Volatile behaviour and magmatic processes leading to the 1257 climatic eruption of Samalas (Indonesia)

The 1257 caldera-forming eruption of Samalas was one of most powerful eruptions of the Holocene (VEI 7). It blasted in the stratosphere tephra and gases which stand as the largest emissions of the Common Era. The sulphuric acid veil engendered a severe decrease of temperatures in the Northern hemisphere during 3 years, favouring crop failures and enhancing deadly famines across Europe in the mid-13th century. Petrological and mineralogical study of the Holocene products of the Rinjani-Samalas complex and their melt inclusions showed that the medieval eruption evacuated 40 km3 of chemically homogeneous trachydacite (64.0±0.4 wt% SiO2; 8.1±0.1 wt% Na2O+K2O, normalized on anhydrous basis). Its mineral paragenesis consists of plagioclase with patchy zoned calcic cores (An82-75) surrounded by sodic bands (An50-43), in association with amphibole (magnesio-hastingsite), orthopyroxene (Mg# 0.66-0.73), titano-magnetite, iron sulphide and apatite. Rare extremely calcic plagioclase (An91-92) records the early stage of crystallization. The trachydacitic magma derived from its parent high alumina basalt (HAB) through 80% of fractional crystallisation in the lower crust involving the peritectic formation of (cryptic) amphibole. Trachydacitic batches of magma were further extracted and transferred to shallow depth (3-4 km). The range of water contents and temperatures measured in melt inclusions (989±10°C 3.7±0.3 wt% H2O) and discrepancies with geothermometers (895-980°C) show that the magma batches crystallized in-situ but isolated under conditions of water-saturation. Volatile concentrations in melt inclusions, minerals, matrix glasses and whole-rocks of the Rinjani-Samalas suite show that prior to the 1257 eruption, volatiles were distributed between the trachydacitic melt, the exsolved C-H-O-S vapour phase, polymetallic sulphides, amphibole and apatite. A total of 158 Tg SO2, 227 Tg Cl and up to 1.3 Tg Br were released during the eruption. The good agreement with sulphur proxies derived from ice core records confirms that the pre-eruptive vapour phase contributed to 80% of the S yield, and suggests that the plume dynamics enhanced the stratospheric injection of climate-impacting gases.