The rheological transition in plagioclase‐bearing magmas

Abstract

Volcanological processes, such as melt segregation, ascent, and eruption, are directly dependent on the rheological behavior of magmatic suspensions. An increase of the crystal fraction of the suspension leads to the formation of a solid‐particle network, which abruptly increases magma viscosity. The crystal fraction at which this rheological transition occurs depends on parameters such as the strain rate and the size, shape, and sorting of particles. To determine the influence of the crystal shape on the rheological transition, suspensions of plagioclase, a representative anisometric crystal, have been investigated at high temperatures and pressures. Synthetic suspensions with crystal fractions (ϕs) ranging from 0.38 to 0.75 were deformed both in compression and torsion in a Paterson apparatus at 300 MPa, 900°C and 800°C, and for strain rates between 1.0 × 10‐5 and 1.0 × 10‐3 s‐1. All suspensions exhibit a non‐Newtonian shear thinning rheological behavior. The experimental results, coupled with existing data and models at low crystal fractions (ϕs < 0.3), allow several rheological domains to be identified, from steady‐state flow to strain weakening, each characterized by a specific microstructure. In particular, a progressive evolution from a pervasive to a strain partitioning fabric is found when increasing the crystal fraction. Our results highlight the influence of both the strain rate and the shape of crystals on the rheological behavior of magmas. During crystallization, magmatic suspensions of anisometric minerals such as plagioclase would develop a solid‐particle network earlier (ϕs ~ 0.3) than suspensions of isometric minerals (ϕs ~ 0.5). Our study shows that localization of strain early in the crystallization history of mushy zones in the magma chamber, near the conduit margins, and at the base of lava flows would facilitate the mobilization, the transfer, and the final emplacement at the surface of highly viscous, feldspar‐rich magmas.