An experimental study on aqueous fluid connectivity in amphibolitic lower crust

Aqueous fluids in the crusts have a prominent role in promoting rock failure and in element transport. Recent advances in magnetotelluric observations have revealed extensive distribution of low electrical resistivity regions in the non-magmatic arc crusts, leading to the interpretation that aqueous fluids have long-range interconnection laterally of dimension comparable to their depth, i.e., tens of kilometers in the lower crust. However, little is known about micro- and macroscopic structures of the fluid-bearing rocks responsible for the long-range interconnectivity, which are also crucial to understand the transport and rheological properties of the crust. In order to constrain the grain-scale pore fluid geometry of the mafic lower crust, we have carried out sintering experiments of aqueous fluid-bearing amphibolite in the lower crustal pressure and temperature conditions. The experiments have been performed by using a piston-cylinder apparatus at 0.7 GPa and 600°C for 1-2 weeks. Natural fine powder (< 2 μm) of amphibolite from Ichinomegarta maar, NE Japan, was used as a starting material. Anorthite powder was also prepared from the coarsely crushed amphibolite by hand-picking and by using polytungstate heavy liquid. The starting materials were loaded into Ag capsule with 0.1- 3.0 wt.% of deionized distilled water. Oxalic acid dihydrate was used to examine the effect of CO2. The polished cut surface of the run products were observed with a FE-SEM. At the triple junctions composed of anorthite and fluid, curved-curved junctions are dominant (>80 %). The dihedral angle between anorthite and fluid in the anorthosite systems is 79° for H2O and 92° for a CO2-rich fluid (H2O : CO2 = 1.0 : 1.6). By contrast, most of the pores (>90 %) are faceted at hornblend-hornblend-fluid triple junctions. At the hornblend-anorthite-fluid triple junctions, hornblend is extensively faceted, while anorthite is mostly curved. The fluid distributed homogeneously in the run products, showing no preference to the interphase boundary nor monomineralic grain boundaries. These experimental results suggest that grain scale fluid distribution in the amphibolitic lower crust is dominantly controlled by the facets of amphiboles in the amphibole-rich composition, while by dihedral angles in the anorthite-rich one. Price et al. (2006) synthesized extensively faceted amphibolite (Fluoro-tremolite rock) and showed that its permeability had an apparent threshold at a porosity of 0.04. Although the dihedral angles between anorthite and fluids obtained in this study are slightly lower than those reported in the previous experiments done at higher pressure and temperature (92° for H2O, 800°C, 0.8GPa and >120° for CO2-rich fluid, 900°C, 1.0GPa; Yoshino et al., 2002), they are clearly higher than the threshold value for interconnection at small fluid fraction (ca. 60°). Therefore, high fluid fraction is necessary for the grain-scale fluid interconnection in the amphibolitic lower crust, as long as crystal distribution is isotropic. The only possible case for the fluid interconnection at a low fluid fraction would be that amphibole-rich crust is strongly foliated and fluids exist parallel to the foliation, in between the facets of amphibole crystals.