Seafloor Magnetism Under Hydrothermal Alteration: Insights From Magnetomineralogy and Magnetic Properties of the Southwest Indian Ridge Basalts

Titanomagnetites in mid-ocean ridge basalt (MORB) experience variable post crystallization alterations associated with seafloor tectonic and environmental processes. Compared to low-temperature oxidation, seafloor hydrothermal alteration is thought to be more destructive but its magnetic aftermaths are insufficiently documented. Here we present comprehensive rock magnetic and electron microscopic analyses of fresh and hydrothermally-altered MORBs dredged from the Longqi and Yuhuang hydrothermal fields, Southwest Indian Ridge. We observe large variations in magnetic properties of fresh MORBs, originated from relative proportions of nano-scale single-domain to vortex state and micron-scale vortex to multi-domain state dendritic titanomagnetites. Progressive hydrothermal alteration produces secondary magnetite through recrystallization of exsolved and dissolved Fe from primary titanomagnetite. Exsolution is evident by a dual Verwey transition signature and coexisting Ti-poor titanomagnetites and sphenes in partially chloritized basalts. A schematic model is proposed to explain the variations in magnetomineralogy and magnetic properties with progressive hydrothermal alteration. Intermediate hydrothermal alteration products retain a secondary chemical remanent magnetization (CRM) which is related to the long-term magnetization variations in oceanic basalts. The established framework allows characterizing MORB hydrothermal alteration and ultimately contributes to resolving the complexity of seafloor magnetism.