科研之友
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摘要
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): nu = (1/rho g)(dP/dT)(dT/dt), where rho 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 (similar to 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 approximate to 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 (similar to 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 similar to 1 m h(-1), in agreement with the slowest rates derived for magma effusion or vapor-driven ascent (similar to 0 center dot 001 to > 0 center dot 2 m s(-1), or 3 center dot 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 approximate to 0 center dot 4 MPa and T approximate to 1200 degrees C for a CO2 content > 1000 ppm.
- 出版日期2013-4
