Comprising ~71% of Earth’s surface, the ocean basins represent perhaps the most fundamental expression of plate tectonics. Yet, questions remain regarding the physical state of the oceanic asthenosphere (temperature, grain size, viscosity), abundance of melt and volatiles (H₂O, CO₂), and degree to which these factors influence plate motion and the scale/vigor of mantle convection. Recent deployments of ocean-bottom seismometers (OBS) in the Pacific aim to fill this observational gap by providing in-situ observations of upper-mantle structure with lateral resolution of ~500 km. Of particular interest are shear attenuation (Q^-1_μ) and velocity (V_S), which offer complementary constraints on the physical and chemical state of the upper mantle.

Here, we present new observations of shear attenuation and velocity at four OBS arrays in the Pacific ranging in seafloor age from ~3–90 Myr from array-based Rayleigh wave attenuation tomography that accounts for elastic focusing/defocusing and local site amplification. We implement an adaptive Bayesian inversion for shear attenuation to ~350 km depth at each of the four locations, revealing a remarkably strong peak in attenuation (Q _μ ~ 20–50) centered at 100–150 km depth. This zone of high attenuation (or low Q _μ) is among the highest observed to date beneath ordinary oceanic plate. The magnitude of the attenuation peak varies spatially but does not simply correlate with seafloor age, consistent with a heterogeneous low-viscosity channel beneath the Pacific plate that persists to at least 90 Myr. By comparing our observations to laboratory-based predictions of Q^-1_μ and V_S, we show that the high-attenuation, low-velocity zone can be explained by a hydrated region (~1000 ppm H₂O) containing some partial melt (< 1%) and reduced grainsizes (1–3 mm). That the most attenuating regions are situated near the South Pacific Superswell may indicate that plumes play a role in hydrating the upper mantle, ultimately lubricating the base of the plate and facilitating plate tectonics.