Epitaxial fluorapatite vein in Northwest Africa 998 host apatite: Clues on the geochemistry of late hydrothermal fluids on Mars
Epitaxial fluorapatite vein in Northwest Africa 998 host apatite: Clues on the geochemistry of late hydrothermal fluids on Mars
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非洲西北部外延氟磷灰石矿脉 998 含有磷灰石：火星晚期热液地球化学线索
DOI：
10.1111/maps.14042
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发表时间：
2023
影响因子：
2.2
通讯作者：
Brearley, Adrian J.
中科院分区：
文献类型：
--
作者：
Martínez, Marina;Shearer, Charles K.;Brearley, Adrian J.
火星赤霞岩中的次生矿物为研究火星地壳中水活动的性质、组成和持续时间提供了强有力的工具。西北非洲（NWA）998结晶早期从nakhlite岩浆源，并有证据表明，最低限度的签名后期热液蚀变事件改变了nakhlite。使用FIB‐TEM技术研究NWA 998中的堆积磷灰石颗粒，我们报告了由氟磷灰石和富含SiO2相组成的亚微米级静脉的第一个证据。氟磷灰石在含氟氯磷灰石的开放解理面的壁上外延生长，富含SiO2的相填充了矿脉的中心。纳米孔隙率和纳米级无定形材料的存在以及脉和宿主磷灰石之间的尖锐界面表明脉代表了耦合溶解-再沉淀过程，该过程产生了不同组成的磷灰石，其与流体更稳定。使用文献中的实验数据和Cl在磷灰石中的扩散系数，我们得出结论，矿脉是由低温（约300°C），微酸性，富F-，Si-的水流体引起的，该流体充当封闭系统。根据矿脉（通过快速注入流体形成）和流体（成分，温度，pH值）的特征，以及我们的SEM和TEM图像中缺乏陆地风化产物，我们推断矿脉是前陆起源的。我们的观察结果支持了这一假设，即触发热液系统的热源是一个低冲击速度的影响，并排除了岩浆起源。最后，矿脉可能是由与其他Nakhlites中报道的不同的晚期流体形成的，但在同一岩浆事件期间形成的，例如，演化程度较低的流体也可能是合理的。
Secondary minerals in martian nakhlites provide a powerful tool for investigating the nature, composition, and duration of aqueous activity in the martian crust. Northwest Africa (NWA) 998 crystallized early from the nakhlite magmatic source and has evidence of minimal signatures of the late hydrothermal alteration event that altered the nakhlites. Using FIB‐TEM techniques to study a cumulus apatite grain in NWA 998, we report the first evidence of a submicron‐scale vein consisting of fluorapatite and an SiO2‐rich phase. Fluorapatite grew epitaxially on the walls of an opened cleavage plane of host F‐bearing chlorapatite and the SiO2‐rich phase filled the center of the vein. The presence of nanoporosity and nanometer‐scale amorphous material and the sharp interface between the vein and the host apatite indicate the vein represents a coupled dissolution–reprecipitation process that generated apatite of a different composition that was more stable with the fluid. Using experimental data and diffusion coefficients of Cl in apatite from the literature, we conclude that the vein was caused by a low temperature (~300°C), slightly acidic, F‐, Si‐rich, aqueous fluid that acted as a closed system. Based on the characteristics of the vein (formation by rapid injection of fluid) and the fluid (composition, temperature, pH), and the lack of terrestrial weathering products in our SEM and TEM images, we infer that the vein is pre‐terrestrial in origin. Our observations support the hypothesis that the heat source triggering a hydrothermal system was a low‐shock velocity impact and rule out a magmatic origin. Finally, the vein could have formed from a late‐stage fluid different from that reported in other nakhlites, but formation during the same magmatic event by, for example, a less evolved fluid might also be plausible.
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