Grain-scale permeability of texturally equilibrated rocks continued: synthetic amphibolite.

Permeability was measured on synthetic, monomineralic aggregates of fluorotremolite, which may be useful analogs for amphibole-dominated rocks in the lower crust. Tremolite aggregates were synthesized in the presence of tremolite-saturated 4M HF, with fluid fractions (φ ) ranging from 0.03 to 0.22, at 950° C and 1.4 GPa for 120 hours in piston-cylinder apparati. Permeability of the quenched and dried materials was measured at near-atmospheric conditions and scaled to a grain-diameter of 1 mm. At low fluid fractions, the tremolite aggregates were substantially less permeable than synthetic quartzites and marbles produced using a similar technique. Materials with φ less than 0.04 did not exhibit detectable permeable flow for periods exceeding 168 hours. Above φ = 0.04, the relationship between permeability (k) and porosity is approximated by the following: k = d² x (φ - 0.04)³ / 270. At φ = 0.22, permeability was similar to that of synthetic quartzite. Scanning electron images of polished sections revealed elongate fluorotremolite crystals with random orientation and an average mean sectional diameter of 10 microns. Pores within these materials were extensively faceted (>90%). Apparent intersections between pore walls averaged 46 degrees, but the small number of curved walls precluded assessing the dihedral angle. Therefore, the exact nature the pore network is not known, but we suspect that the large degree of faceting limits pore connectivity at low φ , reducing permeability relative to other texturally equilibrated, fluid-bearing rocks.