Alteration minerals, fluids, and gases on early Mars: Predictions from 1-D flow geochemical modeling of mineral assemblages in meteorite ALH 84001
Alteration minerals, fluids, and gases on early Mars: Predictions from 1-D flow geochemical modeling of mineral assemblages in meteorite ALH 84001
复制标题
早期火星上的蚀变矿物、流体和气体：陨石 ALH 84001 中矿物组合的一维流动地球化学模型的预测
DOI：
10.1111/maps.12713
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发表时间：
2016
影响因子：
2.2
通讯作者：
Melwani Daswani M
中科院分区：
文献类型：
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作者：
Melwani Daswani M
粘土矿物虽然在火星古代地形上普遍存在，但在火星陨石 Allan Hills (ALH) 84001 中尚未观察到，该陨石是早期火星地壳的斜方辉石岩样本，具有次生碳酸盐组合。我们使用低温 (20 °C) 一维 (1-D) 输运热化学模型来研究可能产生 ALH 84001 碳酸盐组合的水蚀变过程，同时避免粘土矿物的共沉淀。我们发现，ALH 84001 中的碳酸盐可能是在一个过程中产生的，在此过程中，低温（~20°C）流体最初与早期火星大气平衡，穿过表面粘土矿物和富含二氧化硅的层，渗透到陨石的母岩中，并在蒸发时沉淀碳酸盐（从而降低了二氧化碳的分压）。这一发现要求在遇到 ALH 84001 的未风化斜方辉石岩宿主之前，流体渗透到在此过程中风化的岩石中。我们能够预测风化过程中形成的粘土矿物的成分，其中包括双八面体蒙脱石绿脱石、高岭石和绿泥石，所有这些矿物此前都曾在火星上检测到。我们还计算了水合硅酸盐滑石在局部平衡条件下对主岩的替换，水合硅酸盐滑石通常被认为是地球上温度较高的热液相，但可能是通过普遍的水蚀作用形成火星土壤的常见成分。最后，在次生蚀变组合中还发现了针铁矿和磁铁矿的沉淀，后者与氢气的产生有关。显然，尽管有限的水-岩相互作用一定导致了约 3.9 Ga 前碳酸盐的形成，但在 ALH 84001 烃源岩附近，粘土的形成可能很普遍。
Clay minerals, although ubiquitous on the ancient terrains of Mars, have not been observed in Martian meteorite Allan Hills (ALH) 84001, which is an orthopyroxenite sample of the early Martian crust with a secondary carbonate assemblage. We used a low‐temperature (20 °C) one‐dimensional (1‐D) transport thermochemical model to investigate the possible aqueous alteration processes that produced the carbonate assemblage of ALH 84001 while avoiding the coprecipitation of clay minerals. We found that the carbonate in ALH 84001 could have been produced in a process, whereby a low‐temperature (~20 °C) fluid, initially equilibrated with the early Martian atmosphere, moved through surficial clay mineral and silica‐rich layers, percolated through the parent rock of the meteorite, and precipitated carbonates (thereby decreasing the partial pressure of CO2) as it evaporated. This finding requires that before encountering the unweathered orthopyroxenite host of ALH 84001, the fluid permeated rock that became weathered during the process. We were able to predict the composition of the clay minerals formed during weathering, which included the dioctahedral smectite nontronite, kaolinite, and chlorite, all of which have been previously detected on Mars. We also calculated host rock replacement in local equilibrium conditions by the hydrated silicate talc, which is typically considered to be a higher temperature hydrothermal phase on Earth, but may have been a common constituent in the formation of Martian soils through pervasive aqueous alteration. Finally, goethite and magnetite were also found to precipitate in the secondary alteration assemblage, the latter associated with the generation of H2. Apparently, despite the limited water–rock interaction that must have led to the formation of the carbonates ~ 3.9 Ga ago, in the vicinity of the ALH 84001 source rocks, clay formation would have been widespread.