Tectonic mixing of slab-and mantle-derived materials at the interface between converging plates highly enhances fluid-mediated mass transfer from the slab to the overlying mantle. Subduction melanges can provide information about the interaction among different slices accreted at plate interface domains, with implications on the tectonic and geochemical evolution of the plate-interface itself. At Cima di Gagnone, pelitic schists and gneiss enclose chlorite harzburgite and garnet peridotite lenses, like in subduction melanges located in-between downgoing slabs and overlying mantle. These peridotites host MORB-type eclogite and metarodingite, and derive from dehydration of serpentinized mantle protoliths. Their enrichment in fluid-mobile B, As, Sb, U, Th is the result of an early-stage oceanic serpentinization, followed by interaction with host metasediments during subduction burial. Here we define the element exchange process in the Gagnone melange by means of the B, Sr and Pb isotope analysis of its main lithologies (ultramafic, mafic rocks and paragneiss). The Sr-87/Sr-86 and Pb-206/Pb-204 ratios of ultramafic rocks (0.7090-0.7124 and 18.292-18.837, respectively) show enrichments in radiogenic Sr and Pb after exchange with the host paraschist (up to 0.7287 Sr-87/Sr-86; 18.751 Pb-206/Pb-204). The delta B-11 values of peridotites (down to -10 parts per thousand) point to a combined effect of (1) B-11 release to deserpentinization fluids (serpentinized protoliths likely had positive delta B-11 and lower radiogenic Sr, Pb), and of (2) exchange with fluids from the surrounding metasediments. The whole Gagnone rock-suite is finally overprinted by retrograde fluids that essentially bring to an increase in radiogenic Pb (about 19.0 Pb-206/Pb-204) and to values of 0.710 Sr-87/Sr-86 and of -10 parts per thousand delta B-11. The recognition of different stages of interaction between mantle rocks and sedimentary/crustal reservoirs allows us to define the geochemical effects related to the early coupling of such rocks along the plate-interface. Our study shows that ultramafic rocks involved in subduction-zone metamorphism and serpentinization uptake radiogenic Pb and Sr released by associated sedimentary reservoirs. The exchange process envisioned here is not only representative of subduction melanges: it can also be a proxy of mass transfer between slab and serpentinized supra-subduction mantle, as occurs in forearcs. Dehydration of the Gagnone-type serpentinized mantle releases crust-derived components to arcs, without direct involvement of metasediment dehydration and/or melting in subarc environments. The retention of appreciable amounts of fluid-mobile elements, radiogenic Pb and Sr in dehydrated Gagnone peridotites has implications on element recycling in the deep Earth's mantle. (C) 2015 Elsevier Ltd. All rights reserved.
B, Sr and Pb isotope geochemistry of high-pressure Alpine metaperidotites monitors fluid-mediated element recycling during serpentinite dehydration in subduction mélange (Cima di Gagnone, Swiss Central Alps)
Agostini S;Tonarini S;
2015
Abstract
Tectonic mixing of slab-and mantle-derived materials at the interface between converging plates highly enhances fluid-mediated mass transfer from the slab to the overlying mantle. Subduction melanges can provide information about the interaction among different slices accreted at plate interface domains, with implications on the tectonic and geochemical evolution of the plate-interface itself. At Cima di Gagnone, pelitic schists and gneiss enclose chlorite harzburgite and garnet peridotite lenses, like in subduction melanges located in-between downgoing slabs and overlying mantle. These peridotites host MORB-type eclogite and metarodingite, and derive from dehydration of serpentinized mantle protoliths. Their enrichment in fluid-mobile B, As, Sb, U, Th is the result of an early-stage oceanic serpentinization, followed by interaction with host metasediments during subduction burial. Here we define the element exchange process in the Gagnone melange by means of the B, Sr and Pb isotope analysis of its main lithologies (ultramafic, mafic rocks and paragneiss). The Sr-87/Sr-86 and Pb-206/Pb-204 ratios of ultramafic rocks (0.7090-0.7124 and 18.292-18.837, respectively) show enrichments in radiogenic Sr and Pb after exchange with the host paraschist (up to 0.7287 Sr-87/Sr-86; 18.751 Pb-206/Pb-204). The delta B-11 values of peridotites (down to -10 parts per thousand) point to a combined effect of (1) B-11 release to deserpentinization fluids (serpentinized protoliths likely had positive delta B-11 and lower radiogenic Sr, Pb), and of (2) exchange with fluids from the surrounding metasediments. The whole Gagnone rock-suite is finally overprinted by retrograde fluids that essentially bring to an increase in radiogenic Pb (about 19.0 Pb-206/Pb-204) and to values of 0.710 Sr-87/Sr-86 and of -10 parts per thousand delta B-11. The recognition of different stages of interaction between mantle rocks and sedimentary/crustal reservoirs allows us to define the geochemical effects related to the early coupling of such rocks along the plate-interface. Our study shows that ultramafic rocks involved in subduction-zone metamorphism and serpentinization uptake radiogenic Pb and Sr released by associated sedimentary reservoirs. The exchange process envisioned here is not only representative of subduction melanges: it can also be a proxy of mass transfer between slab and serpentinized supra-subduction mantle, as occurs in forearcs. Dehydration of the Gagnone-type serpentinized mantle releases crust-derived components to arcs, without direct involvement of metasediment dehydration and/or melting in subarc environments. The retention of appreciable amounts of fluid-mobile elements, radiogenic Pb and Sr in dehydrated Gagnone peridotites has implications on element recycling in the deep Earth's mantle. (C) 2015 Elsevier Ltd. All rights reserved.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
