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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. Plain Language Summary: Seafloor magnetism is essential for understanding geodynamics and resource prospecting. Near the seafloor high‐temperature hydrothermal fields that host "black smokers", the primary titanomagnetites in oceanic basalts experience complicated alterations that profoundly change seafloor magnetic anomalies. This study investigates the alteration of titanomagnetite through fluid‐rock interactions in such seafloor settings. We find titanomagnetite is disassembled into Fe‐rich and Ti‐rich phases when interacting with fluids, causing a series of magnetic property variations. A schematic model is proposed to explain how high‐temperature fluid‐rock interactions gradually destroy seafloor magnetism. This model improves our understanding of seafloor magnetism from the perspective of hydrothermal alterations, which is important for applying magnetic surveys to prospecting seafloor resources and probing subseafloor structures. Key Points: Magnetomineralogy and magnetic property of MORB vary with progressive high‐temperature hydrothermal alterationExsolution superimposed on titanomagnetite dissolution, producing a dual Verwey transition feature in partially chloritized basaltsHydrothermal alteration may have contributed to the long‐term magnetization variations of oceanic basalts [ABSTRACT FROM AUTHOR]

Magnetic fabrics provide information on the preferred alignment of minerals in rocks. The preferred orientation of ferromagnetic grains specifically can be described by various kinds of remanence anisotropy and partial remanence anisotropy, or isolated from high-field anisotropy of magnetic susceptibility (HF-AMS) measurements. This contribution focuses on comparing results from anisotropy of (partial) anhysteretic remanent magnetization (A(p)ARM), anisotropy of (partial) isothermal remanence (A(p)IRM), anisotropy of (partial) thermal remanence (A(p)TRM), and isolated ferromagnetic sub-fabrics from HF-AMS. Experiments were performed on a collection of igneous, metamorphic and sedimentary rocks. A(p)ARMs and A(p)IRMs can be strongly coercivity-dependent, both in terms of principal directions and in the degree and shape of the anisotropy, indicating different fabrics of different grain sizes. Additionally, remanence anisotropy and the ferromagnetic fabrics from HF-AMS can be largely different, indicating that the large MD grains dominating AMS have different fabrics than those of the smaller remanence-carrying grains. These results indicate that a given sample may display a range of magnetic anisotropy that depend on the experimental parameters and the grains targeted in any specific experiment. Hence, measuring ferromagnetic fabrics with a variety of methods provides a more detailed insight into preferred orientations of sub-populations of ferromagnetic grains. Researchers can use these results to determine the ferromagnetic anisotropy most relevant to their specific applications. [ABSTRACT FROM AUTHOR]