CONCLUSIONS

The principal results of this study are as follows:

  1. Magnetic susceptibility is an extremely useful and precise means of evaluating the lithology and stratigraphy of gabbro recovered in whole-round core.
  2. The distribution of high-susceptibility oxide gabbros on a background of low-susceptibility olivine gabbros and troctolites in Hole 735B is far more intricate than is indicated by designation of lithologic intervals on the core description barrel sheets.
  3. Oxide gabbros on the whole experienced more extreme crystal-plastic deformation than olivine gabbros and troctolites.
  4. Felsic veins and vein nets are strongly but not exclusively associated with oxide gabbros, but usually formed after episodes of crystal-plastic deformation.
  5. The distribution of seams of oxide gabbro is unrelated to the distribution and composition of the olivine gabbros and troctolites that they intrude.
  6. The low magnetic susceptibility of olivine gabbros and troctolites by themselves is still sufficient to reveal compositional differences among these rocks.
  7. The pattern of magnetic susceptibility among olivine gabbros and troctolites is related to the amount of magnetite exsolved from silicate minerals, including plagioclase, and this in turn depends on the average stage of differentiation of these rocks.
  8. There are several minima in the downhole pattern of background magnetic susceptibility of olivine gabbros and troctolites that may be places where fairly large volumes of primitive magma inflated the lower ocean crust at or near the ridge axis.
  9. The block of olivine gabbro and troctolite below a fault zone at 1100 mbsf is more primitive, on the average, than similar lithologies above the fault. It is less deformed, has fewer seams of oxide gabbro, and has more widely separated felsic veins.
  10. Magnetic susceptibility is a useful geochemical log, with the measurements correlating especially strongly with bulk-rock TiO2 contents reflecting a consistent proportion of magmatic ilmenite to magnetite in the rocks.
  11. Taking magnetic susceptibility as an index of TiO2 contents over the whole of the core obtained during Leg 176 confirms estimates for the bulk composition of the section based on lithologic intervals obtained by Dick et al. (2000).

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