulphide and some magnetite in the Fe-rich outer parts of the rosettes. Radiometric dating (K-Ar, Rb-Sr) of the secondary mineral assemblages in one of the nakhlites (Lafayette) suggests that they formed between 0 and 670 Myr, and certainly long after the crystallisation of the host igneous rocks. Crystallisation of ALH84001 carbonate took place 0.5 Gyr after the parent rock. These age ranges and the other research on these assemblages suggest that environmental conditions conducive to near-surface liquid water have been present on Mars periodically over the last similar to1 Gyr. This fluid activity cannot have been continuous over geological time because in that case much more silicate alteration would have taken place in the meteorite parent rocks and the soluble salts would probably not have been preserved. The secondary minerals could have been precipitated from brines with seawater-like composition, high bicarbonate contents and a weakly acidic nature. The co-existence of siderite (Fe-carbonate) and clays in the nakhlites suggests that the pCO(2) level in equilibrium with the parent brine may have been 50 mbar or more. The brines could have originated as flood waters which percolated through the top few hundred meters of the crust, releasing cations from the surrounding parent rocks. The high sulphur and chlorine concentrations of the martian soil have most likely resulted from aeolian redistribution of such aqueously-deposited salts and from reaction of the martian surface with volcanic acid volatiles. The volume of carbonates in meteorites provides a minimum crustal abundance and is equivalent to 50-250 mbar of CO2 being trapped in the uppermost 200-1000 m of the martian crust. Large fractionations in delta O-18 between igneous silicate in the meteorites and the secondary minerals (less than or equal to 30 parts per thousand) require formation of the latter below temperatures at which silicate-carbonate equilibration could have taken place (similar to 400 degreesC) and have been taken to suggest low temperatures (e.g. less than or equal to 150 degreesC) of precipitation from a hydrous fluid. [References: 109]
机译：SNC（Shergotty-Nakhla-Chassigny）陨石记录了火星地壳流体与母体火成岩之间的相互作用。所产生的次生矿物（最多占陨石的1％（体积））提供了有关火星上水合活动的时间和性质以及大气过程的信息。我们建议最合理的次生矿物形成模型涉及低温（25-150摄氏度）盐水的蒸发。这与这些组合的简单矿物学相符-Fe-Mg-Ca碳酸盐，无水石膏，石膏，石盐，粘土-以及AlH84001“ rosettes”中Ca-富Mg碳酸盐的化学分馏。寿命更长，温度更高的热液系统将导致比观察到的更多的硅酸盐蚀变，并且可能导致更复杂的矿物组合。与SNC中存在的碳酸盐组合物相似的实验和相平衡数据表明，地层温度低，冷却时间短（例如几天）。 ALH84001碳酸盐也可能显示出与初始沉淀后的撞击事件有关的部分汽化和脱水的影响。这种冲击事件可能导致玫瑰花结的富铁外部形成硫化物和一些磁铁矿。在其中一个nakhlites（Lafayette）中，次生矿物组合的放射性测年（K-Ar，Rb-Sr）表明，它们形成于0至670 Myr之间，并且肯定在宿主火成岩结晶后很长时间。 ALH84001碳酸盐的结晶在母岩之后0.5 Gyr发生。这些年龄范围和对这些组合的其他研究表明，在最后一次类似于1 Gyr的时期，火星上周期性地存在有利于近地表液态水的环境条件。这种流体活动在整个地质时间内不可能是连续的，因为在那种情况下，陨石母岩中将会发生更多的硅酸盐蚀变，而可溶性盐可能不会被保存。次生矿物可能是从盐水中沉淀出来的，这些盐水具有类似海水的成分，高碳酸氢盐含量和弱酸性。菱铁矿中的菱铁矿（Fe-碳酸盐）和粘土共存表明与母体盐水平衡的pCO（2）水平可能为50 mbar或更高。盐水可能起源于洪水，渗透到地壳的顶部几百米，从周围的母岩中释放出阳离子。火星土壤中的高硫和氯浓度很可能是由于这种水沉积盐的风向再分布以及火星表面与火山酸挥发物的反应所致。陨石中碳酸盐的含量提供了最低的地壳丰度，相当于在火星地壳的最上层200-1000 m内捕获了50-250 mbar的二氧化碳。陨石中的火成硅酸盐和次生矿物之间的O-18δ大分馏（小于或等于千分之三十）要求在低于可能发生硅酸盐-碳酸盐平衡的温度（约400摄氏度）下形成后者。 ），并已被认为暗示低温（例如小于或等于150摄氏度）从含水流体中析出。 [参考：109]

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