Are volcanic melts less viscous than we thought? The case of Stromboli basalt

Abstract

Melt viscosity is one of the most critical physical properties controlling magma transport dynamics and eruptive style. Although viscosity measurements are widely used to study and model the flow behavior of magmas, recent research has revealed that nanocrystallization of Fe–Ti-oxides can compromise the reliability of viscosity data. This phenomenon can occur during laboratory measurements around the glass transition temperature (Tg) and lead to the depletion of iron and titanium in the residual melt phase, with a significant increase in viscosity. Accurate viscosity measurements play a crucial role in determining the reliability of empirical models for magma viscosity, which are used to evaluate eruptive scenarios in hazardous areas. Here, we quantify the reliability of empirical models by elaborating a new viscosity model of Stromboli basalt that relies exclusively on viscosity data obtained from nanocrystal-free samples. We show that empirical models so far used to estimate melt viscosity at eruptive conditions overestimate Stromboli viscosity by a factor ranging between 2 and 5. In the context of numerical modelling of magmatic processes at Stromboli volcano, we analyse and interpret this finding. Based on our findings, we draw the conclusion that Stromboli basalt is anticipated to ascend from the storage area to the vent at a faster rate than previously hypothesized.

Thermal Evolution of the Lithosphere‐Asthenosphere Boundary Beneath Arc and Its Geodynamic Implications: Depth Variation of Thermal Histories of Mantle Xenoliths From Ichinomegata, Northeast Japan

“…Such high water content of the basalt in the subduction zone settings causes a significant reduction of the melt viscosity. According to the viscosity model for the Stromboli basalt (Valdivia et al., 2023), the viscosity of the melt with 7 wt. % H 2 O is estimated to be ∼0.1 Pa・s at 1220°C, which is 2–3 orders of magnitude lower than that of the dry condition (∼100 Pa・s).…”

Nanoscale chemical heterogeneities control magma viscosity and failure

“…The in-situ high-temperature measurements revealed the high reactivity of andesitic glasses and melts at a temperature above T g , emphasizing the need to apply a meticulous experimental methodology in deriving accurate pure melt viscosity 22,23,42 . In light of these considerations, we employed direct viscometry techniques, speci cally micropenetration and concentric cylinder viscometry (Supp.…”

“…Nevertheless, these kind of magmas face challenges in attaining the requisite for magma fragmentation due to their relatively low viscosity [14][15][16][17][18][19] , unless a substantial volume of microlite crystallization occurs 20 . Recent studies have demonstrated that the formation of Fe-Ti-oxide nanocrystals (referred to as nanolites) can signi cantly increase the viscosity of andesitic and basaltic melts during laboratory measurements [21][22][23] . Interestingly, these nanolites can be present in natural volcanic products erupted during explosive events [24][25][26][27][28] .…”

“…It has been argued that such nanocrystals could have a signi cant impact on magma fragmentation, as they increase magma viscosity and provide sites for bubble nucleation, consequently in uencing the eruptive style. However, despite the increasing frequency of discoveries of nanolites in natural products in the literature, the mechanisms and the extent to which they impact magma viscosity continue to be topics of ongoing debate 21,22,27,[29][30][31][32][33][34][35] . In this study, we present the rst in-situ high-temperature nanoscale observation of nanolite formation in a volcanic (andesitic) melt.…”

“…To establish a reliable basis for comparison, we developed new viscosity models exclusively using viscosity data derived from samples devoid of nanolites. This necessity arises from the observed tendency of previous literature, including widespread viscosity models 36 , to overestimate the nanolite-free melt viscosity of compositions prone to nanocrystallization 22,23,25,37 . Ultimately, we demonstrate that nanolite formation results in structural heterogeneities at the nanoscale, creating highly viscous domains that could lead to substantial changes in the physical properties of andesitic plugs and domes.…”