2025/11/19 by Jia Shao, Jason P. Morgan, Jason Phipps Morgan · 1 citation
Earth and Planetary Sciences · #High-pressure geophysics and materials #Geological and Geochemical Analysis #earthquake and tectonic studies
paper · doi:10.1016/j.epsl.2025.119724
Geochemical and petrological observations indicate that mantle plumes consist mainly of peridotite with ∼10–30% embedded pyroxenite veins or blobs. However, the consequences of preferential deeper pyroxenite-rich melting followed by shallower peridotite-rich melting on mantle density and viscosity have yet to be systematically explored. Using 1-D numerical simulations, we investigate density and viscosity changes during upwelling and pressure-release melting of a damp bilithologic mantle aggregate. We find that denser pyroxenite begins melting deeper than its surrounding peridotite. Preferential early pyroxenite melting significantly reduces mantle density by decreasing the volume fraction of this denser component, but only slightly increases mantle viscosity (<3-fold) as the weaker damp peridotite matrix dominates the overall rheology. When peridotite melts and dries at shallower depths, its compositional depletion continues reducing density but to a lesser extent than prior pyroxenite removal, while its dehydration significantly increases the mantle’s viscosity (1–2 orders of magnitude). When dry peridotite melting begins, its higher productivity and latent heat extraction also typically suppresses additional pyroxenite melting. After deep pyroxenite melting, residues of relatively colder material near plume rims have fluid (damp peridotite) viscosity and compositionally buoyant (pyroxenite-poor) density. This tends to form a low-viscosity, compositionally buoyant layer beneath oceanic lithosphere that could be preferentially sampled by subsequent mid-ocean ridge upwelling and melting. In contrast, the hottest plume fractions undergo both pyroxenite and peridotite melting during upwelling beneath oceanic lithosphere. This extensive melting generates a high-viscosity, buoyant restite fraction that adheres to and moves with overlying lithosphere as a hotspot swell root.