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

The Pomici di Mercato (PdM, 8,010±40 a), also
known in the literature as Pomici Gemelle or Pomici di
Ottaviano, 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 more
than 4 ka of repose. The erupted magma is phonolitic in
composition. All the products have very low phenocrysts
content (less than 3%) and show evidence of mineralogical
disequilibria. They contain K-feldspar ± clinopyroxene
(salite and diopside) ± plagioclase ± garnet ± biotite ±
amphibole ± apatite ± Fe-Ti oxides. Pumice fragments
collected at different stratigraphic heights are slightly less
evolved and more enriched in radiogenic Sr composition
upsection. The glass composition is fairly homogeneous in
single pumice fragment and among pumice fragments from
different layers. Glass separated from pumice fragments
collected at different stratigraphic heights is homogeneous
in the Sr-isotope composition (around a value of 0.70717).
Glass is in isotopic equilibrium with salite throughout the
entire sequence and with diopside at the base of the
sequence. Diopside becomes more radiogenic upsection,
reaching a value of 0.707458±7, whereas feldspar is
consistently slightly less radiogenic than glass. Nd-isotope
composition is fairly uniform (ca. 0.51247) through the
whole sequence. The isotopic disequilibria among glass,
feldspar and diopside, together with the homogeneous
isotopic composition of pumice glass in equilibrium with
salite, and the mineralogical disequilibria between plagioclase
and K-feldspar, imply that most of the diopside
and plagioclase crystals are xenocrysts incorporated into
the phonolitic magma during residence in a magma
chamber and/or during ascent towards the surface. The
PdM Tephra are compositionally and isotopically similar to
the phonolitic, first-erupted products of the subsequent
Pomici di Avellino Plinian eruption. On the basis of this
similarity, we suggest that the magma feeding both
eruptions resulted from the tapping of a unique magma
chamber. Prior to the PdM eruption, this chamber was
formed by a large and homogeneous phonolitic magma
body. After the PdM eruption, as a consequence of new
arrivals of more radiogenic in Sr, less-differentiated magma
batches, the magma chamber progressively developed a
slightly stratified phonolitic uppermost portion, capping a
tephriphonolitic layer, both emitted during the subsequent Pomici di Avellino eruption.

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