The effect of melt distribution on the rheology of plagioclase rocks

Synthetic plagioclase aggregates were fabricated by hot isostatic pressing glass powder with An(60)Ab(40) composition at a confining pressure of 300 M Pa and a temperature of 1223 K. The residual glass content ( approximate to 15 vol.%) was reduced to 1-3 vol.% by annealing the specimens at temperatu res between 1423 and 1523 K at 0.1 MPa. High-resolution electron microscopy showed that the residual glass was confined to grain triple junctions. Whe n these samples were annealed at 0.1 MPa and above the solidus (1620 K), th ey also contained silica-rich amorphous phases at grain boundaries. Other s amples were hot pressed at 1423 K; they often have grain boundaries contain ing thin amorphous films 5-50 nm wide. The chemical composition of the film s differs from pure labradorite with up to 80 wt.% SiO2. The volume fractio n of Si-rich glass increases up to approximate to 2 vol.% for samples that contain approximate to 1 wt.% water. Uniaxial creep experiments were perfor med at temperatures of 1373-1638 K and stresses between 2 and 50 MPa. The p lagioclase aggregates deformed by grain size sensitive creep with a stress exponent n approximate to 1. The activation energy for gain boundary diffus ion controlled creep of the partially molten aggregates was 400 +/- 20 kJ m ol(-1). It remained unchanged with varying glass content and glass topology . The strength of samples containing glass along grain boundaries is 4-10 t imes less than without amorphous grain boundary films. The transition betwe en diffusion and dislocation creep is shifted towards higher stresses for s amples that contain glass films at gain boundaries. (C) 2000 Elsevier Scien ce B.V. All rights reserved.