Although injection of oxide gabbros at so many places in the core has greatly fragmented the stratigraphy of the underlying olivine gabbro suite, the spreadsheet for igneous contacts appended as a supplement to the site report (Shipboard Scientific Party, 1999c) and Appendix II of Dick et al. (1991a) allow each group of rocks to be treated separately. There are 504 intervals of the olivine gabbro suite, most of them separated by one, and a few by more than one, narrow seam of oxide gabbro. Stacked end to end and leaving out all seams of the more differentiated gabbros, the olivine gabbro suite totals 1153 m of the core in terms of expanded recovery, as defined earlier. This is 76.4% of the 1508 m drilled. Within the 504 intervals, again neglecting the seams, the principal variations in the olivine gabbro suite are in grain size and percentage of olivine. Often, the two go hand in hand. That is, finer-grained rocks tend to be more olivine rich than coarser-grained rocks. The higher olivine proportion shows up especially in the high MgO and Ni contents of analyses of troctolites and troctolitic gabbros and in corresponding modal data (Shipboard Scientific Party, 1989, 1999c), but not every interval has been chemically analyzed. In many parts of the core, several consecutive intervals of medium- to coarse-grained olivine gabbro, leaving out the oxide-gabbro seams, are bounded by fine-grained olivine gabbro, fine-grained troctolitic gabbro, or fine-grained troctolite, using the terminology of Leg 176. Dick et al. (1991a) term the first two of these olivine microgabbro and troctolitic microgabbro, respectively. Usually grain-size transitions occur across fairly well defined sutured or gradational contacts. In some cases, transitions are more abrupt, as when certain fine-grained troctolitic gabbros have sharp, planar, evidently intrusive boundaries with medium- or coarse-grained olivine gabbro.
Within the olivine gabbro suite, there are 175 recovered internal contacts where one rock adjoins another of different grain size, mode, or mineralogy (Table T1). Add to these all others where contacts were not recovered, and there are 273 separate bodies of olivine gabbro intervals with uniform texture, grain size, and mineralogy that we term composite intervals. Since almost all of the recovered contacts are sutured igneous contacts, each of these might be a separate small intrusion. The average thickness of a composite interval is 4.2 m.
Each alternation of coarse-grained olivine gabbro and finer-grained, more olivine-rich gabbro or troctolite beneath it, combining two or more composite intervals, can be treated as a potential individual injection or series of pulses of magma that contributed to the thickness of the crust of Hole 735B at any one place in the section. Also, coarse- and fine-grained composite intervals do not merely alternate, but they are often separated by members of medium-grained olivine gabbro. Thus general cycles of grain size decreasing downward are common, with the finest-grained and most olivine-rich gabbros lying just above the coarsest-grained and least olivine-rich portion of the next underlying cycle of olivine gabbro. Finally, whereas coarse- and medium-grained members are usually up to several meters thick, fine-grained gabbros are almost always thin. The fine grain size may result from quenching of hot magma in cooler surroundings. Whatever the case, the olivine-charged magmas that supplied the thin, fine-grained intervals likely were fairly dense. For these reasons, we place the most olivine-rich rocks at the base of each alternation or cycle in grain size rather than the top.
Some complications to this pattern near the base of the hole are produced by several thick, graded, even layered intervals. Near the base of the hole as well are several intervals with variable grain size, including some with thin, vertical, pipelike bodies of troctolitic gabbro. These we include in the thicknesses of otherwise uniformly textured coarse olivine gabbro. Nevertheless, most of the primitive gabbros of Hole 735B consist of an orderly succession of coarser olivine gabbros alternating with finer-grained olivine gabbro, olivine microgabbro, or troctolitic microgabbro (Table T1).
In the core, there are 97 such alternations or cycles of the olivine gabbro suite, which we term olivine gabbro sequences. For reasons discussed earlier, we use sequence in place of unit, a word that commonly refers to cyclicity in phase-layered igneous rocks and is the smallest subdivision of a zone within a series (Irvine, 1982). However, we connect sequence here not to phase layering over long distances but to disjunctions in grain size and/or modal proportions of olivine just in this single core. As with cyclic units in layered gabbros and peridotites, olivine is most concentrated at the base of each (e.g., Jackson, 1967, 1969; Irvine, 1969, 1980; Bédard et al., 1988; Emeleus et al., 1996).
This is somewhat arbitrary, but it forms the basis for a hypothesis: if all cycles result from one to several quickly spaced injections or pulses of magma at one place in the core, there were 97 such injections. If the individual composite intervals in a sequence are unrelated, then the olivine gabbro suite is made up of a maximum of 273 separate smaller intrusions. These alternatives are testable using chemistry and mineralogy and are useful to pose at the beginning of an evaluation of chemical stratigraphy. Sequences also provide a convenient shorthand means of assigning the rocks a stratigraphic order, of depicting them graphically, and of comparing one portion of the core to another.
Leaving out intervening oxide gabbros, olivine-gabbro sequences at Hole 735B average 12.0 m in thickness but only 35 of them are >10 m thick. The 35 thickest sequences themselves average 25.5 m thickness and together make up 917 m of the core, or 79% of the length of all the olivine gabbro sequences cored during both Legs 118 and 176. One sequence between 274 and 396 mbsf contains a series of olivine gabbro intervals that are separated by 11 seams of oxide gabbro. Most of this is one massive body of uniform olivine gabbro almost 112 m in composite thickness. This is one composite interval. Below this >9 m of core, the sequence has eight internal contacts of one olivine gabbro against another, all being somewhat more olivine rich than higher up and generally differing in grain size. A narrow olivine microgabbro is at the base. The olivine gabbro intervals, when combined, are 121 m thick, thickest in the entire core. Because the entire sequence is geochemically coherent, becoming more differentiated toward the top (see "Appendix A"), we view it as one intrusive mass with the several internal contacts perhaps being caused by pulses of magma injection or differences in the sizes, proportions, or densities of crystals—mainly olivine—the flowing magma was carrying at any given time, with these more concentrated toward the base of the intrusion. A similar group of composite intervals in another sequence is nearly 57 m in aggregate thickness, and two others are 43 and 40 m thick.
We do not know how any of these sequences thicken or thin in any direction, although they must. Portions of these sequences also are likely to have been displaced by faulting away from this vertical section, although most of the faults noted in the core descriptions are planar features in zones of crystal-plastic deformation and many are present at junctions between oxide and olivine gabbros. Doubtless, the core has numerous small lacunae, and perhaps substantial portions of the original section of olivine gabbros are now missing.
The thin basal fine-grained rocks are only a small proportion of each sequence (Table T1). Of the 96 basal intervals, 91 are fine or medium grained, and the remaining 5 are graded and perhaps are not at the bases of complete sequences, which may have been obscured by or even removed along small faults during deformation. Of the 91 that are not graded, most are <0.5 m thick. The aggregate thickness of all 91 olivine-rich basal intervals is only 65.5 m, or <6% of all the primitive gabbro cored. Some are sill-like with sharp contacts on both sides and may have split preexisting olivine gabbro. However, the basal fine-grained facies of most of these is separated from coarser-grained overlying olivine gabbro by sutured, partially penetrative, or gradational igneous contacts. The rock on both sides thus was at least partially molten when the contacts formed, indicating rapid sequential injection.
Of the 97 potential composite injections of magma, only about a third really mattered, in the sense that they substantially thickened the crust (Fig. F4). The thinnest sequences tend to be strongly grouped. The most olivine-rich intervals are present in clusters of thin sequences, perhaps indicating pulses of variably phenocryst-charged magma. Many of the thicker sequences are deep in the hole, and between 800 and 1100 mbsf there are no very thin sequences. The very thickest stands out not just because it is so thick but also because it is near the top of the hole, and two clusters of narrow coarse-fine sequences bracket it. One small screen of olivine gabbro, an interval 8 cm thick, lies within the oxide gabbro ~1 m above the main body of lithologic Unit V, and there are three other screens of olivine gabbro near the top of the oxide gabbro. The one truly substantial mass of oxide gabbro in the core therefore is a composite body of extensively differentiated material that intruded the section right at the top of the largest mass of olivine gabbro cored in the entire hole. However, Figure F4 shows this as a line, since all intervals of differentiated gabbro have been left out of the diagram. The most important faults and shear zones, annotated to their actual depths in Figure F4, tend to occur where thin sequences of olivine gabbro are clustered. Perhaps they truncate or cause repetitions of sequences of olivine gabbro in these portions of the core.