The “Pomici di mercato” Plinian eruption of Somma-Vesuvius: magma chamber processes and eruption dynamics

摘要

The Pomici di Mercato (PdM, 8,010±40 a), alsoknown in the literature as Pomici Gemelle or Pomici diOttaviano, is one of the oldest Plinian eruptions of Somma-Vesuvius. This eruption occurred after the longest (7 ka)quiescence period of the volcano and was followed by morethan 4 ka of repose. The erupted magma is phonolitic incomposition. All the products have very low phenocrystscontent (less than 3%) and show evidence of mineralogicaldisequilibria. They contain K-feldspar ± clinopyroxene(salite and diopside) ± plagioclase ± garnet ± biotite ±amphibole ± apatite ± Fe-Ti oxides. Pumice fragmentscollected at different stratigraphic heights are slightly lessevolved and more enriched in radiogenic Sr compositionupsection. The glass composition is fairly homogeneous insingle pumice fragment and among pumice fragments fromdifferent layers. Glass separated from pumice fragmentscollected at different stratigraphic heights is homogeneousin the Sr-isotope composition (around a value of 0.70717).Glass is in isotopic equilibrium with salite throughout theentire sequence and with diopside at the base of thesequence. Diopside becomes more radiogenic upsection,reaching a value of 0.707458±7, whereas feldspar isconsistently slightly less radiogenic than glass. Nd-isotopecomposition is fairly uniform (ca. 0.51247) through thewhole sequence. The isotopic disequilibria among glass,feldspar and diopside, together with the homogeneousisotopic composition of pumice glass in equilibrium withsalite, and the mineralogical disequilibria between plagioclaseand K-feldspar, imply that most of the diopsideand plagioclase crystals are xenocrysts incorporated intothe phonolitic magma during residence in a magmachamber and/or during ascent towards the surface. ThePdM Tephra are compositionally and isotopically similar tothe phonolitic, first-erupted products of the subsequentPomici di Avellino Plinian eruption. On the basis of thissimilarity, we suggest that the magma feeding botheruptions resulted from the tapping of a unique magmachamber. Prior to the PdM eruption, this chamber wasformed by a large and homogeneous phonolitic magmabody. After the PdM eruption, as a consequence of newarrivals of more radiogenic in Sr, less-differentiated magmabatches, the magma chamber progressively developed aslightly stratified phonolitic uppermost portion, capping atephriphonolitic layer, both emitted during the subsequent Pomici di Avellino eruption.