Normal grain growth of quartz by experiment and discussion on the effect of grain size reduction by deformation in natural conditions
Résumé
Grain size is an important factor that contributes to rheology. Grain size increases or decreases by grain growth or grain size reduction, in which the latter includes fracturing, dynamic recrystallization, dissolution-precipitation, and reaction. These two opposite mechanisms would occur in nature. Quartz is a major constituent of the Earth's crust. However, little is known about its grain growth parameters. In this study, we experimentally determined grain growth laws of quartz aggregates. We performed experiments using a piston cylinder. We prepared two quartz samples; novaculite as a quartz aggregate whose grain size is ~3 µm and natural quartz powder whose grain size is ~2 µm. We enclosed the two samples with added water of up to 10 wt% in a platinum capsule. Experimental conditions were under pressure of 1.0-2.5 GPa, temperature of 800-1100 °C, and durations of 6-240 hours. Normal grain growth occurred in these two samples, but we did not see differences in grain growth due to differences in amounts of added water. The powder sample showed porosities of ~10 vol %, which caused a slightly slower grain growth rate than that of the novaculite. We obtained the grain size exponents of ~3 and water fugacity exponents of ~2 for the two samples, and activation energies of 50 and 60 kJ/mol for the powder and novaculite, respectively. We applied our grain growth laws to crustal conditions assuming different initial grain sizes. We calculated grain growth rates with time and discussed contribution of plastic strain given under different strain rates which could cause grain size reduction by dynamic recrystallization. The strain-time relationship shows that strain is negligible until ~1000 years when strain rate is <10-12/sec. This means that the contribution of dynamic recrystallization is negligible. In the meantime, grain size can increase to a few tens of µm under the middle crustal condition (at 400 °C where pressure and water fugacity are calculated with a geological temperature and pressure gradients of 25 °C/km and 27 MPa/km, respectively) and to ~a few hundred μm under the lower crustal condition (at 600 °C). Our results indicate that there can be natural conditions where grain growth can be dominant even plastic deformation is operating.