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Published September 15, 1968 | Published
Journal Article Open

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


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.

Additional Information

© 1968 American Geophysical Union. Manuscript Received: 8 APR 1968. It is a pleasure to acknowledge the help of David F. Newbigging who assisted in all phases of the experiment. We are grateful for helpful discussions with Thomas Ahrens, Charles Archambeau, and Samuel Epstein. This research was partially supported by National Science Foundation grant GA 1003.

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