Hole 735B represents the only deep section of lower crust drilled by the Ocean Drilling Program (ODP). Crystallized beneath the ultra slow spreading Southwest Indian Ridge, where normal ocean crust is believed to be typically only ~4 km thick (Bown and White, 1994; Muller et al., 1999; Muller et al., 1997), it likely represents a major portion of the lower ocean crust at the time of accretion. Thus, it is by default the only type section of lower ocean crust formed at slow-spreading ridges. A detailed understanding of how the Hole 735B gabbros were emplaced, how they crystallized, and how this process reflects the unique tectonically active environment of slow-spreading ridges is critical then to constraining the formation of the lower ocean crust. Whereas an abundance of whole-rock major, trace element, and isotopic data has been collected, these data have inherent problems for interpreting the igneous petrogenesis of plutonic rocks. Whole-rock compositions integrate the influence of mineral mode and mineral chemistry as well as the effects of alteration. Moreover, large changes in lithology and texture can often be produced by petrologically minor events in the crystallization history of a pluton, whereas major events may be marked by only minor changes in these parameters. Events in the crystallization of plutonic rocks such as a major new phase of intrusion or a major fault discontinuity in the section are often revealed only in the cryptic variations of the mineral chemistry. While the effects of modal variations during subsolidus reequilibration have to be considered, mineral chemistry is also the best way to understand melt evolution during crystallization. Mineral zonation also provides the best means of addressing postcumulus processes, late-stage melt migration, and melt-rock reaction, which are now viewed as critical to understanding the formation of plutonic rocks.
The first 500 m of Hole 735B was drilled in 1987 during Leg 118, and the lower 1008 m was drilled 10 yr later during Leg 176. Consequently, two scientific parties, among whom there is only partial overlap, have collected mineral data. These data represent a variety of analytical schemes, facilities, and standards. Moreover, because of periodical space limitations, most of the Leg 118 data have been unavailable to the scientific community. This paper presents a large new systematically collected data set for ~200 samples of the Leg 176 gabbros and also synthesizes most of the data collected to date by the Leg 118 and Leg 176 scientists. The data presented in Tables T1, T2, T3, T4, and T5 (available in both ASCII and Excel format [see "Supplementary Material"]) includes 7508 clinopyroxene, 6759 plagioclase, 2955 olivine, 2427 orthopyroxene, and 855 hornblende spot analyses. A preliminary analysis of this data is presented here as well, though with 20,504 analyses, it hardly does justice to the data set. It should also be kept in mind that although this may seem to be a remarkably large data set, there are 952 discrete lithologic intervals in the Hole 735B core described on the basis of clear textural and mineralogic differences. This breakdown did not generally include intervals of less than ~4 cm, and further subdivision of the core could be made. With the analytical data set estimated to represent ~500 discrete samples, it remains inadequate to describe the core in anywhere near its entirety.