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doi:10.2204/iodp.pr.335.2011 Preliminary assessmentPreliminary scientific assessmentExpedition 335 was the fourth ocean cruise of the Superfast Spreading Crust campaign to drill a deep hole into intact oceanic basement and returned to Hole 1256D to deepen this scientific reference penetration a significant distance into cumulate gabbros. Cores and data recovered from the uppermost lower ocean crust and the transition down to cumulate gabbroic rocks would provide hitherto unavailable observations that will test models of the accretion and evolution of the oceanic crust. Although previous cruises to Hole 1256D (Leg 206 and Expedition 309/312) had achieved the benchmark objective of reaching gabbro in intact ocean crust, critical scientific questions remain. These include the following:
At the end of Expedition 312 in 2005, Hole 1256D was clear to its full depth and poised to be deepened into intervals where samples that should conclusively address these questions can be obtained, possibly with only a few hundred meters of drilling. Unfortunately, operational difficulties in Hole 1256D precluded progress toward the scientific objectives with only <15 m of advance achieved in Hole 1256D, and the hole now has a total depth of 1521.6 mbsf (1271.6 msb). The bottom of the hole remains hosted by the dike–gabbro transition zone, dominated by granoblastic basalt. In addition to the few cores drilled, the junk baskets deployed during the successive fishing runs to the bottom of the hole recovered a unique collection of samples including large cobbles (as large as 5 kg), angular rubble, and fine cuttings of principally strongly to completely recrystallized granoblastic basalt with minor gabbroic rocks and evolved plutonic rocks. The large blocks exhibit structural and textural relationships, metamorphic paragenetic sequences, and overprinting hydrothermal alteration, which hitherto have not been observed because of the narrow diameter of drill cores and the very low recovery of the granoblastic basalts cored so far. Including the ~60 m thick zone of granoblastic dikes that reside above the uppermost gabbros, the dike–gabbro transition zone at Site 1256 is >170 m thick, of which >100 m is recrystallized granoblastic basalt. This zone records a dynamically evolving complex thermal boundary layer between the principally hydrothermal domain of the upper crust and the intrusive magmatic domain of the lower crust. An intimate coupling between temporally and spatially intercalated magmatic, hydrothermal, partial melting, intrusive, metamorphic, and retrograde processes is recorded in the samples recovered, which will be subjected to a comprehensive suite of postcruise petrologic, geochemical, and geophysical studies. Expedition 335 left Hole 1256D after making only a very modest advance, and we have yet to recover samples of cumulate gabbros required to test models of ocean ridge magmatic accretion and the intensity of hydrothermal cooling at depth. However, the remarkable sample suite of granoblastic basalt samples, with minor gabbros intruding previously recrystallized dikes, provides a detailed picture of a rarely sampled critical interval of the oceanic crust. Most importantly, the hole has been stabilized, cleared to its full depth, and is ready for deepening in the near future. The archive and working halves of Cores 312-1256D-202R through 234R (1372.8–1507.1 mbsf) from the lower part of the granoblastic dikes to the bottom of the hole at 1507.1 mbsf, including the plutonic section of Hole 1256D, were shipped to the JOIDES Resolution prior to Expedition 335. These cores were made available to the science party for the purpose of familiarization with the recovered plutonic section of Hole 1256D and to establish and test description protocols during transit to Site 1256. The scientists also developed the DESClogik configuration for the capture of plutonic rock descriptions for the first time since the introduction of the LIMS database. The long duration of operations required to open Hole 1256D to its full depth at the beginning of Expedition 335 (see “Operations”; ~16 days) provided sufficient time for the 335 Shipboard Science Party to completely redescribe these cores. This was considerably more time than was available to the shipboard scientists at the end of Expedition 312. Some discrete measurements (paleomagnetism and physical properties) were also performed. A new measuring protocol using a seawater bath was developed onboard that greatly improved the quality of shipboard P-wave velocity measurements of discrete samples. The velocities of gabbro range from 6298 to 6759 m/s, whereas measurements of granoblastic basalt range from 6610 to 6907 m/s, as much as ~800 m/s higher than measurements done under the standard shipboard protocols during Expedition 312 (that are often significantly lower than shore-based laboratory measurements). These high velocities are more consistent with the trends of downhole geophysical logs above 1400 mbsf and may indicate that the lower section of Hole 1256D is close to the seismic Layer 2/3 boundary. The engineering efforts during Expedition 335 (see “Preliminary operational assessment”) have repaired and prepared Hole 1256D for further deep drilling, following 5 years of neglect. Hole 1256D is 1500 m of hard rock coring closer to cumulate gabbros than any other options in intact ocean crust. The already great thickness of granoblastically recrystallized dikes and the tantalizing presence of minor gabbro bodies strongly suggests that the zone of 100% plutonic rocks must be close, perhaps only a few tens of meters below the present bottom of the hole. Hole 1256D is once more poised to answer fundamental questions about the formation of new crust at fast spreading mid-ocean ridges, best achieved by a timely return to the site. Preliminary operational assessmentThe principal goal of Expedition 335’s return to Hole 1256D was to deepen the hole sufficiently into plutonic rocks (a few hundred meters) to obtain definitive answers to long-standing questions about the structure and composition of the oceanic crust and about mechanisms of crustal accretion (these ideas are developed in greater detail in “Background and geological setting”). Operational planning for Expedition 335 was informed by three principal sources:
The recommendations of the Expedition 309/312 Review Task Force indicate an understanding of the nonstandard requirements of deep basement drilling and echo the suggestions made by the Co-Chief Scientists of those expeditions. Three recommendations pertinent to operations during Expedition 335 were
Unfortunately, there has been only modest progress on these recommendations in the 5 years since the ORTF-309/312 meeting, with a half-length expedition initially scheduled by the Science Advisory Structure for the return to Hole 1256D and an absence of new drilling and coring options for hard formations and hole cleaning. The IODP Science Planning Committee requested in 2006 (SPC Consensus 0603-19) that the USIO identify the operational requirements for further drilling in Hole 1256D. TAMU engineers from the USIO presented their operational plan to an audience of scientists and independent drilling engineers at the Mission Moho Workshop (Portland, Oregon, 7–9 Sept 2006) for technical review. There was consensus support for the plan proposed. The USIO considered four deepening scenarios:
Two key points were noted by the USIO and independent engineers at the Mission Moho Workshop:
Regardless of the casing strategy proposed (2 or 3), attempting to open any portion of Hole 1256D to accommodate casing would require significant new hardware, numerous pipe trips, and extend over several expeditions. Such operations would require unproven technology and would be extremely challenging with substantial risk of irreparable damage to Hole 1256D without any further coring or recovery. The USIO and independent experts felt that approaches 2 and 3 are not viable options and recommended that they were not considered further. The preferred USIO strategy was 1—resuming RCB coring with frequent large-volume high-viscosity mud sweeps (ORTF-309/312-12). During Expedition 309/312, large-volume mud sweeps were effective at clearing cuttings from the hole. However, because of the end of Phase I operations, mud stock depletion precluded the vigorous implementation of this approach throughout the whole of Expedition 312. The refitted JOIDES Resolution should be equipped to handle the necessary supplies to maintain an aggressive mud program. Following the recommendations of the IODP Expedition 309/312 Operations Review Task Force, the JOIDES Resolution was stocked with at least 60 T of sepiolite and attapulgite for use during Expedition 335. Assuming reasonable hole conditions and an average rate of penetration comparable to previous coring in this hole, TAMU engineers estimated that this approach should deepen Hole 1256D ~350 m with 39 days on site (Teagle et al., 2010). The engineering and design lessons learned from a few hundred meters deeper penetration of intact ocean crust would be invaluable in planning and exploiting future deep penetration targets. Further deepening of Hole 1256D using large mud sweeps was the only strategy that begins with coring, was the most likely to return samples from deeper in the hole, and had the largest possibility of building on the tremendous success of previous deepening operations on Expedition 309/312 that employed proven drilling techniques. Before conditioning the hole for further deepening, an attempt would be made to acquire an equilibrium temperature profile and recover a water sample before the thermal structure of the crust is perturbed by cleaning and drilling operations (see next section). We would reenter the hole with a tricone drilling bit on a bit release and slowly descend past the zone around 900 mbsf that caused an obstruction during Expedition 312. If the hole was clear, we would withdraw the pipe, drop the bit, and reenter to below the rat hole to allow logging with the triple combo, including the MTT temperature probe. If the wireline conditions were suitably benign, we would attempt to take a borehole water sample from near the bottom of the hole using the WSTP. OperationsExpedition 335 operations are described in “Operations” and summarized in Table T7. The complete record of drilling in Hole 1256D and other deep basement drill holes will be documented in the Expedition reports section of the Expedition 335 volume. Four key issues with operations in Hole 1256D have been identified:
The bridge at 922 mbsfOn the first reentry, a bridge was encountered at ~922 mbsf that required 16 days of operations and nine reentries, including three cementing runs. Details of our best impression of the blockage are in “Operations” (Fig. F24): an inclined massive formation (e.g., base of massive flow) that squeezed the drilling bits against the borehole wall, stopping effective cutting and clearing of the blockage. Cementing and redrilling the cement above the bridge brought the hole back into gauge, restricted the lateral movement of the bit and BHA, and improved the cutting effectiveness of the tricone bit, eventually clearing the bridge and rapidly displacing lose debris in the underlying 10 to 20 m. Once opened and cemented, this interval did not present a problem for operations. It was stabilized with 65 bbl of 14.5 ppg cement at the end of Expedition 335 operations in Hole 1256D to stop further material falling into the hole or borehole wall inflow closing this interval. In hindsight, a similar operation should have been undertaken during Expedition 312, although this may have precluded reaching the benchmark achievement of coring gabbros in intact ocean crust. Clearing cuttings from a deep uncased hole into oceanic basementHole 1256D is a >1520 m deep borehole with >1250 m in uncased basement strata. Approximately 800 m of the hole penetrates through fragile and unstable volcanic formations of mixed competencies. This has led to large intervals being strongly out of gauge, a situation exacerbated by a total number of 62 reentries and pipe trips up and down the hole. The 16 inch casing string only extends ~17 m into basement, below which is a ~7 m rathole ~23 inches in diameter. The long intervals of uncased hole and the wide rathole topped by a ~280 m section of 16 inch casing greatly reduces pumping efficiencies even when using high-viscosity muds. Expedition 335 operations cleared Hole 1256D of a large amount cuttings, some of which had probably been resident in the hole since drilling during Leg 206. At the end of Expedition 335, the hole was completely clear of cuttings, as evidenced by the near complete absence of soft fill (<1 m; compare with Expedition 312, with >60 m of soft fill) and a lack of mud and suspended cuttings in the reentry cone, indicating that cuttings were being successfully swept from the hole and out onto the surrounding sedimentary blanket. Improving the hydrodynamics of Hole 1256D to enable more efficient hole clearing would greatly improve the quality of coring activities. Drilling and coring very hard formationsContact metamorphosed granoblastic basalts were encountered during Expedition 312 and destroyed a C-7 RCB coring bit after only 21 h of rotation. Similar rocks, although with more completely developed granoblastic textures, were encountered and resulted in the absolute destruction and grinding to a smooth featureless stump of a C-9 hard formation RCB coring bit after a maximum of 29 h (coring evidence suggests the bit failed after only 15 h of rotation). This was the first failure of a C-9 RCB bit in ocean drilling. The fine-grained, annealed texture of the granoblastic basalts makes them extremely hard with few fractures. It appears that if the hole is not absolutely clear, the stresses on these bit are such that even the smallest defect will rapidly result in equipment loss. The Expedition 309/312 Co-Chief Scientists recommended the investigation of finding ultra-hard formation drilling and coring bits. Although this recommendation was discussed by the IODP-MI Review Task Force, no progress has been made in this area. Scientific ocean drilling needs a suite of highly armored (gauge-cutters, backreamers, hard surfacing, and thick armored shirttails) drilling and coring bits for opening holes and coring in these ultra-hard intervals that can be at least 100 m thick. LoggingA complete suite of wireline logs of Hole 1256D would have yielded important scientific and operational results. Only the triple combo logging tool was run, and it managed to get to full depth. Geophysical logs would have possibly provided caliper data to the bottom of the hole (FMS and UBI devices are at the bottom of the wireline tool string), as well as imaging of the distribution, size, and orientation of plutonic intrusions into the granoblastic dikes. Unfortunately, the FMS-sonic was not successfully deployed because it would not exit the logging bit, despite two attempts, because the fitting of new centralizer bowsprings significantly increased the external diameter of the logging tool beyond that of the logging bit sub. This necessitated the severance of the logging wireline using the Kinley crimping and cutting tool and the recovery of the logging tool lodged in the logging bit sub by tripping the pipe back to the rig floor. This precluded further logging operations. Going forward in Hole 1256DDuring the expedition, the Co-Chief Scientists built an effective relationship with the Transocean/Overseas Drilling Limited (ODL) drilling crews, and there was open and informative bilateral exchange of information. Communications with the TAMU Operations Superintendent were excellent throughout. Following the completion of site operations, the Co-Chief Scientists organized a formal meeting for an effective debrief and discussion of issues encountered during Expedition 335 while these were still fresh in everyone’s mind. This meeting was attended by the Co-Chief Scientists (Teagle and Ildefonse), past Co-Chief Scientists and leading proponents (Wilson and Alt), the Expedition Project Manager/Staff Scientist (Peter Blum), the Operations Superintendent (Ron Grout), the offshore installation manager (Sam MacLelland), Core Technicians/Tool Pushers (Wayne Malone/Mark Robinson), and Driller (Craig Prosser). The meeting discussed a wide range of future options (casing, cementing, tools, coring, drilling bits, and time on site) and came up with the following comments and recommendations.
Program considerations for the attainment of deep targets by scientific ocean drillingEstablishing the ideal location for drilling is only part of the challenge of successfully drilling moderately deep holes (2–3 km) to recover samples and data necessary to address long-standing primary goals of scientific ocean drilling. Experience from Holes 504B and 1256D indicates that such experiments require multiple cruises to achieve their target depths. Five hundred meters penetration per expedition is an upper limit for coring in the upper crust, with lesser advances and more frequent drilling challenges as these holes get deeper and are drilled into rocks metamorphosed at higher pressures and temperatures. Penetration and core recovery rates have been low to very low in the two sheeted dike complex sections drilled to date (Holes 504B and 1256D). However, experience to date suggests that gabbroic rocks can be cored relatively rapidly at high rates of recovery (e.g., Hole U1309D = penetration rate 2 m/h and >75% recovery), so when the dike–gabbro transition zone is breached, solid progress through the plutonic section can be anticipated. It is very unlikely that without significant good fortune deep targets in intact ocean crust can be achieved in the current science advisory configuration. The peer-review system that has overseen the progress of both Holes 504B and 1256D has required the reevaluation of new proposals following the successful completion of each drilling increment. A system similar to the “complex drilling proposals” (CDPs) used for riser experiments should be extended to riserless targets that require multiple expeditions to achieve important scientific goals. Such is the capriciousness of hard rock coring that scientific ocean drilling may have to consider new approaches if it is to ever successfully address some of the major science questions that remain unanswered after more than 50 years. There are unlikely to ever be “quick wins” with targets that require multiexpedition deep boreholes. Expedition 335 was initially scheduled by the IODP-MI-OTF as a short cruise (~4 weeks), despite the explicit recommendations of the postexpedition 309/312 Operational Review Task Force “to maximize on-site time for deep drilling expeditions” (Recommendation 309/312-03). Flexibility in expedition scheduling may be a low-impact means to achieve deep objectives. Back-to-back cruises to a single target could be scheduled. This approach was successful at drilling Hole U1309D >1400 mbsf during IODP Expeditions 304 and 305. Commonly, the ship has been moved off a deep hole after the significant investment in engineering and cleaning operations that have succeeded in preparing the hole for deep drilling. For example, Expedition 312 drilled >100 m of the dike–plutonic transition zone in Hole 1256D following significant hole remediation operations but left an open clean deep hole. Five years later, most of the scheduled time during Expedition 335 was spent on hole remediation, and the hole is now cleaner than ever. There is a need for mechanisms for revising expedition schedules so that drilling can continue in deep boreholes when progress is actually being made. This would require the movement of crew, scientists, and supplies to and from the rig so that drilling and hole cleaning can continue, along with the temporary postponement of the immediately following expeditions. Clearly, this would be a major departure from the standard operating style of the JOIDES Resolution within ODP and IODP and a challenge to the science advisory and scheduling structure. It would require community acceptance that could be difficult to achieve. However, the present standard “1 proposal = 1 expedition” approach is not an effective process to reach targets that require multiple-expedition deep drilling. Unless the community and the drilling program are able to develop new approaches to achieving deep targets, the lack of closure on science questions that can only be addressed by deep drilling will continue to stain future renewal documents with a perceived lingering staleness due to a continued recycling of unaccomplished goals. |
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