ANALYSIS OF VARIABLES AFFECTING THE HISTORIC VOLCANIC ERUPTIONS OF THE PUYEHUE-CORDON CAULLE VOLCANIC COMPLEX

The historic eruptions of Puyehue-Cordon Caulle are excellent examples of Plinian-hybrid eruptive events. Rhyodacitic eruptions at Cordon Caulle in 1921, 1960 and 2011 started with major explosive phases which were then interspersed with effusive activity. In this study, we determined pre-eruptive magma chamber conditions for these three events to understand the evolution and/or differences in eruptive styles throughout each event. Pre-eruptive temperatures were calculated using two-oxide thermometry (Ghiorso and Evans, 2008) and then used to calculate magmatic water contents using the plagioclase-liquid hygrometer of Waters and Lange (2015). Mineral analysis was done on a suite of well-characterized lava and tephra samples by EPMA at the Florida Center for Analytical Electron Microscopy at FIU. Pre-eruptive temperature and water content data were then compared with measured water contents in lavas to estimate extents of degassing and to calculate pre- and post-eruptive viscosities, in order to assess the evolution from explosive to effusive phases and degassing regime. Since the temperature measurements play a vital role in the calculation of pre-eruptive water contents, the most suitable variable data were selected by evaluating several possible temperature scenarios. The effect of degassing-induced crystallization was also considered. Results indicate that these three historic eruptive events have extremely similar bulk composition and pre-eruptive variables; the 2011 event yielded slightly lower temperature and higher water content. Detailed examination of differences in variables across different samples suggests that the eruptive style does not vary in accordance with the variable changes but instead must be controlled by external factors such as the regional structural and tectonic regime and/or the influence of a deep magmatic heat source.