Mount Sinabung, Sumatra, Indonesia initiated eruptive activity in 2010 with the addition of a magmatic component in 2013, after a 3 year period of quiescence. Observations of magmatic activity began with phreatomagmatic eruption starting July 2013 closely followed by extrusion of andesitic lava in December 2013. Lava effusion has persisted through the eruptive phases (December 2013 – present) with periodic dome building events with partial to complete failure of the dome and production of pyroclastic density currents (PDCs). Since a magmatic component began erupting in 2013, Mount Sinabung has been producing predominately andesite lavas that evolve in composition as the eruptive phases progress (from 57 wt% SiO2 in 2013 to 65 wt% SiO₂ in 2015). In May 2016, when the lava dome collapsed and successively generated PDCs, the resulting magmatic clasts contained intermingled enclaves. This was the first reported observation of these magmatic enclaves and were thought to be evidence for magmatic recharge that may affect eruptive style and longevity of subsequent activity.
This microanalytical forensic investigation was an effort to determine if magmatic enclaves from the May 2016 dome collapse were evidence of mafic recharge in the system. Petrographic analysis was used to for detailed textural description of the samples collected. Phase chemistry was collected on electron microprobe to discern multiple compositional populations of mineral constituents. Sample textures are highly variable are resulted in the division of the samples into four textural units: andesite host (AH), enclave type-I (ET1), enclave type-II (ET2), and enclave type-III (ET3). Mineral compositions were probed to assess if there were multiple mineral populations present and displaying exchange between the enclaves and host. These analyses found that phenocrysts in all four units were broadly similar with small deviations in plagioclase core composition and amphiboles analyzed in ET3.
Multiple geothermobarometers were employed to determine pressure and temperature conditions of pre-eruptive magmas at Mount Sinabung. Temperature estimates for Fe-Ti oxides, pyroxene, and amphibole range from ~825 to 1100 °C. Pressure estimates from pyroxene and amphibole indicate crystallization depths from ~5 to 32 km, with two main regions of crystallization occurring between ~5 to 16 km and ~24 to 32 km. Magmatic inclusions and pyroxene-rich glomerocrysts likely represent magma of a similar composition crystallizing deeper in the system. Ascent of this magma where it intermingled with the AH magma. Mingling was long enough to grew similar composition rims on plagioclase in all four units before it was erupted on the surface during dome formation. The complex data presented here builds an image of the intricate petrological processes occurring beneath Mount Sinabung and contributes to our understanding of pre-eruptive conditions at the volcano.