Single-crystal elastic properties of lawsonite and their variation with temperature

Abstract

We have measured the elastic wave velocities and single-crystal elastic moduli of lawsonite (CaAl 2(Si 2O 7)(OH) 2·H 2O) by Brillouin spectroscopy at high temperatures to 450 °C and at room pressure. Lawsonite is generally characterized by high longitudinal ( v P) and shear ( v S) elastic wave velocities. A number of the elastic properties increase with increasing temperature, including the shear modulus, G ( ∂G/ ∂T=+2.01 GPa/100 K), and the aggregate velocities v P and v S. These anomalous properties are likely related to a displacive phase transition at ∼0 °C. This is supported by the small value of the elastic modulus c 66 (18(1) GPa at room temperature), and the fact that c 66 softens as temperature is decreased approaching the ∼0 °C phase transition. The anomalous behavior in c 66 leads to a decrease in v P/ v S and Poisson’s ratio with increasing temperature at 1 atm pressure. Our results show that the aggregate bulk modulus ( K S=125(2) GPa at 21 °C) decreases monotonically with increasing temperature ( ∂K S / ∂T=−1.8 GPa/100 K), in contrast to the results of Daneil et al. (1999) who observed a minimum in the bulk modulus at ∼230 °C. In general, the aggregate elastic wave velocities of lawsonite under high P– T conditions of a subducted slab are expected to remain comparable to those of anhydrous mafic silicates. This observation reinforces the notion that other hydrous minerals persist in subducted oceanic crust to considerable depth (∼250 km), in order to explain the low velocity layers observed in some subduction zones (e.g. Helffrich, 1996; Abers, 2000).