Petrology, geochemistry, and correlation of tephra deposits from a large early-Holocene eruption of Mentolat volcano, southern Chile

Abstract

Two correlated tephra deposits, each 13 cm thick in the Aisén region of southern Chile, one in a lacustrine sediment core from the Mallín el Toqui (MET) peat bog and another from a subaerial soil exposure ∼10 km to the west in the Río Maniguales (RM) valley, preserve evidence for a large explosive eruption of Mentolat volcano, one of the five stratovolcanoes of the southernmost portion of the Andean Southern Volcanic Zone (SVZ). This eruption is constrained in age to ≥11,728 cal years BP by radiocarbon dating of organic matter from the MET sediment core and is termed the ∼11.7 ka MEN event. The two tephra deposits are identical and based on their petrology, bulk tephra, glass, and amphibole geochemical characteristics, are attributed to an eruption of Mentolat volcano. Both contain pumice lapilli with glass compositions that ranges from 59 to 76 wt. % SiO2, with medium to low-K2O calc-alkaline composition and trace element abundances similar to both lavas and other tephras derived from Mentolat. They have abundant amphibole, plagioclase, orthopyroxene, and clinopyroxene phenocrysts, with a smaller proportion of olivine and Fe-Ti oxides, and a minor amount of distinctive crustal xenoliths with both unfoliated and foliated textures. Amphiboles have low K2O (0.20–0.37 wt. %) and TiO2 (1.5–3.8 wt. %) and are similar geochemically to amphiboles from other Mentolat-derived tephra (K2O = 0.14–0.43 wt. % and TiO2 = 1.9–2.4 wt. %), but distinct from amphiboles in lavas and tephra derived from other volcanoes in the southernmost SSVZ including Cay (K2O = 0.47–0.55 wt. % and TiO2 = 2.4–3.1 wt. %) and Melimoyu (K2O = 0.39–0.52 wt. % and TiO2 = 2.8–4.5 wt. %). Amphiboles from the ∼11.7 ka MEN tephra formed over a broad range of pressures (154–406 MPa), temperatures (834-969 °C), and magma water contents (4.9–7.0 wt. %), which overlap with the physical-chemical conditions for the formation of amphiboles from other Mentolat-derived tephra. The two correlated tephra deposits, which are located ∼95 km southeast of Mentolat, are correlative with tephra of similar age identified in 12 other lacustrine sediment cores from the region, for which, based on their petrology and the geochemistry of their tephra glass and amphiboles, Mentolat is also the likely source volcano. This eruption produced approximately 1.8 km3 of bulk material with an estimated magnitude of 5.2. Mentolat has produced numerous (>18) explosive eruptions since glacial retreat from the region and future explosive eruptions from this center could potentially impact local population centers and the agricultural industry in southern Chile and Argentina.