Temperature derivatives of compressional and shear wave velocities in crustal and mantle rocks at 6 kbar confining pressure

Abstract

Measurements of compressional and shear wave velocities, V_P and V_S, were made in a cubic anvil apparatus up to 700° C at 6 kbar in igneous and metamorphic rocks typical of the crust and mantle. Samples range in composition from acidic to ultramafic, with bulk density of 2.67–3.46 g/cm³ at 6 kbar. Mean atomic weights of the rocks vary between 20.37 and 23.03. A rough dependence of wave velocity on bulk density is apparent. However, there is considerable scatter, and the velocities do not generally follow lines of constant mean atomic weight. Both V_P and V_S increase with increasing amphibole, garnet, pyroxene and olivine content. High quartz content produces relatively low P-wave and high S-wave velocities, giving significantly low Poisson's ratios. In contrast, a high feldspar content is related to relatively high P-wave and low S-wave velocities and consequent high Poisson's ratios. The calculated temperature derivatives of V_P and V_S for the range 20-500° C at 6 kbar confining pressure, using best-fit solutions, cover the range –1.60 to –4.94 x 10^–4 km/s° C and –1.39 to –3.93 x 10^–4 km/s° C, respectively. The results compare fairly well with the few data published so far. For the dunite and peridotite rocks the (dV_P/dT)_p-values are in excellent agreement with published Voigt-Reuss-Hill values calculated from single crystal data. Using the experimental results, P-velocity profiles were calculated along a geotherm of a cold Precambrian shield crust and a warm continental crust. The temperature gradients for the latter cause velocity inversion in numerous rocks, rich in olivine or quartz.

          ARK: https://n2t.net/ark:/88439/y069600

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