Antigorite dehydration fluids boost carbonate mobilisation and crustal CO2 outgassing in collisional orogens

The processes of carbon mobilisation at convergent plate boundaries are hotly debated. Recent findings suggest that release along subduction geothermal gradients may well be more relevant than previously thought; however, it has remained difficult to achieve steady state atmospheric CO2 conditions over geological time scales based on current volatile cycle models. Here, we report meta-ophicarbonate rocks in the contact metamorphic aureole Bergell intrusion, Val Malenco, European Alps, reached T-P least 650 °C 0.35 GPa. We demonstrate by combined field evidence, geochemistry, and closed-system thermodynamic modelling 50% rock carbonate been mobilised locally response reactive fluid flow upon progressive isobaric heating from 350 >650 °C. Despite complete mineral transformation antigorite + calcite devolatilisation reaction, resulting tremolite-ophicarbonate preserves original ophicarbonate texture olivine-chlorite clasts, representing former antigorite-serpentinite fraction, embedded a monomineralic tremolite matrix formed fraction often exceeded 50 vol%. Closed system an composition 80 wt% serpentinite +20 reveals rock-buffered XCO2 values evolve <0.09 as high ?0.16, total H2O-CO2 released 14 wt%; highly sensitive bulk composition. Major trace element geochemistry history peridotite melt depletion followed ocean floor hydration formation, consistent with continent transition (OCT) setting mantle Mesozoic. This is demonstrated positive anomalies B, U, As, Sb, Bi, W, notably primitive normalised REE patterns basically identical precipitated Jurassic seawater except for its negative Eu anomaly. Prograde diopside possess enrichments Sr inherited reactant calcite, also supported good match between measured modelled silicate compositions. Specific geochemical characteristics prograde imply mediated, open processes. Antigorite dehydration ingress, likely produced neighbouring antigorite-serpentinites, have shifted compositions towards higher SiO2/MgO ratios increase B contents mobile elements such Sr. massive, documented here occurred P-T similar those can achieved collisional orogenic settings, whose clockwise P-T-t paths characterised further initial decompression. They thus large angle reactions silicate-carbonate isopleths, fostering peak temperatures moderate low pressures (in region >700 – 1.0 GPa >600 0.3 GPa). Barrovian metamorphism, too, reach 800 1 Amagmatic this way will eventually atmosphere via dispersion groundwater table diffuse outgassing. huge uncertainties associated quantification fluxes, scale global cycling models should rigorously explore significance these contributions.