Petrologic evidence for low-temperature, possibly flood evaporitic origin of carbonates in the ALH84001 meteorite.
Petrologic evidence for low-temperature, possibly flood evaporitic origin of carbonates in the ALH84001 meteorite.
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ALH84001 陨石中碳酸盐的低温、可能是洪水蒸发成因的岩石学证据。
DOI：
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发表时间：
1998
影响因子：
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通讯作者：
Paul H. Warren
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作者：
Paul H. Warren
火星陨石ALH84001中碳酸盐起源的高温模型是不可信的。冲击交代模式通过富CO2流体与寄主正辉石岩发生反应，要求橄榄石转化为正辉石，而ALH84001中橄榄石在富碳酸盐地区未见衰竭；或者将正辉石转化为二氧化硅，这应该会产生更高的二氧化硅/碳酸盐比。冲击熔体模型表明，作为熔体注入的产物，裂缝连接的碳酸盐应该表现为连续的平面脉，但在许多地区并非如此。气相沉积和冲击熔融似乎都不符合许多大型碳酸盐的分带偏位结构。流行的热液模型与ALH84001中几乎不存在次生水合硅酸盐相矛盾。先前的角化作用应该促进了蚀变。热液流体是温暖的，镁基硅酸盐的水化速率遵循阿雷尼乌斯定律，至少达到大约100摄氏度。最重要的是，热液期往往持续多年。古火星地壳的许多地区显示出大规模洪水的证据。我认为碳酸盐是由洪水通过ALH84001裂缝渗透而成的蒸发岩沉积物形成的，但这只是短暂的，因为在洪水后期，蒸发和地下水流动导致地下水位迅速下降到这块岩石的水平以下。它的环境可能是一层巨型岩石层，位于汇集洪水的地表集水区之下，类似于playa湖。碳酸盐沉淀会随着水的蒸发浓度而发生。为了解释ALH84001中硫酸盐的稀缺，必须假设地下水位相对于蒸发速率迅速下降。当ALH84001在地下水位以上时，蒸发会减慢，因为蒸发锋从碎屑层表面下经过，如果地表水的收缩池形成多孔的硫酸盐外壳，蒸发可能会更早。另一种可能是，ALH84001可能发展为火星形式的钙质，也就是说，蒸发的洪水可能完全在地下，因为它（他们）缓慢地通过ALH84001。洪水蒸发岩模型的最大优点是，它使ALH84001暴露于碳酸盐沉淀，而不长时间暴露于水蚀变。该模型似乎也与碳酸盐的重且极不均匀的氧同位素组成相一致。然而，这一假设似乎与McKay等人[1996]提出的碳酸盐是生物成因的观点不太一致。
High-temperature models for origin of the carbonates in Martian meteorite ALH84001 are implausible. The impact metasomatism model, invoking reaction between CO2 rich fluid and the host orthopyroxenite, requires conversion of olivine into orthopyroxene, yet olivine in ALH84001 shows no depletion in carbonate-rich areas; or else conversion of orthopyroxene into silica, which should have yielded a higher silica/carbonate ratio. The impact melt model implies that the fracture-linked carbonates, as products of melt injection, should appear as continuous planar veins, but in many areas they do not. Both vapor deposition and impact melting seem inconsistent with the zoned poikilotopic texture of many large carbonates. The popular hydrothermal model is inconsistent with the virtual absence of secondary hydrated silicates in ALH84001. Prior brecciation should have facilitated alteration. Hydrothermal fluids would be warm, and rate of hydration of mafic silicates obeys an Arrhenius law, at least up to approximately 100 degrees C. Most important, hydrothermal episodes tend to last for many years. Many areas of the ancient Martian crust show evidence for massive flooding. I propose that the carbonates formed as evaporite deposits from floodwaters that percolated through the fractures of ALH84001, but only briefly, as evaporation and groundwater flow caused the water table to quickly recede beneath the level of this rock during the later stages of the flood episode. The setting might have been a layer of megaregolith beneath a surface catchment of pooled floodwater, analogous to a playa lake. Carbonate precipitation would occur in response to evaporative concentration of the water. To explain the scarcity of sulfates in ALH84001, the water table must be assumed to recede quickly relative to the rate of evaporation. During the period when ALH84001 was above the water table, evaporation would have slowed, as the evaporation front passed beneath the surface of the debris layer, and possibly earlier, if the shrinking pool of surface water developed a porous sulfate crust. Alternatively, ALH84001 may have developed as a Martian form of calcrete, i.e., the evaporating flood(s) may have been entirely below ground as it (they) passed slowly through ALH84001. The greatest advantage of the flood evaporite model is that it exposes ALH84001 to carbonate precipitation without prolonged exposure to aqueous alteration. The model also seems consistent with the heavy and extremely heterogeneous oxygen isotopic compositions of the carbonates. However, this hypothesis seems no more than marginally consistent with the suggestion of McKay et al. [1996] that the carbonates are biogenic.