Changes in Secondary Alteration Mineralogy and Brine Chemistry through Hypersaline Hydrothermal Alteration of Basalt and Mafic Glass

Hydrothermal conditions are commonly found in a variety of terrestrial and extraterrestrial systems, such as deep ocean hydrothermal vents, geothermal basins, magmatic fluids from volcanic activity, and impact craters. Heat increases both mineral solubilities and reaction rates, increasing the likelihood of brine formation and changing dissolved elemental species. Differences in brine chemistries likely result in different secondary mineral assemblages formed during alteration, particularly differences in oxides and clay minerals, and impacts availability of nutrients for potential microbial processes. We further investigate these hydrothermal processes by reacting basaltic materials and glass-rich samples with different endmember brines at 350K to determine what secondary alteration products form. We reacted 20 g of basalt and mafic glass with 200 mL of near-saturated brines of NaCl, MgCl₂, Na₂SO₄, MgSO₄, 10% dilutions of each saturated brine,and ultra-pure water (UPW) for 50 consecutive days at 350K, mixing the reactors frequently. X-ray diffraction analyses show that pyroxene, apatite, and amorphous phases showed the largest relative wt% changes. Compared to the unreacted mafic glass, samples reacted with NaCl, MgCl₂, and MgSO₄ exhibited a relative increase in amorphous content, whereas UPW and Na₂SO₄ samples showed a slight decrease.  Apatite phases decreased below detection limits for most samples except for near-saturated MgSO₄. Additional alteration phases such as Fe and Ti oxides formed at low concentrations following reaction with most brine solutions. While aqueous Si concentrations (measured via ICP-OES) increased in most brine samples reacted with basalt and mafic glass, both near-saturated MgSO₄ samples, basalt+10% saturated MgCl₂, and basalt+10% saturated Na₂SO₄ exhibited a decrease in aqueous Si. Changes in dissolved Si concentrations were more prominent with mafic glass samples than basalt samples, despite basalt having greater relative amounts of Si-bearing species compared to glass as observed in XRD relative wt%. Despite the commonly assumed trend of increased dissolution at higher temperatures, smaller net changes in dissolved Si in brines were observed at 350K compared to lower temperature brine experiments. While apatite decreased in glass-rich samples during the alteration, aqueous P was not detected in any of the brine samples. Instead, phosphorus adsorption to preexisting mineral surfaces and new amorphous phases may have removed P from solution. The formation of new amorphous phases with higher surface areas likely increases the amount of reactive adsorption sites which results in greater sequestration of aqueous P and other potential biological nutrients in solution. Because amorphous phase content is impacted by differences in brine chemistries, dissolution and phase formation mechanisms that make nutrients available for microbial metabolic functions may therefore differ and suggest potential enhancements or limitations for habitability in extreme and extraterrestrial hydrothermal systems.