The geochemical evolution of an extreme rejuvenated lava: A case study on Aitutaki of the Cook-Austral island chain

Aitutaki is a basaltic Ocean Island in the South-Central Pacific Ocean, a region densely populated with age-progressive islands and seamounts considered to be a result of three or more overlapping mantle plumes. Recent (~1.9 Ma) volcanic activity at Aitutaki is difficult to reconcile with direct melting of a mantle plume, and may instead be an example of rejuvenation, whereby volcanism reappears after an extended hiatus. We have analysed samples of 21 lavas collected from Aitutaki for whole-rock major and trace elements and radiogenic isotopes, with electron probe microanalysis (EPMA) of olivine crystals and xenocrysts. Whole-rock MgO in these samples ranges from 10.3 to 13.9 wt. %. Petrographic examination and EPMA data indicate olivine accumulation is not the cause of these high values. Evidently, very little fractionation occurred during melt ascent through the volcanic plumbing network. Direct eruption of high-MgO (>10 wt.%) primitive magma is unusual in Ocean Island basalts, except among rejuvenated volcanics.

Our radiogenic isotope data closely overlap in Pb-isotope space with the nearby Samoan rejuvenated lavas from Savai’i and Tutuila, and otherwise have an EM1-FOZO signature. The major elements, trace elements and radiogenic isotopes all distinguish two populations of lavas, indicating the lavas are sourced from different regions or source materials in the asthenosphere. By analysing the spatial distribution of these chemical anomalies on the island, we observe the least evolved and most trace-element enriched samples (lowest SiO₂, highest Th/Y) concentrate in geographically distinct regions on the island. Without a geochemical continuum between these two populations, we suggest the surface distribution of the enriched and depleted lavas may reflect spatial isolation of the mantle sources. Aitutaki produced multiple pulses of geochemically diverse lavas with extreme compositions throughout a short-lived (~50 ka) rejuvenated eruption cycle, exemplifying the processes responsible for producing rejuvenated lavas and challenging our understanding of the petrogenesis of such volcanism.