Experimental deformation of a glassy basalt

Deformation experiments were conducted with the intention of identifying how the mechanical behavior and deformation microstructures of a basalt are influenced by the presence of glass that can crystallize to metamorphic minerals or flow. A glassy plagioclase-rich basalt was tested for periods of up to 649 hours at temperatures of 675-875°C, a confining pressure of 1.0 GPa, and strain rates of 10-4-10-7 S-I. Over the whole range of experiments, the basalt deformed ductilely in mechanical and textural steady state. At low temperatures (675-775°C), plagioclase crystals deformed by microcracking and twinning, and the glass did not flow into intracrystalline cracks. At higher temperatures (825-875°C), the samples were notably weaker, plagioclase crystals deformed by twinning and dislocation glide, and the glass flowed into cracks. In all samples that were tested for more than 30 hours, neoblasts of dark mica and quartz crystallized at the expense of K-rich glass + enstatite + H2O. This neomineralization did not affect the strength of the basalt, presumably because it affected only 50 vol. % of each sample. Changes in plagioclase deformation mechanism from twinning and microcracking at fast strain rates to dislocation glide at slow strain rates, may, however, reflect an increase in effective pressure caused by crystallization of the glass.