The ocean crust, which covers ~70% of the Earth's surface, is the net product of the magmatic, tectonic, and hydrothermal processes taking place along the world ocean ridges. Although this concept is well conceived, important details of these processes and the structure and composition of the bulk ocean crust remains poorly understood because of the inaccessibility of lower ocean crust. This incomplete knowledge of ocean crust not only hinders our understanding of ocean crust accretion at ridges, but also leaves models of some fundamental geodynamic problems unconstrained. For example, the nature and composition of primary mantle melts parental to mid-ocean-ridge basalt (MORB) has been in debate for over three decades (e.g., O'Hara, 1968; Elthon, 1989). Whereas this debate is now less acute, considering the polybaric (vs. isobaric) melting processes beneath ocean ridges (e.g., Klein and Langmuir, 1987; McKenzie and Bickle, 1988; Niu and Batiza, 1991), it remains unknown what primary mantle melt should look like until the bulk composition of the entire ocean crust is obtained. Furthermore, ocean crust is widely accepted to be an important geochemical reservoir. Popular models of chemical geodynamics use average composition of MORB as proxy for ocean crust (e.g., Hofmann, 1988), but this use neglects the fact that MORB is only a compositional end-member and constitutes no more than ~10%-15% of the total crustal mass (e.g., Niu, 1997). Therefore, characterization of the actual composition of lower ocean crust is needed.
Ocean Drilling Program (ODP) Hole 735B, Southwest Indian Ridge, drilled during Leg 118 (Robinson, Von Herzen, et al., 1989; Dick et al., 1991) and Leg 176 (Dick, Natland, Miller, et al., 1999) provides an unprecedented opportunity to study in detail the mineralogy, lithology, and composition of lower ocean crust formed at slow-spreading ridges. Hole 735B may not yet provide solutions to the above problems but has yielded important observations toward addressing these problems, as already well demonstrated by Dick, Natland, Miller, et al. (1999). In this paper, we report mineral chemistry, whole-rock major element compositions, and trace element analyses of samples selected during Leg 176. We discuss these data, together with Leg 176 shipboard data and data from Leg 118 samples found in the literature, in terms of primary igneous petrogenesis.