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

Abstract

Grain growth of quartz was investigated using two quartz samples (powder and novaculite) with water under pressure and temperature conditions of 1.0–2.5 GPa and 800–1100 ∘C. The compacted powder preserved a substantial porosity, which caused a slower grain growth than in the novaculite. We assumed a grain growth law of <i>dⁿ−dⁿ₀=k</i>₀<i>f^r_H2O</i>exp<i>(−Q/RT)t</i> with grain size <i>d</i> (µm) at time <i>t</i> (seconds), initial grain size <i>d</i>₀ (µm), growth exponent <i>n</i>, a constant <i>k</i>₀ (µm<i>^n</i> MPa<i>^−r</i> s<i>^−1</i>), water fugacity <i>f</i>_H2O (MPa) with the exponent <i>r</i>, activation energy <i>Q</i> (kJ mol⁻¹), gas constant <i>R</i>, and temperature <i>T</i> in Kelvin. The parameters we obtained were <i>n</i>=2.5±0.4, <i>k</i>₀=10^−8.8±1.4, <i>r</i>=2.3±0.3, and <i>Q</i>=48±34 for the powder and <i>n</i>=2.9±0.4, <i>k</i>₀=10^−5.8±2.0, <i>r</i>=1.9±0.3, and <i>Q</i>=60±49 for the novaculite. 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 (novaculite), even if the differences between powder and novaculite 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 < 10⁻¹² s⁻¹.