Magma evolution inside the 1631 Vesuvius magma chamber and eruption triggering

Abstract

Vesuvius is a high-risk volcano and the 1631 Plinian eruption is a reference event for the next episode of explosive unrest. A complete stratigraphic and petrographic description of 1631 pyroclastics is given in this study. During the 1631 eruption a phonolite was firstly erupted followed by a tephritic phonolite and finally a phonolitic tephrite, indicating a layered magma chamber. We suggest that phonolitic basanite is a good candidate to be the primitive parental-melt of the 1631 eruption. Composition of apatite from the 1631 pyroclastics is different from those of CO 2 -rich melts indicating negligible CO 2 content during magma evolution. Cross checking calculations, using PETROGRAPH and PELE software, accounts for multistage evolution up to phonolite starting from a phonolitic basanite melt similar to the Vesuvius medieval lavas. The model implies crystal settling of clinopyroxene and olivine at 6 kbar and 1220 ∘ C, clinopyroxene plus leucite at a pressure ranging from 2.5 to 0.5 kbar and temperature ranging from 1140 to 940 ∘ C. Inside the phonolitic magma chamber K-feldspar and leucite would coexist at a temperature ranging from from 940 to 840 ∘ C and at a pressure ranging from 2.5 to 0.5 kbar. Thus crystal fractionation is certainly a necessary and probably a sufficient condition to evolve the melt from phono tephritic to phonolitic in the 1631 magma chamber. We speculate that phonolitic tephrite magma refilling from deeper levels destabilised the chamber and triggered the eruption, as testified by the seismic precursor phenomena before 1631 unrest.

Experimental constraints on pre-eruption conditions of the 1631 Vesuvius eruption

“…The 1631 products show a compositional variability from more evolved, tephritic-phonolites at the base of the lapilli fallout sequence up to less evolved phonolitic-tephrites at top (Rosi et al, 1993;Santacroce et al, 2008;Stoppa et al, 2017). Similar gradients have been classically interpreted at Vesuvius as the result of the interplay be-tween the eruption dynamics and the tapping of a vertically zoned magma reservoir (Civetta et al, 1991;Cioni et al, 1998).…”

“…v40 -Crystal rich basal sand layer (Rosi et al, 1993); AS94-314 -Crystal rich basal sand layer (Santacroce et al, 2008); 1631 SG -Starting glass for phase-equilibrium experiments (EPMA analyses; average and standard deviation, in italic, of 30 analyzed spots). recharge of the reservoir and/or from the disruption of the crystallized margins of the magma reservoir and of the thermometamorphic-metasomatized host rocks (Cioni et al, 1995;Stoppa et al, 2017). This compositional variation is also clearly visible from the analysis of single phenocrysts, in which Mg-rich phenocrysts may, or may not, be rimmed by Mg-poor compositions (Figs.…”

“…Contrary to other eruptions, the products of the 1631 event have not received dedicated petrological studies, apart from the recent paper of Stoppa et al (2017) dealing with an extensive presentation and discus-sion of compositional data of juvenile material and skarn fragments from the metasomatic halo around the magma chamber. The tephra fallout deposit shows a general compositional zoning, ranging from tephritic phonolite at the base to phonolitic tephrite at top (MgO vari-able from 2.4 to 6.2 wt% upsection).…”

Helium isotopes in Plinian and inter-Plinian volcanic products of Vesuvius, Italy

“…Sialic minerals, leucites from 1631 to 1944 period (Present Vesuvius), and sanidines from magma-skarn interface of 79 AD eruption, show the lowest 3 He/ 4 He values, in the range 0.7-1.8 R A . The 3 He/ 4 He values for fumarolic gases range Stoppa et al, 2017). Volcanic activity is driven by the recurrent arrival of primitive mafic magma batches from the mantle into an extended deep-seated reservoir ("deep reservoir"), where homogenization occurs with resident, pre-existing magmas.…”

Archaeometry of a Roman Millstone from Santa Maria Arabona, Manoppello (Abruzzo, Central Italy)

“…Although the origin of the materials used in Roman times to produce millstones is generally known, identifying the quarries remains problematic. In fact, due to nearly 2000 years of continuous volcanic activity from Vesuvius and Etna, many of these quarries were completely obliterated [7]. Another problem is that the volcanic formations do not have lateral continuity and are often relatively small, so that the quarrying sites may have been abandoned after a relatively short period of use.…”