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Evolution petrologique des lithospheres en subduction: approche experimentale in situ des transformations mineralogiques et de leurs cinetiques

Abstract : The evolution of a subduction zone is intimately linked with the petrological reactions in the subducting plate and their kinetics. Several examples showing this relation have been experimentally investigated using HP-HT devices (large volume press, diamond anvil cell) and synchrotron in situ X-ray diffraction. (i) The transformation rates of coesite into its low-pressure polymorph, quartz, have been determined. These kinetic data are used to discuss the preservation of coesite during its ascent to the Earth's surface, and the tectonic processes related to the exhumation of ultra-high pressure rocks. The use of the percentage of retrogression of natural coesite inclusions to constrain P-T-t paths is examined. (ii) The breakdown of antigorite under low H2O activity conditions results in a fluid discharge rate of the order of 3.10-6 to 3.10-8 m3fluid.m-3rock.s-1. This is faster than the viscous relaxation of serpentinites, and could lead to brittle failure or weakening of pre-existing faults. The dehydration of antigorite could thus provide an explanation for the seismicity in the lower plane of double seismic zones. (iii) The petrological assemblage of a mid ocean ridge basalt (MORB) under lower mantle conditions consists mostly of Mg-rich perovskite, Ca-rich perovskite and stishovite. Between 800 and 1150 km, two Al-rich phases occur, the Ca-ferrite type and the new aluminum-rich phase (NAL), and represent 20 wt% of the assemblage. At 1200 km approximately, the NAL phase disappears whereas all other phases are stable up to 1400 km depth. From 800 to 1400 km, the density of a thermally equilibrated oceanic crust is higher than that of the surrounding mantle. Moreover, the NAL disappearance triggers a 1% density rise, increasing the dragging effect of the oceanic crust in the uppermost lower mantle. This petrological change might be related to the seismic heterogeneities detected at ~1200 km depth beneath Pacific subduction zones. (iv) Finally, investigations on the chemical analysis of experimental samples, at a sub-micron scale, using a nano-SIMS ion probe are presented.
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Contributor : Jean-Philippe Perrillat <>
Submitted on : Wednesday, January 4, 2006 - 11:12:42 PM
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Jean-Philippe Perrillat. Evolution petrologique des lithospheres en subduction: approche experimentale in situ des transformations mineralogiques et de leurs cinetiques. Géologie appliquée. Université Claude Bernard - Lyon I, 2005. Français. ⟨tel-00011301⟩

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