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

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 h. 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 H 2 O 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.

Rheological change and degassing during a trachytic Vulcanian eruption at Kilian Volcano, Chaîne des Puys, France

“…Such extensive crystallization ( Fig. 6) and water loss estimates agree with previous studies suggesting that both of these processes are relatively rapid in trachytic melts (e.g., Arzilli and Carroll 2013;Fanara et al 2013;Shea et al 2017;Arzilli et al 2020). Effusive and explosive events in andesitic and rhyolitic magmas are typically associated with ascent rates lower and higher than approximately 3.6 MPa h -1 , respectively (Cassidy et al 2018).…”

“…On the other hand, large-volume, highly explosive eruptions of trachytic magma like the Plinian phase of the Campanian Ignimbrite eruption in the Phlegrean Fields, Italy, or the Povaçao ignimbrite in Azores, also occur (Rosi et al 1999;Gottsmann et al 2002;Polacci et al 2003). This broad spectrum of eruptive style for trachytic magmas, even during a single eruption, is likely related to (i) the low viscosity of trachyte compared to rhyolite (Giordano et al 2004;Andújar and Scaillet 2012), (ii) a slightly higher melt water diffusivity of trachytic magma compared to rhyolitic composition (Fanara et al 2013), and (iii) the faster crystallization kinetics of trachytic magmas (e.g., D'Oriano et al 2005;Arzilli and Caroll 2013;Shea et al 2017;Arzilli et al 2020). These idiosyncrasies cause first-order rheological changes in the magma and affect ascent rate, closed-system degassing, outgassing efficiency, and fragmentation behavior.…”

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

“…The best-estimate alkali feldspar and clinopyroxene liquidus curves for two pantelleritic melts are shown in Figure 1b at temperatures of 700-850°C and P (H 2 O) of 25-150 MPa. The two melts differ slightly in composition, with slightly higher wt% concentrations of FeO and Na 2 O for the melt composition used by Arzilli et al [2020], as well as a small difference in the P.I. of 1.46 vs. 1.40 for the Di Carlo et al [2010] study (see Table 1).…”

“…See Romano et al [2021] for more detailed discussion on water solubility in pantelleritic melts. (b) Phase diagram for alkali feldspar (Afs) and clinopyroxene (Cpx) under water-saturated conditions at NNO + 0.8 and NNO − 1,2 in pantelleritic compositions from Arzilli et al [2020] and Di Carlo et al [2010], respectively. lower redox conditions investigated in Romano et al…”

“…Many variables, including melt composition, crystallinity, temperature (T ), pressure (P ), undercooling (∆T = T liquidus − T subliqidus ), time, melt water content, oxygen fugacity ( f O 2 ), and cooling and decompression rates, can influence magma crystallization at depth and during ascent to surface [e.g., Couch et al, 2003, Martel and Schmidt, 2003, Hammer, 2006, Brugger and Hammer, 2010, Mollard et al, 2012, Martel, 2012, Shea and Hammer, 2013, Arzilli and Carroll, 2013, Arzilli et al, 2016. Crystallization has commonly been investigated in pantelleritic melts under equilibrium conditions [Scaillet and MacDonald, 2001, 2003, 2006, Di Carlo et al, 2010, Romano et al, 2020, but disequilibrium crystallization kinetics deserve additional attention because of possible consequences for conduit flow processes and eruptive dynamics of peralkaline magmas [Arzilli et al, 2020].…”