The Northwest Africa ( NWA ) 5790 meteorite: A mesostasis-rich nakhlite with little or no Martian aqueous alteration
The Northwest Africa ( NWA ) 5790 meteorite: A mesostasis-rich nakhlite with little or no Martian aqueous alteration
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西北非洲 (NWA) 5790 陨石：富含中稳态的方辉石，很少或没有火星水蚀变
DOI：
10.1111/maps.12424
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发表时间：
2015
影响因子：
2.2
通讯作者：
Tomkinson T
中科院分区：
文献类型：
--
作者：
Tomkinson T
西北非洲（NWA）5790是最近发现的Nakhlite组成员。它的矿物学不同于其他的钠闪石，具有高丰度的中稳态（38.1 ± 3.6体积%）和缺乏橄榄石（4.0 ± 2.2体积%）。此外，辉石斑晶的分区，以及其他岩石学和化学特征表明，NWA 5790样品的母熔岩流/岩床的冷冻边缘。NWA 5790含有方解石和罕见的粘土矿物，这是其暴露于液态水的证据。方解石形成了一种水泥，覆盖在发现物外表面的灰尘上，并延伸到陨石内部的矿脉中。涂层内微生物残留物的存在证实了粉尘及其碳酸盐胶结物是陆地来源，与方解石的碳和氧同位素组成一致。粘土矿物是精细结晶的，占陨石体积的约0.003%。粘土矿物的δD值介于−212 ± 109‰至−96 ± 132‰之间，不能用来区分地球或火星的起源。由于岩相学结果也不是决定性的，我们得出结论，火星地下水产生的次生矿物在NWA 5790中充其量是非常罕见的。因此，陨石对熔岩流/岩床的一个区域进行了采样，很少或根本没有暴露于改变其他Nakhlites的水溶液中。这种隔离可能与NWA 5790中橄榄石的稀缺有关，因为火星地下水溶解其他Nakhlites中的橄榄石增强了它们的孔隙度和渗透性，并为次生矿物提供了溶质。
Northwest Africa (NWA) 5790 is the most recently discovered member of the nakhlite group. Its mineralogy differs from the other nakhlites with a high abundance mesostasis (38.1 ± 3.6 vol%) and scarcity of olivine (4.0 ± 2.2 vol%). Furthermore, zoning of augite phenocrysts, and other petrographic and chemical characteristics suggest that NWA 5790 samples the chilled margin of its parent lava flow/sill. NWA 5790 contains calcite and rare clay minerals that are evidence for its exposure to liquid water. The calcite forms a cement to coatings of dust on the outer surface of the find and extends into the interior of the meteorite within veins. The presence of microbial remains within the coating confirms that the dust and its carbonate cement are terrestrial in origin, consistent with the carbon and oxygen isotope composition of the calcite. The clay minerals are finely crystalline and comprise ~0.003 vol% of the meteorite. δD values of the clay minerals range from −212 ± 109‰ to −96 ± 132‰, and cannot be used to distinguish between a terrestrial or Martian origin. As petrographic results are also not definitive, we conclude that secondary minerals produced by Martian groundwaters are at best very rare within NWA 5790. The meteorite has therefore sampled a region of the lava flow/sill with little or no exposure to the aqueous solutions that altered other nakhlites. This isolation could relate to the scarcity of olivine in NWA 5790 because dissolution of olivine in other nakhlites by Martian groundwaters enhanced their porosity and permeability, and provided solutes for secondary minerals.