The magmatic conditions, from storage to surface preceding effusive and explosive eruptions at Kelud Volcano

Abstract:

Kelud volcano, (Indonesia) has in recent years demonstrated the classical dichotomy of eruptive styles, with an effusive dome eruption in 2007-08 and an explosive Plinian eruption in 2014, causing widespread ash fall and the evacuation of 100,000 people. This study compares both the deep (magma reservoir) and shallow (conduit feeder system) magmatic processes that occurred before both eruptions, using a combination of petrography as well as analytical and experimental petrology. The first results from this study show that despite their significant differences in eruptive style, these eruption products had identical magma compositions, comprising of basaltic andesite. Furthermore, the phenocryst assemblage and mineral compositions of the explosive and effusive products are also similar, pointing towards equivalent pre-eruptive storage conditions. The key difference lies within the microlite populations and matrix glass chemistry. These suggest that the eruptive style is determined by the ascent history, with different P(H₂O)-T-t paths leading to either effusive or explosive behavior. Interestingly, some plagioclase phenocrysts preserve progressive capture of small melt inclusions during different stages of crystal growth. Raman spectroscopic analysis of these small melt inclusions from crystal core to rim, provides a unique view of magmatic water contents during magma reservoir evolution. We speculate that water content variations (~0.6 – 2.3%) may exist due to a complex interplay between the fractionating basaltic andesite and fresh mafic injections. This study will also show the preliminary results from the first phase experiments done on Kelud volcanics, constraining the magmatic storage conditions preceding both the 2014 explosive and 2007 effusive eruptions.