Elastic Anomalies Across the α ‐ β Phase Transition in Orthopyroxene: Implication for the Metastable Wedge in the Cold Subduction Slab

Abstract Single‐crystal elasticity of both α ‐ and β ‐orthopyroxene was determined up to 20 GPa and 300 K by Brillouin scattering. Using the derived full elastic moduli ( C ij ), we investigated the contribution of the metastable pyroxene to the seismically observed 3%–5% low‐velocity anomalies along the subducting slab in the top transition zone. Our modeled results show that a harzburgite wedge with a 1000‐K colder geotherm and metastable α ‐orthopyroxene and olivine displays compressional ( V P ) and shear‐wave ( V S ) velocities 3.0%–3.6(6)% and 2.0%–2.8(6)% lower than the surrounding mantle at 410–460 km depth, respectively. At deeper depth up to 520 km, V P and V S of this metastable wedge with β ‐orthopyroxene and olivine are 3.6%–4.4(6)% and 2.8%–4.3(6)% lower than the pyrolitic mantle, respectively. The presence of both metastable orthopyroxene and olivine instead of metastable olivine alone helps better explain the origin of the low‐velocity anomalies within the subduction slab in the top transition zone.