Petrological forensics of the Curacautín Magma(s)

Abstract:

In basaltic systems, plinian eruptions are the most hazardous and yet enigmatic style of volcanic activity. The Curacautin ignimbrite (Ci; ~12.6 ka) is a large-volume (proximal estimate of 3.4-3.9 km³ DRE), mafic (50.7-57.6 wt% SiO₂) plinian deposit erupted by Llaima volcano, Chile. We are conducting a petrologic study involving > 50 samples of the Ci to understand the conditions that gave rise to the large-volume explosive activity at the predominately effusive mafic center. We propose that the amalgamation of many isolated magma reservoirs provided the volume of magma necessary to produce the large-volume, Curacautin eruption. If this were the case, we would expect to observe heterogeneity in major and trace element concentrations in samples with no coherency within and between disparate outcrops. To test this hypothesis, we targeted four fresh, tens-of-meters thick, exposures of mafic ignimbrite to the north (n = 1), east (n = 1), and west (n = 2) of the volcano and sampled them at high vertical resolution (2 m increments) for major and trace element analysis. There is little to no systematic variation in the major elements between the four exposures. SiO₂ and CaO/Al₂O₃ increase ~1.5 wt. % and 0.3, respectively, with increasing sample height, while MgO decreases by 0.4 wt. %. Rare earth element patterns are similar for most samples across the four exposures, with the LREE showing enrichment with respect to HREE abundances. On average, samples from the two western outcrops have the highest trace element abundances (e.g., La = 6.3 ppm), intermediate at the northern outcrop (La = 5.3 ppm) and lowest at the eastern outcrop (La = 5.1 ppm). For all outcrops, trace element concentrations, on average, increase as a function of stratigraphic height. However, excursions within each outcrop convolute this relationship. Progressive fractional crystallization of the magma is preserved in the volcanic stratigraphy by both increases in SiO₂ and CaO/Al₂O₃, as well as a decrease in MgO with increasing stratigraphic height. Also, major and trace element variations between the four exposures indicate heterogeneity in the erupted magma and potentially variable degrees of fractional crystallization. The above observations are consistent with a magma derived from a series of isolated magma reservoirs and geochemical modeling is ongoing.