SUMMARY

Comparison of microprobe work on glass cuttings and thin sections with the shipboard ICP-AES and ORI microprobe work has shown the following:

  1. The cuttings came mostly, if not entirely, from the lower part of Hole 1243B because geochemically distinct Units 3 and 4 of the upper part of the hole are not present in the analyzed cuttings. Distinctive, alkalic Unit 4 is represented by several cores from high in the drilled basement section but it is not represented by even one of the 582 cuttings analyzed from the bottom of the hole. Tholeiitic Units 1, 3, and 5, from the upper to middle part of the section, resemble each other, but although Group M is similar to those units in MgO and CaO content, its lower SiO2, Al2O3, and FeO indicate that Group M, representing more than one-half of the cuttings, is not from a Unit 1, 3, or 5 source.
  2. Group K is close to Analyses U6&8K and fairly close to Analysis U6Na in nearly all components (SiO2, TiO2, K2O, Al2O3, CaO, FeO, and MgO). Therefore, Group K probably came from ~160 mbsf and deeper and may extend as deep as Section 203-1243R-18R-1 (Piece 7) at 190.67 mbsf (the "8" in U6&8K), a piece so small it may have caved and fallen from that higher level. The substantial fraction of Group K in the cuttings, in spite of the intensive effort to sweep the hole clean and the thick section of cavings, wide caliper readings, and lower sonic velocities, indicates that a significant part of the lower part of Site 1243 is of Group K composition.
  3. Group M, because of its abundance, probably came from below 170 mbsf, deep in the hole where recovery was especially poor and analyses are few.
  4. Group A probably matches Unit 5, which was sampled at ~156 mbsf.
  5. Nothing plots close to Group B, and it may be a slight differentiate of Group M.
  6. Group H may match Unit 6 near 167 mbsf or perhaps Unit 8 at 190 mbsf if only a few elements or ratios are considered. Group H may also be a slight differentiate of Group M.
  7. Considering the total evidence of the cores, the cuttings and their likely depths of source, the numbers of analyses, the gamma ray log for Unit 4, and the apparent cavings history for Group K, approximately two-thirds of the penetrated basement is N-MORB and one-third (Unit 4, Group K, and perhaps Unit 5/Group A) is enriched or T-MORB.
  8. The combination of normal and enriched basalts, indicating two distinct parental magmas, may be a record of the Galápagos plume, or it may be a consequence of spreading rate and mantle thermal regime.

NEXT