ALBERT

Description: Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle-ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective associations with ductile and cataclastic deformation, and an overall decrease downhole with hydrothermal alteration generally 〈=5% in the bottom kilometer. Hole 735B provides evidence for a strongly heterogeneous lower ocean crust, and for the inherent interplay of deformation, alteration and igneous processes at slow-spreading ridges. It is strikingly different from gabbros sampled from fast-spreading ridges and at most well-described ophiolite complexes. We attribute this to the remarkable diversity of tectonic environments where crustal accretion occurs in the oceans and to the low probability of a section of old slow-spread crust formed near a major large-offset transform being emplaced on-land compared to sections of young crust from small ocean basins.

Description: Hole 735B provides a unique opportunity to compare the in situ properties of a thick sequence of oceanic gabbros with the properties of oceanic Layer 3 observed seismically worldwide. For example, the correlation of laboratory tests, sonic log, and vertical seismic profile (VSP) compressional wave velocities from Hole 735B with typical refraction velocities of oceanic Layer 3 (6.5-6.8 km/s) was well established after the early work during Ocean Drilling Program (ODP) Leg 118 (Shipboard Scientific Party, 1989, doi:10.2973/odp.proc.ir.118.107.1989; Iturrino et al., 1991, doi:10.2973/odp.proc.sr.118.151.1991; Swift et al., 1991, doi:10.2973/odp.proc.sr.118.141.1991; Von Herzen et al., 1991, doi:10.2973/odp.proc.sr.118.165.1991). It was also observed, however, that there was anomalously high attenuation in the Site 735 gabbros (Goldberg et al., 1991, doi:10.2973/odp.proc.sr.118.143.1991, 1992; Swift and Stephen, 1992, doi:10.1029/92GL01452). If oceanic Layer 3 consisted largely of the types of gabbros observed at Site 735, then so much seismic energy would be absorbed on propagation through the crust that we would not observe mantle and Moho arrivals. Because we clearly do observe lower crustal and upper mantle arrivals on refraction seismic experiments, it was hypothesized that oceanic gabbros could compose only a small percentage of seismic Layer 3 material. A major objective of the physical properties program during Leg 176 was to test this hypothesis with measurements on additional cores from deeper in the section. Other objectives were to identify the nature of the reflections observed below the borehole during Leg 118 at depths of 560 and 760-825 meters below seafloor (mbsf) and to check for seismic anisotropy in the gabbros. The "Physical Properties" section of the "Site 735" chapter in the Leg 176 Initial Reports volume (Shipboard Scientific Party, 1999, doi:10.2973/odp.proc.ir.176.103.1999) presents the results of shipboard measurements of compressional wave velocity. The average value of measurements on 217 minicores was 6.777 ± 0.292 km/s, which is in agreement with the Leg 118 results of 6.713 ± 0.383 km/s based on 228 minicores (Shipboard Scientific Party, 1989, doi:10.2973/odp.proc.ir.118.107.1989). These measurements were made at room temperature and pressure. We have carried out shore-based laboratory measurements on a small subset of the Leg 176 cores to (1) confirm the compressional wave velocities under in situ conditions of pressure, (2) measure shear wave velocities, (3) measure compressional and shear wave attenuation, and (4) check for anisotropy. For the last objective, we specifically acquired three sets of three mutually orthogonal whole-core samples.