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

Principal results

Measurements of borehole properties form the majority of Expedition 340T results. VSP data extend the new information out to a region including a few hundred meters distance from the 1415 mbsf deep hole at Site U1309. In addition to logging in Hole U1309D, observations of the nearby seafloor were made with the VIT camera. One previously unrecognized feature generated sufficient interest that a brief sampling effort was made there. A small amount of material was successfully recovered, and the location was designated Site U1392. A map of these IODP sites on the Central Dome of Atlantis Massif shows the spatial relationship between Expedition 304/305 Holes U1309A–U1309E and Expedition 340T Hole U1392A (Fig. F5).

Site U1309

All components of the planned logging program in Hole U1309D were successfully conducted during Expedition 340T. The triple combo, sonic, and magnetic susceptibility runs produced high-quality data. The majority of the VSP data are noisy and will require substantial postprocessing. However, a few stations in the upper 150 m of the section recorded clear, strong seismic arrivals, thus providing in situ constraints on average properties across the zone inferred to be most strongly affected by detachment processes at the Central Dome. The details and timing of logging operations in Hole U1309D are given in Table T2.

The reentry cone for Hole U1309D was approached slowly as the ship positioned to get the logging bit into the borehole. Several observers carefully watched the VIT video for indication of possible seawater flow from the opening, but none was seen, so reentry proceeded without delay.

Data quality

The main logs recorded by the triple combo, sonic, and magnetic susceptibility tool strings in Hole U1309D are displayed in Figures F6, F7, and F8. Borehole size and shape measured by the calipers are general indicators of data quality. Hole U1309D is larger and more irregular in the upper 750 m, where borehole diameter ranges from ~11 to 18 inches, whereas the lower ~650 m is generally more regular in shape with a diameter closer to bit size (Fig. F6). Anomalously low density values in the wider, irregular sections of the hole are a consequence of the inability of the tool sensors to make full contact with the borehole wall. The gamma ray and deep resistivity measurements should not be affected by the size of the borehole.

The clear arrivals in the monopole waveforms and the high coherence in the compressional and shear velocity tracks shown in Figure F7 indicate that the DSI was able to measure reliable velocity values.

Magnetic susceptibility, measured with the deep-reading sensor of the MSS, should be fairly insensitive to standoff from the borehole wall. The MSS is a relatively new logging tool that was not available during Expedition 304/305; however, the reliability of the magnetic susceptibility log can be assessed by comparison with measurements made on Expedition 304/305 core pieces with the multisensor track (MST) system. Figure F8 shows good agreement between the two data sets, tracking meter-scale high-amplitude features at the same depths. MSS data are not yet calibrated for temperature, and the offset between the down, main, and repeat passes is most likely due to the variation in internal tool temperature between the runs (see temperature track, Fig. F8).

The comparison between hole size from Expeditions 304/305 and 340T in Figure F9 shows that the hole has not changed appreciably in the 7 y since it was last occupied. Intervals with a rough, irregular shape are indicated in the same areas, and distinct features are clearly repeated between the data sets. During the triple combo deployment of Expedition 340T, the caliper measured hole diameter to be <6 inches between 1387 and 1404 mbsf, suggesting that the lower ~20 m of the hole may contain some fall-in material. The small-diameter interval recorded between 630 and 655 mbsf in Expedition 305 data was due to tool failure and should not be interpreted as the hole having changed in diameter. Gamma radiation, density, resistivity, and sonic velocity data show good repeatability between the three sets of logs where coverage overlaps.

Scientific results

A steady increase in borehole fluid temperature with depth was documented, and a value of 146.2°C was recorded at 1405 mbsf (Fig. F10). This is >20°C hotter than the maximum temperature recorded in the hole at the end Expedition 305, when drilling and flushing had altered conditions considerably. The present temperature profile is quasilinear as a simple conductive model would predict for equilibrium conditions, but modest deviations do exist, as discussed in “Discussion.” The few-degree dips in temperature observed in the Expedition 305 Temperature/Acceleration/Pressure data (black curve, Fig. F10) are not apparent in the 340T data until the dominant linear trend is removed; then dips of a fraction of a degree Celsius are visible near 1100 and ~750 mbsf.

The Expedition 340T VSP data are noisy enough that the automated stack computed during acquisition produced only a few reasonable quality traces. A preliminary assessment was made of the quality for individual shots using a 10–60 Hz band-pass filter and a sliding short time window centered on the predicted arrival time (see “Appendix”). Arrival time predictions were generated from vertical integration of the sonic log data and also one-dimensional velocity modeling for the hole taken from a 3-D tomography model of Atlantis Massif developed from surface MCS data (Henig et al., in press) (Fig. F11). The maximum difference in predicted arrival times from these two models is 8 ms. Predicted times were adjusted for the gun firing delay (~28 ms), gun depth (5 ms), and a predicted time advance due to the sloping seafloor (~13 ms). First arrivals from 93 out of the 659 shots, corresponding to 33 of the 55 station depths, were graded as excellent to very good on at least one channel, primarily the vertical component (Tables T4, T5). Although necessarily subjective, an arrival was graded as excellent (value of 4 in Table T4) if the trace was quiet before the first break and a reliable traveltime could be picked directly from the trace. A “very good” arrival (value of 3 in Table T4) was one for which a reliable traveltime is expected to be obtainable by stacking and/or waveform cross-correlation. The good VSP stations for Expedition 340T thus range from 86 to 1360 mbsf, extending coverage beyond the Expedition 305 data interval of 272–792 mbsf (Collins et al., 2009). An example of excellent quality data on all three components is displayed in Figure F12 for the new station at 150 mbsf. If relative station amplitudes are reliable, these traces indicate that the first arrival arrives with direction ~30° from the vertical. The single good arrival for the deepest station at 1360 mbsf is displayed in Figure F13, arriving just before the model prediction.

The most significant new data are the sonic logs recorded below 820 mbsf where no velocities were measured at the end of Expedition 305. These data are the first in situ measurements of the velocity of gabbros typical of oceanic lower crust, with V_P reaching values > 7000 m/s. V_P and V_S show little variation in the main gabbroic zone between ~760 and ~1070 mbsf, reflecting the mostly uniform composition in this zone (Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 304/305 Scientists, 2006). Deeper in the hole, velocity seems to reflect the variation in composition, with high variability in the olivine-rich troctolite interval between 1070 and 1230 mbsf and steady values below this depth where gabbros are again dominant (Fig. F7).

In addition to the high-frequency (~12 kHz) waveforms used for the velocity logs, the same transducers were used to generate low-frequency (~500 Hz) Stoneley waveforms that can be used to identify fractures or permeable intervals. When they encounter fractures, Stoneley waves propagating in the borehole are reflected, with the reflectivity dependent on the openness and permeability of the fracture (Hornby et al., 1989). The chevron-shaped patterns that can be seen at various depths in the Stoneley waveforms in Figure F7 are generated by such fracture-induced reflectivity. Figure F14 shows that the fracture responsible for these patterns around 1345 mbsf is a 30 cm thick northeast-dipping fracture that is clearly identified in the Formation MicroScanner (FMS) images recorded during Expedition 305 logging operations.

This expedition marked the first sea trial of the deep-reading sensor of the newly rebuilt LDEO MSS. Figure F15 shows finer scale variations of core and log susceptibility with lithology from Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 304/305 Scientists (2006) between 170 and 400 mbsf. The background gabbroic rocks in Hole U1309D have relatively low susceptibility, whereas highest values (or highest amplitude features) are associated with intervals of oxide gabbro and highly serpentinized ultramafic rocks (e.g., olivine-rich troctolite, dunite, and harzburgite). There is a very good correlation between the magnetic susceptibility logs and the lithologies that have a high content of ferro- and ferrimagnetic minerals.

Site U1392

A seafloor feature located 3 m south and 2 m east of the reentry cone for Hole U1309D caught our interest during Expedition 340T. It was first seen shortly after the seafloor came into view on the VIT camera, as the positioning for reentry was starting. The circular shape of the feature and distinct coloring relative to surrounding seafloor gave the impression that it was the (still distant) reentry cone, so we dynamic positioned to it. Additional characteristics became evident with closer view (Fig. F16). A distinct rim separates the center of the feature from surrounding material. Outside the rim, concentric or stacked intervals are distinguishable from the observed shadow pattern. Inside the rim, partial darker shadow is suggestive of an opening, but otherwise the imagery there lacks structure as might be typical for unconsolidated sediment. The VIT imagery is not high quality, but these characteristics were clearly and repeatedly observed at the feature, being more evident in video than in single frames. This feature was named Decoy mound due to our initial misinterpretation that it was the reentry cone for Hole U1309D with sediment encrusted on/around/below it.

The ~2 m diameter, 1–2 m high Decoy mound was initially interpreted as a deposit formed since IODP was last at Site U1309 (February 2005). Its solid, possibly cylindrical wall, of thickness comparable to the diameter of the drill bit (~25 cm), has outside morphology that is characteristic of geothermal deposition: rounded or bulbous layers that vary in thickness (distance?) from the main rim diameter. The rim (top of the wall?) has somewhat irregular shape but is clearly distinct from the interior material. The height of the feature varies around its circumference. The logging bit could only penetrate 1–2 m below the rim before encountering a solid interface that could not be pushed through. It is this interface that was sampled as Hole U1392A.

The motivation for sampling was to test the inference that Decoy mound included (or was solely) geothermal deposits, whose composition might provide insight into any fluids responsible for the growth or, perhaps, simply lithification of preexisting or concurrently deposited sediments. The fact that the actual reentry cone for Hole U1309D was relatively sediment free (Fig. F16) indicates that background marine deposition since 2005 has not been significant.

There was not sufficient time after the primary work of Expedition 340T (logging) to run pipe, change the logging bit to an APC bit, and return to the seafloor for a sample, so an alternate strategy was devised. A modified APC bit was lowered in the pipe to 1500 mbrf and held there while the ship was positioned over Decoy mound. The drill string was pushed into contact with the impenetrable surface below the rim and held there. The cable tension was freed, allowing the barrel to obtain a gravity core, with intent that the flapper valve could retain material penetrated. Cable was reeled in to position the barrel again near 1500 mbrf and then tension released for a repeat sampling attempt. The core barrel was recovered after this second drop. The core catcher contained a small amount of material that was catalogued as Sample 340T-U1392A-1M-CC, the “M” indicating miscellaneous sample type.

Core 340T-U1392A-1M contained ~17 g of mixed rock fragments and some microfossils (Fig. F17A). Four types of material were recognized during visual inspection by nonexpert geoscientists onboard. About a third of the sample consists of sharp-edged, platy fragments (Fig. F17B) that are black on one side when wet and rust-colored on the other side. The black side is commonly finely striated. A few angular grains of possible fault rock are milky or translucent and white-blue in color (Fig. F17C), reminiscent of the talc-tremolite schist obtained in prior Atlantis Massif studies (Boschi et al., 2006; Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 304/305 Scientists, 2006). Lithified carbonate pebbles also make up a few percent of the sample, and these pieces incorporate a variety of other material as tiny grains (Fig. F17D). The majority of the sample consists of dark green-gray angular subcentimeter-sized fragments (Fig. F17E) mixed with tiny grains of the previously described rock chip types. Microfossils also were obtained in this sample, as can be seen in this image.

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