Constraints from Phase Equilibrium Experiments on Pre-eruptive Storage Conditions in Mixed Magma Systems: a Case Study on Crystal-rich Basaltic Andesites from Mount Merapi, Indonesia

Abstract

Mount Merapi is one of Indonesia’s and the world’s most hazardous volcanoes. Existing constraints on the location and the crystallization conditions of its pre-eruptive magma reservoir based on mineral and glass thermobarometry and geophysical surveys are inconclusive, yet of immediate importance for future hazard mitigation. Here, we use two series of phase equilibrium experiments to quantify the conditions of pre-eruptive magma storage and magma recharge in Merapi’s upper-crustal reservoir: (1) to characterize crystallization at total equilibrium conditions; (2) to characterize crystallization at local equilibrium conditions between crystal rims and host melt. We demonstrate that this experimental approach can constrain pre-eruptive crystallization conditions and their variation in crystal-rich, mixed magma systems, such as that of Merapi, for which standard crystallization experiments and mineral thermobarometry fail. In agreement with geophysical estimates, we infer that Mount Merapi’s pre-eruptive reservoir partially crystallizes at ≥100–200 ± 75 MPa and thus at relatively shallow depths of <it>c.</it> ≥4·5 to ∼9 km. Magmas are stored at ≥925–950 ± 25°C with a melt H<inf>2</inf>O content of ∼3–4 wt % and a vapour phase with an <it>X</it>H<inf>2</inf>O [H<inf>2</inf>O/(H<inf>2</inf>O + CO<inf>2</inf>)] of ∼0·5–0·6 ± 0·1. Pre-eruptive recharge magmas, in contrast, have a temperature >950 to <1000°C, a higher melt H<inf>2</inf>O content of ∼4–5 wt % and a vapour phase with a higher <it>X</it>H<inf>2</inf>O of ∼0·8 ± 0·1. We hypothesize that these variations in melt H<inf>2</inf>O content and vapour <it>X</it>H<inf>2</inf>O between resident and pre-eruptive recharge magmas relate to variable degrees of open-system degassing of magma parcels en route into the reservoir rather than to variations in volatile fluxing of the stored magmas.