Acoustic emissions accompanying the compressive ductile-brittle transition in highly- crystalline lavas.

Understanding of the ductile-brittle transition in dome lavas may well contain the key to an adequate description of dome growth and stability. To elucidate this transition in dome lavas, a series of experiments were performed to characterize microcracking during compressive deformation of crystal-rich lavas. Multiphase lavas behave as visco-elastic fluids with a strain-rate dependence of viscosity across the ductile-brittle field. In order to map out the onset of brittle failure across the transition, we have deformed large volume samples (80 mm long by 40 mm diameter) in a high-load, high-temperature uniaxial press equipped with acoustic emission (AE) monitoring sensors. Our apparatus has been calibrated using an NBS717a standard glass. The absence of cracking and associated AE during deformation of this standard, which behaves as a homogeneous viscous melt under our experimental conditions, allows us to calibrate and filter out extraneous background noise. Samples from each of the five volcanoes chosen for this study (Colima, Unzen, Bezimianny, Krakatau, and Tungurahua) were deformed at two temperatures (940 and 980°C) and at stresses from 1 to 50 MPa. At low stresses (1-10 MPa), only a few AE events were detected and the AE rate decreased with increasing strain. Occasional high-energy events were recorded, and attributed to cracking of single crystals. Increasing the stress to 20-30 MPa resulted in an increased AE rate that stayed essentially constant with increasing strain. Occasional high-energy events persisted. At 40 and 50 MPa, the AE rate was higher still, and increased with increasing strain (overwhelming the few high energy events that continued to occur). Preliminary evaluation of the seismic b-value shows a decreasing trend from >3.0 at low stress to <1.5 at high stress, suggesting a shift from distributed small-scale cracking to more localized larger-scale cracking as stress is increased. These results will be discussed in terms of the deformation of dome lavas during extrusion and residence at the surface.