A deep mantle origin for the primitive signature of ocean island basalt
Abstract
Seismological observations have identified large-scale compositional heterogeneities in the Earth's deep mantle(1-5). These heterogeneities may represent reservoirs of primitive material that differentiated early in Earth's history(6-8). The volcanic rocks that make up ocean islands are thought to be sourced, in part, from these deep reservoirs, with the primitive material transported to the surface via mantle plumes. Geochemical signatures within the erupted ocean island basalts further support the idea that the regions of heterogeneity are composed of primitive, undegassed mantle material(7,9-14). Here we perform numerical experiments of thermo-chemical convection to simulate the entrainment of primitive material by plumes generated at the top of primitive reservoirs in the deep mantle. We vary the chemical density contrast between the primitive, undegassed and regular, degassed mantle materials. We find that the simulations that reproduce the observed geometry of the heterogeneous regions also explain the geochemical signatures measured in ocean island basalts. In these simulations, the entrainment of primitive material into the mantle plume does not exceed 9%. We conclude that the presence of primitive reservoirs in the deep mantle is dynamically feasible and satisfies both seismological and geochemical constraints.