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Pyroxenite causes fat plumes and stagnant slabs

Conventional wisdom holds that there is a change in the pattern of mantle convection between 410 and at 660 km, where structural transformations convert olivine into its high‐pressure polymorphs. In this regard, recent tomographic studies have been a complete surprise, revealing (i) rapid broadening of slow seismic anomalies beneath hotspots from hundreds of kilometers wide at shallow depths to 2000–3000 km wide deeper than ~800 km, and (ii) fast seismic anomalies associated with subducted lithosphere that appear to flounder at 800–1000 km. It is difficult to reconcile these observations with the conventional view of a mantle that experiences limited mineralogical change below 660 km. Here we propose that plumes and slabs contain significant proportions of lithologies that experience an entirely different suite of mineral reactions, demonstrating that both subducted basalt and pyroxenite upwelling in plumes experience substantial changes in mineralogy and thus physical properties at ~800 km depth. We show the importance of this for mantle rheology and dynamics and how it can explain hitherto puzzling mantle tomographic results.