The aim of this work is to understand the formation of primary evaporites—sulfates, borates, and chlorides—in Gale crater using thermochemical modeling to determine constraints on their formation. We test the hypothesis that primary evaporites required multiple wet-dry cycles to form, akin to how evaporite assemblages form on Earth. Starting with a basalt-equilibrated Mars fluid, Mars-relevant concentrations of B and Li were added, and then equilibrated with Gale lacustrine bedrock. We simulated cycles of evaporation followed by groundwater recharge/dilution to establish an approximate minimum number of wet-dry cycles required to form primary evaporites. We determine that a minimum of 250 wet-dry cycles may be required to start forming primary evaporites that consist of borates and Ca-sulfates. We estimate that ~14,250 annual cycles (~25.6 k Earth years) of wet and dry periods may form primary borates and Ca-sulfates in Gale crater. These primary evaporites could have been remobilized during secondary diagenesis to form the veins that the Curiosity rover observes in Gale crater. No LiCl salts form after 14,250 cycles modeled for the Gale-relevant scenario (approximately 106 cycles would be needed) which implies Li may be leftover in a groundwater brine after the time of the lake. No major deposits of borates are observed to date in Gale crater which also implies that B may be leftover in the subsequent groundwater brine that formed after evaporites were remobilized into Ca-sulfate veins.

Various recent studies have shown that basalt formations have the capacity for long-term secure CO2 storage through carbon mineralization. Many of these studies have demonstrated extremely rapid rates of mineralization, but the underlying mechanism enabling these elevated reaction rates, and their relation to the processes occurring in proposed basaltic reservoirs, remain poorly constrained. In this work, a 3D micro-continuum reactive transport model was designed to investigate the impact of alkalinity on basalt interactions with CO2-rich fluids. Reactive transport models were developed in PFLOTRAN based on 3D imaging data from high-temperature, high-pressure flow-through experiments (Luhmann et al. (2017) Chemical Geology, Water Resources Research). Mineral reactive surface areas in the model were adjusted to produce agreement with chemistry of output fluids sampled during the experiments. The benchmarked model showed that no considerable carbonate was formed during interaction with the relatively low alkalinity, low pH solutions, regardless of the enrichment of basalt-derived Na+, Mg2+, and Fe2+ ions in the reactant fluid. Increasing the alkalinity of the injected fluids consistently yielded higher rates of carbon mineralization. Similarly, introducing a small initial volume fraction of carbonate minerals into the system contributed to increased carbon mineralization, because of the increased fluid alkalinity. These results thus reinforce a conceptual understanding of carbonate mineralization in basalt-hosted CO2 storage reservoirs that emphasizes the importance of aquifer fluid alkalinity, and caution against extrapolating results from elevated-alkalinity CO2 storage reservoirs and experiments to others where this is less likely to be representative.

Serpentinization ubiquitously affects ultramafic rocks that interact with water, with strong implications for the origin of ancient microbial ecosystems, the chemical budget of the ocean, and the rheology of the oceanic crust. The increase in volume associated with this reaction, and its consequences for reaction progress, have been debated for over a century. Serpentine minerals are ~40% more voluminous than olivine, which suggests that fully serpentinized peridotite should have negative, or at least very low, porosity. Recent studies have proposed that self-propagating, reaction-driven fracturing can facilitate serpentinization reactions, but the nanoscale mechanisms by which fluid is transported through serpentinized fractures in order to react with fresh olivine surfaces remain poorly understood. To address this issue, we studied a sample of serpentinized harzburgite collected during ODP expedition 209 at site 1274, using Focused Ion Beam – Scanning Electron Microscopy (FIB-SEM)-assisted tomography. We specifically targeted the interface between the serpentinized peridotite and the unaltered primary mineralogy. The resultant images illustrate the presence of nanopores within the serpentine alteration products, primarily at the interface with the unreplaced minerals. Importantly, no pores were observed in the serpentine away from the grain boundaries with olivine, suggesting that these nanopores form during the initial stage of reaction, and then disappear with further reaction progress. We argue that the observed nanoporosity is an intrinsic feature of the serpentinization reaction, and that its transient presence during serpentinization is vital for facilitating reaction progress. We suggest that the transient nature of the pores arises from the opposing kinetics and thermodynamics of the replacement reaction. The former promotes the formation of voids that enhance the advective transport of fluids to the reaction front, while the latter drives the reduction of pore space by means of recrystallization of the serpentine aggregates and minimization of the interfacial energy.

Basalt carbonation has gained traction as a key technology for avoiding the worst consequences of human-driven climate change. However, our understanding of this method’s promise is likely inflated by the specialized conditions used in many of the most well-known laboratory studies and demonstration projects. For technological, hydrogeologic, and energetic simplicity, many basalt CO2 storage projects will likely inject supercritical, not dissolved, CO2. Thus, fluids in these systems are likely to have low alkalinity and low pH, in contrast to many experimental and demonstration studies. Here, we present a series of geochemical models that explore the dependence of carbon mineralization efficiency on alkalinity and therefore pH at conditions relevant to these proposed operations. We modelled the interaction of basalt with CO2 enriched, seawater-derived aquifer fluid with varying initial alkalinities at 60°C using a custom thermodynamic database incorporating updated thermodynamic data for relevant primary and secondary minerals. The results reinforce the notion that alkalinity is an important driver for carbonate precipitation, ultimately because carbonate minerals are up to an order of magnitude more soluble at pH <5 than they are at pH >6. Alkalinity increases of 5 to 10% proportionally increase carbonate precipitation in the models. Our results thus demonstrate that the elevated alkalinity found in many of the most well-known basalt carbonation studies yield disproportionately high rates of carbon mineralization, which, in turn, frames basalt carbonation as an extremely rapid and exceptionally effective CO2 storage method. Although supercritical CO2 injection operations such as those we explore here are likely to achieve high fractions of CO2 mineralization over their lifetimes, this will likely take considerably longer and potentially be ultimately less effective, due to sluggish rates of CO2 dissolution and alkalinity generation.