One consequence of the development of high-resolution measurements of core properties by ODP using the MST and some other techniques has been the gradual relinquishment of the primacy of visual core descriptions in evaluation of core lithology. Descriptions of igneous rocks have thus far escaped this fate, largely because of low recovery. For whatever technical reason, however, rotary coring with large bits penetrated massive and unfractured gabbro in Hole 735B like butter, relatively speaking, and recovered so much of it that, for the first time, high-resolution techniques can be applied to at least this one igneous lithology at this one place. This bodes very well for future deep drilling in the lower ocean crust, even if it means that the eyes of igneous petrologists turn out to be less trustworthy than, say, measurement of magnetic properties of the rocks.
The gabbros of Hole 735B have extraordinary lithologic complexity. This was evident on the first description of them during Leg 118, and in this respect Leg 176 did not disappoint. Many potential relationships among the rocks, anticipated both in comparison to ophiolites and to layered intrusions, simply did not show up (Natland and Dick, 2001). Instead, other kinds of relationships—to fractures, faults, patterns of alteration, and zones of deformation—became important, if not indeed all-important. The full extent of this complexity, however, is revealed less by core descriptions than by integrating the descriptions with the high-resolution measurement of magnetic susceptibility. One result of this, however, is that the initial basis for interpreting the chemical stratigraphy of the gabbros (e.g., Shipboard Scientific Party, 1999b; Dick et al., 2000) is inadequate. The existence of five "plutons" drilled during two legs and definable on the basis of, for example, MgNo vs. depth, with each contributing to a substantial thickening of the ocean crust at this location, is no longer tenable in view of the breakdown between "background" olivine gabbros and troctolites and "peak" disseminated-oxide and oxide gabbros. Whereas XRF analyses suggest that each of the three plutons penetrated during Leg 176 is increasingly differentiated upward, in fact each instead represents the consequence of sampling "representative" lithologies for chemical analysis, with intrusive oxide-bearing and oxide-rich seams becoming more prominent with elevation in each "pluton." However, this is entirely accidental. Fluctuations in "background" compositions have nothing to do with the location of oxide gabbros; thus, they should be treated separately as rocks that correspond to an underlying, fundamental, and very different igneous stratigraphy. These rocks were later, and in uncertain sequence, intruded and modified on an intimate scale by much more differentiated liquids. The deeper primitive gabbros may have contributed to those later liquids by a continuing process of differentiation, with expulsion of buoyant interstitial liquids upward during compaction and deformation (Natland and Dick, 2001), or the later liquids may have come in from somewhere else altogether (Dick et al., 2000).
Concurrently, although the core as a whole can be treated as a sequence or series of igneous units with composition as the fundamental variable, perhaps instead the core should be treated as a series of structural units, in which the attributes of crystal/plastic deformation in the presence of interstitial melts (Fig. F25) figure at least as strongly as bulk composition. Oxide gabbros are intimately interleaved with more primitive olivine gabbros and troctolites. The relationships between the two are equally intimately connected to the late-stage high-temperature deformation of the rocks. All of the oxide gabbros should be conceptually stripped out of the section before the underlying intrusive relationships among olivine gabbros and troctolites—in particular all of the contacts and variations in grain size and texture just among these lithologies are considered, if we want to understand the initial stages of construction of ocean crust at this slowly spreading ridge. That is the foremost lesson of this initial evaluation of the high-resolution igneous and deformational stratigraphy of Hole 735B. This is what was done in the evaluation of chemical stratigraphy for the synthesis of this volume (Natland and Dick, Synthesis Chap., this volume).