HR: 0830h
AN: T11C-0416    [PDF]
TI: Stabilities of Dense Hydrous Magnesium Silicates in the Earth's Upper Mantle: From Water-Undersaturated Experiments
AU: * Komabayashi, T
EM: tkomabay@geo.titech.ac.jp
AF: Dept. Earth and Planet. Sci., Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AU: Omori, S
EM: omori@geo.titech.ac.jp
AF: Dept. Earth and Planet. Sci., Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AU: Maruyama, S
EM: smaruyam@geo.titech.ac.jp
AF: Dept. Earth and Planet. Sci., Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AB: Water (H$_{2}$O) has significant effects on the mantle dynamics such as melting temperature, rheological property, electrical conductivity, and possible origin of subduction zone seismicity. A group of dense hydrous magnesium silicates (DHMSs) are stable in the mantle system and, therefore, are possible candidates as water storages in the Earth_fs mantle. Most previous experimental studies showed the phase relations of the DHMSs in water-oversaturated condition, although excess fluid is not expected in deep upper-mantle condition. The post-antigorite (serpentine) assemblage is hydrous phase A, enstatite, and water at relatively cold region in the deep mantle, i.e. subduction zone. This assemblage should become water-absent assemblage of hydrous phase A and enstatite, due to the free water penetration to the surface. Our Schreinemakers analysis on the previous water-excess experiments predicted that water-absent solid-solid reaction is a key for the determination of the stability field of the DHMS in actual subduction process. In the present study, we investigated stability relations after phase A and enstatite assemblage in a water-undersaturated condition by the high-pressure experiments in a multi-anvil apparatus from 10 to 15 GPa and from 600 to 1100$\deg$C, in order to clarify the stabilities of the DHMSs in the deep upper-mantle conditions. The starting material is a mixture of reagent powders (MgO: 48.3$%$, SiO$_{2}$: 41.5$%$, and Mg(OH)$_{2}$: 10.2$%$, in mole), which is the composition of phase A and enstatite with water content of 3.67 wt.$%$ (phase A : enstatite = 7 : 71, in mole). The results show that phase A + enstatite transformed to forsterite + water at 10 GPa, 1000$\deg$C and at 11.5 GPa, 1100$\deg$C and to hydrous phase E + forsterite at 12.5 GPa, 800$\deg$C. The high-pressure limit of phase A and enstatite is represented by the solid-solid reaction: phase A + enstatite = phase E + forsterite. As the low-pressure limit of this assemblage by the dehydration reaction: forsterite + water = phase A + enstatite is located at around 5 GPa (Komabayashi et al., 2003 submitted), the assemblage of phase A and enstatite is stable over 7 GPa, correspondingly from 150-km to 380-km depth. During subduction, whole water in phase A is transported to phase E by the solid-solid reaction described above. Since the phase A and enstatite assemblage occurs over 200 km-depth, the physical properties of this mixture may have importance on both the dynamics and the seismological observation of the deep subduction zone. We will further discuss phase relations of the DHMSs in water-undersaturated condition down to the bottom of the upper mantle.
DE: 1025 Composition of the mantle
DE: 1655 Water cycles (1836)
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting