Characterising the magma supply system of Agung and Batur volcanoes on Bali, Indonesia

Volcanic and magmatic processes are controlled by the composition of the magmas involved and the nature and structure of their underlying plumbing systems. To understand and predict volcanic behaviour, it is of critical importance to characterize the associated magmatic plumbing and supply system. This study investigates the magma plumbing system beneath Bali, Indonesia by employing several thermobarometric models using mineral phases in lavas from the simultaneous eruptions of Agung and Batur volcanoes in 1963 and the 1974 eruption of Batur. Compositional data were acquired from feldspar, pyroxene, and olivine crystals, using electron microprobe analysis, as well as from whole-rock samples using inductively coupled plasma mass spectrometry. Clinopyroxene-melt and clinopyroxene composition thermobarometers were then applied to equilibrated clinopyroxene-melt couples, while plagioclase-melt thermobarometry was employed on equilibrated plagioclase-melt pairs. The results were used to construct comprehensive magmatic plumbing models for Agung and Batur and are compared with geochemical, geophysical and petrological data on these volcanoes and others in the region. For the 1963 Agung eruption, results from clinopyroxene-melt thermobarometry suggest dominant crystallisation levels between 18 and 22 km depth. Clinopyroxene from the 1963 eruption of Batur record crystallisation depths between 12 and 18 km, whereas clinopyroxene from the 1974 Batur eruption show a main crystallisation level between 15 and 19 km. Furthermore, plagioclase-melt thermobarometry indicates the existence of shallow level magma reservoirs, with depths between 3 and 7 km for the 1963 eruption of Agung, between 2 and 4 km for the 1963 Batur eruption and between 3 and 5 km for the 1974 Batur event. The deep magma storage regions notably coincide with lithological boundaries in the crust and mantle beneath Bali, while the shallow reservoirs are consistent with recent geophysical studies that point to regional shallow-level magma storage. An along-arc comparison reveals this trend to be characteristic of Sunda arc magma storage systems and highlights the utility of a thermobarometric approach to detect multi-level systems beneath recently active volcanic systems.