Selective melt extraction of an enriched lithospheric boundary explains rejuvenated OIB

Rejuvenated volcanism on ocean islands represents a resurgence of volcanic activity that postdates mantle plume magmatism, however the mantle source and melting conditions of the generated lavas remain elusive. Aitutaki Island in the central Pacific is an exceptional case of rejuvenated volcanism ∼8 Myr after shield-stage volcanism. Here we present new olivine data, radiogenic isotope and major-trace element data that show a previously undocumented geochemical bimodality in the island’s lavas. Major-trace element data support low-degree melt extraction from the base of the lithosphere, typical of alkalic OIB. Our mineral data and trace element and isotopic modelling indicate an enriched pyroxenitic source fluxed by carbonatitic melt at 3 GPa best reproduces the rejuvenated lavas. This depth of melting corresponds to the breakdown of pargasitic amphibole, and the appearance of a slow seismic velocity anomaly. Our investigation of the rejuvenated lavas from Aitutaki suggests this boundary layer contains enriched pockets of partial melt amongst depleted restite from plume volcanism. Hafnium-Nd systematics imply carbonatite may be a common component of this boundary layer. We argue that under hotter thermal regimes the volatile-depleted restite is more effectively sampled, leading to greater isotopic similarities between shield and rejuvenated lavas than under cooler regimes. Selective sampling of stored components also explains the isotopic kinship between separate islands along a hotspot trace, including at Aitutaki, Rarotonga and Samoa. These results support the presence of a long-lived, enriched partial-melt boundary layer at the base of the lithosphere and show how its reactivation can generate rejuvenated volcanism millions of years after volcanic extinction.