We propose a method to estimate deep-level magma ascent rates, and potentially, the depths of initial magma dehydration, using pressure and temperature (P-T) estimates from mineral-liquid thermobarometers, and cooling rates inferred from Crystal Size Distribution theory. Ascent rates can be ascertained by recognizing that the slope of a given P-T path (dP/dT), rates of cooling (dT/dt), and magma ascent rates (dP/dt) are interrelated, and ascent velocity, v, is given as: v = 1 ⎛dP⎞ ⎛dT⎞, where Ú is magma density and Ú g ⎝dT⎠ ⎝dt ⎠ g is the acceleration due to gravity. Preliminary applications of this method are provided for Mt. Etna lava flows, where both dP/dT and dT/dt have been well characterized based on, respectively, clinopyroxene thermobarometry, and clinopyroxene CSDs (the latter yields dT/dt = 2×10-6). Deep-level (>20 km) magma ascent rates range from effectively 0 (where clinopyroxene P-T estimates form a cluster, and so dP/dT ≈ 0), to about 10 m/hr 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, as calculated by pMELTS (Ghiorso et alii 2002), which closely approximates water contents obtained from melt inclusions; these slopes yield ascent rates of ~1 m/hr, and are comparable to the very slowest rates derived for magma effusion or vapor-driven ascent (~0.001 to > 0.2 m/s, or 3.6 to 720 m/hr). The initiation of such upward movements, however slow, may be key to understanding eruption triggering mechanisms. At Mt. Etna, certain flows exhibit two kinds of clinopyroxene crystallization behavior: from a single flow, those clinopyroxenes that form at the greatest depths either follow a clinopyroxene saturation surface (as calculated from pMELTS) or cluster along such a curve, but clinopyroxenes from these same flows that yield more shallow depth estimates fall on near-vertical P-T paths. Such changes in slope appear to indicate an acceleration of upward magma transport, which may be due to the initiation of deep-level magma dehydration.
A new model for estimating deep-level magma ascent rates from thermobarometry: an example From Mt. Etna and implications for deep-seated magma dehydration
ARMIENTI, PIETRO;
2010-01-01
Abstract
We propose a method to estimate deep-level magma ascent rates, and potentially, the depths of initial magma dehydration, using pressure and temperature (P-T) estimates from mineral-liquid thermobarometers, and cooling rates inferred from Crystal Size Distribution theory. Ascent rates can be ascertained by recognizing that the slope of a given P-T path (dP/dT), rates of cooling (dT/dt), and magma ascent rates (dP/dt) are interrelated, and ascent velocity, v, is given as: v = 1 ⎛dP⎞ ⎛dT⎞, where Ú is magma density and Ú g ⎝dT⎠ ⎝dt ⎠ g is the acceleration due to gravity. Preliminary applications of this method are provided for Mt. Etna lava flows, where both dP/dT and dT/dt have been well characterized based on, respectively, clinopyroxene thermobarometry, and clinopyroxene CSDs (the latter yields dT/dt = 2×10-6). Deep-level (>20 km) magma ascent rates range from effectively 0 (where clinopyroxene P-T estimates form a cluster, and so dP/dT ≈ 0), to about 10 m/hr 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, as calculated by pMELTS (Ghiorso et alii 2002), which closely approximates water contents obtained from melt inclusions; these slopes yield ascent rates of ~1 m/hr, and are comparable to the very slowest rates derived for magma effusion or vapor-driven ascent (~0.001 to > 0.2 m/s, or 3.6 to 720 m/hr). The initiation of such upward movements, however slow, may be key to understanding eruption triggering mechanisms. At Mt. Etna, certain flows exhibit two kinds of clinopyroxene crystallization behavior: from a single flow, those clinopyroxenes that form at the greatest depths either follow a clinopyroxene saturation surface (as calculated from pMELTS) or cluster along such a curve, but clinopyroxenes from these same flows that yield more shallow depth estimates fall on near-vertical P-T paths. Such changes in slope appear to indicate an acceleration of upward magma transport, which may be due to the initiation of deep-level magma dehydration.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
