Ocean Drilling Program (ODP) Leg 118 at the Southwest Indian Ridge recovered the first vertically oriented continuously cored gabbros from in situ ocean crust. Magnetic properties and oxide petrography of drillcore samples were analyzed in order to determine the processes which control the remanent magnetization of oceanic layer 3 and the ability of gabbros to record reversals of the Earth's magnetic field. Primary, igneous magnetite is common only in evolved, Fe-Ti oxide-rich gabbros and averages, by volume, 2% of the rock, although substantially higher contents are common. Secondary magnetite, produced by high-temperature exsolution and hydrous alteration of olivine and pyroxene, is observed in all gabbros studied and, is the most important magnetic phase in the majority of recovered gabbros (cumulate olivine gabbros). Because of the high temperatures associated with this alteration, it is likely that the primary magnetization of this crustal section was a thermal remanent magnetization, or a very high temperature chemical remanent magnetization acquired very near the ridge crest. Hysteresis loop parameters indicate that all of the studied samples are capable of carrying significant, stable remanent magnetization. Finally, previously published drilling results and field studies from ODP Site 735B indicate that although this crustal section has been tectonically uplifted, it formed at a mid-ocean ridge spreading center, similar to much of the oceanic crust. Therefore, the results of this magnetic study can be extrapolated to include the oceanic gabbro section, in general, and argue strongly that layer 3 crustal rocks are capable of recording reversals of the Earth's magnetic field. ¿ American Geophysical Union 1993