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We focused study on primary carbonate-bearing hydrothermal structures in undeformed pillow metabasalts (e.g. carbonate-filled cooling cracks in pillow cores) that are exceptionally well preserved in Alpine Corsica. The rock mainly consists of carbonate (both calcite and relict aragonite) + Ca-silicates, typically lawsonite, epidote and garnet, part of which likely represent relicts of the primary seafloor assemblage (epidote). The rocks exhibit clear evidence of interaction between carbonates and silicates. Modeling of redox reactions and time-integrated fluid fluxes yields estimated fluid composition and fluxes that are surprisingly rich in carbon. This result indicates that carbonic fluids percolating the subducting oceanic crust enhance additional carbonate devolatilization. A manuscript is in preparation and should be submitted by the end of the year (Vitale Brovarone et al., in prep.).
Recent work has shown the fundamental contribution of carbonate dissolution into the carbon fluxes at subduction zones (Ague and Nicolescu, 2014; Frezzotti et al., 2011; Kelemen and Manning, 2015). Owing to the relatively recent diffusion of this concept in the Earth Science communities, little is known on the fate of carbonic fluids produced by carbonate dissolution. Migration towards volcanic arc and volcanic degassing has been proposed in the attempt of reconciling the carbon input and output budgets of subduction zones. However, interaction of these fluids with shallower rocks may potentially generate new carbon reservoirs. PhD work of Francesca Piccoli (funded in part by this grant) focuses on peculiar carbonate-bearing rocks found along major lithologic boundaries in Alpine Corsica, which may have formed by precipitation of carbonate by external fluids bearing considerable dissolved carbon. Whole rock strontium isotope analyses will be performed by the end of the year in order to better constrain the source of Ca associated with the rock carbonation, and possibly the carbon source. A rough mass balance indicated that the amount of carbon that can be stored in the rock by high-pressure carbonation is comparable to some estimated carbon fluxes produced by carbonate dissolution. These results have the potential to identify a new important piece of the subduction zone carbon cycle after additional studies, most notably thermodynamic calculations. Initial results were presented at Goldschmidt2015 and a first paper is in preparation to be submitted soon (Piccoli et al., in prep.).
Several small grants and in-kind support in the form of personnel effort have leveraged Sloan funding by about 6:1.