Detection and quantification of low levels of carbonate mineral species using thermogravimetric-mass spectrometry to validate CO2 drawdown via enhanced rock weathering

Enhanced rock weathering (ERW) is a promising technology being actively investigated to ameliorate anthropogenic climate change. ERW accelerates atmospheric carbon dioxide (CO2) drawdown in agricultural soils following the addition of crushed, silicate rocks. The application of basalt, an abundant silicate rock, has widely been proposed for ERW. In order to measure and model the efficacy of ERW, it is critical to fully characterise the mineralogical composition of the basaltic materials being deployed in laboratory to catchment scale experiments. As previously demonstrated on analogous basaltic rock and soil materials on Mars, thermal analytical methods such as coupled thermogravimetric analysis-mass spectrometry (TGA-MS), provide a potentially rapid, effective technique to identify and also quantify the carbonate mineral species present in ERW basalts and their weathering products. Our study of six different basaltic materials used in ERW experiments, from widely different geological and geographical settings, suggests that TGA weight losses can provide useful information on the low-levels of carbonate minerals present. Furthermore, carbonate mineral identification and quantification derived from the accompanying MS-detected CO2 evolution and corroborated by SEM-EDS mapping, is capable of detecting even lower concentrations with detection limits below 100 ppm for calcite. TGA-MS therefore offers significant advantages over the traditionally-applied inorganic carbon analyses. Carbonate mineral weight losses encountered during TGA and their accompanying CO2-evolutions have previously been ascribed to relatively small, defined temperature ranges. However, crucially, this study has revealed the concentration-dependent, large-scale variation in the mass spectrometric peak CO2-evolution for a range of carbonate minerals. A knowledge of such parameters is imperative for the accurate interpretation of basalt TGA-MS output, the characterisation of these materials and assessment of ERW technologies.