Recycled gabbro signature in hotspot magmas unveiled by plume–ridge interactions

Abstract

The geochemistry of lavas erupted at locations where mantle plumes interact with mid-ocean ridges reflects the mixing between the two sources. Analysis of lavas erupted above the Foundation hotspot, near the Pacific–Antarctic Ridge, reveal a geochemical signature indicative of both a primitive mantle plume source and a recycled oceanic lithosphere source. Lavas erupted within plate interiors above upwelling mantle plumes have chemical signatures that are distinct from mid-ocean ridge lavas. When a plume interacts with a mid-ocean ridge, the compositions of both their lavas changes, but there is no consensus as to how this interaction occurs1,2,3. For the past 15 Myr, the Pacific–Antarctic mid-ocean ridge has been approaching the Foundation hotspot4 and erupted lavas have formed seamounts. Here we analyse the noble gas isotope and trace element signature of lava samples collected from the seamounts. We find that both intraplate and on-axis lavas have noble gas isotope signatures consistent with the contribution from a primitive plume source. In contrast, near-axis lavas show no primitive noble gas isotope signatures, but are enriched in strontium and lead, indicative of subducted former oceanic lower crust melting within the plume source5,6,7. We propose that, in a near-ridge setting, primitive, plume-sourced magmas formed deep in the plume are preferentially channelled to and erupted at the ridge-axis. The remaining residue continues to rise and melt, forming the near-axis seamounts. With the deep melts removed, the geochemical signature of subduction contained within the residue becomes apparent. Lavas with strontium and lead enrichments are found worldwide where plumes meet mid-ocean ridges6,7,8, suggesting that subducted lower crust is an important but previously unrecognised plume component.