HR: 1330h
AN: T42A-0285    [PDF]
TI: Grain-Scale Distribution of Aqueous Fluid in Wherlites
AU: * Ouchi, T
EM: ouchitmh@ganko.tohoku.ac.jp
AF: Inst. Mineral. Petrol. Econ. Geol., Graduate School of Science, TOHOKU Univ., Aobaku, Sendai, 9808578 Japan
AU: Nakamura, M
EM: nakamm@mail.tains.tohoku.ac.jp
AF: Inst. Mineral. Petrol. Econ. Geol., Graduate School of Science, TOHOKU Univ., Aobaku, Sendai, 9808578 Japan
AB: Aqueous fluid is one of the most important geological fluids in the Earth's upper mantle. The distribution of aqueous fluid affects not only to rheological properties and seismic attenuation, but also to material transport. Recently, many dihedral angles between minerals and fluids have been reported in order to evaluate connectivity of the fluids in rocks. Watson (1999) proposed the concept of lithologic partitioning of fluids. Grain size also affects the fluid distribution in rocks (Wark \& Watson, 2000). Fujii et al. (1986) pointed out that in the Ol - Opx - basalt system, the connectivity of melt through grain edges was interrupted by Opx grains whose dihedral angle with melt is larger than 60 degree. Processes that control the fluid distribution in multi-phase rocks, however, are not fully understood. In this paper, we report preferential distribution of pore-fluid in the phase-boundaries (PBs), which is caused by the fact that migration of grain boundary of the same phases (GBs) dragging the fluid ceases at PB. We have synthesized wherlites with various forsterite (Fo) / diopside (Di) ratios with 1 - 1.5 wt percent water at 1.2 GPa and 1200 deg.C for 3 days and a week. Preference of aqueous fluid for one phase relative to the other, due to the differences in relative mineral-fluid interfacial energies, was not expected, because dihedral angles of Fo-fluid and of Di-fluid are similar at the experimental conditions. Most of the fluid was distributed in the GBs as pore fluids and no fluid inclusion was observed. We classified the pore fluids into two types, those surrounded by GBs (A-type) and by PBs (B-type). Regardless of Fo/Di ratios, proportion of B-type was larger in 1 week runs than in 3 days runs. The 80-90 percent of pore fluids were B-type in the 1 week runs. This result can be explained by the difference in mobility between GB and PB: namely, the pore fluids are dragged by the GBs much more rapidly than the PBs. During grain-growth of multi-phase rocks, pore fluids dragged by the GBs bump against the other mineral surface. This result leads to the implication that the connectivity of fluid in multi-phase rocks would be strongly affected by properties of the triple junctions composed of two different mineral phases and fluid, i.e., dihedral angle, faceting, and spatial distribution.
DE: 1236 Rheology of the lithosphere and mantle (8160)
DE: 5104 Fracture and flow
DE: 8045 Role of fluids
DE: 8434 Magma migration
SC: Tectonophysics [T]
MN: 2003 Fall Meeting