The Effect of Temperature and Partial Melting on Velocity and Attenuation in a Simple Binary System

Abstract

A possible explanation of the low-velocity, low-Q zone in the upper mantle is partial melting, but laboratory data are not available to test this conjecture. As a first step in obtaining an idea of the role that partial melting plays in affecting seismic variables, we have measured the longitudinal and shear velocities and attenuations in a simple binary system that is completely solid at low temperatures and involves 17% melt at the highest experimental temperature. The system investigated was NaCl • H_2O. At temperatures below the eutectic the material is a solid mixture of H_2O (ice) and NaCl • 2 H_2O. At higher temperatures the system is a mixture of ice and NaCl brine. In the completely solid regime the velocities and Q change slowly with temperature. There is a marked drop in the velocities and Q at the onset of melting. For ice containing 1% NaCl, the longitudinal and shear velocities change discontinuously at this temperature by 9.5 and 13.5%, respectively. The corresponding Q's drop by 48 and 37%. The melt content of the mixture at temperatures on the warm side of the eutectic for this composition is about 3.3%. The abrupt drop in velocities at the onset of partial melting is about three times as much for the ice containing 2% NaCl; for this composition, the longitudinal and shear Q's drop at the eutectic temperature by 71 and 73%, respectively. If these results can be used as a guide in understanding the effect of melting on seismic properties in the mantle, we should expect sharp discontinuities in velocity and Q where the geotherm crosses the solidus. The phenomena associated with the onset of melting are more dramatic than those associated with further melting.