Time-scales of recent Phlegrean Fields eruptions inferred from the application of a ‘diffusive fractionation’ model of trace elements
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The variation of chemical element compositions in two pyroclastic sequences (Astroni 6 and Averno 2, Phlegrean Fields, Italy) is studied. Both sequences are compositionally zoned indicating a variability of melt compositions in the magma chamber prior to eruption. A clear dichotomy between the behaviour of major vs. trace elements is also observed in both sequences, with major elements displaying nearly linear inter-elemental trends and trace elements showing a variable scattered behaviour. Together with previous petrological investigations these observations are consistent with the hypothesis that magma mixing processes played a key role in the evolution of these two magmatic systems. Recently it has been suggested that mixing processes in igneous systems may strongly influence the mobility of trace elements inducing a ‘diffusive fractionation’ phenomenon, whose extent depends on the mixing time-scale. Here we merge information from 1) numerical simulations of magma mixing, and 2) magma mixing experiments (using as end-members natural compositions from Phlegrean Fields) to derive a relationship relating the degree of ‘diffusive fractionation’ to the mixing time-scales. Application of the ‘diffusive fractionation’ model to the two studied pyroclastic sequences allowed us to apply the relationship derived by numerical simulations and experiments to estimate the mixing time-scales for these two magmatic systems. Results indicate that mixing processes in Astroni 6 and Averno 2 systems lasted for approximately 2 and 9 days, respectively, prior to eruption.
KeywordsPhlegrean Fields Magma interaction Trace elements Chaotic mixing Numerical models Experiments Eruption time-scales
This work was funded by INGV (Istituto Nazionale di Geofisica e Vulcanologia), MIUR (Ministero Italiano dell’Università e della Ricerca), and University of Perugia grants. The authors would like to express their gratitude to Mauro Di Vito, Lucia Civetta and Giovanni Orsi for their suggestions and assistance during field work. D.B. Dingwell acknowledges the support of a Research Professorship (LMUexcellent) of the Bundesexcellenziniziativ. Constructive comments by R. Cioni and M. Jellinek are gratefully acknowledged.
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