High-temperature creep of partially molten plagioclase aggregates

We have investigated the high-temperature creep of synthetic labradorite (An₆₀Ab₄₀) between 1323 K and 1523 K at atmospheric pressure and low stresses (2 - 65 MPa). Average grain size varies from 12 to 16 μm. Samples contained up to 12 vol % melt (An₆₀Ab₄₀ glass). Fourier transform infrared measurements indicate that trace amounts of water (∼0.1 - 0.15 wt % H₂O) were incorporated during crystallization. This water was only partly released at high temperature and atmospheric pressure. At low stresses and temperatures, deformation of labradorite was controlled by grain boundary diffusion creep yielding a stress exponent of n ∼ 1 and an activation energy of 365 ± 25 kJ/mol (regime I). With increasing stress and temperature, mechanical data and microstructural observations indicate a transition to power law creep with n ∼ 3 (regime II). This transition occurs in both partially molten and melt-free aggregates. In regime I the strain rate of samples containing as much as 12 vol % melt is between 3 and 5 times higher than in fully crystalline specimens. The activation energy is only slightly affected by the melt content between 0 and 10 ± 2 vol %. Transmission electron microscopy shows that the amorphous phase resides in connected triple junctions or in unconnected pockets. Grain boundaries are not wetted except surrounding large melt pockets. The melt distribution of undeformed and deformed samples is similar. The strength and activation energy of synthetic labradorite are comparable to those for pure anorthite with trace amounts of water. Both plagioclase compositions are slightly weaker than synthetic diopside.