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doi:10.2204/iodp.pr.340T.2012

Discussion

Preliminary findings for Expedition 340T fall into the following categories: (1) seismic structure of the intrusive crust within the domal core of Atlantis Massif and correlation of meter-scale velocity with general lithology, alteration, and fault zones; (2) inferences about localized fluid flow and deformation at the site based on seafloor imagery and downhole temperature; and (3) magnetic susceptibility of the borehole rock and potential for insights on relative timing of serpentinization. Most results were obtained by logging Hole U1309D, but brief camera surveys and opportunistic sampling in the new Hole U1392A added to our findings.

We have obtained the first in situ measurement of intrusive oceanic crust, which typically comprises seismic Layer 3. The Expedition 340T sonic logs indicate that the little-altered section from 800 to 1400 mbsf has mean compressional velocity of 6.6 km/s and mean shear velocity of 3.7 km/s (Fig. F18). This average excludes the olivine-rich troctolite interval at 1070–1220 mbsf that has several highly serpentinized intervals. The multimeter-scale sonic log average is an appropriate value for the inherent seismic properties of a gabbroic section. When postprocessing of the VSP data is complete, we will obtain a site average V_P for this intrusive crustal section that will include any effects of fracturing at the 100 m scale such as may be associated with OCC development.

Useful VSP stations bracket the range of lithologies and alteration that occur in Hole U1309D (Fig. F19). Only a single station from Expedition 305 is located in the interval above the upper olivine-rich troctolites (310–350 mbsf) but well below the diabase units that are common in the upper 130 mbsf.

The Expedition 340T sonic log confirms that olivine-rich troctolite intervals have sufficient velocity contrast with surrounding rock to be responsible for reflectivity observed in surface seismic data (Fig. F18). The 750 mbsf fault zone also has significant seismic and density contrast, but its 20–30 m thickness is at the margin of observability, relative to subseafloor seismic wavelengths. However, if pore fluid presently exists there, as suggested by the temperature dip measured in this zone (Fig. F20A), this could enhance the impedance contrast and produce a reflector despite the narrow interval. An additional reflector occurs at 1340 mbsf, as is most easily seen in the Stoneley wave results (Fig. F7). This is the first recognition of this zone as having distinctive properties—Expedition 304/305 core/logging analyses did not highlight this interval, but a retrospective review of FMS, borehole density, porosity, and photoelectric data does show it as a distinctive, narrow interval.

Small deviations from a downhole conductive thermal gradient provide another indication that narrow depth intervals within the domal core of Atlantis Massif have distinctive properties, and, likely, currently active processes. In addition to the temperature dip associated with the 750 mbsf fault zone mentioned above, a similar small dip (0.3°–0.5°C, relative to the local linear gradient) is observed at the 1100 mbsf fault zone (Fig. F20). Our preliminary interpretation of these signals is that slow percolation of seawater (cooler temperature [T] when it initially enters a fracture network at the seafloor, whether at or laterally displaced from Site U1309) occurs, made possible by modest porosity within these zones. The maximum vertical extent of each zone is 10–20 m based on the limit of the T deviation. Michibayashi et al. (2008) determine a ~6 m thickness for the fault zone at 750 mbsf on the basis of borehole resistivity, gamma ray, and density anomalies. Borehole structure imaged in Expedition 304/305 FMS data tends to dip east in a central 1 m interval of this zone, in contrast to the general north–south dip of the structures above and below. Low resistivity and positive gamma ray and neutron porosity signatures in the fault zone are consistent with the presence of a conductive phase such as seawater.

The downhole temperature profile has 2–3 modest breaks in slope; characterizing their location and investigating potential reasons for the changes in gradient will be addressed in postcruise research. In general, the increase in temperature with depth is greater in the lower half of the hole than at shallower depths.

The absolute value of the Expedition 340T downhole magnetic susceptibility correlates very well with the measured susceptibility of the core obtained during Expeditions 304 and 305. Essentially every MST peak (for smoothed data that reduces core edge artifacts) corresponds to an MSS anomaly (Fig. F8). Beyond the multimeter-scale correlation that we determined via shipboard analysis, more detailed documentation of the borehole thickness of oxide gabbro units will be possible. Some of the cores with this lithology had low recovery during Expeditions 304/305, as their extremely large grain size was conducive to breakup of the core into biscuit-size pieces and resulted in an unknown amount of material loss. More relevant for the goals of Expedition 340T, it appears there may be information on the relative timing of serpentinization of olivine-rich troctolite units. Using just the basic rock name from the Expedition 304/305 unit log, there is a clear association of relative lows in recorded MSS value and the olivine-rich troctolite units in the 300–350 mbsf section (Fig. F15). This warrants further investigation, and more detailed information on both primary and alteration mineralogy should be brought to bear in analyses to determine how/whether it is serpentinization that is key or some other factor(s). If it is the serpentinized intervals that correlate with the submeter-scale drops in MSS values, then inference of an opposite polarity of the magnetic field during alteration, relative to the reversed polarity that dominated throughout cooling of the igneous section, might be appropriate.

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