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The island of Bali in Indonesia is home to two active stratovolcanoes, Agung and Batur, but relatively little is known of their underlying magma plumbing systems. Here we define magma storage depths and isotopic evolution of the 1963 and 1974 eruptions using mineral-melt equilibrium thermobarometry and oxygen and helium isotopes in mineral separates. Olivine crystallised from a primitive magma and has average delta O-18 values of 4.8%. Clinopyroxene records magma storage at the crust-mantle boundary, and displays mantle-like isotope values for Helium (8.62 R-A) and delta O-18 (5.0-5.8%). Plagioclase reveals crystallisation in upper crustal storage reservoirs and shows delta O-18 values of 5.5-6.4%. Our new thermobarometry and isotope data thus corroborate earlier seismic and InSAR studies that inferred upper crustal magma storage in the region. This type of multi-level plumbing architecture could drive replenishing magma to rapid volatile saturation, thus increasing the likelihood of explosive eruptions and the consequent hazard potential for the population of Bali.

Water is a key parameter in magma genesis, magma evolution, and resulting eruption styles,because it controls the density, the viscosity, as well as the melting and crystallization behavior of a melt. Theparental water content of a magma is usually measured through melt inclusions in minerals such as olivine, amethod which may be hampered, however, by the lack of melt inclusions suitable for analysis, or postentrapmentchanges in their water content. An alternative way to reconstruct the water content of a magma is touse nominally anhydrous minerals (NAMs), such as pyroxene, which take up low concentrations of hydrogenas a function of the magma’s water content. During magma degassing and eruption, however, NAMs maydehydrate. We therefore tested a method to reconstruct the water contents of dehydrated clinopyroxene phenocrystsfrom the Western Canary islands (n=28) through rehydration experiments followed by infrared andM€ossbauer spectroscopy. Employing currently available crystal/melt partitioning data, the results of the experimentswere used to calculate parental water contents of 0.71±0.07 to 1.49±0.15 wt % H2O for WesternCanary magmas during clinopyroxene crystallization at upper mantle conditions. This H2O range is in agreementwith calculated water contents using plagioclase-liquid-hygrometry, and with previously published datafor mafic lavas from the Canary Islands and comparable ocean island systems elsewhere. Utilizing NAMs incombination with hydrogen treatment can therefore serve as a proxy for pre-eruptive H2O contents, which weanticipate becoming a useful method applicable to mafic rocks where pyroxene is the main phenocryst phase.