Somma-Vesuvius Plinian Eruptions fed by mafic magma: insights from bubbles in melt inclusions

Abstract:

Mt. Somma-Vesuvius Plinian eruptions were first described by Pliny the younger in 79 AD during the infamous eruption that destroyed Pompeii. Today, such eruptions are still a concern to the nearly 3 million people living in the Naples metropolitan area. Understanding the source for Mt. Somma-Vesuvius magma and the coexisting volatile phase is vital to better constrain the long-term eruptive behavior of this volcano.

In the present study, ~ 50 olivine phenocrysts were selected from lavas and pumices produced during mild effusive events referred to as inter-Plinian eruptions, and from highly explosive Plinian eruptions that occurred at Mt. Somma-Vesuvius between 33000 ka and 1631 AD. Selected olivine phenocrysts containing MI were examined petrographically and analyzed for Fo content. Fo varies from 69 to 73 mole% for inter-Plinian olivine crystals and from 84 to 90 mole% with one zoned olivine containing 76-81 mole% Fo, for Plinian olivine crystals. Investigated MI vary from slightly crystallized to highly crystallized.

Selected crystallized MI were reheated using the Vernadsky stage, and quenched to a homogeneous glass (Group 1) or glass plus a vapor bubble (Group 2). On one hand, MI of Group 1 are hosted in olivine ranging from Fo₇₂ to Fo₇₆ and were all erupted from the Pompeii eruption (white pumice deposit). On the other hand, MI of Group 2 are trapped in olivine ranging from Fo₆₉ to Fo₈₁ and from Fo₈₄ to Fo₉₀, and the hosts are representative of both Plinian and inter-Plinian events. The only eruption where Group-1 and Group-2 MI coexist is the Pompeii eruption.

Group 2 MIs were further analyzed by Raman to test for the presence of volatiles (CO₂ or H₂O) in the vapor bubbles. CO₂ was detected in all MI analyzed. CO₂ density was determined using the distance between the two Fermi-diad peaks, and ranges between 0.14 and 0.55 g/cm³. Six MI also showed evidence for H₂O in the vapor bubble. In addition, carbonates were detected at the glass-vapor interface of five MI.

This study shows that the CO₂-rich fluid phase, which might exsolve from Plinian melts contain a significant amount of H₂O. In addition, the first melting temperature of the fluids in the vapor bubble (~ -58ºC) suggests that volatile components, other than CO₂, are included in the vapor bubbles.