Compositional layering within the large low shear-wave velocity provinces (LLSVPs) in the lower mantle
Seismic tomography reveals two antipodal LLSVPs in the Earth’s mantle, each extending from the core-mantle boundary (CMB) up to ∼1000 km depth. The LLSVPs are thought to host primordial mantle materials that bear witness of early-Earth processes, and/or subducted basalt that has accumulated in the mantle over billions of years. A compositional distinction between the LLSVPs and the ambient mantle is supported by anti-correlation of bulk-sound and shear-wave velocity (Vs) anomalies as well as abrupt lateral gradients in Vs along LLSVP margins. Both of these observations, however, are mainly restricted to the LLSVP bottom domains (2300∼2900 km depth), or hereinafter referred to as “deep distinct domains” (DDD). Seismic sensitivity calculations suggest that DDDs are more likely to be composed of primordial mantle material than of basaltic material. On the other hand, the seismic signature of LLSVP shallow domains (1000∼2300 km depth) is consistent with a basaltic composition, though a purely thermal origin cannot be ruled out.