The effect of pores (fluid-filled vs. drained) on magma rheology

• Published in: Chemical geology Vol. 569; p. 120147
• Main Authors: Sicola, Stefania, Vona, Alessandro, Ryan, Amy G., Russell, James K., Romano, Claudia
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20.05.2021
• Subjects: Drained pores; Fluid-filled bubbles; Magma viscosity; Porous volcanic material; Rhyolite; Vesicularity; Magma viscosity; Porous volcanic material; Rhyolite; Fluid-filled bubbles; Vesicularity; Drained pores
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2021.120147

The presence of pore space strongly affects the rheological behavior of magma and thus influences all volcanic processes (pre-, syn- and post- eruptive). The effects of porosity on magma rheology are, however, unresolved and subject to debate. Here, we present new high-temperature experiments designed to constrain the rheological properties of variably porous melts (0.09–0.66 fractional porosity) deforming at high temperature (750–800 °C) and low strain rates (10−4 — 10−7 s−1). The starting materials are cylindrical cores of natural vesicle- and crystal-free rhyolitic obsidian from Krafla (Iceland) initially containing 0.114 wt% of dissolved H2O. Our experiments comprise two steps. First, cores are heated above the glass transition temperature (700 °C) to 900–1050 °C; second, the cores are deformed at lower temperatures (750 or 800 °C) under a constant low load (1.5 N). We have employed two different strategies for the second step: i) samples are deformed in situ directly after foaming (single-stage, SS); or ii) samples are quenched then reheated and deformed at 750 °C after 15 days repose at room conditions (double-stage, DS). Our experiments provide data that inform on the effects of porosity on the viscosity of natural rhyolitic deposits (e.g., ignimbrites, lavas, domes). Discordant results between SS and DS experiments (~0.6 log10 Pa s for 0.5 fractional porosity) suggest that the rheology of porous volcanic materials depends on whether pore spaces are isolated, fluid-filled bubbles (e.g., magmas in the conduit) or are interconnected, drained voids (e.g., domes, lavas, pyroclastic deposits). •High-temperature experiments inform on rheological behavior of bubble-rich melts•Effective viscosity of melts depends on whether pores are fluid-filled or “drained”•An empirical model for effective viscosity is presented that accounts for abundance and nature of bubbles•Experiments and model are relevant to lavas (filled pores) and pyroclastic deposits (drained pores).