Electromagnetic Imaging of Silicic Magma Bodies

Monday, 8 January 2018: 15:10
Salon Quinamavida (Hotel Quinamavida)
Martyn Jonathan Unsworth, University of Alberta, Department of Physics, Edmonton, AB, Canada, Darcy Raymond Cordell, University of Alberta, Edmonton, AB, Canada and Matthew Joseph Comeau, University of Münster, Institute of Geophysics, Münster, Germany
Abstract:
Silicic magmatism is responsible for some of the most destructive volcanic eruptions, and has also played a key role in the formation of the continental crust. A full understanding of this type of magmatism requires the study of magma bodies, where a parent magma differentiates to produce magmas with a range of compositions that may either be erupted, or crystallize to form intrusions. Geological studies of erupted lavas and crystallized magma bodies exposed at the surface have given valuable information about processes occurring in magma bodies. Numerical modelling has given important insights into the complex processes occurring within these bodies.

However, geophysical studies are required to complement geological observations and give real-time images of magma bodies. Seismic studies have delineated a number of magma bodies and can constrain the melt fraction through studies of velocity and attenuation, while geodetic data have detected time variations in size through the associated surface deformation. Electromagnetic (EM) methods offer an alternative view since both magma bodies and associated hydrothermal systems are characterized by electrical resistivity values that are lower than the surrounding crystalline rock. Magnetotellurics (MT) is one of the most widely used EM methods and can image the subsurface resistivity structure in 3-D using natural EM signals. The resistivity of the magma body depends on the amount, geometry and composition of the melt. Interpretation of the electrical resistivity of partially molten zones is inherently non-unique. However, when resistivity models are combined with (1) other geophysical data, (2) petrological constraints of melt composition and (3) laboratory measurements of the resistivity of partial melts, the non-uniqueness can be greatly reduced.

This presentation will give a critical review about what can be determined about crustal magma bodies using EM methods. The MT method will be reviewed with an emphasis on which resistivity model features of magmatic and hydrothermal systems are well resolved by EM surveys. The approach outlined above to reduce the uncertainty in resistivity interpretation will be illustrated with examples from a number of volcanic systems, both in the Andes and elsewhere.

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