In-situ Measurement of Dislocation-induced Anelasticity by Using a Rock Analogue

We developed a new experimental system with which anelasticity of a rock analogue sample (polycrystalline borneol) can be measured in situ during dislocation creep. We attached a piezo-electric actuator to a triaxial deformation apparatus, to add a small cyclic load to a large constant load. We also attached a load cell and two laser displacement meters to measure the small cyclic load and displacement accurately. Using this new system, we deformed a polycrystalline borneol sample under diffusion creep (σ = 0.27 MPa) for about 1 day, intermediate creep (σ ≈ 1.1 MPa) for about 0.5 day and dislocation creep (σ ≈ 2.2 MPa) for about 1 day continuously, and performed an in-situ measurement of Young’s modulus and attenuation at frequencies of 2.5 and 1.0 Hz. During the first diffusion creep, Young’s modulus increased, probably due to the improved contact between the sample and the piston, and reached a constant value. Although the modulus did not change during the second intermediate creep, it gradually decreased during the third dislocation creep. The final modulus reduction was about 20%. The present result supports our previous result obtained from the anelasticity measurement of a pre-deformed sample (Sasaki et al., 2017, AGU fall meeting; Sasaki et al., 2019, submitted to JGR). In the previous study, we showed that detailed form of the dislocation creep curves can be explained well by considering a gradual increase in dislocation density during the dislocation creep. Also, reduction of Young’s modulus by about 10% was observed after the dislocation creep under the similar condition to this study. The gradual decrease in Young’s modulus observed in this present study provides a supporting evidence for these previous results. Frequency dependence of the dislocation-induced anelasticity obtained from our previous study is a peak at much higher frequency than the grain boundary-induced anelasticity (Sasaki et al., 2017, AGU fall meeting; Sasaki et al., 2019, submitted to JGR). In contrast, dislocation-induced anelasticity obtained from single crystal forsterite or polycrystalline olivine is a broad absorption band (Guéguen et al., 1989; Farla et al., 2012). It is important to clarify the reason for this discrepancy.