Numerical study of the origin and stability of chemically distinct reservoirs deep in earth’s mantle

Abstract

Seismic tomography is providing mounting evidence for large scale compositional heterogeneity deep in Earth’s mantle, and also the diverse geochemical and isotopic signatures observed in oceanic basalts suggest that the mantle is not chemically homogeneous. Isotopic studies on Archean rocks indicate that mantle inhomogeneity may have existed for most of the Earth’s history. One important component may be recycled oceanic crust, residing at the base of the mantle. We investigate, by numerical modeling, if such reservoirs may have been formed in the early Earth, before plate tectonics (and subduction) were possible, and how they have survived – and evolved – since then. During Earth’s early evolution, thick basaltic crust may have sunk episodically into the mantle in short but vigorous diapiric resurfacing events. These sections of crust may have resided at the base of the mantle for very long times. Entrainment of material from the enriched reservoirs thus produced may account for EM and HIMU signatures in oceanic basalts, whereas deep subduction events may have shaped and replenished deep mantle reservoirs. Our modeling shows that (1) convective instabilities and resurfacing may have produced deep enriched mantle reservoirs prior to the era of plate tectonics, that (2) such formation is qualitatively consistent with the geochemical record showing multiple distinct OIB sources, and that (3) reservoirs thus produced may be stable for billions of years.