Abstract
Listvenites form by metasomatic transformation of variably serpentinized peridotites to carbonate-quartz rocks due to extensive reaction with CO$_2$-bearing aqueous fluids. This transformation sequesters large amounts of carbon since listvenites commonly contain >30 wt% CO$_2$. Although volumetrically a rare rock type, they occur in many ophiolites throughout much of the geological record, with preserved examples from the Archean to the present. Listvenites are highly interesting because they can form in the forearc mantle wedge, modulating deep carbon cycling. They further are natural analogues for optimal carbon sequestration by mineral carbonation.Here we elucidate the influence of different controlling variables and feedback mechanisms on natural listvenite formation, investigate which prerequisites and geodynamic settings are favorable, and discuss related implications for the deep carbon cycle and engineered CO2 storage by mineral carbonation. Using thermodynamic fluid infiltration-fractionation models that simulate idealized, step-wise carbonation flow-through experiments, we quantify expected changes in volume, mass, solute transfer (e.g. Mg, Si mobility), redox conditions and pH with reaction progress in dependence of protolith composition, infiltrated fluid composition, temperature and pressure. The models agree well with experiments and natural observations of high-T carbonation but have limitations at low temperature (T ≲ 130–150 °C) where kinetic effects in experiments limit the approach to equilibrium. Our modeling and assessment of typical CO$_2$ concentrations in metamorphic/hydrothermal fluids highlight that listvenite formation requires high time-integrated fluid flux, which in turn requires dynamic renewal of permeability despite reactive volume expansion. As most known listvenites crop out along tectonic contacts between crustal and ultramafic rocks in ophiolites that delineate major orogenic sutures, key factors controlling CO$_2$ supply are deviatoric stress and related deformation. Listvenite microstructures indicate that important hydro-mechanical-chemical feedback processes allowing continued fluid flux during carbonation are brittle fracturing and vein formation, reaction-enhanced ductile deformation, and local mass redistribution due to solute transfer. These feedback processes critically influence carbon flux pathways and the extent of CO$_2$ sequestration, but are not yet well understood as they are challenging to reproduce experimentally and by modeling. The CO$_2$ source for listvenite formation is most commonly inferred to be metamorphic devolatilization and/or dissolution of carbonate ± organic carbon bearing meta-sediments. Less commonly reported are mantle or magmatic CO$_2$ sources. First-order thermodynamic modeling indicates that listvenites may be a direct consequence of meta-sediment devolatilization in subduction zones, while their formation at low pressure conditions requires rapid, large scale ascent and cooling of deep sourced fluids along shear or fault zones. The latter setting is reminiscent of induced mineral carbonation by subsurface CO$_2$ injection. The microstructural and chemical record of listvenites thus provides a means to investigate hydro-mechanical-chemical feedbacks during carbonation reaction progress at scales that cannot be achieved in laboratory experiments. Understanding these mechanisms is crucial for developing and validating strategies of engineered subsurface CO$_2$ sequestration by mineral carbonation.