Geochemical variations during Kilaueas Pu'u 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 Puu Oo eruption allows us to probe the crustal and mantle magmatic processes beneath Kilauea 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 Puu Oo 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 Sr-87/Sr-86 and incompatible trace element ratios, and MgO-normalized major element abundances document rapid changes in the parental magma composition delivered to Puu Oo. Recent (1998-2003) lavas display a systematic temporal evolution towards an intermediate area between the compositional fields of historical Kilauea and Mauna Loa lavas. At least three distinct mantle source components are required to explain the overall isotopic and chemical variability of Puu Oo lavas. Two of these source components observed in pre-1998 Puu Oo 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 Kilauea- 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 Kilaueas typical melting region (important for the 1985-1998 lavas) towards Mauna Loa, where a similar proportion of Kilauea- and Mauna Loa-like mantle components might exist. The Puu Oo data suggest that Kea and Loa mantle components are distributed on a fine-scale within the Hawaiian plume, and both are present beneath Kilauea volcano. Based on the geochemical and isotopic variations during the Puu Oo eruption, the estimated volume for Kilauea and Mauna Loa compositional heterogeneities is < 10-35 km(3).