Keywords:Mantle Convection, Primordial Reservoir
The large low shear-wave velocity provinces (LLSVP) are thermochemical anomalies in the deep Earth’s mantle, thousands of km wide and ~1,800 km high. This study explores the hypothesis that the LLSVPs are compositionally subdivided into two domains: a primordial bottom domain near the core-mantle boundary and a basaltic shallow domain extending from 1,100~2,300 km depth. This hypothesis reconciles published observations in that it predicts that the two domains have different physical properties (bulk-sound vs. shear-wave speed vs. density anomalies), the transition in seismic velocities separating them is abrupt, and both domains remain seismically distinct from the ambient mantle. We here report underside reflections from the top of the LLSVP shallow domain, supporting a compositional origin. By exploring a suite of two-dimensional geodynamic models, we constrain the conditions under which well-separated “double-layered” piles with realistic geometry can persist for billions of years. Results show that long-term separation requires density differences of ~100 kg/m3 between LLSVP materials, providing a constraint for origin and composition. The models further predict short-lived “secondary” plumelets to rise from LLSVP roofs and to entrain basaltic material that has evolved in the lower mantle. Long-lived, vigorous “primary” plumes instead rise from LLSVP margins and entrain a mix of materials, including small fractions of primordial material. These predictions are consistent with the locations of hotspots relative to LLSVPs, and address the geochemical and geochronological record of (oceanic) hotspot volcanism. The study of large-scale heterogeneity within LLSVPs has important implications for our understanding of the evolution and composition of the mantle.