Compositional layering within the large low shear-wave velocity provinces (LLSVPs) in the lower mantle

Abstract:

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 primitive mantle materials that bear witness of early-Earth processes, and/or subducted basalt that has cumulated 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 lateralgradients in Vs along LLSVP margins. Both of these observations, however, are mainly restricted to the LLSVP bottom domains (2300~2900 km depth). Comparison of seismic observations with mineral-physics data suggests that these bottom domains are more likely to be composed of primitive mantle than of basaltic material. On the other hand, the seismic signature of the LLSVP shallow domains (1000~2300 km depth) is consistent with a basaltic composition, though a purely thermal origin cannot be ruled out.

Here, we explore the dynamical, seismological, and geochemical implications of the hypothesis that the LLSVPs are compositionally layered with a primitive bottom domain and a basaltic shallow domain (see Fig.). We test this hypothesis using 2D thermochemical mantle-convection models. Depending on the density difference between primitive and basaltic materials, the materials either mix or remain separate as they join to form thermochemical piles in the deep mantle. Separation of both materials within the piles provides an explanation for LLSVP seismic properties, including substantial internal vertical gradients in Vs observed at 400-700 km height above the CMB. Geodynamic models predict short-lived “secondary” plumelets to rise from the roofs of these compositionally layered piles while entraining basaltic material that has evolved in the lower mantle. Long-lived “primary” plumes rise from LLSVP margins and entrain a mix of materials, including small fractions of primitive mantle material. These predictions address the geochemical and geochronological record of intraplate hotspot volcanism in the Pacific. LLSVP compositional layering has indeed important implications for our understanding of heat and material fluxes through mantle reservoirs, as well as bulk Earth chemistry and evolution.