Obsidian pyroclasts: storytellers of the 2008 Chaitén volcano eruption, Chile.

FORTE, P.; CASTRO, J.

Congreso; AGU Fall Meeting; 2020

American Geophysical Union

The rhyolite eruption of Chaitén volcano in 2008 marked the end of an approximate100 year-long hiatus in major silicic events and showed for the first time the capacity of these systems to produce volcanic impacts on a global scale. The eruption itself lasted for more than two years and involved explosive (~10 days) and effusive styles. Notably, a protracted ?transitional? phase was observed, characterized by simultaneous tephra emission and extrusion of degassed obsidian lava. Chaitén produced vast amounts of glassy rhyolite pyroclasts, ranging from very fine ash to metric-size bombs. In this work, we used this pyroclastic material as a tracer of the eruptive dynamics of 2008 Chaitén eruption. We specifically measured magmatic H2O (FTIR), one of the key parameters controlling explosivity of volcanic eruptions. Deposits formed during each of the three eruptive phases were sampled in field from 2010 to 2016. Our samples comprise: Plinian tephra fallout, PDC deposits, obsidian bombs, vent-proximal tephra cone developed during hybrid activity and the lava dome. We analyzed more than 500 pyroclastic obsidians from time-constrained stratigraphic sections and mapped the H2O content of the obsidian bomb field. Results show a well-defined trend of H2O depletion from the early explosive to the later effusive deposits. Bulk H2O contents range from 2.8 to 0.5 wt.% across the pyroclastic sequence, while values of the lava dome are consistently < 0.3 wt.%. In addition, we have detected water content windows within stratigraphic horizons of the explosive and transitional phases, defined by upper and lower H2O content values that are offset from the effusive H2O values. These water content windows shift with stratigraphic position -becoming narrower and moving towards lower H2O with eruption progress- suggesting an H2O-content limit on explosive fragmentation.