Constraining Magma Reservoir Conditions by Integrating Thermodynamic Petrological Models and Bulk Resistivity From Magnetotellurics

Magnetotelluric (MT) data image the bulk resistivity of the subsurface which can be used to infer magma reservoir conditions beneath volcanoes. The bulk resistivity of magma depends primarily on the melt volume fraction, temperature, and water content. These variables are coupled thermodynamically, yet mixing relations for bulk resistivity implicitly treat them as independent. Here, we use a parameterization of the rhyolite-MELTS thermodynamic model to constrain relationships between melt fraction, temperature, dissolved water content and bulk resistivity for rhyolitic magmas. This method results in MT interpretations which are (a) thermodynamically consistent at near-equilibrium conditions, (b) independent of temperature and water content estimates derived from erupted products, and (c) able to consider saturated melts containing a volatile (i.e., aqueous fluid) phase. The utility of the method is demonstrated with three case studies of silicic systems: Mono Basin, Newberry volcano and the Laguna del Maule Volcanic Field (LdMVF). The moderately conductive feature at Mono Basin can be explained by under-saturated partial melt (6–15 vol%) at <775°C, indicating relatively stable magma storage conditions since the last eruption. However, a relatively resistive feature at Newberry Volcano requires lower temperatures (<750°C) than previous estimates, suggesting that the system has cooled since the last eruption. A conductive feature at the LdMVF cannot be explained by saturated or under-saturated rhyolitic melt and requires additional conductive phases. These results demonstrate the potential of this new method to reduce uncertainty in MT interpretations and highlight the need for additional coupling strategies between petrology, geophysics, and thermo-mechanical models to better understand magmatic systems.