Ophicarbonate evolution from seafloor to subduction and implications for deep-Earth C cycling

Cannao, E.; Scambelluri, M.; Bebout, G.E.; Agostini, S.; Pettke, T.; Godard, M.; Crispini, L. (2020). Ophicarbonate evolution from seafloor to subduction and implications for deep-Earth C cycling. Chemical geology, 546(119626), pp. 1-29. Elsevier 10.1016/j.chemgeo.2020.119626

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The chemical and physical processes operating during subduction-zone metamorphism can profoundly influence the
cycling of elements on Earth. Deep-Earth carbon (C) cycling and mobility in subduction zones has been of particular
recent interest to the scientific community. Here, we present textural and geochemical data (CeO, Sr isotopes and bulk
and in-situ trace element concentrations) for a suite of ophicarbonate rocks (carbonate-bearing serpentinites) metamorphosed
over a range of peak pressure-temperature (P-T) conditions together representing a prograde subduction
zone P-T path. These rocks, in order of increasing peak P-T conditions, are the Internal Liguride ophicarbonates (from
the Bracco unit, N. Apennines), pumpellyite- and blueschist-facies ophicarbonates from the Sestri-Voltaggio zone (W.
Ligurian Alps) and the Queyras (W. Alps), respectively, and eclogite-facies ophicarbonates from the Voltri Massif. The
Bracco oceanic ophicarbonates retain breccia-like textures associated with their seafloor hydrothermal and sedimentary
origins. Their trace element concentrations and δ18OVSMOW (+15.6 to +18.2‰), δ13CVPDB (+1.1 to
+2.5‰) and their 87Sr/86Sr (0.7058 to 0.7068), appear to reflect equilibration during Jurassic seawater-rock interactions.
Intense shear deformation characterizes the more deeply subducted ophicarbonates, in which prominent
calcite recrystallization and carbonation of serpentinite clasts occurred. The isotopic compositions of the pumpellyitefacies
ophicarbonates overlap those of their oceanic equivalents whereas the most deformed blueschist-facies sample
shows enrichments in radiogenic Sr (87Sr/86Sr=0.7075) and depletion in 13C (with δ13C as low as −2.0‰). These
differing textural and geochemical features for the two suites reflect interaction with fluids in closed and open systems,
respectively. The higher-P-metamorphosed ophicarbonates show strong shear textures, with coexisting antigorite and
dolomite, carbonate veins crosscutting prograde antigorite foliation and, in some cases, relics of magnesite-nodules
enclosed in the foliation. These rocks are characterized by lower δ18O (+10.3 to 13.0‰), enrichment in radiogenic Sr
(87Sr/86Sr up to 0.7096) and enrichment in incompatible and fluid-mobile element (FME; e.g., As, Sb, Pb). These data
seemingly reflect interaction with externally-derived metamorphic fluids and the infiltrating fluids likely were derived
from dehydrating serpentinites with hybrid serpentinite-sediment compositions. The interaction between these two
lithologies could have occurred prior to or after dehydration of the serpentinites elsewhere. We suggest that decarbonation
and dissolution/precipitation processes operating in ancient subduction zones, and resulting in the mobilization
of C, are best traced by a combination of detailed field and petrographic observations, C, O and Sr isotope
systematics (i.e., 3D isotopes), and FME inventories. Demonstration of such processes is key to advancing our understanding
of the influence of subduction zone metamorphism on the mobilization of C in subducting reservoirs and
the efficiency of delivery of this C to depths beneath volcanic arcs and into the deeper mantle.

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