Single-crystal elastic properties of lawsonite and theirvariation with temperature

We have measured the elastic wave velocities and single-crystal elastic moduli of lawsonite (CaAl₂(Si₂ O₇)(OH)₂·H₂O) 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).