Epitaxial fluorapatite vein in Northwest Africa 998 host apatite: Clues on the geochemistry of late hydrothermal fluids on Mars

Abstract

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.

Early Thermal Histories of IAB Main Group Irons: Insights From Ca‐Phosphates in Campo del Cielo and Nantan

“…Integration of in situ secondary ionization mass spectrometry (SIMS) U‐Pb/Pb‐Pb dating with petrographic study has been widely used to reconstruct the complex thermal histories of extraterrestrial samples. Ca‐phosphates have been observed in many chondrites (e.g., Jones et al., 2014; McCubbin et al., 2023), achondrites (e.g., Martínez et al., 2023; Reger et al., 2023), as well as some nonmagmatic iron meteorites (e.g., Benedix et al., 2000; Kirby et al., 2022; S Li et al., 2021). The petrography and compositions of Ca‐phosphates and associated phases could provide a record to trace a specific geological process, such as the subsolidus metamorphism, shock heating, liquid‐crystallization and fluid‐assisted metasomatism (e.g., Kirby et al., 2022; S Li et al., 2021; McKibbin et al., 2019; Van Roosbroek et al., 2017; Yin et al., 2014; Zhang et al., 2020).…”