Rheological evolution of eruptible Basaltic-Andesite Magmas under dynamic conditions: The importance of plagioclase growth rates

The effects of crystal nucleation and growth on the viscosity of andesitic magmas are investigated at 1 atm. Pressure. The data are used to describe the rheological evolution of andesitic magmas and to derive empirical equations to model the crystallization dynamics. Viscosity experiments were performed at temperatures of 1473, 1483, and 1493 K with shear rates of 0.5 s(-1) by using wide-gap concentric cylinder viscometry. We focused on eruptible magmas (i.e., crystal contents well-below similar to 50%) by following the time evolution of crystal nucleation and growth of a natural andesitic magma, while continuously monitoring the changes in viscosity. At near equilibrium conditions, the basaltic-andesite contains 6,13, and 25 area % crystals at 1493,1483, and 1473 K, respectively, resulting in a viscosity increase of ca. 1 log unit. We show that the timescale of viscosity increase is mainly dictated by the delay time preceding crystallization and markedly decreases with increasing undercooling (from 22 to 42 K) and shear rates (from 0.1 to 1 s(-1)). The plagioclase growth rates estimated from our data are in the order of similar to 3-5 x 10(-6) cm/s, which is much faster than the rates estimated for crystallization in static conditions. We conclude that the effect of the shear rate must be taken into account in modelling magmatic and volcanic processes, especially when transient changes in viscosity need to be estimated. Our results are important for understanding the dynamics of lava flows where deformation plays a significant role in promoting crystallization. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study investigates the impact of crystal nucleation and growth on the viscosity of andesitic magmas at 1 atm pressure. It is found that both shear rate and undercooling significantly affect the timescale of viscosity increase. The plagioclase growth rates estimated from the data are much faster than those under static conditions, highlighting the importance of considering shear rate effects in modeling magmatic and volcanic processes.