Compressional wave velocity of granite and amphibolite up to melting temperatures at 1 GPa
Introduction
Seismic observations have provided key information on the structure of the continental crust in the form of velocity–depth relations. In order to interpret the velocity profiles in terms of petrology, laboratory measurements of elastic properties have been conducted for a variety of rock types (e.g., Birch, 1960, Birch, 1961, Christensen, 1979, Christensen and Mooney, 1995, Ito and Tatsumi, 1995, Jackson et al., 1990, Kern et al., 1999). However, owing to technical difficulty, the effect of temperature high enough to cause partial melting on elastic wave velocities is not sufficiently understood. For instance, considering the phase relations of plausible constituents of continental crust, such as granite and amphibolite at high P–T conditions corresponding to high heat flow (Lachenburch and Sass, 1977), these rocks would suffer partial melting at depths (e.g., Huang and Wyllie, 1981, Wolf and Wyllie, 1995). Therefore, although the presence of low velocity zones have been proposed in the continental crust with anomalously high heat flow (e.g., Gutenberg, 1951, Kind et al., 1996, Swenson et al., 2000), the origin of such low velocity zones is still in debate. In this study, we report laboratory measurements of compressional wave velocities (VP) at 1 GPa for granite and amphibolite at temperatures beyond the solidus (800–950 °C) in order to investigate the influence of partial melting on seismic wave velocity.
Section snippets
Experiments
The experiments were carried out using a piston-cylinder apparatus with a 24-mm inner diameter and 80-mm thickness. The assembly is shown in Fig. 1. Talc was used as the pressure-transmitting medium. Nickel tubes (each 23 mm long) placed on the top and base of a carbon heater (20 mm long), act to lower the temperature around the transducer and facilitate experiments under high temperature conditions (∼1000 °C). The sample was wrapped in Pt foil in order to avoid direct contact between the
Granite
Fig. 4(a) illustrates the temperature variation of the observed waveforms. Compared to previous measurements (Aizawa et al., 2001), the signals were degraded and sample echoes were slightly ambiguous, probably because the ultrasonic waves were scattered at the grain boundaries. This is probably caused by the relatively large grain size (0.3–1.5 mm) of the natural rock sample. At high temperatures, specifically immediately above the solidus (from 750 to 850 °C), the amplitudes of signals
Conclusion
The temperature variation of compressional wave velocities showed that the VP values of granite decreased with rising temperature, but substantially increased beyond the melting temperature (850–900 °C). Such an increase may be caused by the α–β transition of quartz. The present results suggest that the presence of felsic crust with sufficiently high temperature to cause partial melting, for example, beneath southern Tibet, yield not only low seismic wave velocity but also relatively high
Acknowledgements
We thank Dr. T. Sano and Ms. R. Tsurudome for their help in EPMA and XRF analyses. We are grateful to Profs. S. Ji and N.I. Christensen for providing useful suggestions. This study was partly supported by Tenma Educational Institution under the Grant TEI-9602 and TEI-9702 to K. Ito.
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