High remanent magnetization measured in hydrothermally altered lavas

Magnetic surveys are used to find regions in volcanoes where hot fluids alter the rocks and weaken them, locate ore bodies, and for geothermal exploration. These regions are expected to show low total magnetization on the surveys because the alteration process could consume iron-bearing magnetic minerals in these rocks. Here, we report a different observation from our laboratory experiments on rocks from Whakaari (White Island) volcano in New Zealand. The total magnetization of rocks is the sum of magnetization from (a) minerals that temporarily become magnetized when an external magnetic field is applied (induced magnetization) and (b) minerals that stay magnetized even without an external magnetic field (remanent magnetization). We find that sometimes altered rocks, despite low induced magnetization, can have higher remanent magnetization than fresh rocks. We discuss the potential causes for this. Given the importance of magnetic surveys in volcanic slope stability assessment and mineral and geothermal exploration, we suggest exercising caution in assuming that hydrothermally altered regions would necessarily be associated with low total magnetization.