ATTENUATION DUE TO PARTIAL MELTING - AN EXPERIMENTAL-STUDY ON A MODEL SYSTEM, USING THE LAB CODA METHOD

Partial melting at the grain boundaries of mantle rock is a source of attenuation often invoked in the interpretation of teleseismic data. However, related experimental studies are extremely difficult and hence are scarce. We have used a convenient metallurgical system to model a rock sample with a layer of fluid at grain boundaries: the analog of the rock sample is a cylinder of fine-grained, pure aluminum (diameter is 6 cm; height is 6 cm), and the analog of the fluid is provided by a thin layer of aluminum-gallium eutectic al the grain boundaries. The metal gallium, liquid at room temperature, diffuses extremely rapidly into the grain boundaries of aluminum where it forms a soft eutectic that melts at 28-degrees-C. The total amount of gallium in the sample represents about 0.2 wt %. Ultrasonic (1 MHz) pulses were transmitted through the sample by a transducer, and the full waveform of the reflected and scattered signals, including the coda, were recorded. Analyses of the delay parameter of the coda envelope yield values of Q-1. Preliminary results show that the thin film of aluminum-gallium eutectic at the grain boundaries noticeably increases attenuation. For pure aluminum at room temperature, Q-1 = 0.095 x 10(-3); after invasion of gallium into the grain boundaries, the value measured at 26-degrees-C (in solid state) was Q-1 = 0.62 x 10(-3); at 28-degrees-C, when the eutectic became liquid, Q-1 = 1.01 x 10(-3), an order of magnitude higher than for pure aluminum.