Geochemical variations during Kīlauea's Pu'u 'Ō'ō eruption reveal a fine-scale mixture of mantle heterogeneities within the Hawaiian plume

Long-term geochemical monitoring of lavas from the continuing 25-year-old Pu'u 'Ōō eruption allows us to probe the crustal and mantle magmatic processes beneath Kīlauea volcano in unparalleled detail. Here we present new Pb, Sr, and Nd isotope ratios, major and trace element abundances, olivine compositions, and petrographic data for Pu'u 'Ōō lavas erupted from 1998 to 2005. Olivine fractionation and accumulation are important crustal processes for the eruption, with minor clinopyroxene fractionation observed in the most recent lavas. Small, yet systematic variations in ⁸⁷Sr/⁸⁶Sr and incompatible trace element ratios, and MgO-normalized major element abundances document rapid changes in the parental magma composition delivered to Pu'u 'Ōō. Recent (1998-2003) lavas display a systematic temporal evolution towards an intermediate area between the compositional fields of historical Kīlauea and Mauna Loa lavas. At least three distinct mantle source components are required to explain the overall isotopic and chemical variability of Pu'u 'Ōō lavas. Two of these source components observed in pre-1998 Pu'u 'Ōō lavas have similar Pb, Sr, and Nd isotope ratios, although one underwent a recent (< 8 ka) small-degree melting event and became depleted in incompatible trace elements. This recently depleted component was an increasingly important source for lavas erupted between 1985 and 1998. The third component is a hybrid mixture of nearly equal portions of Kīlauea- and Mauna Loa-like mantle source compositions. It was progressively tapped in greater amounts from 1998 to 2003 and then subsequently decreased. The increasing importance of the hybrid source can be explained if melt pathways migrated from an area within Kīlauea's typical melting region (important for the 1985-1998 lavas) towards Mauna Loa, where a similar proportion of Kīlauea- and Mauna Loa-like mantle components might exist. The Pu'u 'Ōō data suggest that Kea and Loa mantle components are distributed on a fine-scale within the Hawaiian plume, and both are present beneath Kīlauea volcano. Based on the geochemical and isotopic variations during the Pu'u 'Ōō eruption, the estimated volume for Kīlauea and Mauna Loa compositional heterogeneities is < 10-35 km³.