PETROGRAPHY AND GEOCHEMISTRY OF LAVAS FROM HYANG-ARGOPURO VOLCANO, EAST JAVA, SUNDA ARC: A WINDOW INTO QUIESCENT VOLCANIC COMPLEX OF AN ACTIVE ARC

Volcanoes can experience centuries of quiescence before a catastrophic eruption, leaving local populations unaware of the potential for hazardous eruptions. This phenomenon was illustrated most famously by Mount Pinatubo’s 1991 VEI-6 eruption. Hyang-Argopuro (HAV) is another Southeast Asian volcano in East Java, Indonesia, an area characterized by volcanic activity. Despite being bounded by two active caldera complexes (Bromo and Ijen), HAV is part of an inactive volcanic chain. There has not been a recorded eruption of HAV in modern times, although unconfirmed reports suggest an eruption in 1597. Here, we present some of the first analyses of basaltic lava flows from HAV. These samples will provide some of the first insights into the nature of HAV’s past eruptions, recent magmatic activity, and future eruptive potential. Whole rock geochemical data has been collected using X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICPMS). These data show HAV lavas are calc-alkaline basaltic-andesites with noticeable large-ion lithophile elements (LILE) enrichments compared to other volcanoes in the region. To complement our whole rock data, we plan to employ microanalytical techniques to provide a more detailed insight into the system’s behavior. Optical petrographic analyses of thin sections will provide an estimate of mineral abundances and an understanding of the textures consistent with different kinds of igneous processes. Additional data, like the degree of phenocryst zoning and microlite textures, will be collected with a scanning electron microscope (SEM). Energy dispersive spectroscopy (EDS) will be done jointly to collect semi-quantitative chemical data and help to confirm or refute petrographic findings. Electron microprobe analysis (EPMA) will then be used to acquire quantitative high-precision chemical data on glass and minerals in each sample which can be used to calculate the temperatures and pressures of magma. These models will be the first attempt to determine pre-eruptive conditions in the volcano. Results will allow for insight into the pre-eruption processes of HAV and could potentially explain why HAV and the volcanic chain in this region have been inactive in recent centuries, providing a better understanding of how a future eruption of HAV may impact East Java.