Self‐Reversed Magnetization in Oceanic Basalts Regulated by Disparate Titanomagnetite Hydrothermal Alteration

Abstract The occurrence of self‐reversal in oceanic basalts complicates the interpretation of paleomagnetic records and oceanic crustal magnetization. Self‐reversed magnetization in aged oceanic basalts is primarily carried by oxidized titanomagnetite (titanomaghemite) with a narrow composition range (i.e., high Ti contents and high oxidation states), but the underlying processes controlling self‐reversed magnetization are not well understood. Paleomagnetic measurements on submarine basalts from the International Ocean Discovery Program (IODP) Site U1506 of Expedition 371 in the southwest Pacific reveal varying degrees of self‐reversed magnetization, obscuring paleomagnetic interpretation on primary remanent magnetization. Electron microscopic and rock magnetic analyses indicate distinct titanomaghemite grain sizes and magnetic domain states among the studied oceanic basalts with variable self‐reversal components. The field‐aligned samples contain primarily submicron single‐domain (oxidized) titanomagnetites occurring as exsolved lamellae and recrystallized nanoinclusions embedded in silicified plagioclases, whereas those with self‐reversed remanence components are dominated by micron non‐single‐domain titanomaghemites. We suggest that hydrothermal activity is critical for acquiring a self‐reversed magnetization in oceanic basalts, and decimeter‐scale disparate hydrothermal conditions can explain this grain size and microstructure‐dependent self‐reversal. Alteration of self‐reversed basalts occurs at relatively low temperature (<200°C), while field‐aligned basalts are altered by high‐temperature (>200–350°C) fluids. Documentation of this underlying grain size and microstructural control of the self‐reversed remanence acquisition is important for the paleomagnetic investigations of submarine basalts in tectonically active mid‐ocean ridges and subduction zones, where hydrothermal alteration is pervasive.