RHEOLOGY OF DACITIC MAGMAS I: WHAT CONTROLS MAGMA RHEOLOGY IN THE CONDUIT?

Dacitic magmas in subduction zone settings form diverse eruptive products including lava domes, thick lava flows, and pyroclastic deposits. All three kinds of activity are ongoing at Santiaguito volcano, Guatemala. Each style reflects different regimes of rheology and strain rate. Domes and flows represent low strain rates affecting crystalline magmas in the brittle (spines) and ductile (flows) regimes, respectively. Explosions reflect high strain rates that exceed the relaxation rates of the silicate melt, crossing the glass transition and leading to brittle behavior. The primary controls on melt viscosity are melt composition, temperature, volatile content and crystallinity.

The glassy matrix of Santiaguito lavas is rhyolitic in composition, with ~40 volume % crystals. These include mm-sized complexly zoned plagioclase and subordinate pyroxene phenocrysts, and feldspar microlites interpreted to reflect crystallization during ascent. The magma chamber probably contains rhyodacitic melt, with water contents less than about 2 wt.% (below amphibole stability), and ~20% antecrysts. The viscosity of a series of seven remelted dacitic liquids, containing up to 4.62 wt.% dissolved water, was measured by parallel-plate viscometry. The temperature of the 10¹¹ Pa.s isokom decreases from ~725˚C for the anhydrous liquid, to ~670˚C and ~515˚C for water contents of 0.44 and 1.73 wt.% H₂O respectively. Dacites are less viscous then rhyolites at magmatic temperatures (≥900˚C) and low water contents, but dissolved water decreases the viscosity and glass transition temperature of rhyolitic liquids more than for dacitic melts.

During ascent, degassing results in increased melt viscosity, crystallization, and bubble growth. Crystallization results in increased bulk magma viscosity due to both physical and chemical effects (matrix evolution towards rhyolitic melt) but also releases latent heat. Temperature changes between chamber and vent are minor. The dominant controls on conduit rheology are degassing and crystallization. As viscosity increases, the strain rate at the conduit margins eventually exceeds the relaxation rate and fragmentation occurs. Explosions are fed from conduit margins and lava flows are fed from the central plug.