Crystallization Kinetics of Alkali Feldspar in Peralkaline Rhyolitic Melts: Implications for Pantelleria Volcano

Fabio Arzilli,Alessandro Fabbrizio,Patrizia Landi,Paola Stabile,Michael R Carroll,Bruno Scaillet,Eleonora Paris

doi:10.3389/feart.2020.00177

Abstract

Peralkaline rhyolites, associated with extensional tectonic settings, are medium to low viscosity magmas that often produce eruptive styles ranging from effusive to highly explosive eruptions. The role of pre-eruptive conditions and crystallization kinetics in influencing the eruptive style of peralkaline rhyolitic magmas has been investigated and debated considering equilibrium conditions. However, experimental constraints on the effect of disequilibrium in crystallization in such magmas are currently lacking in the literature. Therefore, we performed isobaric cooling experiments to investigate alkali feldspar crystallization kinetics in peralkaline rhyolitic melts. Experiments were performed under water-saturated, water-undersaturated and anhydrous conditions between 25 and 100 MPa, at 670-790 °C and with experimental durations ranging from 0.5 to 420 hours. Here we present the first data on crystallization kinetics of alkali feldspar, which is the main crystal phase in peralkaline rhyolitic melts, in order to improve our understanding of the evolutionary timescales of these melts and their ability to shift between effusive and explosive activity. Our experimental results indicate that the alkali feldspar nucleation delay can range from hours to several days as a function of undercooling and H2O content in the melt. Thus, a peralkaline rhyolitic magma can be stored at the pre-eruptive conditions for days without important variations of its crystal fraction. This suggests that crystallization may not necessarily play the main role in triggering fragmentation during explosive eruptions of peralkaline rhyolitic magmas.

Highlights

This study shows the role of T and water in controlling the alkali feldspar nucleation delay in peralkaline rhyolitic melts
Our results indicate that the nucleation delay can reach up to a few days under small T, whereas, it decreases from days to hours with increasing T and H2O content dissolved in the melt
Small T can drastically reduce the crystallization in natural peralkaline rhyolitic magmas, and it explains the low crystal content observed in the present experimental study and in previous works (e.g., Campagnola et al, 2016; Hughes et al, 2017; Clarke et al, 2019)

Summary

Introduction

Peralkaline rhyolitic magmas erupt in diverse geological settings, including continental rifts, intraplate ocean islands, subduction zones, and back-arc basins (e.g., Morra et al, 1994; Barclay et al, 1996; Heumann and Davies, 2002; Lustrino et al, 2004; Macdonald and Scaillet, 2006; White et al, 2006, 2009; Ren et al, 2006; Marshall et al, 2009; Bhushan et al, 2010; Rooney et al, 2012; Crystallization of Peralkaline Rhyolitic MeltsParker et al, 2012; Renna et al, 2013; Hong et al, 2013). Hydrous peralkaline rhyolites with concentration of H2O ≤ 4 wt.% have relatively low viscosities (102 to 105 Pa s; Di Genova et al, 2013), their volcanic activity is characterized by a wide range of eruptive styles These can vary from lava flow to lava fountaining to Plinian-type eruptions associated with pyroclastic flows and ignimbrites (e.g., Schmincke, 1974; Mahood and Hildreth, 1986; Duffield, 1990; Lowestern and Mahood, 1991; Houghton et al, 1992; Stevenson et al, 1993; Webster et al, 1993; Wilding et al, 1993; Barclay et al, 1996; Stevenson and Wilson, 1997; Horn and Smincke, 2000; Gottsmann and Dingwell, 2002)

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Conclusion