Recycled oceanic crust in the Hawaiian Plume: evidence from temporal geochemical variations within the Koolau Shield

The subaerial surface of Koolau volcano is composed of lavas that define the distinctive endmember composition for Hawaiian shield lavas, known as the Koolau component, now designated as the Makapuustage. The geochemical characteristics of lavas recovered by the Koolau Scientific Drilling Project (KSDP) show that this distinctive composition forms a <300-m thick veneer. Below this veneer, from 300m to 470 m below sea level, Koolau shield lavas transition to a composition similar to Mauna Loa lavas, now designated as the Kalihi-stage. This transition was gradual, occurring over >80 ka; therefore it was not caused by an abrupt event, such as a landslide. Among all Koolau shield lavas, there are correlations between radiogenic isotopic ratios of Sr, Nd and Pb and compositional characteristics, such as SiO2 content (adjusted to be in equilibrium with Fo90 olivine), Sr/Nb, La/Nb and Th/La. These long-term compositional and isotopic trends show that as the shield aged, there was an increasing role for an ancient recycled marine sediment component (<3% of the source) accompanied by up to 20% SiO2-rich dacitic melt. This melt was generated by partial melting of garnet pyroxenite, probably kilometers in size, that formed from recycled basaltic oceanic crust. In detail, time series analyses of depth profiles of Al2O3/CaO, Sr/ Nb, La/Nb and Th/La in the KSDP drill core show correlations among these ratios indicating that recycled oceanic crust contributed episodically, 29 ka period, to the magma source during the prolonged transition from Kalihito Makapuu-stage lava compositions. The longterm geochemical trends show that recycled oceanic crust was increasingly important as the Koolau shield moved away from the plume and encountered lower temperature.