HR: 16:15h
AN: V24A-02    [Abstracts]
TI: Effect of Water Activity on Mineral Reaction Overstepping at High P and T: Experimental Approach and Petrological Implications
AU: * Brunet, F
EM: brunet@geologie.ens.fr
AF: Ecole Normale Superieure - CNRS, Laboratoire de Geologie, 24 rue Lhomond, Paris, 75005, France
AU: Gasc, J
EM: gasc@geologie.ens.fr
AF: Ecole Normale Superieure - CNRS, Laboratoire de Geologie, 24 rue Lhomond, Paris, 75005, France
AU: Verlaguet, A
EM: verlaguet@geologie.ens.fr
AF: Ecole Normale Superieure - CNRS, Laboratoire de Geologie, 24 rue Lhomond, Paris, 75005, France
AU: Findling, N
EM: findling@geologie.ens.fr
AF: Ecole Normale Superieure - CNRS, Laboratoire de Geologie, 24 rue Lhomond, Paris, 75005, France
AU: Lathe, C
EM: christian.lathe@desy.de
AF: DESY HASYLAB, Notkestr. 85, Hamburg, 22607, Germany
AU: Goffe, B
EM: bruno.goffe@ens.fr
AF: Ecole Normale Superieure - CNRS, Laboratoire de Geologie, 24 rue Lhomond, Paris, 75005, France
AB: It has long been considered, on the basis of laboratory experiments, that mineral transformation kinetics are high compared to the rates of P-T changes responsible for metamorphic processes. Natural examples, however, clearly show that average mineral transformation kinetics are much slower than expected. One way to reconcile natural and experimental data is to consider fluid availability (especially water) as the limiting factor. There is a growing number of field and geophysical evidences for prograde metamorphic reaction overstepping, especially with respect to eclogite formation which is of importance for the Earth's dynamics due to the associated density changes. The role of water availability on reaction overstepping in metamorphic processes is a challenging experimental field. Reaction overstepping means that nucleation is delayed and therefore the role of water upon passing mineral nucleation barriers is the issue to be addressed. A way to favor, experimentally, nucleation over growth is to investigate systems which are far from equilibrium. Using the MAX80 cubic multi-anvil press installed on the German synchrotron (HASYLAB-DESY, Hamburg), we have monitored using in-situ X-ray diffraction, the progress of reaction: Ca(OH)₂ + MgCO₃ = CaCO₃ + Mg(OH)₂, at 1.7 GPa for temperatures below 600°C (i.e., in the field of aragonite). These experiments were performed either under controlled dry conditions (low H₂O activity) or under the intrinsec humidity of the pressure assembly; the Ca(OH)₂ + MgCO₃ starting material, composed of fine powder, being initially submitted to the ambient air moisture. Under these pressure and temperature conditions, the free energy of this exchange reaction is of -30 to -35 kJ.mol.^-1. Additional experiments were performed under excess water in a piston-cylinder apparatus (starting material sealed together with water in a gold capsule) at 1.7 GPa and 150°C for five different run durations. For all experiments, the plot of the reaction progress as a function of time displays shapes which are typical of nucleation-dominated reactions. In order to attain the same nucleation kinetics under dry and excess-water conditions, two highly contrasted temperatures are required (550 and 150°C, respectively). Furthermore, under dry conditions, Arrhenius plot shows a high activation energy of ca. 570 kJ.mol^-1 (in the 550 - 600°C range), in line with the notion of overstepping.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3630 Experimental mineralogy and petrology
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting