Low-δ18O magmas at Batur, Indonesia: Evidence for slab-window melting beneath arc volcanoes

Low-δ 18 O magmas (δ 18 O < +5.5 ‰) record the assimilation of rocks that underwent high-temperature hydrothermal alteration. This study presents an integrated analysis of oxygen isotopes and major elements of olivines, along with whole-rock geochemistry and Sr–Nd–Hf isotopes for basaltic andesites and andesites from the Quaternary Agung and Batur volcanoes in Bali, Indonesia. Agung and Batur lavas exhibit similar trace element and Sr–Nd–Hf isotopic signatures but differ in their olivine oxygen isotopic compositions. Olivine phenocrysts from Agung lavas show δ 18 O values ranging from +4.65 ‰ to +5.38 ‰, with an average of +5.08 ± 0.21 ‰ (2SD, n = 20). Olivines from Batur lavas exhibit lower δ 18 O values (+3.70 ‰ to +5.15 ‰), with an average of +4.50 ± 0.14 ‰ (2SD, n = 120), and most are distinctly below the mantle olivine values. The low δ 18 O values observed in both Agung and Batur olivines are unlikely to result from assimilation of pre-existing hydrothermally altered crustal or syn -magmatic rocks. Instead, they provide evidence for the assimilation of recycled altered oceanic crustal materials. We propose that these assimilated oceanic crustal materials could have been derived from the partial melting of low-δ 18 O lower oceanic crust, possibly induced by hot asthenospheric flow through a slab window. This study highlights that slab windows may facilitate not only the formation of rear-arc ultrapotassic lavas but also significantly influence magmatism at the volcanic front. • The Discovery of low-δ 18 O olivines in Bali volcanoes. • Low-δ 18 O olivines may point to recycled oceanic crust. • Slab windows may facilitate crustal recycling beneath Bali, Indonesia.