The Hawaiian Melting Pot Beneath Ko'olau Volcano, O'ahu: Crystal Record Evidence of Silicic Melt Mixing

Abstract

The Hawaiian-Emperor chain is an alignment of 129 volcanoes in the Pacific Ocean, formed as the Pacific Plate moves across the Hawaiian Mantle Plume. Since ~5 Ma, it has developed two spatially organized mantle geochemical signatures: Loa-type volcanoes on the southwestern side of the chain and Kea-type volcanoes on the northeastern, defined by distinct isotopic and trace element compositions. Ko‘olau, a Loa-type volcano on O‘ahu, began erupting isotopically enriched lavas at the end of the shield volcanic stage (Ko‘olau-Makapu‘u). This “Enriched Loa” geochemical endmember comprises higher 208*Pb/206*Pb, 87Sr/86Sr, and SiO2 and lower 143Nd/144Nd, 176Hf/177Hf, CaO and TiO2 compared to other Loa- and Kea-type lavas, which suggests distinct, time-integrated compositional variations in the magmatic source that may require non-peridotite lithologies such as pyroxenite or eclogite. Ko‘olau also is one of only two Hawaiian Island volcanoes that lacks a postshield volcanic stage, implying a potential magmatic plumbing system response to these variations in mantle source composition. The investigation of a geologically constrained stratigraphic sequence of ~0.22 Ma of eruptive history of Ko‘olau-Makapu‘u reveals physical and chemical evidence of a complex history of magma mixing. The presence of orthopyroxene reaction rims, cyclic enrichment and depletion of the Ko‘olau-Makapu‘u isotopic signature, cyclic increase and decrease in olivine and orthopyroxene crystallization temperatures, olivine Fo, and pyroxene Mg#, and complex pyroxene chemical zoning provides evidence that Ko‘olau-Makapu‘u cyclically rejuvenated the plumbing system at least three times and heavily reacted with the partially fractionated Loa-type cumulate mush in the shallow magma storage reservoir. This study provides a model for the petrogenesis of Ko‘olau-Makapu‘u basalts and evolution of Ko‘olau volcano from the shield to late shield informed by textural analysis of mineral populations, mineral chemistry, olivine and orthopyroxene thermometry, and Pb-Nd-Sr-Hf isotopic signatures.