Elastic wave velocity anomalies of anorthite in a subducting plate: In situ experiments

Abstract

To understand the origin of observed low velocities in the crustal portion of subducting plates, we performed in situ measurements of elastic wave velocities of anorthite at temperatures up to 1373 K at pressure of ~1 GPa and up to 773 K at 2.0-7.0 GPa. A fine-grained polycrystalline anorthite, which was synthesized using a gas pressure apparatus, was used for the measurements. The high-pressure experiments were performed using the multi-anvil apparatus installed on beamline BL04B1 at SPring-8. The elastic wave velocity was measured by the ultrasonic pulse method with synchrotron X-ray radiographic imaging and X-ray diffraction techniques. At ~1.0 GPa, elastic wave velocities exhibited a sharp temperature-induced kink at ~500 K. Below 500 K, the elastic wave velocities decrease with increasing temperature. In contrast, above 500 K, the elastic wave velocities show an increasing trend in the range of 500-900 K, and then revert back to a decreasing trend at above 900 K. We also found a pressure-induced velocity anomaly of anorthite. At 300-373 K, v P is constant up to 4 GPa, but decrease above 4 GPa with increasing pressure, while v S decreases monotonously with increasing pressure. These elastic anomalies are considered to be attributable to the tilting behavior of the corner-sharing TO 4 (T = Al, Si) tetrahedra in three-dimensional frameworks of anorthite. Our results suggest the presence of plagioclase feldspar has the potential to causes low-velocity anomaly in the subducting oceanic crust when it survives as a metastable phase in the slab at higher pressure and lower temperature conditions.

Elastic anomalies of anorthite: Molecular dynamics simulations

“…2). The bulk modulus appears to be higher than the value given by experimental data (Matsukage et al, 2015); however, the pressure dependence of unit cell volume is qualitatively reproduced. As observed in Fig.…”

“…5). The bulk modulus is higher and the changes in elastic moduli are larger than the experimental values (Matsukage et al, 2015). However, the trends for pressure and temperature are qualitatively reproduced.…”

“…A temperature-induced anomaly of the elasticity and elastic wave velocities of anorthite is reported by Matsukage et al (2015). The bulk modulus and shear modulus decrease at temperatures up to 510 K, increase from 510 K to 900 K, and then decrease in the domain above 900 K.…”

“…To reproduce these geophysical observations theoretically, we need to know the properties of minerals. Matsukage et al (2015) investigated the elastic properties of anorthite (CaAl 2 Si 2 O 8 ) using in situ experiments under high pressure and high temperature. Anorthite is a calcium end-member of plagioclase that is the one of the major rock-forming minerals in the crust.…”

“…The former is a stable phase, and the latter is a meta-stable phase (Hariya and Kennedy, 1968). These three polymorphs have the same topology, that is, there is no breaking and reconnection of bonds and the network topology is preserved throughout the phase transition, while there are pressure and temperature induced anomalies in elastic wave velocity (Matsukage et al, 2015). At the lower pressure, S wave and P wave velocities decrease at temperatures up to 510 K, increase between 510 K and 900 K, and then decrease above 900 K. Under high pressure, both velocities decrease above 3 GPa.…”