H2O evolution in rhyolitic systems: the case of the 2008 Chaitén eruption, Chile

PABLO FORTE; JONATHAN CASTRO

Portland

Conferencia; IAVCEI General Assembly; 2017

IAVCEI

H2O is one of the key parameters controlling explosivity of volcanic eruptions. Studies performed on several Holocene rhyolite eruptions throughout the Pacific Northwest have elucidated degassing systematics of silicic systems, and generally show that H2O content of the magma evolves with eruption progress. The 2008 eruption at volcán Chaitén was the first opportunity to scientifically observe a complete rhyolitic eruption cycle and therefore relate volcanic activity to H2O contents preserved in deposits. The eruption started on 1 May 2008 and lasted for almost 3 years. The first ten days were characterized by explosive eruptions that were followed by lava effusion over a period of several months.  Interestingly, a protracted transitional phase between the purely explosive and effusive stages was identified, with the simultaneous emission of tephra and the emplacement of a degassed obsidian lava body. We sampled deposits formed during each of the three eruptive phases in several field campaigns spanning 2010 to 2016. Our samples comprise: distal Plinian tephra fallout, PDC deposits, an obsidian bomb field, a tephra cone developed around the vent during the hybrid activity and the lava dome. We characterized how H2O concentrations varied during the eruption by applying Fourier transform infrared spectroscopy (FTIR). We analyzed more than 500 pyroclastic obsidians from time-constrained stratigraphic sections and mapped the H2O content of the obsidian bomb field. We also evaluated cooling rates using hydrous speciation and textural variations (crystalinity and vesicularity). Preliminary results show a well-defined trend of H2O depletion from the explosive to the effusive deposits. Bulk H2O contents range from 2.5 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 transitional cone, defined by upper and lower H2O content values. These water content windows shift with stratigraphic position and demarcate clear H2O gaps with respect to effusive obsidians. We are currently investigating the meaning of such chemical patterns in a broader set of Chaitén pyroclastic samples.