Deformation-Enhanced Fluid and Mass Transfer Along Central and Western Alps Paleo-Subduction Interfaces: Significance for Carbon Cycling Models

Abstract The interplay between fluid activity and deformation along subduction zone interfaces is poorly understood, and largely unknown is the relationship of this fluid activity to the efficiency of decarbonation and carbonate dissolution in mobilizing subducted carbon. The significance of this interplay, and its influence on subduction C flux, can be evaluated through study of well-exposed paleo-subduction interfaces in the Italian and Swiss Alps. Recently published research on decarbonation history in the Western Alps has indicated that, in intact volumes of metasedimentary, metabasaltic, and meta-ultramafic exposures away from major shear zones and containing fewer through-going veins, C as carbonate is largely retained to depths of up to 80-90 km (Cook-Kollars et al., 2014; Collins et al., 2015). This is in part due to a lack of infiltration by externally-derived H2O-rich fluids necessary to drive more extensive decarbonation and carbonate dissolution (Collins et al., 2015). Results from this study suggest that deformation-enhanced fluid infiltration can lead to mass transfer along subduction interface shear zones and in more densely fractured domains in which rocks experience high fluid-to-rock ratios. Along these interfaces, the abundance of carbonate-bearing veins and other metasomatic assemblages, and the C and O isotope compositions of carbonate minerals, are certainly consistent with enhanced fluid flow and related C mobility. At the three field localities examined in this study, there is a strong suggestion of infiltration by a far-traveled H2O-rich fluid with δ18OVSMOW of +9 to +11‰. These values could reflect mixtures of fluids emanating from metabasaltic, meta-ultramafic, and metasedimentary sources at deeper depths in the subducting slab and along the interface. The loss of C along such zones, by decarbonation and carbonate dissolution, could disproportionately contribute to the C loss from the slab/interface section and thus strongly influence subduction zone CO2 outputs via arc volcanism. However, recently published carbonate solubilities indicate that flow of H2O-rich fluid upward along such interfaces should result in the precipitation of carbonate, not dissolution, and that the large amount of carbonate precipitated in veins at all three localities could reflect fluids moving along the corresponding P-T trajectories. Thermal gradients in the uppermost parts of subducting sections and interfaces (particularly at depths of >80 km) could result in flow paths that are initially up-T, thus conceivably promoting carbonate dissolution. This flow would presumably be followed by flow down-T and down-P, and thus down the solubility gradient for calcite in H2O (potentially precipitating carbonate), as the fluids then move toward the surface along the interface.