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

Lava flows are a common hazard at basaltic to intermediate volcanoes and have
30 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 32 basaltic to intermediate 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, 34 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 36 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 38 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 40 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 42 increases until reaching a specific sub-liquidus temperature, the so-called “rheological cut- off temperature” (Tcutoff), at which the lava becomes rheologically immobile and flow 44 ceases. This departure from the pure liquid viscosity curve to higher viscosity is a consequence of rapid crystallization and its variability for a given lava is found to be 46 primarily controlled by the imposed cooling rate. Based on these experimental data, we adapt the failure forecasting method (FFM) - commonly used to describe the self- 48 accelerating nature of seismic signals to forecast material failure - to predict the rheological cut-off temperature (Tcutoff). The presented data substantially expand the 50 modest experimental database on non-equilibrium rheology of lavas and represent a step towards understanding the underlying process dynamics.