Sammendrag
The mechanical properties of quartz are fundamental to control the plastic behaviour of the continental crust. Our understanding of quartz rheology is still limited in the following respects: i) the large variability of flow law parameters in the earlier literature (stress exponent n = 4 to ≤ 2 and activation energy Q = 120 to 242 kJ/mol), and ii) the difficulty to identify the rate-limiting deformation mechanism, if several mechanisms are operating simultaneously. These two issues are connected and cannot be resolved separately. The present study has carried out constant-load experiments to constrain the flow law parameters of quartz. A new generation hydraulically-driven Griggs-type apparatus has been employed, resulting in reproducible mechanical data, even at very low strain rates (10−8 to10−9 s−1; so far, closest to the natural ones). Furthermore, the Q-value in constant load experiments can be estimated without prior knowledge of the n value. Our new n (= 2) and Q values (= 110 kJ/mol) are fairly low. We calculated an A-value of 1.56 × 10−9 /MPa/sec. Microstructural analysis suggests that the bulk sample strain in our experiments is achieved by crystal plasticity, i.e., dislocation glide with minor recovery by sub-grain rotation, accompanied by grain boundary migration. Micro-cracking helps to nucleate new grains. It is inferred that strain incompatibilities induced by dislocation glide are accommodated by grain boundary processes, including dissolution-precipitation creep and grain boundary sliding. These grain boundary processes are responsible for the n-value that is lower than expected for dislocation creep ( 3). The new flow law can consistently estimate strain rates (especially at low stresses) in excellent agreement with documented natural case studies and predicts a rapid drop in strength of quartz-bearing rocks in the continental crust below a depth of ∼10 km or at a temperature of ∼300 °C and higher.
Vis fullstendig beskrivelse
