Petrologic imaging of silicic to intermediate magma chambers through amphibole barometry.

Abstract:

Aluminum substitution in amphibole is one of the few barometers available for differentiated volcanic rocks. Most calibrations, however, fail to take into account the fact that aluminum concentration in amphibole is a function of both pressure and temperature. We are developing a new equation based on aluminum substitution in the octahedral site of the amphibole structure, which provides a temperature-independent barometer that can be used for petrological imaging of silicic to intermediate magma chambers. A preliminary calibration based on published experimental data for amphibole in equilibrium with plagioclase and biotite in rhyolitic to dacitic magmas at 650-950 °C records the thickness and depth of magma reservoirs with uncertainties of 0.4 and 2.2 km, respectively. Pressures are given by the equation: P (MPa) = 892 · ^VIAl + 101, where ^VIAl is the octahedral aluminum content of the amphibole. Further experiments are underway to extend the calibration to different compositions (low-K2O rhyolites and dacites, andesites, trachytes...).

As an example, we investigated the Kizilkaya ignimbrite in central Turkey. Using a detailed statistical barometry analysis of the juvenile pumice, and a field-based estimate of the erupted volume, we show that the reservoir feeding this large silicic eruption is pancake-shaped, in agreement with recent conceptual models of the magma systems that feed large silicic eruptions. Similar work can be performed on active silicic systems, where the petrological study of possible precursory small eruptions can highlight the shape and depth of a large silicic reservoir. Pressure estimates for small recent silicic domes around the active Laguna del Maule complex in Chile indicate storage pressures in excellent agreement with geophysical estimates of the active magma chamber. Our new barometer can thus be used in combination with volcanological and geophysical data to infer the size, shape and depth of magma reservoirs at large silicic volcanoes and may serve as a tool for monitoring future activity.