Equilibrium Viscosity and Disequilibrium Rheology of a high Magnesium Basalt from Piton De La Fournaise volcano, La Reunion, Indian Ocean, France

Abstract

Lava flows are a common hazard at basaltic to intermediate volcanoes and have posed a significant threat to La Reunion Island over the past centuries. In sustained flow units, the efficiency of lava transport away from the vent is dominated by cooling. For basaltic to inter− mediate lavas, it is the ability of the lava to solidify during cooling which exerts a first−order control on spatial extent and flow distance. As a consequence, understanding the sub−liquidus rheology of lavas has become a key focus in lava flow research in the past decade. To date, the development of a systematic understanding of lava rheology during emplacement conditions has been significantly hampered by a lack of experimental data. Here we present new data on the rheological evolution of crystallizing high−Mg basalt from Piton de la Fournaise. Sub−liquidus experiments were performed at constant cooling rates ranging from 0.5 to 5 K/min. Those rates mimic thermal conditions experienced 1) by lava during flow on the surface and 2) by magma during dike and sill emplacement. Our data show that the effective viscosity of the crystallizing suspension increases until reaching a specific sub−liquidus temperature, the so−called "rheological cut−off temperature" (T cutoff), at which the lava becomes rheologically immobile and flow ceases. This departure from the pure liquid vis− cosity curve to higher viscosity is a consequence of rapid crystallization and its variability for a given lava is found to be primarily con− trolled by the imposed cooling rate. Based on these experimental data, we adapt the failure forecast method (FFM) − commonly used to describe the self−accelerating nature of seismic signals to forecast material failure − to predict the rheological cut−off temperature (T cutoff). The presented data substantially expand the modest experimental database on disequilibrium rheology of lavas and represent a step towards understanding the underlying process dynamics.

Viscosity of Pyroxenite Melt and Its Evolution During Cooling

“…Finally, the mentioned T cutoff (or the temperature at which the system reaches its rheological death) and relative modeling were successfully tested for basaltic systems by Kolzenburg et al (). Applying our cooling rate, it provides a value of approximately 1510 K where viscosity for TF1‐1 and TF0.1‐2 will be on the order of 10 2 Pa·s (far off from a possible rheological death of Theo's lava flow).…”

“…Recently, Kolzenburg et al (2017), following the work of Giordano et al (2007), highlighted the importance of monitoring the viscosity during cooling, suggesting that it is an important influence also of the applied shear rates. Data from Kolzenburg et al (2017) and Kolzenburg, Di Genova, et al, 2018, Kolzenburg et al, 2019 show that the effective viscosity of the lava can drastically increase, as a function of temperature and time, until reaching a specific subliquidus temperature that, revising the concept first introduced by Giordano et al (2007), they named T cutoff , suggesting that the viscosity increase is a consequence of the onset of crystallization, which is mainly controlled by the cooling rate. In such a context these authors also highlighted a second-order shear rate effect on the rheological departure.…”

“…• Figure S2 • Figure S3 • Table S1 • Table S2 • Table S3 • Table S4 • Table S5 • Table S6 Correspondence to: F. Vetere, francesco.vetere@unipg.it approaches Kolzenburg et al, 2019) and direct measurements in the field (Chevrel et al, 2018). These are extremely important mechanisms that deserve further investigation as most of the magmatic and volcanic environment are controlled by nonequilibrium processes.…”

“…Following the pioneering work of Carmichael (), reporting the rheological importance of the coexistence of crystals and liquid material, melt solidification processes have been investigated via experiments under different applied cooling rates (see Hammer, , for a general summary). Diverse studies at crystal‐melt disequilibrium conditions have been performed (e.g., Lofgren, ; Cashman, ; Lange et al, ; Arzilli & Carroll, ; Vetere et al, , and reference therein), but only recently has even the deformation acting on lavas been taken into account with both laboratory approaches (Kolzenburg et al, ; Kolzenburg et al, ; Kolzenburg et al, ) and direct measurements in the field (Chevrel et al, ). These are extremely important mechanisms that deserve further investigation as most of the magmatic and volcanic environment are controlled by nonequilibrium processes.…”

“…In both approaches, temperature and melt composition are required in order to calculate viscosity and estimate the lava flow velocity and lava flux. To our knowledge, only very few experimental data sets on planetary magma/lava viscosity introduce cooling rate and shear rates as potential parameters affecting magma mobility (e.g., Kolzenburg et al, ; Kolzenburg, Giordano, et al, ). Presently, only a few general models and combined equations (e.g., Mader et al, ; Vona et al, ) are able to track viscosity variation taking into account the crystalline phases in terms of their volume, aspect ratio, and applied shear stress.…”