EFFECT OF MELT ON QUARTZITE STRENGTH

Quartzite rheology has been determined by performing experiments on natural quartzites, which contain minor amounts of muscovite and feldspars. Almost all of these experiments were performed at temperatures and pressures outside the stability field of quartz-muscovite aggregates (e.g. T > 800° C), which indicates that these experiments may have contained melt. In order to determine the effect of melt on the water fugacity (ƒ_H2O) dependence of quartzite strength, I performed both increasing and decreasing pressure-stepping experiments on two natural quartzites: Tana quartzite and Arkansas novaculite. By either increasing or decreasing pressure during these experiments I was able to determine if the melt formation caused a path dependence of the strength of quartzite.

Differential stresses of Tana quartzite samples ranged from 93 to 470 MPa and increased as a function of decreasing pressure in decreasing-pressure experiments but increased as a function of increasing pressure in increasing-pressure experiments. Differential stresses of Arkansas novaculite ranged from 269 to 394 MPa as pressure decreased, but the strength did not decrease as a function of pressure. Microstructures observed in all samples were consistent and included undulatory extinction, bulging recrystallization, recrystallized grains, deformation lamellae, and grain flattening. The maximum amount of melt observed was ~1%, while the minimum amount of melt observed was < 0.1%; this melt was oriented along grain boundaries parallel to σ₁. The slope of this relationship between strength and ƒ_H2O in Tana quartzite during decreasing-pressure stepping experiments is negative and generally consistent with previous experimental determinations of this relationship. However, the slope of the relationship between strength and ƒ_H2O during increasing-pressure stepping experiments is positive. These results indicate that the presence of melt affects the strength of quartzite and causes the strength to increase as a function of strain rather than decreasing ƒ_H2O. This effect may be due to melt preferentially absorbing water on grain boundaries, limiting the rate of recovery processes. As a result, the strength of natural quartzites predicted by flow laws derived from melt present experiments may be underestimated.