Leg 176 was the second of two legs to occupy one hole, Ocean Drilling Program (ODP) Hole 735B, which was drilled during both 1987 and 1997 into the gabbroic portion of the ocean crust in the Indian Ocean. The drilling was enormously successful, providing great insight into interactive processes of crustal accretion, igneous differentiation, high-temperature crystal-plastic deformation, and cooler static hydrothermal alteration of the lower ocean crust at a very slowly spreading ridge. Results have been presented in Proceedings of the Ocean Drilling Program, Initial Reports volumes for Legs 118 and 176 (Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999), the Scientific Results volume for Leg 118 (Von Herzen, Robinson, et al., 1991), and a number of articles in the general scientific literature. Some are now presented in this volume. Five other surveys have been conducted in the vicinity of Hole 735B, one prior to Leg 118 in 1986 and one prior to and three following Leg 176. These investigations form an important backdrop to the drilling. Additional drilling was carried out a little more than a kilometer from Hole 735B during engineering Leg 179 (Pettigrew, Casey, Miller, et al., 1999).
There is consequently material aplenty to synthesize. The planning for the drilling of Hole 735B involved many people, many documents, and many meetings. Proper understanding of the scientific results requires a summary of why the drilling was carried out in the first place and of how and in what ways it was either successful or not successful. Such an evaluation often is part of the presentation of the Initial Reports volume of the Proceedings of the Ocean Drilling Program; in this case two of those were prepared, but neither was an overview of the entire project. Consequently, we believe that the scientific reports in this volume and elsewhere are well served by a general prefatory chapter. We hope this will be useful to anyone who wishes to propose and carry out any project of similar scale in scientific ocean drilling in the future.
We have also prepared a general scientific synthesis reviewing the chemical stratigraphy of the entire core recovered during both Legs 118 and 176, integrating all shipboard and postcruise geochemical data (Natland and Dick, Synthesis Chapter, this volume). The summaries here of survey results and of concepts of ocean crust structure are important background for that chapter.