Magnesium and carbon isotope fractionation during hydrated Mg-carbonate mineral phase transformations

The fractionation of carbon and magnesium isotopes is a potentially useful tracer of natural weathering in ultramafic catchments and engineered CO2 storage. To evaluate the use of carbon and magnesium isotopes as tracers of ultramafic weathering and CO2 storage, we assessed the carbon and magnesium isotope fractionation between hydrous Mg-carbonate minerals and fluid during a mineral phase transformation from nesquehonite [MgCO3 center dot 3H(2)O] to dypingite [Mg-5(CO3)(4)(OH)(2)center dot similar to 5-8H(2)O], two common products of ultramafic rock weathering. Batch reactor experiments containing nesquehonite were conducted at 5 degrees C, 25 degrees C, and 35 degrees C and the evolution of mineralogical composition, fluid composition, and isotopic composition were tracked over time. At 5 degrees C, the solid remained nesquehonite throughout the experiments, and isotopic equilibrium did not appear to be achieved between the solid and the fluid phase for either carbon or magnesium. At 25 degrees C, and 35 degrees C a transformation from nesquehonite to dypingite occurred by dissolution and re-precipitation, which resulted in extensive exchange of Mg and C between solid and fluid. The phase transformation caused the initial C and Mg isotopic composition of the solid phase to be overwritten. The extensive isotopic exchange during the phase transformation suggests C and Mg isotopes likely obtained approximate isotopic equilibrium between dypingite and fluid. For dypingite, the Delta(13)C(dyp-DIC )was 4.74 +/- 0.12 parts per thousand (VPDB) and 4.47 +/- 0.17 parts per thousand (VPDB) at 25 and 35 degrees C, respectively. The Delta Mg-26(meas)dyp-fluid between solid and the bulk fluid was -0.76 parts per thousand, and -0.98 +/- 0.08 parts per thousand for the 25 and 35 degrees C experiments, respectively. There was no clear impact of temperature on Mg or C isotope fractionation. The calculated Delta Mg-26(calc)dyp-Mg2+ between dypingite and the Mg2+ aquo species rather than bulk aqueous Mg values were positive. This indicates that if dypingite is formed by the incorporation of the free Mg2+ ion in the solid, the solid is preferentially enriched in the isotope of higher mass (Mg-26). This is opposite to anhydrous Mg-bearing carbonate minerals, which tend to be depleted in 26 Mg relative to the forming fluid. These data will help improve interpretation of carbon and magnesium isotope compositions measured in natural and engineered ultramafic weathering environments, and may help to trace the fate of anthropogenic CO2 during engineered CO2 storage efforts. (C) 2020 Elsevier Ltd. All rights reserved.