HR: 11:20h
AN: T32B-05    [Abstracts]
TI: True Equilibrium Experiments on Dense Hydrous Magnesium Silicates (DHMS) in the MgO-SiO$_{2}$-H$_{2}$O System, and the Stability of Phase A, E, and Clinohumite in Mantle Compositions
AU: * Melekhova, E
EM: elena.melekhova@erdw.ethz.ch
AF: Institute for Mineralogy and Petrology ETH Zentrum, Sonneggstrasse,5, Zurich, 8092 Switzerland
AU: Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: Institute for Mineralogy and Petrology ETH Zentrum, Sonneggstrasse,5, Zurich, 8092 Switzerland
AU: Ulmer, P
EM: ulmer@erdw.ethz.ch
AF: Institute for Mineralogy and Petrology ETH Zentrum, Sonneggstrasse,5, Zurich, 8092 Switzerland
AB: Although none of the so-called Dense Hydrous Magnesium Silicates (DHSM) have been found as minerals in nature, they are of crucial importance for understanding both hydration and dehydration processes in the Earth's mantle. Since Ringwood and Major (1967), numerous experimental studies have focused on the determination of phase relations involving DHMS-phases and their stability fields within MgO-SiO$_{2}$-H$_{2}$O. However, most of these results are based on synthesis and unreversed experiments. In addition, fluid-saturated high pressure experiments suffer from high solubilities of the solid components in the fluid, which result in chemical zonations through Soret-diffusion. As a result, phases in excess of those allowed by the phase rule (in a homogeneous system) are commonly present in run products, and strong zonations in the run products may cause erroneous interpretations of the phase relations and equilibria. We overcome the problem of zonation and 'excess' phases by inverting the capsule at experimental conditions within the gravity field ('rocking' multi-anvil). Rocking and tilting of a chemically stratified fluid induces Rayleigh-Taylor instabilities, causing chemical re-homogenisation of the fluid. If rocking of the multi-anvil is frequent enough (1 turn/min) the chemical gradient in the fluid is reset before dissolution/reprecipitation processes cause zonations, we thus obtain a homogeneous run product containing exactly 2 solid phases (+fluid), which represent chemical equilibrium throughout the entire capsule. Reversed equilibrium experiment have been carried out with the 'rocking' multi-anvil at conditions near $900\deg$C, 11 GPa, around the invariant point where the stability fields of phase A+enstatite, phase E+forsterite, and forsterite+enstatite (+fluid, in model mantle compositions) were thought to encounter. Mineral compositions of the experimental charges were determined by electron microprobe, and phase identities confirmed by Raman spectroscopy. The results show that the above invariant point is metastable and that there is a narrow stability field of clinohumite (for mantle bulk compositions). The stability relations between phase A, phase E, enstatite, forsterite and clinohumite have been revised.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3630 Experimental mineralogy and petrology
DE: 3694 Instruments and techniques
DE: 3924 High-pressure behavior
DE: 1025 Composition of the mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting