Magnetic properties Tc), saturation isothermal remanent magnetization (IRMs), and K¿enigsberger ratio (Q)> and opaque mineralogy were determined for basalts, diabases, gabbros, periodites, and serpentinites collected by dredging and submersible from the rift valley at five hydrothermal sites <15 ¿N, 17 ¿N, 23 ¿N (the Snake Pit hydrothermal field), 26 ¿N (the TAG hydrothermal field) on the Mid-Atlantic Ridge, and 42 ¿N (the Sea Cliff hydrothermal field) on the northern Gorda Ridge>. The magnetomineralogy is interpreted in terms of deuteric, hydrothermal, metasomatic, and ambient seawater alteration. Evidence for unequivocal magnetic mineral modification by hydrothermal action is present only in a small percentage of extrusive basalts but is pervasive in diabases, gabbros, and ultramafic rocks. This lithologic distribution suggests a progressive increase in alteration intensity with depth in the oceanic crust and upper mantle from minor low-temperature alteration in young surface basalts to more pervasive high-temperature alteration in diabases, gabbros, and periodites. The studies reveal distinct correlations between magnetization intensity, thermomagnetic behavior, and magnetic mineralogy, grain size, style, and intensity of alteration and rock type. Basalts have the highest NRM intensities and the lowest Curie points (105 ¿--294 ¿C); basalts with NRM/IRMs ratios less than 10-2 may have been remagnetized and have NRM intensities and Q values much lower (one-third and one-half, respectively) than basalts with NRM/IRMs ratios of 10-2 or greater. In the oceanic lithospheric suite examined, remanent (Q>1) magnetization is present in basalts and induced magnetization (Q<1) occurs in metabasalts and the other rock types.

Layer 2A basalts are the source of median valley magnetic anomalies. The magnetic source may shift from the surface to deeper horizons with progressive seafloor aging and enhanced deuteric oxidation in layer 2B dikes and layer 3 gabbros and the formation of magnetite in serpentinized peridotites. This shift is influenced by fluctuations in the Curie isotherm related to the duration of active magma chambers and convective hydrothermal cells. The magnetic and mineralogic properties of oceanic rocks determined show the effects of a wide range of alteration processes that are variously related to depth in oceanic lithosphere, magmatic cooling history, and hydrothermal circulation. ¿ American Geophysical Union 1990