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.
Effects of Charged Solute-Solvent Interaction on Reservoir Temperature during Subsurface CO2 Injection
A short-term side-effect of CO2 injection is a developing low-pH front that forms ahead of the bulk water injectant, due to differences in solute diffusivity. Observations of downhole well temperature show a reduction in aqueous-phase temperature with the arrival of a low-pH front, followed by a gradual rise in temperature upon the arrival of a high concentration of bicarbonate ion. In this work, we model aqueous-phase transient heat advection and diffusion, with the volumetric energy generation rate computed from solute-solvent interaction using the Helgeson–Kirkham–Flowers (HKF) model, which is based on the Born Solvation model, for computing specific molar heat capacity and the enthalpy of charged electrolytes. A computed injectant water temperature profile is shown to agree with the actual bottom hole sampled temperature acquired from sensors. The modeling of aqueous-phase temperature during subsurface injection simulation is important for the accurate modeling of mineral dissolution and precipitation because forward dissolution rates are governed by a temperature-dependent Arrhenius model.
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- Award ID(s):
- 2019194
- NSF-PAR ID:
- 10354628
- Date Published:
- Journal Name:
- Minerals
- Volume:
- 12
- Issue:
- 6
- ISSN:
- 2075-163X
- Page Range / eLocation ID:
- 752
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/min12060752
