Metamorphic controls on seismic velocity of subducted oceanic crust at 100–250 km depth

Most circum-Pacific subduction zones at 100–250 km depth contain layers in which seismic velocities are ca. 5% slower than in the adjacent mantle. We compute seismic velocities from thermodynamic data for equilibrium metabasalt mineralogies, determined by free energy minimization, at subduction zone conditions. Lawsonite stability has a profound effect on seismic velocities of subducted oceanic metabasalts. Velocity reductions of 3–7% are estimated for lawsonite–eclogites derived by metamorphism of hydrothermally altered oceanic basalt subducted along relatively cool geotherms, whereas a 2–4% velocity increase is characteristic of anhydrous eclogites within the coesite stability field. The restricted depth extent of low-velocity layers is explicable through the influence of the coesite–stishovite transition, which reduces lawsonite stability at high pressure. This transition also increases the positive velocity anomaly in anhydrous eclogites to 4–6%, an effect that may account for deep high-velocity layers. The quality of the match between the properties of lawsonite–eclogite and low-velocity layers supports the contention that significant quantities of volatiles are retained within the oceanic crust beyond sub-arc depths. Because the velocity anomalies are explicable in terms of equilibrium phase relations, we find no reason to invoke metastability of metamorphic reactions to explain the low-velocity layers.