Crystallization of peralkaline rhyolitic magmas: pre- and syn-eruptive conditions of the Pantelleria system

Abstract

Pantelleritic magmas are low-viscosity peralkaline rhyolites which exhibit large differences in eruptive style (explosive to effusive). The processes that promote fragmentation and explosive eruptions of pantelleritic magma remain subject to debate, but undoubtedly variations of magma viscosity during magma ascent and degassing contribute to differences in eruptive style. Because crystallization can significantly influence magma rheology, we present a review of equilibrium and disequilibrium crystallization experiments of pantellerites, focusing on the crystallization of the main phases, alkali feldspar, and (lesser) clinopyroxene. Our analysis of data for several explosive pantelleritic eruptions on Pantelleria suggests pre-eruptive pressures of 50-100 MPa, temperatures of 700-800°C for water-saturated conditions. Given these conditions, we show that the low pre-eruptive crystal fractions (0.08 to 0.15), temperatures between 700 and 800°C, and the decrease of melt H 2 O content during magma ascent/decompression can promote a significant change in viscosity (up to 10 6 -10 7 Pa•s), leading to magma brittle fragmentation and explosive eruptions. Because of their typical range of viscosity, pantelleritic magmas may show greater variations in eruptive style due to differences in ascent (decompression) rate when compared with metaluminous rhyolites.

Contrasting Styles of Inter-Caldera Volcanism in a Peralkaline System: Case Studies from Pantelleria (Sicily Channel, Italy)

“…According to White et al [22], the observed mineral assemblages of these pantelleritic samples are compatible with redox conditions between ∆FMQ-1.5 and -0.5 for a temperature in the range 794-991 • C. Considering this f O 2 range, we can further constrain the magma temperature from olivine composition by using the equation of Romano et al [23], which relates fayalite content in olivine with f O 2 , T, and H 2 O melt . As reported by several studies [27][28][29][30][31], melt water content in pantellerite ranges between 2.5-4.5 wt%.…”

Peralkaline silicic extrusive rocks: magma genesis, evolution, plumbing systems and eruption

“…The viscosities of peralkaline silicic melts have been determined experimentally [Stevenson and Wilson, 1997, Stevenson et al, 1998, Di Genova et al, 2013, Stabile et al, 2016 and the results have been incorporated into various models of melt viscosity [Dingwell et al, 1998, Giordano et al, 2008. The results have confirmed that peralkaline silicic melts have lower viscosities than their metaluminous equivalents, by as much as 2-3 orders of magnitude, e.g., 10 10 -10 11 Pa•s, Stevenson and Wilson, 1997;10 9.6 -10 13.6 Pa•s, Stabile et al, 2016Stabile et al, , 2021. On Pantelleria, for example, the volcanological consequences of the low viscosities include the ubiquity of welded and rheomorphic pumice falls and high-grade to extremely high-grade ignimbrites [Rotolo et al, 2021].…”

“…Lanzo et al [2013] used the Papale et al [2006] model and a measured water content of 4.2 wt% to show that the Green Tuff, Pantelleria, had a saturation pressure of 65 MPa, equivalent to ∼2.5 km depth. According to Romano et al [2021] and Stabile et al [2021], the Papale et al model tends to slightly overestimate water solubility in such melt compositions. On the basis of new experimental determinations of water solubility in trachytic and pantelleritic rocks from Pantelleria, Romano et al [2021] estimated the depth of the felsic reservoirs on the island to deepen from 2.4 to 3.5 km.…”