Part I. The Effect of Temperature and Partial Melting on Velocity and Attenuation in a Simple Binary System. Part II. Effect of Temperature and Pressure on Elastic Properties of Polycrystalline and Single Crystal MgO

Citation

Spetzler, Hartmut A. W. (1969) Part I. The Effect of Temperature and Partial Melting on Velocity and Attenuation in a Simple Binary System. Part II. Effect of Temperature and Pressure on Elastic Properties of Polycrystalline and Single Crystal MgO. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/CZFN-2N63. https://resolver.caltech.edu/CaltechTHESIS:04022018-124017475

Abstract

A possible explanation of the low-velocity, low-Q zone in the upper mantle is partial melting, but laboratory data has not been available to test this conjecture. As a first step in obtaining an idea of the role that partial melting plays in affecting seismic variables, the longitudinal and shear velocities and attenuations were measured 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•H₂O. At temperatures below the eutectic the material is a solid mixture of H₂O (ice) and NaCl•H₂O. 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.

The theory for randomly oriented fluid-filled penny-shaped cracks satisfactorily explains the velocity data. The anomalous behavior on the warm side of the eutectic temperature is attributed to thermochemical effects associated with interaction of the sound wave with the phase equilibria. This phenomenon is not observed when supercooling is possible.

A laboratory has been constructed to measure the elastic properties of solids to 12 kbar and 1200°K by ultrasonic interferometry techniques. The elastic constants and their temperature and pressure derivatives have been measured to high temperature and pressure for both single crystal and polycrystalline MgO. A pseudoresonance technique involving pulse superposition and a lapped buffer rod without bond were used in order to obtain the necessary precision. The results for the single crystal are tabulated below.

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