Compositional Layering and Syn-eruptive Mixing of a Periodically Refilled Shallow Magma Chamber: the AD 79 Plinian Eruption of Vesuvius

A detailed study of the pyroclastic deposits of the AD 79 'Pompei' Plinian eruption of Vesuvius has allowed: (1) reconstruction of the thermal, compositional and isotopic (Sr-87/Sr-86) pre-eruptive layering of the shallow magma chamber; (2) quantitative definition of the syn-eruptive mixing between the different magmas occupying the chamber, and its relationships with eruption dynamics; (3) recognition of the variability of mafic magma batches supplying the chamber During the different Phases of the eruption similar to 25-30% of the magma was ejected as white K-phonolitic pumice, and 70-75% as grey K-tephri-phonolitic pumice. The white pumice results from the tapping of progressively deeper magma from a body (T = 850-900 degrees C) consisting of two distinct layers mainly formed by crystal fractionation. The grey pumice results from syn-eruptive mixing involving three main end-members: the phonolitic 'white' magmas (salic end-member, SEM), mafic cumulates (cumulate end-member, GEM) and a crystal-poor 'grey' phono-tephritic magma (mafic end-member, MEM) which was never erupted without first being mixed with 'white' magma. Evidence is provided that mixing occurred within the chamber and was characterized by a transition with time from physical mixing at a microscopic scale to chemical hybridization The MEM magma had a homogeneous composition and constant Sr-87/Sr-86 isotopic ratio, possibly as a result of sustained convection. No unambiguous liquidus phases were found suggesting that the MEM magma was superheated (T = 1000-1100 degrees C); its very low viscosity was a main cause in the establishment of a physical discontinuity separating the white and the grey magmas. The white-grey boundary layer possibly consisted of a multiply diffusive interface, periodically broken and recreated, supplying the phonolitic body through mixing of moderate amounts of fractionated grey melts with the overlying white magma. The presence of a large overheated mass indicates the young, growing stage of the AD 79 chamber whose main engine was the periodic arrival of hot mafic magma batches. These were characterized by K-tephritic to K-basanitic compositions, high temperatures (>1150 degrees C), high volatile contents (2.0-2.5% H2O + Cl + F + S), low viscosities [(1-2x10(2) poises)] and relatively low densities (2500-2600 kg/m(3)). The birth of the Pompei chamber followed the repeated arrival of these batches (on average characterized by Sr-87/Sr-86 approximate to 0.70729) into a reservoir containing a tephritic-phonolitic, phonolitic, crystal-enriched magma, a residue from the preceding 'Avellino' Plinian eruption (3400 BP). In fact, about half of magma ejected during the Ao 79 eruption could have been inherited from pre-Avellino times.