Preserving Deep Mantle Structure in Hotspot Lavas

Abstract:

Studies of the bilateral asymmetry recorded in Hawaii, and other Pacific hotspots, have been used to infer the structure and origin of their underlying mantle source(s). Recent models suggest that isotopic signatures carried to the surface in plumes can be used to infer the isotopic structure of the deep mantle. In this study we use numerical modelling framework, Fluidity, to test if these conclusions may be extended to heterogeneous zones with distinct density and rheology. To reduce computational requirements, we utilise Fluidity's adaptive mesh capability, allowing mesh resolution to vary with the solution complexity. We explore the parameter space over which bilateral asymmetry in Pacific hotspots can (and cannot) be explained by large-scale structures at the base of the mantle by varying the density and viscosity of such features. The existence of large-scale chemical structure in the mantle has traditionally been supported by seismic observations that define two Large Low Shear-wave Velocity Provinces (LLSVPs) beneath Africa and the Pacific. Therefore, we include in our parameter search the range of values predicted for LLSVPs as possible long-lived, chemically distinct 'piles'. This study has implications for: (i) the interpretation of isotopic patterns at hotspot locations; (ii) the thermal and compositional structure of mantle plumes and; (iii) our ability to relate the geochemistry of hotspot lavas to the nature of their deep mantle source.