ALBERT

Description: Amphibole plays a crucial role in the study of several earth and planetary processes. One of its most common applications is thermobarometry, especially for volcanic systems. Many amphibole thermobarometers require the input of the melt composition which is not always available in volcanic products (e.g. partially crystallized melts or devitrified glasses) and/or show rather high errors for characterizing the pressure and depth of crustal magma chambers. By contrast, Amp-TB2 (Ridolfi 2021) presents the advantage of being a single-phase model with relatively low errors (P±12 %; T±22 °C) and is valid for most of the calcic amphiboles in equilibrium with calcalkaline or alkaline melts across a wide P-T range. In order to understand the limitations of Amp-TB2, we have performed a series of experiments in P-T ranges of 120-1250 MPa and 730-1100°C. Our results show that this method can be confidently used whenever amphibole crystallization occurs at equilibrium conditions. By contrast, its application to amphiboles crystallized during kinetic (in particular cooling) experiments could result in large pressure overestimations (up to 200%). In this section, we present case studies and methodological suggestions for detecting compositional and textural features of amphiboles crystallized at equilibrium conditions, hence optimizing the accuracy of depth determinations. We show that the depths of the main magma storage zones obtained following these recommendations are confirmed by independent methods such as seismic tomography and pre-eruptive seismicity, and can thus be used to reliably characterize the magma feeding systems of active volcanoes. Ridolfi, F. (2021): Minerals, 1, 324.