A New Model to Estimate Deep-level Magma Ascent Rates, with Applications to Mt. Etna (Sicily, Italy)

Magma ascent velocities, v (dH/dt; where H is depth and t is time), can be determined from decompression rates (dP/dt), and rates of cooling (dT/dt): Graphic, where ρ is magma density, P is pressure, T is temperature and g is the acceleration due to gravity. This equation for v provides a key to investigating the relationships between the initial ascent velocity of magmas and the depths of magma dehydration. Ascent velocities can be calculated using pressure and temperature (P–T) estimates from mineral–liquid thermobarometry and cooling rates inferred from crystal size distribution (CSD) theory. For recent Mt. Etna lava flows, both dP/dT and dT/dt have been characterized for the portion of the feeding system between the Moho (∼27 km) and 6 km based, respectively, on clinopyroxene thermobarometry and clinopyroxene CSDs. Deep-level (>6 km) magma ascent velocities range from practically zero (where clinopyroxene P–T estimates form a cluster, and so dP/dT ≈ 0), to about 10 m h–1 for flows that yield very steep P–T trajectories. Many lava flows at Mt. Etna yield P–T paths that follow a hydrous (∼3% water) clinopyroxene saturation surface, which closely approximates the water content inferred from melt inclusions. Independent assessments of deep-level water contents have been obtained by means of a new geohygrometer and yield ascent rates of ∼1 m h–1, in agreement with the slowest rates derived for magma effusion or vapor-driven ascent (∼0·001 to >0·2 m s–1, or 3·6–720 m h–1). Changes in P–T slope, as determined by pyroxene thermobarometry, indicate an upward acceleration of magma, which may be due to the onset of deep-level magma dehydration linked to the non-ideal behavior of water and CO2 mixtures that induce a deep-level maximum of water loss at P ≈ 0·4 MPa and T ≈ 1200°C for a CO2 content >1000 ppm.