Geothermometry From Bimodal Eruption Suites

Geothermometry from bimodal eruption suites: a study of the Cuicuiltic Member at Los Humeros Caldera, Mexico

Student : Marco Rebecchi

Supervisor Prof. Dr. Olivier Bachmann, Dr. Ben Ellis , Juliana Troch Geologisches,  Institut:

Holocene trachytic and trachyandesitic volcanic deposits with sharp stratigraphic boundaries from the bimodal Cuicuiltic Member eruptions at Los Humeros Caldera were analysed using geothermometers and thin section microscopy to determine pre-eruptive conditions (temperatures, pressures and water contents) and assess the eruptional models of the heterogeneous magmatic system. Thermometric inputs such as pressures and water contents were assumed based on previous studies and the analysed mineral phases were plagioclase, orthopyroxene, clinopyroxene and Fe-Ti-oxides. The results confirmed two different groups which belong to each one of these layers. While the trachyandesitic samples range around 1100 °C with pressures of 0.5 GPa and low water contents of 0.5 wt. %, the trachytic ones range around 910 °C with lower pressures of 0.3 GPa and higher water contents of up to 5 wt. % showing two different chemical magma compositions in the crust. A possible crystallization sequence is proposed resulting in a first crystallization of plagioclase followed by the pyroxenes and Fe-Ti-oxides. Thermometric comparisons of the Cuicuiltic Member with prior erupted Los Humeros deposits resulted in similar mineral compositions and showed a close fitting for the trachytic samples, whereas for the trachyandesitic samples the results scattered a lot and were less reliable as indicated by the limited mineral-melt equilibria. Nevertheless, the results show that the Los Humeros magmatic system has a bimodal nature not only based on stratigraphic but also on melt analysis and is able to erupt chemically contrasting deposits from independent vents. The thermometric results cannot exactly explain the heterogeneous structure of the magmatic reservoir, but still allow the application of prior developed eruptional models and provide constraints on the nature of the magmas erupted.