HR: 16:30h
AN: V24A-03    [Abstracts]
TI: Fast Kinetics of Dehydration of Hydrous minerals at Subduction Zone Conditions
AU: * Daniel, I
EM: isabelle.Daniel@univ-lyon1.fr
AF: Laboratoire des Sciences de la Terre, CNRS UMR5570, Ecole normale superieure de Lyon, Universite Claude Bernard Lyon 1, Bat. Geode - R6, 2 rue Raphael Dubois, Villeurbanne, 69622, France
AU: Chollet, M
EM: melanie.chollet@ens-lyon.fr
AF: Laboratoire des Sciences de la Terre, CNRS UMR5570, Ecole normale superieure de Lyon, Universite Claude Bernard Lyon 1, Bat. Geode - R6, 2 rue Raphael Dubois, Villeurbanne, 69622, France
AU: Koga, K T
EM: Kenneth.Koga@opgc.univ-bpclermont.fr
AF: Laboratoire Magmas et Volcans, CNRS UMR6524, Université Blaise Pascal, 5 rue Kessler, Clermont-Ferrand, 63038, France
AU: Petitgirard, S
EM: sylvain.petitgirard@esrf.fr
AF: Laboratoire des Sciences de la Terre, CNRS UMR5570, Ecole normale superieure de Lyon, Universite Claude Bernard Lyon 1, Bat. Geode - R6, 2 rue Raphael Dubois, Villeurbanne, 69622, France
AU: Petitgirard, S
EM: sylvain.petitgirard@esrf.fr
AF: European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, Grenoble, 38043, France
AU: Morard, G
EM: morard@esrf.fr
AF: European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, Grenoble, 38043, France
AB: In a number of subduction zones, earthquakes are not located on a single upper Benioff plane, but on two dipping planes separated by 20-40 km, defining a double seismic zone (DSZ). The likelihood origin for those earthquakes is hydraulic embrittlement, due fluids released by dehydration reactions. Such a model relies on the coincidence between the loci of the dehydration reaction and hypocenters, on the rheology of the rocks, and on their dehydration rate, which needs to be fast enough to locally raise fluid pressure. From phase equilibrium experiments, antigorite, talc, chlorite and 10Å phase appear as likely candidate minerals, since their dehydration occur also at P-T conditions compatible with those of the DSZ earthquakes. However, little was known about the speed of those reactions. Hence, we have measured the kinetics of dehydration of antigorite, talc, and 10Å phase, under H₂O unsaturated and saturated conditions, in order to decide whether their dehydration might trigger earthquakes in subduction zones. Measurements were done by real-time in situ X-ray diffraction at HP and HT in the Paris-Edinburgh press at the European Synchrotron Radiation Facility (ESRF, ID27), with time resolution shorter than 1min. Hydrous minerals were loaded in a cold-sealed double wall capsule of h-BN outside, and Ti inside closed by a thin Au foil at both ends. They were pressurized, and then subsequently heated in the Paris-Edinburgh press. P and T conditions were calculated from the unit-cell parameters of Au and NaCl, according to the cross- calibration method. While approaching the dehydration reactions, X-ray diffraction pattern were acquired and inspected for new diffraction lines to identify the onset of reaction. As dehydration began, P-T conditions were held constant, until transformation was significant, if not completed. The extent of transformation as a function of time was deduced from the relative intensity of the diffraction peaks of the reaction products vs the reactants. The transformation-time data were analysed in terms of the Avrami's theory. Grain size and shape were inferred from SEM observations, on the recovered samples. This allowed to calculate the rate of release of aqueous fluids by the dehydration of the various minerals as a function of depth, to compare dehydration and viscous deformation rates, and to discuss the possibility of fluid embrittlement.
DE: 3630 Experimental mineralogy and petrology
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting