Experimental high pressure investigation of partial melting in natural rocks and their influence on V-P and V-S

Plutonic and high-grade metamorphic rocks from the crystalline complexes of the Saxonian Erzgebirge and Granulitgebirge in Germany were used for exper iments up to 0.5 GPa at laboratory temperature to measure the elastic wave velocities (v(p)) and (v(s)), under the minimized influence of pore space a nd cracks in an oil pressure chamber. The obtained high pressure data were also used to select samples for further experiments under the conditions of mineral dehydration and partial melting. We also applied a high performanc e gas pressure apparatus to investigate v(p) and v(s) on cylindrical sample s up to 2 GPa and temperatures up to 1200 degreesC. This paper shows the re sults of the elastic wave velocities v(p) and v(s), measured simultaneously with encapsulated samples of granite, pyroxene granulite and pyroxenite un der different partial melting conditions. Post-experimental microscopic ana lyses of the quenched samples including digital image processing were under taken to determine the material and/or structural cause of a change in the physical properties, resulting in empirical relationships between elastic w ave velocities and mineral content, dissipation and orientation of melt alo ng grain boundaries. In a granite melt starts to form at 650 degreesC with an amount of about 4 % at 900 degreesC and reaching massive melting at temp eratures above 1000 degreesC. For basic to ultrabasic rocks at a pressure o f 2 GPa a much smaller amount of melt has formed in the temperature interva l above 900 degreesC. At lower pressures the Poisson's ratio increases in t he range of partial melting dramatically from 0.25 to about 0.4. For basic to ultrabasic rocks at 2 GPa pressure it keeps nearly unchanged up to 1000 degreesC. (C) 2001 Elsevier Science Ltd. All rights reserved.