HR: 0800h
AN: T41B-1196    [Abstracts]
TI: Kinetics of Antigorite Dehydration by in Situ X-ray Diffraction
AU: * Perrillat, J
EM: Jean-Philippe.Perrillat@univ-lyon1.fr
AF: Laboratoire de Sciences de la Terre UMR5570-CNRS-UCBLyon1-ENSLyon, Bat 402 Geode 2 Rue Raphael Dubois, Villeurbanne, 69622 France
AU: Daniel, I
EM: Isabelle.Daniel@univ-lyon1.fr
AF: Laboratoire de Sciences de la Terre UMR5570-CNRS-UCBLyon1-ENSLyon, Bat 402 Geode 2 Rue Raphael Dubois, Villeurbanne, 69622 France
AU: Koga, K
EM: Kenneth.Koga@ens-lyon.fr
AF: Laboratoire de Sciences de la Terre UMR5570-CNRS-UCBLyon1-ENSLyon, Bat 402 Geode 2 Rue Raphael Dubois, Villeurbanne, 69622 France
AU: Reynard, B
EM: Bruno.Reynard@ens-lyon.fr
AF: Laboratoire de Sciences de la Terre UMR5570-CNRS-UCBLyon1-ENSLyon, Bat 402 Geode 2 Rue Raphael Dubois, Villeurbanne, 69622 France
AU: Cardon, H
EM: Herve.Cardon@ens-lyon.fr
AF: Laboratoire de Sciences de la Terre UMR5570-CNRS-UCBLyon1-ENSLyon, Bat 402 Geode 2 Rue Raphael Dubois, Villeurbanne, 69622 France
AU: Crichton, W A
EM: crichton@esrf.fr
AF: European Synchrotron Radiation Facility, BP220, Grenoble, 38043 France
AB: The dehydration of serpentine minerals during the burial of oceanic slabs plays a significant role in the origin of magmatism and sismicity at subduction zones. Although recent high-pressure experiments studies show that antigorite is the only stable serpentine mineral at mantle conditions, the mechanisms and kinetics of its breakdown are still misunderstood. The dehydration of antigorite under pressure (1-5.5 GPa) and temperature (until 800øC) has been studied in situ by XRD, using a Paris-Edinburgh press installed at ESRF. Three kinds of starting materials were used: (a) a powder of natural antigorite, (b) a mix of antigorite powder (95 wt%) and crystal seeds of forsterite + enstatite (5 wt%), (c) samples a or b saturated in water, to study the influence of the water-content on the P-T conditions for dehydration. The X-rays being absorbed by metallic capsules (Au, Pt), the samples were loaded in boron nitride (BN) containers. BN capsules do not fully prevent fluids to escape from the sample, so that the water activity during the experiments was lower than one. The samples were first pressurised, and then slowly heated (10øC/min). The collection of diffraction spectra every 1-2 minutes during the temperature increase enables to follow, in real-time, the mineralogical reactions of antigorite breakdown. The sequence of XRD spectra reveals that the high-temperature stability limit of antigorite is reduced by 50-100øC under water-unsaturated conditions. Moreover, the decomposition of antigorite to the high-temperature products, forsterite + enstatite, proceeds via an intermediate assemblage of forsterite + "talc-like" phase, observed within a temperature interval of 130 +/- 20øC. The analysis of the transformation-time data using the Avrami model suggests that the breakdown of antigorite and the "talc-like" phase is kinetically controlled by surface growth processes at the edges of grains. The overall transformation rates are 10 to 100 times faster than those observed in water-saturated experiments, and show that the water activity is a strong driving force for dehydration. In the light of these kinetic results, we examine the possibility of dehydration induced seismicity in subducting oceanic slabs.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3630 Experimental mineralogy and petrology
DE: 3660 Metamorphic petrology
DE: 3900 MINERAL PHYSICS
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting