Rock Anelasticity Studies at High Pressure and Temperature

Abstract

Geophysical techniques used in the exploration for the thermal structure of the Earth include heat flow measurements, seismic studies and electric and electromagnetic soundings. The goals of these geophysical explorations are to detect and interpret the anomalies defined in the physical quantities measured, and to derive thermal models of the Earth. To this end, knowledge of the physical properties of in-situ rocks is required. In particular, seismic methods, which provide the most direct definitive information, have the maximum potential for probing high temperature and partial melt in the Earth. Thus, from the standpoint of successful seismic exploration, an understanding of the seismic properties of rocks is an important requisite. The parameters of direct interest here are the seismic velocity and attenuation (Q-1) ; the variables are pressure, temperature, frequency, fluid content, chemical composition (including volatile content) , grain size and preferred mineral orientation. It is anticipated that laboratory anelasticity data are valuable for the interpretation of seismic structure in terms of composition, temperature, and the degree of fluid or partial melt under regions where high attenuation (low Q) has been reported. The laboratory results are particularly applied to the locations of geophysical interest, the midocean ridge, the hot spot and the island arc, where seismic models have been extensively studied. The recent progress in anelasticity study shows that the low Q asthenosphere beneath ocean basins is hot but solid, and that seismic wave absorption mostly occurs at the mineral grain boundaries. Partial melt exists only in the extremely low Q zones right beneath ocean ridges or hot spots. For island arc volcanism, the descending slab is thought to be a possible source of volatiles. A role of volatiles in arc volcanism will be investigated from laboratory Q studies on hydrous rocks.