The 79 ad Plinian eruption of Vesuvius produced first a white pumice fallout from a high steady eruptive column, and then a grey pumice fallout originating from an oscillatory eruptive column with several partial column collapse events after which there was a total column collapse. This first total collapse was followed by renewed Plinian activity and produced the last grey pumice (GP) fallout deposit of the eruption. Textural characteristics (vesicularity and microcrystallinity) of a complete sequence of the pumice fallout deposits are presented along with the major element compositions and residual volatile contents (H2O, Cl) to constrain the degassing processes and the eruptive dynamics. Large variations in residual volatile contents exist between the different eruptive units. Textural features also strongly differ between white and grey pumices, but also within the grey pumices. The degassing processes were thus highly heterogeneous. We propose a new model of the 79 ad eruption in which pre-eruptive conditions (H2O saturation, magma temperature and viscosity) are the critical controls on the diversity of the syn-eruptive degassing processes and hence the eruptive dynamics. Cl contents measured in melt inclusions show that only the white pumice and the upper part of the grey pumice magma were H2O saturated prior to eruption. The white pumice eruptive units represent a typical closed-system degassing evolution, whereas the first grey pumice one, stored under similar pre-eruptive saturation conditions, follows a particular open-system degassing evolution. We suggest that the oscillatory regime that dominated the grey pumice eruptive phase is linked to pre-eruptive water undersaturation of most of the grey magma, and the associated time delays necessary for H2O exsolution. We also suggest that the high residual H2O content of the last grey pumice, deposited after the renewal of Plinian activity following the first total column collapse event, is due to syn-eruptive saturation of GP magma and reduced H2O exsolution efficiency resulting from speciation of dissolved H2O in the melt.
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