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

Preliminary scientific assessment

Most components of Expedition 340T were successful, and the new data will allow us to address aspects of lithospheric hydration associated with oceanic core complex formation and evolution, as was our aim at the outset.

Two aspects of the temperature log data are relevant and are expected to help constrain new models of slow flow within narrow subsurface fault zones and broader scale (several tens to hundreds meter scale) cooling of the uplifted core of Atlantis Massif: (1) the small dips in value associated with two documented faults and (2) the distinct intervals of linear thermal gradient that differ downhole.

Sources of seismic reflectivity within the intrusive core of the massif have been identified in the Expedition 340T sonic logs. Notable impedance contrasts are associated with differences in physical properties of the olivine-rich troctolite intervals compared to surrounding lithologies. The fact that portions of these intervals are highly serpentinized means that MCS and waveform inversion methods can be expected to provide insight into the distribution of these types of subsurface hydration pathway. The combination of sonic and downhole temperature data suggests that the 750 mbsf fault zone could reflect comparable amounts of seismic energy. This will need to be assessed more carefully by modeling the volume of percolative flow needed to produce the observed temperature drop within that interval and estimating associated pore fluid volumes within the zone. Logged porosity and postcruise Stoneley wave analysis may also provide constraints in this regard.

With the data we obtained at the quietest VSP stations, an average velocity will be determined for the upper 150 m of the Central Dome, and this will allow us to address properties of the exposed detachment zone, which include hydration (alteration mineralogy) as well as deformation (porosity, in this case) that wave speeds are sensitive to.

Some of the information that we hoped to obtain as part of the zero-offset VSP experiment could not be obtained due to operational limitations.

It is likely not possible to record reflectors in this type of geologic setting with the approach used on the JOIDES Resolution for obtaining VSPs. The borehole seismometer needs to be decoupled from cable motion, and perhaps, environmental noise (ship? pipe against hole?) may need to be reduced in order to succeed at this next level of seismic investigation. We recommend that this information be made available to all scientists expressing interest in (at least hard rock) VSP work within IODP. Decoupling the sensor from cable motion would also reduce the likelihood of the clamping arm bending or breaking, as was experienced during two of the three VSP runs during Expedition 340T. Seas were moderate, not high, at the times of our VSP work, so our problems in this regard do not appear to reflect use in extreme conditions.

We recognize the need to comply with the designated JOIDES Resolution protected species mitigation plan. The PSO watches and VSP work were carried out well and in accordance with this plan. Because future IODP expeditions may benefit, we note that if we had been able to continue air gun shots into the evening hours on 24 February, our VSI data set may have been significantly better than it is. For whatever combination of reasons (clamping arm not bent? less ship/cable noise?) the 24 February VSP run was the only one to produce consistently good quality data. Acquisition might have extended to additional (and deeper) stations if we had been able to continue under those conditions. For projects where VSP work is of high priority, IODP may want to explore developing a mitigation plan designed to allow evening seismic source operations.

Less-than-optimum selection of design, possibly material, of the centralizer arms impacted our operations. If we had not arrived on site early, due to very rapid transit, and if the USIO had not been willing to allow us to use this extra time for our work, we would have failed in our attempt to document the seismic properties of the lower portion of Hole U1309D. About 2 days were required for our team to recognize and learn how to address this problem in order to have successful DSI and VSI runs.

Higher than usual (for IODP) temperature in this hole may be a factor in the significant wear and failure of the centralizer arms. It is not out of the question that borehole chemistry also played a role (but this has not been tested), since Lost City hydrothermal vent field a few kilometers south of Site U1309 is known to have high-pH fluid. Measured fluid resistivity in the borehole does deviate from standard seawater values in the lower parts of the hole.

The rapid transit from Lisbon, Portugal, to Site U1309 introduced an unexpected set of activities—the potential for extra time on site meant that additional science related to both Expeditions 340T and 304/305 might be addressed. The ship’s management, technical and drill crews, and onshore personnel were all very helpful as we explored this possibility. Their willingness to adjust onboard workflow during Expedition 340T to accommodate a modest amount of unexpected coring is recognized and very much appreciated. In the end just a single sampling effort was possible; even this reflected the onboard group’s ability to be creative in order to take advantage of a scientific opportunity. By rigging a modified APC bit/barrel and deploying it from 150 m above seafloor within the drill pipe, we obtained what was essentially a gravity core. The fragments of rock and gravel recovered show promise for providing worthwhile geochemical and microstructural information on the uppermost surface of the detachment exposed on the dome of Atlantis Massif.

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