High Remanent Magnetization Measured in Hydrothermally Altered Lavas

Abstract

Magnetic surveys are used to identify and monitor hydrothermally altered regions on volcanoes. Commonly such magnetic data are interpreted on the premise that hydrothermal alteration consumes Fe-Ti oxides in the host rocks, reducing their total magnetization. Here, we report a contrasting observation from Whakaari (White Island) volcano in New Zealand. We study the magnetic properties of 42 conduit-filling and surficial lithologies that have undergone varying degrees of acid-sulfate alteration. We find that while the induced magnetization of lavas decreases with hydrothermal alteration, some altered lavas have an order of magnitude higher remanent magnetization than fresh lavas. We discuss plausible mechanisms by which altered lavas can retain high remanent magnetization including the importance of magnetic mineralogy and grain size. Our results urge caution in correlating reduced magnetization with hydrothermally altered regions. Furthermore, they highlight the importance of measuring both the induced and remanent magnetization of samples used to interpret field-scale data.