SUMMARY AND CONCLUSIONS

  1. Basement at Site 1224 is separated into three lithologic units: Unit 1 (massive flow; ~26–62 mbsf), Unit 2 (pillow breccia; ~62–135 mbsf), and Unit 3 (intermixed pillow and thin flows; ~135 mbsf). Unit 1 is divided into two subunits (upper and lower flow units) based on grain size of groundmass and distribution of alteration halos. Unit 2 is divided into two subunits based on recovery and differences in alteration effects.
  2. Differences in HFSE and compositions among the three units are remarkable. Unit 3 has the highest content of these elements, and Unit 2 has the lowest. Units 1 and 2 are divided into two subunits on the basis of these elements, and this classification is consistent with lithologic characteristics. It is thought that these chemical differences reflect differentiation process (i.e., fractionation of clinopyroxene).
  3. Variations in physical properties and downhole logging data correspond to the lithologic and chemostratigraphic changes that define the units and subunits.
  4. The basement rocks have high incompatible element contents (e.g., higher HFSE content than N-MORB), and some element contents are similar to OIB (cf. Sun and McDonough., 1989). Based on their high abundances of immobile HFSE (Zr and Y) and high, MORB-like Zr/Nb, we conclude that these are MORB that fractionated the mineral phases, olivine, clinopyroxene, and plagioclase within the oceanic crust.

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