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

Eberhard, Lisa; Pettke, Thomas (2021). Antigorite dehydration fluids boost carbonate mobilisation and crustal CO2 outgassing in collisional orogens. Geochimica et cosmochimica acta, 300, pp. 192-214. Elsevier Science 10.1016/j.gca.2021.02.030

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

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