PETROLOGICAL FORENSICS OF THE MOUNT SINABUNG, SUMATRA, INDONESIA MAGMA RESERVOIR PRIOR TO MAY 2016 DOME COLLAPSE

Mount Sinabung, Sumatra, Indonesia began erupting in 2010 and resumed activity in 2013. The eruptions transitioned from phreatic (August 2010 – September 2010) to phreatomagmatic (starting July 2013) with extrusion of andesitic lava (starting 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 (57-65 wt. % SiO₂). In May 2016 when the lava dome collapsed and successively generated PDCs, the resulting magmatic clasts contained intermingled enclaves. Usually, magmatic enclaves are interpreted as evidence for recharge and/or magma mingling prior to eruption. To test this hypothesis, we used forensic microanalytical investigation of the host lava, intermingled enclaves, and glomerocrysts. Petrographic analysis and phase chemistry collected on electron microprobe were used to decipher the relationship between these three constituents. The host andesite (59-61 wt% SiO₂) generally has restricted phenocryst compositions and lower calculated temperatures and pressures. Petrographic analysis revealed three types of glomerocrysts varying in mineral assemblage and presence of glass and/or pore space and three major enclave types based on observed crystal content and groundmass variation. Chemically, there is broad overlap between the host, enclaves, and glomerocrysts. The similarity in phase chemistry and calculated intensive parameters suggest the host and enclaves are crystal cumulates from different regions of one magma reservoir and the glomerocrysts represent disaggregates of these cumulates.