ALBERT

Beschreibung: Chemical exchange between oceanic lithosphere and seawater is important in setting the chemical composition of the oceans. In the past, budgets for chemical flux in the flanks of mid-ocean ridges have only considered exchange between basalt and seawater. Recent studies have shown that lower crustal and upper mantle lithologies make up a significant fraction of sea floor produced at the global mid-ocean ridge system. Moreover, the rugged topography of slow spread crust exposing lower crust and upper mantle facilitates prolonged fluid circulation, whereas volcanic ridge flanks are more rapidly isolated from the ocean by a sediment seal. Hence, elemental fluxes during lower crust-seawater reactions must be assessed to determine their role in global geochemical budgets. ODP Hole 735B penetrates more than 1500 m into lower ocean crust that was generated at the very slow spreading Southwest Indian Ridge and later formed the 5-km-high Atlantis Bank on the inside corner high of the Atlantis II Fracture Zone. The gabbroic rocks recovered from Hole 735B preserve a complex record of plastic and brittle deformation and hydrothermal alteration. High-temperature alteration is rare below 600 m below seafloor (mbsf), but the lowermost section of the hole (500-1500 mbsf) has been affected by a complex and multistage low-temperature (〈250°C) alteration history probably related to the tectonic uplift of the basement. This low-T alteration is localized and typically confined to fractured regions where intense alteration of the host rocks can be observed adjacent to veins/veinlets filled with smectite, smectite-chlorite mixed layer minerals, or chlorite +/- calcite +/- zeolite +/- sulfide +/- Fe-oxyhydroxide. We have determined the bulk chemistry and O and Sr isotope compositions of fresh/altered rock pairs to estimate the chemical fluxes associated with low-temperature interaction between the uplifted and fractured gabbroic crust and circulating seawater. The locally abundant low-temperature alteration in crust at Site 735 has significantly changed the overall chemical composition of the basement. The direction of these changes is similar to that defined for volcanic ridge flanks, with low-temperature alteration of gabbroic crust acting as a sink for the alkalis, H2O, C, U, P, 18O, and 87Sr. The magnitudes of element fluxes are similar to volcanic ridge flanks for some components (C, P, Na) but are one or two orders of magnitude lower for others. The flux calculations suggest that low-temperature fluid circulation in gabbro massifs can result in S uptake (3% of riverine sulfate input) in contrast to the S losses deduced for volcanic ridge flanks.

Beschreibung: 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.

Beschreibung: The mineralogy and stable (O and C) and Sr isotopic compositions of low-temperature alteration phases were determined in Hole 735B gabbroic rocks in order to understand the processes of low-temperature alteration in this uplifted block of lower oceanic crust. Phyllosilicates include smectite (saponite, Mg montmorillonite, and nontronite), chlorite/smectite, chlorite, talc, and serpentine. Other phases include prehnite, albite, K-feldspar, analcite, natrolite, thompsonite, pyrite, and titanite. The low-grade mineral assemblages mainly represent zeolite facies and lower-temperature "seafloor weathering" processes. Phyllosilicates formed over a range of temperatures but may also reflect variable reaction progress. Alteration temperatures were probably somewhat greater below 1300 meters below seafloor. Mineralogy and isotopic data indicate that conditions were mostly reducing and that seawater solutions were rock dominated. Carbonates formed late from cold and generally oxidizing seawater solution, however, as seawater penetrated downward as the result of fracturing and faulting in the uppermost portion of the uplifted crustal block.