Journal Article

Compositional variations and magma mixing in the 1991 eruptions of Hudson volcano, Chile

Carey, Steven, Naranjo, Jose-Antonio, Kratzmann, David J, Scasso, Roberto

2009

Overview

The August 1991 eruptions of Hudson volcano produced ~2.7 km 3 (dense rock equivalent, DRE) of basaltic to trachyandesitic pyroclastic deposits, making it one of the largest historical eruptions in South America. Phase 1 of the eruption (P1, April 8) involved both lava flows and a phreatomagmatic eruption from a fissure located in the NW corner of the caldera. The paroxysmal phase (P2) began several days later (April 12) with a Plinian-style eruption from a different vent 4 km to the south-southeast. Tephra from the 1991 eruption ranges in composition from basalt (phase 1) to trachyandesite (phase 2), with a distinct gap between the two erupted phases from 54–60 wt% SiO 2 . A trend of decreasing SiO 2 is evident from the earliest part of the phase 2 eruption (unit A, 63–65 wt% SiO 2 ) to the end (unit D, 60–63 wt% SiO 2 ). Melt inclusion data and textures suggest that mixing occurred in magmas from both eruptive phases. The basaltic and trachyandesitic magmas can be genetically related through both magma mixing and fractional crystallization processes. A combination of observed phase assemblages, inferred water content, crystallinity, and geothermometry estimates suggest pre-eruptive storage of the phase 2 trachyandesite at pressures between ~50–100 megapascal (MPa) at 972 ± 26°C under water-saturated conditions (log f O 2 –10.33 (±0.2)). It is proposed that rising P1 basaltic magma intersected the lower part of the P2 magma storage region between 2 and 3 km depth. Subsequent mixing between the two magmas preferentially hybridized the lower part of the chamber. Basaltic magma continued advancing towards the surface as a dyke to eventually be erupted in the northwestern part of the Hudson caldera. The presence of tachylite in the P1 products suggests that some of the magma was stalled close to the surface (<0.5 km) prior to eruption. Seismicity related to magma movement and the P1 eruption, combined with chamber overpressure associated with basalt injection, may have created a pathway to the surface for the trachyandesite magma and subsequent P2 eruption at a different vent 4 km to the south-southeast.

• Publisher: Springer Berlin Heidelberg,Springer,Springer Nature B.V
• Subject: Basalt / Calderas / Crystalline rocks / Crystallization / Earth and Environmental Science / Earth Sciences / Earth, ocean, space / Engineering and environment geology. Geothermics / Exact sciences and technology / Geology / Geophysics / Geophysics/Geodesy / Igneous and metamorphic rocks petrology, volcanic processes, magmas / Lava / Lava flows / Magma / Mineralogy / Natural hazards: prediction, damages, etc / Research Article / Sedimentology / Seismology / Volcanic eruptions / Volcanoes / Volcanology / Water content