Experimental ‘syntectonic’ hydration of basalt

Abstract

This paper outlines in general terms the role of metamorphic transformations during rock deformation. Long duration, triaxial stress relaxation tests at 600°C. 160 MPa confining pressure on wet crushed basalt, sheared in sawcuts made in intact basalt cylinders are reported. Mechanical data are interpreted in terms of a material which shows a small strain-rate sensitivity to large variations in applied stress, but which work hardens dramatically as a result of marked microstructural changes which occur during straining. Permanent distortions of c. 20% were produced in the samples. Tests at 100 MPa pore-water pressure resulted in the formation of oriented overgrowths of amphibole, preferentially developed on less stressed interfaces between original grains and in pore spaces. Substantial redistribution of feldspar also occurred. At low pore pressures no amphibole formed and only feldspar was recrystallized. Much of the original porosity of the rock was eliminated in all cases. The mechanical and microstructural data are interpreted in terms of flow by some combination of grain sliding and diffusional accommodation, with the reprecipitated, diffused material being the product of a chemical reaction involving all original solid phases plus water. From a consideration of the effects of the reaction on the chemical potential gradients driving diffusion, it is concluded that although the syntectonic hydration exerts a profound effect on the microstructural and mineralogical development of the rock during deformation, there is no substantial enhancement of the rate of strain. Similar microstructural developments also occur in natural rocks deformed during retrogressive metamorphic events, and examples are described for comparison.