Carbon mobility and exchange in a plate-interface subduction mélange: a case study of meta-ophiolitic rocks in Champorcher Valley, Italian Alps

Abstract

Ultramafic and carbonate-rich rocks juxtaposed in an oceanic sedimentary mélange that experienced Alpine subduction (Champorcher, Aosta Valley, Italy) show evidence of metasomatic alteration at their contacts. The reactions that occurred at these contacts afford an assessment of the sources and compositions of fluids associated with the alteration, the degree and scale at which these reactions mobilized carbon. At these contacts, carbonate-rich rocks display calcite replacement by diopside and tremolite along foliations and pressure-dissolution planes as the result of decarbonation and/or carbonate dissolution. The associated ultramafic bodies record serpentine replacement by carbonates and formation of metamorphic veins hosting carbonate and carbonate + diopside + chlorite. These two sets of observations point to coeval, coupled decarbonation and carbonation reactions resulting in C mobility along the subduction interface (at about 60 km depth) but conceivably without considerable net loss and large-scale transport.

The δ18OV-SMOW values of all the samples analyzed in this study are lower than expected for oceanic protoliths (for marine limestone, carbonate with values of +28 to +30‰; for seafloor ophicarbonate somewhat lower), suggesting pervasive interaction of these rocks with externally derived fluids, as has been observed throughout the region (for calcschists such as the Schistes Lustrés, lowering values to +20 ± 2‰). The calcschist unit at Champorcher has δ18O (+21.9 to +23.6‰) falling into this regionally developed range; however, the other rocks at this locality (carbonate mélange/broken formation, metasomatic rinds, and veins) tend to have lower values (to as low as +13.5‰). These lower values appear to require interaction with a fluid with δ18O lower than that affecting the calcschist on a regional scale and more consistent with derivation from a mafic/ultramafic (ophiolitic) source. Some metamorphic veins showing carbonate δ18O values 1 to 2‰ lower than their hosts and the occurrence of metamorphic veins and metasomatic horizons with anomalously high 87Sr/86Sr point to circulation of isotopically distinct external fluids enriched in radiogenic Sr within the Champorcher suite, perhaps involving a source in devolatilizing terrigenous rocks.

Juxtaposition of rocks at scales observed in these mélange units could enhance the mobilization of C via decarbonation reactions if this deformation is accompanied by the infiltration by H2O-rich fluids capable of driving the reaction process. Information regarding the metasomatism within this hybrid carbonate-ultramafic unit bears on the question of C mobilization along subduction interfaces and whether the magnitude of any loss or gain at the scales investigated could significantly influence whole-margin C cycling.

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ORCID for Francesca Rotondo: orcid.org/0000-0002-3922-9254

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