Lower mantle scattering profiles and fabric below Pacific subduction zones

The Earth's mantle is compositionally evolving principally through the process of mid-ocean ridge basalt production (95% by lithospheric area, but also by way of arc magmatism (> 5%) and continental growth (< 1%) (Reymer, A., Schubert, G., 1984, Phanerozoic addition rate to the continental–crust and crustal growth, Tectonics, 3, 63–77; Rowley, D.B., 2002, Rate of plate creation and destruction: 180 Ma to present, Geol. Soc. Am. Bull., 114, 927–933). Convection eventually must mix these products back into the mantle, but the details of the mixing process are unknown. We report here new observational data documenting the location of differentiated material in the present-day lower mantle and scattering power profiles with depth. Small-scale (~ 10 km) objects which efficiently scatter short period S waves are present in the mantle lying below the Pacific Ocean rim at depths between 1100 and 1800 km. We also observe a reduction in scattering power with depth indicating that the objects represent bodies richer in differentiated basalt than the surrounding mantle. The scattering objects appear to be deformed to various degrees and usually form clusters separated from each other by a distance of 100 to 200 km. The clusters probably represent folds of the order of 100 km, probably corresponding to early stage of mixing process. Because the scattering power does not increase with depth, neither does a permanent barrier to flow exist in the mid-mantle, nor does the intrinsic density of basalt prevent re-entrainment into mantle flow from the bottom of the mantle.