Mechanisms of secondary carbonate precipitation on felsic, intermediate and mafic igneous rocks: a case study for NW Scotland

Abstract

Silicate weathering is a natural regulator of atmospheric carbon dioxide (CO 2 ), where the majority of carbonate is stored in the oceans. In some instances, carbonates may form via bedrock weathering in terrestrial landscapes, but this is commonly noted in ultramafic rock owing to its desirable mineralogy. In NW Scotland, carbonate minerals surround felsic, intermediate and mafic igneous bedrock. Their abundance suggests that these rocks could potentially be targeted for carbonation, given the absence of other known cation sources nearby. We present geochemical and mineralogical data for bedrock samples and mineralogy and stable carbon and oxygen isotope data for the carbonate samples. Whole-rock geochemistry demonstrates that major abundances of CaO and MgO are present in the bedrock samples and would be sourced from plagioclase and pyroxene, which are target minerals for CO 2 mineralization. The stable carbon and oxygen isotope data demonstrate that there are probably mixed carbon sources (atmospheric and organic), and other environmental factors including temperature, CO 2 degassing or evaporation and nearby waters influence the isotopic composition. Results suggest that rocks traditionally overlooked in CO 2 mineralization studies have the potential to serve as a feedstock for carbonation, given the abundance of secondary carbonates found in NW Scotland.