Experimental grain growth of quartz aggregates under wet conditions and its application to deformation in nature

The grain growth of quartz was investigated using two samples of quartz (powder and quartzite) with water under pressure and temperature conditions of 1.0–2.5 GPa and 800–1100 °C. The compacted powder preserved a large porosity, which caused a slower grain growth than in the dense quartzite. We assumed a grain-growth law of dn-d0n = k0 fH2Or exp⁡(−Q/RT)t with grain size d (µm) at time t (second), initial grain size d0 (µm), growth exponent n, a constant k0 (µmn MPa−r s−1), water fugacity fH2O (MPa) with the exponent r, activation energy Q (kJ/mol), gas constant R, and temperature T in Kelvin. The parameters we obtained were n = 2.5 ± 0.4, k0 = 10−8.8 ± 1.4, r = 2.3 ± 0.3, and Q = 48 ± 34 for the powder, and n = 2.9 ± 0.4, k0 = 10−5.8 ± 2.0, r = 1.9 ± 0.3, and Q = 60 ± 49 for the quartzite. The grain-growth parameters obtained for the powder may be of limited use because of the high porosity of the powder with respect to crystalline rocks, even if the differences between powder and quartzite vanish when grain sizes reach ~ 70 µm. Extrapolation of the grain-growth laws to natural conditions indicates that the contribution of grain growth to plastic deformation in the middle crust may be small. However, grain growth might become important for deformation in the lower crust when the strain rate is