HR: 10:35h
AN: V52A-02    [Abstracts]
TI: High pressure behaviour of hydrates and carbonates and the generation of C-O-H fluids in subduction zones: experimental constraints in mafic systems up to 5 GPa
AU: * Poli, S
EM: Stefano.Poli@unimi.it
AF: Universita' di Milano Dipartimento Scienze della Terra, Via Botticelli 23, Milano, 20133 Italy
AB: Volatile recycling in subduction zones is mostly controlled by transformations in phase assemblages bearing hydrates, carbonates and/or graphite/diamond. Experiments were carried out on a MORB bulk composition using piston-cylinder and multianvil machines at P from 1 to 5 GPa and T from 550 °C to 800 °C, in the presence of a fluid at variable C-O-H ratios ranging from pure H₂O to mixtures of oxalic acid dihydrate and silver oxalate. fH₂ was buffered at Ni-NiO(-H₂O) (NNO) and hematite-magnetite(-H₂O) (HM) buffers using a double capsule technique. Experiments in the presence of a C-O-H fluid show a large amphibole-carbonate phase field up to P < 2 GPa at NNO hydrogen fugacities, with calcite or dolomite at low pressure and with dolomite and magnesite above 1.6 GPa. Amphibole breaks down at 2.5-2.6 GPa, epidote persists to 2.7 GPa, 730°C and talc to 3.3 GPa 800 °C. Graphite is ubiquitous above 1.8 GPa at both hydrogen fugacities. Carbonates disappear at pressures above 2.0 GPa, being graphite the only C-bearing solid phase at fH₂ buffered by NNO. On the contrary, at HM hydrogen fugacities (oxygen fugacities close to GCO buffer in the inner charge), carbonate phase fields get wider with pressure: aragonite and/or dolomite are stable up to 2.4 GPa and at higher pressure they are replaced by coexisting magnesite and dolomite. Mg-calcite forms at 5.0 GPa 800 °C. Unexpectedly, lawsonite was found to coexist with magnesite at temperatures as high as 730 °C at 4.2 GPa and 700 °C at 3.0 GPa, revealing a thermal stability in C-O-H bearing systems exceeding that found in mafic assemblages in the presence of hydrous species only. In C-O-H bearing systems, complex mass-balance relations govern the partitioning of volatiles between fluid, hydrates, carbonates, and graphite/diamond at fixed fH₂. In natural high pressure rocks, the relative amounts of ferric/ferrous iron in garnet and clinopyroxene versus C/carbonate may impose internally buffered oxygen fugacities and therefore fluid speciation. Relatively CO₂-rich compositions are expected only at high fO₂ in the presence of graphite/diamond.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3630 Experimental mineralogy and petrology
DE: 8410 Geochemical modeling (1009, 3610)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005