Elucidating geochemical heterogeneity and evolution of the explosively erupted Curacautin magma, Llaima volcano, Chile

Explosive mafic eruptions are among Earth's most hazardous volcanic phenomena due to the rapid ascent rates of mafic magma limiting time for early detection and warning systems. To date, work on explosive mafic eruptions has primarily been in the context of conduit and mafic magma fragmentation processes. Our work uses geochemistry and numerical modeling to establish magma source and system dynamics that led to an explosive mafic eruption at Llaima volcano, Chile. This study compares major and trace elements of 70 whole rock ignimbrite samples collected from 17 different outcrops of the Curacautin ignimbrite, including four extensive outcrops sampled vertically every 1-3 m to examine chemical variations and changes in magma source prior to and during the eruption. The bases and tops of the four stratigraphic sections and four additional samples with unique chemical signatures were analyzed for Sr-Nd-Pb isotopes to investigate mantle source variation. Eruptive products of the Curacautin eruption define a medium-K calc-alkaline suite of basalt to andesite (SiO2 50-58 wt. %). Both regionally and stratigraphically, trace element patterns produced remarkably similar, parallel patterns with the most enriched sample in the west where the base of the ignimbrite is exposed (LaN/SmN = 1.63). The Curacautín ignimbrite is chemically heterogeneous [e.g., MgO=2.5-6 wt. %, Ce = 11-39 ppm, and Ba/Th= 170-263], but Pb isotopes are homogeneous (207Pb/204Pb = 15.59-15.61), indicating the magma source was influenced by slab inputs or secondary process (e.g., AFC). However, Sr-Nd isotopes still need to be collected. To constrain mantle source heterogeneity, we utilize hierarchical clustering analysis (HCA). HCA results indicate four chemically distinct groups in the north, two in the east, and two in the west. To explore secondary processes, Magma Chamber Simulator (MCS; Bohrson et al., 2014) modeling will be used to produce thermodynamic models to constrain the chemical variability resulting from fractional crystallization (FC) processes.