Volatiles, major and trace element in quartz-host melt inclusions from ignimbrites in the Central Andes

Studies of ignimbrites provide valuable insight into the evolution processes occurring within silicic magma bodies. Two groups of eruptions producing these ignimbrites have been identified in the Central Andes. Small to moderate volume eruptions (<50 km3) associated with arc volcanism (ex. Cerro Blanco), and supereruptions of 102 to 103 km3 in systems operating in flare-up mode (ex. Altiplano Puna). In this work, we investigate these two modes of eruptions focusing on the pre-eruptive volatile contents and the characterization of the conditions and physical processes involved in the pre-eruptive magmatic differentiation. Volatiles, major and trace elements in quartz-hosted melt inclusions and matrix glass of pumice clasts from the Cerro Blanco caldera complex (CBCC), Argentina are compared with the massive ignimbrites of the Altiplano-Puna Volcanic Complex (APVC). Trace element compositions of matrix glasses of the CBCC are similar to or more evolved than quartz-hosted melt inclusions consistent with in situ crystallization of the quartz. The trace element content of the melt inclusions and the matrix glass shows enrichment in light rare earth elements and Rb, and strong Ba and Sr depletion. This depletion in is not seen in the APVC. Moreover, comparison of the trace elements variations show that CBCC rhyolites are less contaminated fractionates from andesitic magmas that typically erupt from the arc, while APVC dacites are much more crustal in origin. Infrared spectroscopy analyses of the melt inclusions trapped in quartz phenocrysts from CBCC have dissolved H2O contents varying between 2.6 and 8.6 wt% and dissolved CO2 ranging from below detection limit up to 173 ppm (saturation pressure of 39 to 325 MPa). These values are in marked contrast with data from the APVC where dissolved volatile content in quartz-hosted melt inclusions have a narrower range of H2O content (3.0 to 4.5 wt%) for a much higher range of CO2 content (from below detection limit up to 400 ppm). In a closed system environment, these high water concentrations can be produce by high degree of plagioclase and feldspars fractionation as suggest by the depletion of Ba, Sr, and Eu in the melt inclusions. Our data are consistent with the previous hypothesis that the smaller volumes eruptions in the Central Andes can be triggered by volatile oversaturation within the chamber in a closed system environment, while those from the supereruptions evolve in an open system environment where passive degassing may take place and require an external trigger.