The role of fluid-inclusion composition on dynamic recrystallization in experimentally deformed quartz single crystals

Joyce Schmatz, Ineke Dijkstra, Alice Post, and Janos Urai

In order to study the effect of fluid composition on rheology and recrystallization we have deformed and annealed single crystals of natural quartz with different fluid inclusion compositions. Two samples contained mainly aqueous inclusions; one sample contained more gaseous inclusions. The samples were deformed in a Griggs-type solid medium apparatus at 1200 MPa confining pressure, temperatures ranging from 710 to 820°C and strain rates between 0.9 x 10–6 s-1 and 11 x 10-6 s-1. After deformation samples were held between 800-900°C and annealed for 2-279 hours. The gas-rich samples have a significantly higher strength than samples containing aqueous inclusions, resembling dry quartz samples. We relate the difference in strength to an effect of fluid composition on dislocation climb and/or glide.

Quartz recrystallized in all samples but recrystallization is much slower in the gas-rich samples. Recrystallized grains are elongated perpendicular to the shortening direction. They contain many fewer fluid inclusions than the starting material and many inclusions are found in their grain boundaries. This suggests that the fluid segregates to the grain boundaries during migration. The elongated shape may result from differences in thickness of a fluid film on the grain boundaries parallel and perpendicular to the shortening direction.