Leg 147 was the first drilling leg that specifically followed the offset drilling strategy to test models for magmatic accretion at the fast-spreading East Pacific Rise. At Site 894, a sequence of gabbroic rocks was recovered from a tectonic exposure of the lower crust and provided critical new data to characterize the nature of the upper part of axial magma chambers and the processes that shape them. Within 9 km of Site 894, a sequence of harzburgite, dunite, and gabbros representative of the shallow mantle was recovered at Site 895. This unique suite of cores demonstrated spatial heterogeneity of melt percolation in the upper mantle.
The drilling strategy for Site 894 was based on the success of Hole 735B, where gabbroic rocks were successfully drilled with high recovery. We anticipated that, once a hole was initiated, drilling would proceed smoothly and that hole conditions would be stable. We did not anticipate the degree to which the intra-rift ridge would be brittlely fractured and the difficulties that this would present for drilling. In addition, the predrilling survey with the JOIDES Resolution showed that the intra rift ridge was covered with a more continuous and thick sediment cover than expected and that in many locations the sediments were underlain by talus. Both of these factors posed significant challenges for drilling. This was primarily a consequence of the fact that the submersible survey observations on which the sites were selected were not made with the intent of selecting drilling targets. A more appropriate submersible survey may not have prepared us for the degree of fracturing but would have provided information about sediment thickness and the slope of outcrops, which would have modified our operational plans.
Six pilot holes were drilled to test the drillability and determine the rock types. These holes provided important information concerning the neotectonics of Hess Deep, as well as the representativeness of the hole we committed for deep drilling. At Hole 894G, we deployed a hard-rock guide base and despite guide base and hole instabilities, a 154-m-deep hole was drilled. Drilling problems resulted from: the guide base slipping due to placement on a steep sedimented slope; considerable hole instability due to brittlely fractured rocks cemented with soft minerals; difficulty in deploying casing; and the decision not to cement, which caused circulation problems. We decided not to cement so that the HRB could be moved if we lost the hole.
At 894C, the first attempt to deploy a guide base failed because we were unable to accurately place the guide base on the basis of the X-Y map and we were unable to look around the site to confirm the location with the camera before we started drilling. The accuracy of the X-Y map was insufficient for deploying the guide base on a very small target area. The result was that the HRB flipped over during drilling due to slope failure after misplacement on a talus slope. On the basis of our pre-cruise meeting, we were led to believe that it would be possible to confirm the site location prior to drilling. We were surprised to learn that this would not be possible as the HRB was being lowered to the seafloor. This experience demonstrates the importance of continuous communication between the co-chiefs, staff scientist, and engineers. The operational process was modified before deployment of the second HRB so that the site could be marked and viewed prior to setting down the guide base.
At Site 895, our strategy was to drill a series of unsupported holes to bit destruction as we no longer had a HRB to deploy. We drilled seven holes along a N S transect. Although drilling conditions were quite variable, holes became unstable within 30-90 mbsf. Drilling at several holes was terminated due to the loss of the BHA rather than hole conditions. Although on-bottom observations suggested that we had selected outcrop to drill, paleomagnetic data from most holes indicated that we had drilled large blocks that may have contributed to hole problems. This array of holes, however, emphasized the feasibility of unsupported spud in to drill holes with a very high scientific return.
To Scientists' Perception of Leg 153 Operations
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