Tectonically driven carbonation of serpentinite by mantle CO2: Genesis of the Castiglioncello magnesite deposit in the Ligurian ophiolite of central Tuscany (Italy)

Abstract

Carbonation of ultramafic rocks is a key piece of the global carbon cycle taking place from the seafloor to the surficial environment and becoming particularly efficient in the genesis of ultramafic-hosted magnesite deposits in the shallow crust. Even though these are exceptional occurrences for the study of carbonation reactions and the implementation of engineered CO2 storage solutions, their genetic model, in particular the role of tectonics and source of CO2, is still debated. In this study, we focus on the Castiglioncello magnesite deposit hosted in the Ligurian ophiolite of central Tuscany (Italy). Textural, mineralogical, and isotopic data indicate that here carbonation is the result of a serpentinite-hosted epithermal system where mantle CO2 mixed with meteoric water, became enriched in Mg2+ through serpentine alteration and precipitated magnesite in response to tectonic activity. More specifically, carbonation was driven by subsequent tectonic events during the Apennine orogenesis which: i) developed structural traps that allowed the concentration of CO2 into serpentinite lenses; ii) created crustal-scale pathways for the rising of mantle CO2 into the shallow crust; and iii) opened dilatational fault jogs where magnesite precipitated. Cyclical fracturation guaranteed the necessary permeability for the ingress of multiple batches of fluids, prolonged alteration, and repeated precipitation of magnesite, dolomite, and opal-CT/chalcedony. Our results highlight a fundamental control of tectonics on carbonation of ultramafic rocks in the shallow crust and the importance of appropriate crustal architecture for developing epithermal carbonation systems. Syn-tectonic epithermal carbonation systems provide an alternative view on CO2 storage in ultramafic rocks compared to the better-known post-tectonic supergene systems. The mantle origin of CO2 indicates that in extensional settings, transfer zones can be conveyors of CO2, and possibly other volatiles, from the Earth’s mantle to the surface.