Secondary magnetite (SM) is the dominant Fe–Ti oxide over a 3–4 km thick vertical interval in a profile through the Troodos, Cyprus, ophiolite. SM is important in dikes and flows where dike density exceeds about 30% within the transition from extrusives to sheeted dikes and remains important until dike density falls below about 30% in the transition from sheeted dikes to plutonics. The association of SM with high dike density suggests that it is a product of the thermal metamorphism of hydrothermally metamorphosed dikes and flows. The most common occurrence of SM is at the site of primary titanomagnetites that had been altered earlier through deuteric, low-temperature, and regional hydrothermal alteration processes. SM often closely pseudomorphs the form of primary magnetite (PM) grains, but it can be distinguished from PM using a series of criteria. SM is low in TiO2 and other impurities. Estimated and observed Curie temperatures are undistinguishable, which, combined with optical evidence, indicates that SM is probably approximately stoichiometric.SM may also be present at depth in in situ oceanic crust, in other ophiolites, and in other hydrothermally altered volcanic sequences where the density of minor intrusions is sufficiently high.The formation of SM leads to the annealing of the features of altered PM that provide the ability to retain strong, stable remanent magnetization. In appropriate combinations of the timing of formation of SM and of geomagnetic field reversals, the different parts of a volcanic sequence may be magnetized with opposite polarities.