Shau et al. (this volume) determined the magnetic properties and oxide mineralogy of a large suite of samples. The dominant magnetic carrier in pillow basalts is maghemite, presumed to have formed by low-temperature oxidation of primary titanomagnetite. A significant proportion of the magnetized material is in the form of small (<0.5 µm) single-domain Fe-Ti oxide grains within interstitial glass. NRM values are typical for MORB and decrease with increasing age across the 14- to 28-Ma interval spanned by the Leg 187 sites. This evidence for ongoing alteration contrasts with that of S. Krolikowska-Ciaglo et al. (pers. comm., 2003) (see "Regional Alteration Patterns" in "Alteration Studies"), who concluded that low-temperature alteration of pillow basalts was essentially complete by 14 Ma.
In the massive basalts of Hole 1160B, which are less altered than coexisting pillow basalts (Christie, Pedersen, Miller, et al., 2001), the dominant magnetic carrier is primary titanomagnetite, which occurs as homogeneous grains a few tens of micrometers in size. In the single greenschist-facies metadiabase clast from Hole 1162C, the dominant carrier is magnetite, formed by hydrothermal alteration of primary subhedral titanomagnetite grains as large as a few hundred micrometers.
The magnetic characteristics of pillow basalt samples do not change from east to west across the region, indicating that these properties are insensitive to mantle source signature and topographic style, as discussed in the preceding section. Noting that aeromagnetic anomaly amplitudes (Vogt et al., 1983) are lower within the AAD than in Zone A, Shau et al. (this volume) point out that the magnetic uniformity of the pillow basalts across the region requires a significant contribution from the lower crust to the total magnetic field. This observation is consistent with the extensive exposures of Layer 3 and Layer 4 materials at the seafloor that can be inferred from the association of chaotic terrain with extensive megamullions in Segments B4 and B3 (Christie et al., 1998).