Jade Bowers

and 2 more

The low viscosity of mafic magmas generally promotes effusive to mildly explosive eruptions; however, highly explosive mafic volcanic activity does occur. Recent models suggest that rapid ascent rates leading to significant disequilibrium drastically impact the rheology leading to fragmentation of the mafic magma. However, the magmatic conditions preceding magma ascent initiation are primarily unknown. There is a rich rock and mineral record for understanding melt generation in the mantle wedge and open system processes stored in the juvenile products of these highly explosive mafic eruptions. This study uses the pyroclasts from the Curacautin ignimbrite, a large volume (3.5–4.5 km3 DRE), mafic (50.4-57.3 wt% SiO2) ignimbrite at Llaima volcano in Chile.Of the 68 ignimbrite samples collected and previously analyzed, we targeted four for small volume (5 mg vs. standard 50 mg) characterization via ICPMS: (1) the top and (2) base of the ignimbrite, and the most (3) depleted and (4) enriched samples. We prepped and analyzed ten randomly selected pyroclasts from the highlighted samples (a total of 40 analyses). This study aimed to determine how sampling (e.g., composite vs. individual and the number of samples) impacts the signal of geological processes in trace element compositions. We used the χ2 distribution to test if the observed standard deviation (corrected for noise by subtracting analytical error) of all 40 samples, as well as each sample set’s ten analyzed pyroclasts, is larger than the analytical error. We identified true variability for at least 13 of the 30 trace elements measured for the base of the ignimbrite and enriched end member sample sets and the combined 40 analyses. For the top of the ignimbrite sample and the depleted end member sample, there is true variability in almost all 30 trace elements. Small volume whole rock geochemical characterization of individual pyroclasts detects previously unseen small-scale geochemical variability in compatible and REE elements and constrains new enrichment end-member compositions for geochemical models. Using principal component analysis, we investigate melt source heterogeneity and fractional crystallization as controls on the observed variability.

Jade Bowers

and 4 more

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.