Ocean Drilling Program (ODP) Hole 735B, located on Atlantis Bank on the Southwest Indian Ridge, penetrated 1508 meters below seafloor with an average recovery of 87%, providing a nearly continuous sample of a significant part of oceanic Layer 3. Based on variations in texture and mineralogy, 12 major lithologic units are recognized in the section, ranging from 39.5 to 354 m thick. The principal lithologies include troctolite, troctolitic gabbro, olivine gabbro and microgabbro, gabbro, gabbronorite and Fe-Ti oxide gabbro, gabbronorite, and microgabbro. Highly deformed mylonites, cataclasites, and amphibole gneisses are locally present, as are small quantities of pyroxenite, anorthositic gabbro, and trondhjemite. Downhole variations in mineral composition, particularly for olivine and clinopyroxene, show a number of cyclic variations. Plagioclase compositions show the widest variations and correspond to different degrees of deformation and alteration as well as primary processes. Downhole chemical variations correspond reasonably well with variations in mineral compositions. Iron and titanium mainly reflect the presence of Fe-Ti oxide gabbros but show some cyclical variations in the lower part of the core where oxide gabbros are sparse. CaO is highly variable but shows a small but consistent increase downhole. MgO is more uniform than CaO and shows a very small downward increase. Sulfur and CO2 contents are generally low, but S shows significant enrichment in lithologic Unit IV, which consists of Fe-Ti oxide gabbro, reflecting the presence of sulfide minerals in the sequence. The lithologic, mineralogical, and geochemical data provided here will allow detailed comparisons with ophiolite sections as well as sections of in situ ocean crust drilled in the future.
1Hertogen, J., Emmermann, R., Robinson, P.T., and Erzinger, J., 2002. Lithology, mineralogy, and geochemistry of the lower ocean crust, ODP Hole 735B, Southwest Indian Ridge. In Natland, J.H., Dick, H.J.B., Miller, D.J., and Von Herzen, R.P. (Eds.), Proc. ODP, Sci. Results, 176 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/176_SR/chap_06/chap_06.htm>. [Cited YYYY-MM-DD]
2Afdeling Fysico-chemische geologie, University of Leuven, Celestijnenlaan, 200C1, B-3001 Heverlee, Belgium.
3GeoForschungsZentrum, Telegrafenberg, D-14473 Potsdam, Germany.
4Permanent address: Centre For Marine Geology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5. Correspondence author: robinso@is.dal.ca
Initial receipt: 24 September 2000
Acceptance: 4 December 2001
Web publication: 21 June 2002
Ms 176SR-003