We present the results of studies of magnetic mineralogy and magnetic properties of Holes 504B and 896A upper crustal rocks recovered during Leg 148. The ratio of saturation remanence to saturation magnetization (J_RS/J_S) ranges from 0.012 to 0.154 (mean = 0.06) in samples from Hole 504B, consistent with pseudo-single-domain behavior. Hole 896A samples display a variable ratio of J_RS/J_S that ranges from 0.012 to 0.154 (mean = 0.076), reflecting the differences in grain morphologies and magnetic properties between pillow and massive units.

Intensity of natural remanent magnetization (NRM) ranges from 0.01 to 0.24 A/m (mean = 0.09 A/m) in Hole 504B samples, less than that measured in samples recovered during other legs because the drilling-induced remanence imparted by the bottom-hole assembly was substantially less during Leg 148. Volume susceptibility averages 0.032 ± 0.029 SI, similar to that from upper parts of the dike sequence. In all samples examined from Hole 504B, high-temperature oxidation of primary titanomagnetite has produced lamellae or pods of magnetite and ilmenite. Hydrothermal alteration has further altered the minerals in some samples to a mixture of magnetite, ilmenite, titanite, and a high-titanium mineral (either rutile or anatase). Electron microprobe analyses show that magnetite lamellae are enriched in the trivalent oxides Cr₂O₃ (mean = 0.076 wt%), Al₂O₃ (mean = 0.566 wt%), and V₂O₃ (mean = 0.757 wt%), whereas divalent oxides are concentrated in ilmenite lamellae (mean MnO = 1.997 wt% and mean MgO = 0.073 wt%).

In samples from Hole 896A, NRM intensity ranges from 0.14 to 22.59 A/m (mean = 5.06 ± 4.91 A/m). Volume susceptibility averages 0.0272 ± 0.0254 SI. Low-temperature oxidation of primary titanomagnetite has produced titanomaghemite exhibiting shrinkage cracks. Ulvöspinel content averages 0.508 and ranges from 0.217 to 0.647. Titanomaghemite appears slightly enriched in Al₂O₃ (mean = 1.697 wt%), Cr₂O₃ (mean = 0.064 wt%), V₂O₃ (mean = 0.673 wt%), MnO (1.257 wt%), and MgO (0.577 wt%).

Date of initial receipt: 15 August 1994
Date of acceptance: 28 February 1995

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