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