PETROLOGICAL FORENSICS OF MAGMA MINGLING DURING 2013 TO PRESENT ERUPTIONS OF SINABUNG VOLCANO, SUMATRA

The possible link between Sinabung Volcano and Toba Caldera, Sumatra, makes investigation of the magma dynamics of the ongoing eruptions at the former an imperative. Since it began erupting in September 2013, the Sinabung Volcano has erupted predominantly light grey, frothy, medium to coarse-grained andesitic (59-61% SiO₂) lava that hosts abundant black, porphyritic basaltic andesite (56.9% SiO₂) enclaves. The presence of these intermingled enclaves suggests that recharge of a more mafic magma mixed with a cooler, andesitic magma and triggered the eruptions. To test this hypothesis, we have embarked on a forensic microanalytical investigation of recent eruption products of Sinabung Volcano to compare the phenocryst populations. Petrographic analysis reveals at least three enclave types based on observed crystal content and groundmass variation: (1) large cumulate clots of crystal fragments and minor interstitial amphibole, (2) a group with a dominant dark glassy groundmass with microlites, and (3) a group with a groundmass of abundant microlites with an interstitial amphibole phase. The first enclave type is found in the samples of combined enclave and host lava while the other two are found exclusively in the enclave only samples. The crystal population is largely homogeneous, with plagioclase being the most abundant in all samples, followed by two pyroxenes (clinopyroxene and orthopyroxene) and amphibole having similar percentages, and rare olivine. Plagioclase zoning of the crystals in the enclaves may have originated in the andesitic host lava based on host plagioclase cores compositions (An 75-81%) and enclave plagioclase cores (An 83-92%). While the physical evidence of enclaves indicates mingling occurred prior to eruption of the host lava, the sharp boundaries between the enclave and the host suggests that mixing may have been incomplete. Furthermore, plagioclase normal zoning could suggest that either (1) the composition or temperature variations between the two magmas was not significant enough to affect equilibrium, or (2) the eruption occurred too quickly after the recharge event for rim growth to record the change. Continuing analysis will focus on the entire mineral assemblage record of interaction of these magmas.