Elastic-wave velocity in crustal and mantle rocks at high pressure and temperature: the role of the high-low quartz transition and of dehydration reactions

Abstract

The compressional- and shear-wave velocities, V P and V S, in typical crustal and mantle rocks were determined in a cubic anvil apparatus over a range of constant confining pressure up to 6 kbar and temperatures ranging from 20–750°C. Samples range between acidic and ultramafic with mean atomic weights between 20.4 and 23.0 and bulk densities of 2.66–3.50 g cm −3 at 6 kbar. Compressional-wave velocities are only roughly related to densities and do not generally follow lines of constant mean atomic weights. However, the rocks typical for the upper mantle, the continental and oceanic crust clearly separate into three distinct areas in the velocity versus density plot. In plutonic and metamorphic rocks velocities are strongly dependent on the mineral composition. Both V P and V S increase with increasing amphibole, garnet, pyroxene and olivine content. High feldspar content produces high Poisson's ratios as a consequence of relatively high compressional wave velocities compared to the respective shear wave velocities. Increasing quartz content results in a decrease of V P and an accompanying increase of V S, thereby significantly lowering Poisson's ratio. Elastic-wave propagation is affected to a large degree by flat cracks and pore spaces. High confining pressures act to reduce microcracks; heating causes an increase in porosity. However, microfracturing is increasingly suppressed by the simultaneous action of confining pressure. A pressure increase of at least 10 bar per 1°C is needed to prevent the opening of cracks. Velocities are highly affected by the α-β quartz transition. Due to stress concentrations in polycrystalline material the quartz transition temperature is shifted to higher values than might be expected from single crystal measurements. Dehydration reactions in zeolite-bearing basalts cause the velocities to decrease as a consequence of lowering the effective pressure and of a reconstitution of pore geometry. Velocity anisotropies observed in some plutonic rocks, and most metamorphic rocks, at high confining pressures and high temperatures correlate well with preferred lattice orientation of the constituent major minerals. The effect of dimensional orientation and microcracks seems to be of minor importance for the directional dependence of wave velocities under conditions of high pressure.