Widely-spaced Double Hotspot Chains due to Forked Plumes sample Lower Mantle Geochemical Structure

Age-progressive volcanic “hotspot” chains result from the passage of a tectonic plate over a thermochemical mantle plume, thereby sampling the otherwise-inaccessible lowermost mantle. A common feature in oceanic hotspot tracks is the occurrence of two parallel volcanic chains with an average separation of ~50 km (e.g., Loa and Kea chains in Hawaii). Some other tracks (including Tristan-Gough, Shona, the Line Islands, Wake seamounts, Tuvalu and Cook-Austral) feature a 200-400 km spacing, but the origin of such widely-spaced melting zones in the mantle remains unknown. Here, we explore 3D Cartesian geodynamic models of thermochemical plume ascent through the upper mantle. We explore various distributions of intrinsically-dense eclogitic material across the plume stem. For a wide range of eclogite distributions, the plume pools in the depth range of 300~410 km, where the excess density of eclogite is greater than above and below, as also predicted by Ballmer et al., EPSL 2013. This “Deep Eclogitic Pool” then splits up into two lobes that feed two separate shallow plumelets, particularly at high eclogite contents in the center of the underlying plume stem. The two plumelets feed two separate melting zones at the base of the lithosphere, which are elongated in the direction of plate motion due to interaction with small-scale convection. This “forked plume” morphology can account for hotspot chains with two widely-spaced (250~400 km) tracks and with long-lived (>5 My) coeval activity along each track. Forked plumes may also provide an ideal opportunity to study geochemical zonation of the lower-mantle plume stem, as each plumelet ultimately samples the opposite side of a deep plume conduit that potentially preserves spatial heterogeneity from the lowermost mantle. We compare this model to geochemical asymmetry evident along the Wake, Tuvalu and Cook-Austral double-chain segments, which make up the extensive (>100 Ma) Rurutu-Arago hotspot track. The preservation of a long-lived NE-SW geochemical asymmetry along the Rurutu-Arago double chain indicates a deep origin, likely from the southern boundary of the Pacific large low shear-velocity province. Our findings highlight the potential of the hotspot geochemical record to map lower-mantle structure over space and time, complementing the seismic-tomography snapshot.