Fluids released from the slab by progressive dehydration reactions during deep subduction can have strong petrological and geochemical effects, inducing metasomatism into the mantle wedge. Deep-subduction fluids must be considered an integral part of the HP-UHP phase assemblages and, in the last twenty years, their chemical-physical properties have been subject of experiments, thermodynamic models, and field studies. Deep-subduction fluids are almost ubiquitously trapped in eclogite-facies crustal suites as inclusions that still preserve firsthand information on their chemical composition and origin. Thus, data from natural fluid inclusions can be combined with those from experiments and thermodynamics to investigate the chemical-physical properties of fluids released during subduction, their solvent and transport capacity, and the subsequent implications for the element recycling. Three distinct populations of fluid inclusions have been observed in HP and UHP metamorphic rocks: i) chloride-bearing aqueous fluid inclusions and/or gaseous fluid inclusions, ii) multiphase-solid inclusions, and iii) melt inclusions. Their study reveals that the chemical composition of natural deep-subduction fluids is more complex than that considered by experiments. At forearc depths, moderate concentrations of chlorides, Si, Al, and alkalis, ± non-polar gases are present in water-dominated solutions; at subarc depths, mobile water-rich phases gradually increases the amounts of aluminosilicate components (e.g., Si, Al, Ca, Fe, alkalis, Ti, Zr, (SO4)2-, (CO3)2-) and trace elements at rising temperature. Trace element patterns show enrichments in LILE, U, Pb, LREE, and depletion in HFSE, and result chemically controlled by stabilization/destabilization of some hydrous and accessory minerals. These data are in agreement with experiments of mineral dissolution and element fractionation at deep subduction and also support experiments and thermodynamics on physical properties of deep-subduction fluids. At forearc depths, fluids are supposed to have properties similar to crustal fluids, i.e. dominated by halide ligands. In contrast, element solubility and transport of aqueous fluids released at subarc depths are supposed to be governed by polymerization, which is directly proportional to pressure and temperature. On the contrary, the finding of substantial amounts of dissolved oxidized carbon in natural fluid inclusions from deep-subducted rocks reveals that the deep transport of carbonates by aqueous fluids is a relevant process, and implies a reconsideration of the petrological models supporting liberation of C into the mantle wedge as CO2. Although hampered by some analytical difficulties, in the coming years the research on fluid inclusions in HP-UHP metamorphic rocks will provide exciting new results, supplying the added value to experimental and thermodynamic data.
Fluid inclusions as tracers for the chemical-physical behaviour of deep-subduction fluids.
FERRANDO, Simona;
2014-01-01
Abstract
Fluids released from the slab by progressive dehydration reactions during deep subduction can have strong petrological and geochemical effects, inducing metasomatism into the mantle wedge. Deep-subduction fluids must be considered an integral part of the HP-UHP phase assemblages and, in the last twenty years, their chemical-physical properties have been subject of experiments, thermodynamic models, and field studies. Deep-subduction fluids are almost ubiquitously trapped in eclogite-facies crustal suites as inclusions that still preserve firsthand information on their chemical composition and origin. Thus, data from natural fluid inclusions can be combined with those from experiments and thermodynamics to investigate the chemical-physical properties of fluids released during subduction, their solvent and transport capacity, and the subsequent implications for the element recycling. Three distinct populations of fluid inclusions have been observed in HP and UHP metamorphic rocks: i) chloride-bearing aqueous fluid inclusions and/or gaseous fluid inclusions, ii) multiphase-solid inclusions, and iii) melt inclusions. Their study reveals that the chemical composition of natural deep-subduction fluids is more complex than that considered by experiments. At forearc depths, moderate concentrations of chlorides, Si, Al, and alkalis, ± non-polar gases are present in water-dominated solutions; at subarc depths, mobile water-rich phases gradually increases the amounts of aluminosilicate components (e.g., Si, Al, Ca, Fe, alkalis, Ti, Zr, (SO4)2-, (CO3)2-) and trace elements at rising temperature. Trace element patterns show enrichments in LILE, U, Pb, LREE, and depletion in HFSE, and result chemically controlled by stabilization/destabilization of some hydrous and accessory minerals. These data are in agreement with experiments of mineral dissolution and element fractionation at deep subduction and also support experiments and thermodynamics on physical properties of deep-subduction fluids. At forearc depths, fluids are supposed to have properties similar to crustal fluids, i.e. dominated by halide ligands. In contrast, element solubility and transport of aqueous fluids released at subarc depths are supposed to be governed by polymerization, which is directly proportional to pressure and temperature. On the contrary, the finding of substantial amounts of dissolved oxidized carbon in natural fluid inclusions from deep-subducted rocks reveals that the deep transport of carbonates by aqueous fluids is a relevant process, and implies a reconsideration of the petrological models supporting liberation of C into the mantle wedge as CO2. Although hampered by some analytical difficulties, in the coming years the research on fluid inclusions in HP-UHP metamorphic rocks will provide exciting new results, supplying the added value to experimental and thermodynamic data.
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