Olivine phosphorus zonings reveal distinct magmatic processes of shield and volatile-rich rejuvenated lavas in Hawaiian Maui Island

The Hawaiian islands, a typical hot-spot related volcanic chain, were formed by shield-stage tholeiite lavas followed by the rejuvenated-stage, or post-erosion stage, CO₂- rich alkaline lavas. However, the fine-scale and possible distinct magmatic history of the two magmatic stages remain unclear. Olivine crystallizes early in basaltic magmas and is ideal for investigating magmatic fractionation and evolution processes. Here, we analyze in-situ geochemistry (e.g., Mg–Fe–Ni–P) of olivine phenocrysts from shield tholeiites and rejuvenated alkaline basalts from Maui Island. The shield tholeiites generally lack compositional zoning or exhibit normal zoning of olivine Fo (molar Mg/(Mg+Fe)) and Ni contents. In contrast, olivine grains from rejuvenated alkaline basalts display “shoulder” zoning in Fo and Ni, indicative of initial olivine growth followed by diffusion. Olivine grains in tholeiites exhibit dendritic, skeletal, patchy P-rich zones with well-defined oscillations and high sharpness, whereas olivine grains in alkaline basalts show overall P-poor zones, which are formed by the dissolution-reprecipitation process. The oscillatory P zoning in alkaline basalts is less distinct, with blurred boundaries, possibly attributed to the effect of slower cooling and growth rates. Therefore, our results on compositional zonings in olivine reveal different magmatic evolutions between shield and rejuvenated lavas from the Hawaiian Maui Island.