Oxygen Isotope Variations before and during the Pu‘u‘ō‘ō Eruption (1982–2018): A Case of Recurring Crustal Contamination at Kīlauea Volcano

Abstract Pu‘u‘ō‘ō is Kīlauea’s longest lived (1983–2018) and most voluminous (~4.4 km3) historical eruption. New oxygen isotope data are presented for matrix material from lava erupted from 2006 to 2018, olivine from 1997 to 2018, clinopyroxene from 2013 to 2018 (first such data for Kīlauea), and matrix from two 1982 summit eruptions. These data are integrated with our previously published oxygen isotope results for the first part (1983–1997) of the Pu‘u‘ō‘ō eruption and 19–20th century Kīlauea summit eruptions and compared with published whole-rock XRF and Sr isotope data. This study builds on our previous work (using whole-rock and mineral chemistry) that indicated strong evidence for mixing of evolved magmas, which had undergone shallow storage, crystal fractionation, and crustal contamination, with new, mantle-derived magma. Pu‘u‘ō‘ō δ18O values range from 4.4 to 4.9‰ for olivine and 4.6–5.7‰ for matrix. Both are ~0.1–0.9‰ lower than the canonical mantle δ18O value. Olivine δ18O values are relatively constant during the eruption (4.7 ± 0.1‰), whereas matrix δ18O values progressively increased reflecting the flushing of contaminated magma from the feeder system. The δ18O results from Pu‘u‘ō‘ō and other Kīlauea historical lava provide evidence of ~200 years of recurring crustal contamination within the volcano. Equilibrium in O isotopes between the matrix and coexisting olivine was periodically established after major changes in the Pu‘u‘ō‘ō eruption and for summit eruptions. New, mantle-derived magma was contaminated in the summit reservoir after several major collapses (100+ m) and eruptive hiatuses, and in the east rift zone as magma was stored for weeks to decades. Poor correlations (R2 ≤ 0.1) were found between olivine and matrix δ18O values with Nb/Y and Sr isotopes for Pu‘u‘ō‘ō and historical Kīlauea summit lavas (1820–1982) exhibiting O isotope equilibrium. Thus, the ranges in olivine and matrix δ18O values in Kīlauea’s historical lava are not indicative of δ18O heterogeneity in its mantle source. Kīlauea’s mantle source has an apparently narrow range of δ18O values (5.6 ± 0.1‰) as evidenced by matrix O isotope values for its more primitive summit lavas and those from the latter part of the Pu‘u‘ō‘ō eruption. Olivine from young Hawaiian shield volcanoes with KEA-like (Kīlauea and Mauna Kea) and LOA-like (Kama’ehuakanaloa and Mauna Loa) sources have nearly identical δ18O values (5.0–5.1) suggesting the Hawaiian plume source is essentially homogeneous in δ18O during shield stage for recent volcanism.