We present a synthesis of some 20,504 mineral analyses of ~500 Hole 735B gabbros, including 10,236 new analyses conducted for this paper. These are used to construct a mineral stratigraphy for 1.5-km-deep Hole 735B, the only long section of the lower crust drilled in situ in the oceans. At long wavelengths, generally >200 m, there is a good chemical correlation among the principal silicate phases, consistent with the in situ crystallization of three or four distinct olivine gabbro bodies, representing at least two major cycles of intrusion. Initial cooling and crystallization of these bodies must have been fairly rapid to form a crystal mush, followed by subsequent compaction and migration of late iron-titanium-rich liquids into shear zones and fractures through which they were emplaced to higher levels in the lower crust where they crystallized and reacted with the olivine gabbro host rock to form a wide variety of ferrogabbros. At the wave lengths of the individual intrusions, as represented by the several olivine gabbro sequences, there is a general upward trend of iron and sodium enrichment but a poor correlation between the compositions of the major silicate phases. This, together with a wide range in minor incompatible and compatible element concentrations in olivine and pyroxene at a given Mg#, is consistent with widespread permeable flow of late melt through these intrusions, in contrast to what has been documented for a 600-m section of reputedly fast-spreading ocean crust in the Oman Ophiolite. This unexpected finding could be related to enhanced compaction and deformation-controlled late-stage melt migration at the scale of intrusion at a slow-spreading ocean ridge, compared to the relatively static environment in the lower crust at fast-spreading ridges.
1Dick, H.J.B., Ozawa, K., Meyer, P.S., Niu, Y., Robinson, P.T., Constantin, M., Hebert, R., Maeda, J., Natland, J.H., Hirth, J.G., and Mackie, S.M., 2002. Primary silicate mineral chemistry of a 1.5-km section of very slow spreading 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_10/chap_10.htm>. [Cited YYYY-MM-DD]
2Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole MA 02543, USA. Correspondence author: hdick@whoi.edu
3Geological Institute, Faculty of Science, University of Tokyo, Tokyo 113, Japan.
4Department of Earth Sciences, The University of Queensland, Brisbane QLD 4072, Australia.
5Institute of Geochemistry, GeoForschungsZentrum, Telegrafenberg, D-14473 Potsdam, Germany.
6Département de géologie et de génie géologique, Pavillon Pouliot, Université Laval, Québec G1K 7P4, Canada.
7Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8 Kita, Sapporo, Hokkaido 060-0810, Japan. [N1]
8School of Oceanography, University of Miami, 4600 Rickenbacker Causeway, Miami FL 33149, USA.
Initial receipt: 7 September 2000
Acceptance: 6 June 2002
Web publication:
16 October 2002
Ms 176SR-001