TY - JOUR
T1 - Ophicarbonate evolution from seafloor to subduction and implications for deep-Earth C cycling
AU - Cannaò, E.
AU - Scambelluri, M.
AU - Bebout, G. E.
AU - Agostini, S.
AU - Pettke, T.
AU - Godard, M.
AU - Crispini, L.
N1 - Funding Information:
EC acknowledges funding by the Italian Society of Mineralogy and Petrology (SIMP award “Borsa di Studio per l'estero 2015”) for supporting his visit to Lehigh University, and the CNR for providing a Short-Term Mobility grant to visit the Bern University for in-situ analyses. The project has been supported by funds of the University of Genova and of the Italian MIUR to MS (MIUR PRIN projects 2012R33ECR_002 and 2017ZE49E7 ), of the USA- National Science Foundation to GEB (grant EAR-1119264) and of the IGG-CNR-P0000514 to SA. Constructive reviews by J. Alt and anonymous reviewer, and manuscript handling by Editor C. Chauvel, greatly improved the presentation of the manuscript and have been much appreciated.
Funding Information:
EC acknowledges funding by the Italian Society of Mineralogy and Petrology (SIMP award ?Borsa di Studio per l'estero 2015?) for supporting his visit to Lehigh University, and the CNR for providing a Short-Term Mobility grant to visit the Bern University for in-situ analyses. The project has been supported by funds of the University of Genova and of the Italian MIUR to MS (MIUR PRIN projects 2012R33ECR_002 and 2017ZE49E7), of the USA-National Science Foundation to GEB (grant EAR-1119264) and of the IGG-CNR-P0000514 to SA. Constructive reviews by J. Alt and anonymous reviewer, and manuscript handling by Editor C. Chauvel, greatly improved the presentation of the manuscript and have been much appreciated.
Publisher Copyright:
© 2020 Elsevier B.V.
PY - 2020/7/20
Y1 - 2020/7/20
N2 - 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 (C[sbnd]O, 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 pumpellyite-facies 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.
AB - 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 (C[sbnd]O, 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 pumpellyite-facies 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.
KW - C-O-Sr isotopes
KW - Deep carbon cycle
KW - High-pressure ophicarbonates
KW - Oceanic ophicarbonates
KW - Subduction zone
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U2 - 10.1016/j.chemgeo.2020.119626
DO - 10.1016/j.chemgeo.2020.119626
M3 - Article
AN - SCOPUS:85085269488
VL - 546
JO - Chemical Geology
JF - Chemical Geology
SN - 0009-2541
M1 - 119626
ER -