New Mexico Geological Society Annual Spring Meeting — Abstracts

The reduction of atmospheric CO₂ is a major global challenge in mitigating climate change [1,2]. Among various strategies, geologic CO₂ storage through mineralization has emerged as a promising long-term solution. By injecting CO₂-bearing fluids into reactive rock formations, carbon can be permanently sequestered as stable carbonate minerals [3–5]. Mafic and ultramafic rocks are ideal candidates due to their high concentrations of Ca, Mg, and Fe, which form carbonates such as calcite, dolomite, ankerite, and siderite. Prior studies have shown that basaltic glass and mafic minerals dissolve relatively quickly, releasing cations necessary for carbonate formation within decades [6,7].

This study evaluates the potential for basaltic rocks and mine waste in New Mexico, USA, to sequester CO₂ and explores the kinetics of fluid-rock interactions, changes in fluid chemistry, and the mobility of critical elements (Cu, Ni, Co, Zn). Batch-type experiments were conducted at up to 40°C under low dissolved CO₂ concentrations (<60 mmol/kg), varying rock composition, grain size, and time. The experimental setup aimed to simulate fluid-rock interaction paths to inform models of reaction kinetics and buffering mineral assemblages.

Basalt, trachybasalt, and trachyandesite from the Taos Plateau and Carrizozo, NM, were crushed into three grain size fractions: <0.15 mm, 0.15–0.5 mm, and 2 mm. The materials were repeatedly washed in deionized water and acetone to remove fine particles. BET surface area analysis revealed values of 0.90, 0.55, and 0.43 m²/g for the fine, intermediate, and coarse fractions, respectively. Experiments were conducted in 2 L polypropylene bottles with gas-tight PEEK fittings. A synthetic Taos groundwater was interacted with crushed mafic rock at a fluid-to-rock ratio of 4:1. The synthetic Taos groundwater was prepared with high-purity Ca, Mg, Na, and K chloride salts to approximate the major element chemistry of Taos groundwater and was charged with either 40 or 60 mmol/kg dissolved CO₂. The headspace was purged with N₂ gas.

Experiments were conducted for up to ~200 days at room temperature. The solution was periodically samples in situ and CO₂ concentrations were measured using titration and an aliquot was acidified for ICP-OES and ICP-MS analyses for determination of major (e.g., Ca, Mg, Na, K, Si, Al) and trace elements (e.g., Fe, Cu, Zn, Co, Ni). Dissolved CO₂ concentrations were determined by titration with 0.1 M HCl after reaction with NaOH.

The CO₂ concentrations decreased from ~2200 ppm in the initial fluid to ~800 ppm at the time the experiment was quenched and pH increased from 4.5 to 6.8, indicating that CO₂ is sequestered as the fluid equilibrates with the basaltic rocks. The grain size of the rocks had a large impact on the reaction rates with the finest grain fraction (<0.15 mm) exhibiting the fastest response showing CO₂ concentrations decrease from ~28 to 21.1 mmol/kg over 100 days. The intermediate (0.15–0.5 mm) and coarse (2 mm) grain fractions showed slower rates, with final CO₂ concentrations of 21.8 and 21.2 mmol/kg, respectively. On average, 56% of dissolved CO₂ was removed across all experiments, and 70–80% was mineralized after 100 days. In the experiments, ~3 g of carbonate precipitated per kg of solution and 240 g of basalt. These findings align with prior research and confirm the viability of rapid CO₂ mineralization in mafic rock systems. Major cation concentrations (Ca, Mg, Fe) tracked the dissolution and subsequent precipitation of primary and secondary minerals. Calcium and Mg levels initially increased due to the dissolution of clinopyroxene and subsequently decreased after 20–40 days due to the precipitation of secondary carbonates. The fine-grained fraction maintained higher Ca and Mg levels for longer durations, consistent with faster dissolution kinetics. Iron was most abundant in fine-grained experiments and concentrations sharply decreased after 20 days, due to rapid dissolution of Fe-Ti oxides and precipitation of secondary Fe hydroxides. Copper concentrations at the finest grain size fraction initially increased quickly within the first day, followed by an asymptotic decrease which stabilize after 10 days, thereafter Cu concentrations slightly increase between 50 – 100 days. The behavior of Cu overlaps with the Fe concentrations indicating that Cu may adsorb onto Fe hydroxides. In coarser grain fractions, Cu increased initially, then slightly decreased after ~30 days. Zinc and Ni concentrations increased after ~50 days, consistent with the onset of olivine dissolution, as observed in Mg trends. Cobalt concentrations were generally low and steadily increased in fine-grained systems but mirrored Ca and Mg trends in coarser grain fraction experiments, indicating that clinopyroxene was likely the mineral releasing Co.

These experiments demonstrate that basaltic rocks and local groundwater in New Mexico can support significant CO₂ sequestration over relatively short timescales. Results show that finer grain sizes enhance reaction kinetics and improve CO₂ drawdown efficiency. The observed mobilization of critical elements (Cu, Ni, Zn, Co) from silicate minerals is particularly noteworthy for mine waste applications. Since mafic mine waste often retains relict hydrothermal alteration from ore-forming fluids, these materials may serve dual roles in CO₂ sequestration and critical metal recovery. These findings suggest a promising pathway for both climate mitigation and resource upcycling in mine-impacted regions.

References:

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