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Variations in the Ba isotopic composition of seawater are largely driven by the extent of barite precipitation in the marine photic zone and replenishment of Ba by upwelling and/or continental inputs. Pelagic barites offer a robust tool for tracing sources and sinks of Ba in the (paleo)ocean as they record these isotopic variations. Knowledge of the Ba isotope fractionation between barite and ambient waters is therefore imperative. Here, the Ba isotope fractionation between barite and Ba2+ (aq) under equilibrium conditions has been estimated by the three-isotope method with a 135Ba-enriched reactive fluid. The estimated Ba isotope fractionation was Δ137/134BaBarite-Ba2+ = −0.07 ± 0.08‰. Textural observations of barite crystals recovered up to 756 days of reaction reveal smoothing of solid surfaces but also typical dissolution features such as development of pits and cracks. Thus, dissolution/re-precipitation is likely the mechanism controlling the observed isotope exchange that is facilitated by the further development of porosity in the crystals. Additionally, the isotope exchange in the experimental runs fits a second-order law yielding a surface normalized isotope exchange rate of ∼2.8 × 10−10 mol/m2/s. This exchange rate could theoretically result in complete isotope exchange between pelagic barite with a typical edge size of 1 μm and ambient seawater or pore fluid within years, altering the barite's Ba isotopic composition during settling towards the seafloor and/or after deposition in marine sediments. Although there is considerable uncertainty in extrapolating experimental results to natural conditions and longer time scales, the rapid rates of exchange observed experimentally over short timescales suggest that isotope exchange in pelagic barite should be considered during interpretation of the Ba isotope composition as a paleoarchive.

Mineral-fluid interactions underpin element cycles, mobility of hazardous and valuable metals, and removal of CO2 from the atmosphere. The fractionation of isotopes provides insights into the processes controlling mineral-fluid reactions in both natural and human-influenced environments. Similarly, the isotopic compositions of minerals reflect the environmental conditions, such as temperature and fluid composition, at the time of mineral formation. Interpreting environmental conditions of the Earth’s past and designing strategies for remediation of pollutants and greenhouse gases requires an understanding of the mechanisms of mineral-fluid interaction. We use experimental approaches and analysis of stable isotopes (Mg, C, O, Ca, Si) to investigate processes controlling mineral-fluid interactions and the isotopic composition of minerals at Earth’s surface conditions, with a focus on the weathering reactions that remove CO2 from the atmosphere. Our experiments demonstrate that element exchange between carbonate minerals and fluids continues at bulk mineral-fluid equilibrium potentially resulting in alteration of isotopic signatures and immobilization/remobilization of metals. Similarly, isotope fractionation during non-stoichiometric wollastonite [CaSiO3] dissolution suggests mass exchanged is not unidirectional even far from chemical equilibrium. On the other hand, mineral dissolution-precipitation reactions are revealed to be dependent on water activity, controlled by relative humidity, with reactions effectively arrested below a threshold relative humidity. Together, our research explores the processes controlling mineral-fluid interactions at the mineral-surface scale, with implications for the global scale element cycles operating over geologic timescales.

The stability and longevity of carbonate minerals make them an ideal sink for surplus atmospheric carbon dioxide. Biogenic magnesium carbonate mineral precipitation from the magnesium-rich tailings generated by many mining operations could offset net mining greenhouse gas emissions, while simultaneously giving value to mine waste products. In this investigation, cyanobacteria in a wetland bioreactor enabled the precipitation of magnesite (MgCO3), hydromagnesite [Mg5(CO3)4(OH)2·4H2O], and dypingite [Mg5(CO3)4(OH)2·5H2O] from a synthetic wastewater comparable in chemistry to what is produced by acid leaching of ultramafic mine tailings. These precipitates occurred as micrometer-scale mineral grains and microcrystalline carbonate coatings that entombed filamentous cyanobacteria. This provides the first laboratory demonstration of low temperature, biogenic magnesite precipitation for carbon sequestration purposes. These findings demonstrate the importance of extracellular polymeric substances in microbially enabled carbonate mineral nucleation. Fluid composition was monitored to determine carbon sequestration rates. The results demonstrate that up to 238 t of CO2 could be stored per hectare of wetland/year if this method of carbon dioxide sequestration was implemented at an ultramafic mine tailing storage facility. The abundance of tailings available for carbonation and the anticipated global implementation of carbon pricing make this method of mineral carbonation worth further investigation.