Magnetic minerals in six samples of oceanic basalts of the transition zone and upper sheeted dikes from Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) Hole 504B, Leg 83, were studied by methods of rock magnetism and transmission electron microscopy (TEM). TEM observations showed that the magnetic mineral in these basalts is end-member magnetite (TM0) of extremely fine-grain size (30--100 nm) primarily in the range of pseudo-single-domain magnetite, consistent with the rock magnetic properties including hysteresis parameters, Curie temperature, and low-temperature measurements (Verwey transition). Magnetite formed by two different processes: (1) oxidation-exsolution, true exsolution, and hydrothermal alteration, and (2) oxidation-exsolution, a second stage of oxidation-exsolution, and hydrothermal alteration. The primary titanomagnetite (TM60--70) that crystallized from the melt thus evolved to end-member magnetite coexisting with titanite (sphene), kassite, ulv¿spinel (TM~87), and ilmenite on a submicroscopic scale. On the basis of the formation mechanisms of the magnetic carrier, the primary titanomagnetite (TM~60) with Curie temperature of ~180 ¿C did not acquire thermoremanent magnetization (TRM) in these basalts. Instead, the Ti-bearing magnetite (TM~10--20) that formed as oxidized or exsolved lamellae acquired its first thermal chemical remanent magnetization (CRM) at ~500--400 ¿C during subsolidus cooling. Upon the onset of hydrothermal alteration the recrystallized end-member magnetite acquired a second CRM. The natural remanent magnetization of the basalts from the transition zone and upper sheeted dikes is therefore characteristic of CRMs that were acquired when titanomagnetite altered, in part, to magnetite during subsolidus cooling and hydrothermal alteration close to the ridge axis. ¿ 2000 American Geophysical Union