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Preliminary assessments

Preliminary operational assessment of Expedition 309

The drilling objective of Expedition 309 was to deepen Hole 1256D as much as possible during the operating time allowed. The target depth of 1350 mbsf was based on a ROP of 1.5 m/h. This was to be accomplished while leaving the hole free of junk so that it could be reentered again during Expedition 312. Hole 1256D was reentered for the first time on 16 July 2005. After an initial water sample was taken with the WSTP, logging runs with the triple combo and FMS-sonic tool strings were completed to determine the condition of the hole. During Expedition 309, the hole was cored to a depth of 1255.1 mbsf, a total of 503.1 m penetration, using nine C-9 coring bits. There was an overall recovery rate of 36.3%, with the recovery for the final bit run at 73.2%. On two occasions while coring, drops in pump pressure were noticed by the Transocean crew and the drill string was pulled out of the hole. On both occasions, cracks were found, once in the bit sub and once in the 5 inch drill pipe. Quick action and good decisions by the crew averted junking the hole on both occasions. After coring, the hole was again logged using the triple combo, FMS-sonic, and UBI tool strings. An attempt to do a seismic study using Schlumberger’s WST tool was not successful because the tool could not be lowered to the bottom of the hole. Operations ended in Hole 1256D on 25 August at 1300 h.

Preliminary operational assessment of Expedition 312

The primary coring objective for Expedition 312 was to extend Hole 1256D as deeply as possible during the allocated operational time. The target depth of 1740 mbsf was based upon an average ROP assumption of 1.0 m/h and an average bit life of 50 rotating hours. An important criterion for success was to leave the hole free of metal debris at the end of the expedition so that future missions could extend the depth of the hole.

Hole 1256D was deepened 252.0 m at an average ROP of 0.8 m/h for all bits. Average recovery for the cored interval was 18.5%, which is less than the two previous cruises to this hole (Leg 206 = 47.8% and Expedition 309 = 36.3%). To put this into perspective, the average recovery for the last ODP cruise (Leg 148) to Hole 504B was 8.6% (Alt, Kinoshita, Stokking, et al., 1993).

The average bit life during the expedition was 44.3 h. If the C-7 bit that suffered a premature demise is discounted, the average bit life for the six remaining coring bits is 48.1 rotating hours. Note that the first coring bit was used in an unsuccessful attempt to wash to the bottom of the hole and does not enter into the calculations.

The hole was deepened to a total depth of 1507.1 mbsf and was exited without hardware in the bottom of the hole. A total of 9.5 operating days were lost in hole maintenance activities: 5.2 days were expended cleaning out the hole at the beginning of the expedition and another 4.3 days were consumed with fishing and milling cones from the bottom of the hole following bit failure. The 9.5 days are exclusive of the additional time that was spent washing down to the bottom of the hole following all reentries subsequent to each coring bit trip.

The hole was successfully logged with the triple combo, VSI, FMS-sonic, and UBI tool strings. An additional logging suite composed of the TAP, DLL, and Scintillation Gamma Ray Tool (SGT) instruments obtained a temperature profile of the hole prior to departure for Balboa.

Preliminary scientific assessment of Expedition 309

The primary operational objective of Expedition 309 was to drill Hole 1256D as deeply and as cleanly as possible to attain the first continuous sampling of the uppermost ocean crust. Despite >30 y of scientific ocean drilling, this fundamental objective remains an unattained ambition. Such a section will provide hitherto unavailable knowledge about the geological, geochemical, and geophysical structure of the oceanic crust and the processes responsible for its accretion and evolution. This drilling campaign will confirm the nature of axial low-velocity zones, thought to be high-level magma chambers, as well as establish the relationships between such magma chambers and the overlying dikes and eruptive lavas. It will provide critical samples to understand the interactions between axial and ridge flank magmatic, hydrothermal, and tectonic processes and ground-truth regional seismic and magnetic measurements.

As the critical middle leg of this combined mission, Expedition 309 was highly successful in all respects. Hole 1256D was deepened to a total depth of 1255 mbsf (1005 msb) and, following a comprehensive program of wireline logging, was exited cleanly. Hole 1256D was left in good condition, clear of junk and ready for deepening during Expedition 312. At the end of Expedition 309, the bottom of Hole 1256D was in a region of sheeted intrusives (below 1061 mbsf), after sampling ~754 m of eruptive lavas and a ~57 m thick lithologic transition zone. Reconsideration of cores recovered during Leg 206 identified two lava subdivisions that appear to have been erupted on the flanks of the ridge axis with a ~100 m thick massive ponded lava overlying ~184 m of lava flows with rare inflation textures that require eruption onto a subhorizontal surface. This total thickness of ~284 m of off-axis lavas is very close to our preferred estimate (~300 m; see Table T2) for the lavas that buried the axial magma chamber on the ridge flanks and agrees well with geophysical interpretations (e.g., Hooft et al., 1996; Carbotte et al., 1997a). Accounting for this thickness of off-axis lavas and 250 m of sediments, our best estimate of the depth where gabbros will occur, at the end of Expedition 309, was between 1275 and 1550 mbsf (Table T2). At a total depth of 1255 mbsf, Hole 1256D was nearing a depth where gabbros are predicted to occur if our precruise predictions remain valid. It was thought that gabbros are certainly within range of drilling during Expedition 312, assuming progress similar to Expedition 309. A relatively thick extrusive sequence (~470 m of on-axis lavas; the sheet and massive flows) (Table T4) and thin sheeted dike complex with a predicted thickness of between 215 and 490 m (from Expedition 309 drilling combined with the estimated depths to gabbro) is in agreement with theoretical models of the accretion of fast spreading rate ocean crust (Phipps Morgan and Chen, 1993; Wilson, Teagle, Acton, et al., 2003).

Core recovery during Expedition 309 was 36%, although the final bit run sampled 40 m of massive basalts at a recovery rate of 73%. The overall recovery rate of 36% is less than that achieved in the upper portion of Hole 1256D drilled during Leg 206 (48%), but that figure is skewed by very high rates of recovery in the ponded lava flow (93%; 250–350 mbsf); recovery of lavas beneath this unit (39%) was similar to that of Expedition 309. These recovery rates are far superior to those achieved in Hole 504B, with average core recovery of ~30% in volcanic rocks and a miserly 14% from the dikes. Poor core recovery of hard, fractured formations such as MORB continues to be a major operational obstacle to scientific progress by ocean drilling. Many critical questions require high-recovery continuous cores such as can be obtained on land. Presently, the integration of incomplete core with wireline logs remains extremely difficult and time consuming.

As expected for crust formed at a fast spreading rate (>80 mm/y), sheet and massive flows are the dominant extrusive rocks drilled during both Leg 206 and Expedition 309. However, deeper in the drilled section the exact nature of the sheeted dikes may be open to debate. Subvertical chilled margins are common from ~1061 mbsf (Fig. F15), and wireline acoustic and electric images indicate numerous steeply dipping fractures suggestive of dike margins in this zone. Our preferred interpretation is that the lower part of Hole 1256D (below 1061 m) has entered a sheeted dike complex. However, because of only partial recovery of core inherent to upper crustal ocean drilling (~36%), there is the possibility that some of the massive basalts sampled from this zone could be subvolcanic sills crosscut by thin dikes. The absence of recovered subhorizontal chilled contacts weighs against the presence of sills, but the possibility exists that such contacts were preferentially lost because of low core recovery. Further close inspection of wireline images postcruise should validate our interpretation that these rocks are sheeted dikes.

The intimate association of brecciation, dike intrusion, hydrothermal alteration, and mineralization becomes increasingly common below ~1000 mbsf and is a new observation. In these cores, there is a clear linkage between the intrusion of magmas and the penecontemporaneous incursion of mineralizing fluids during dike injection at a magmatically robust spreading ridge, as has been suggested from recent seismic anisotropy experiments undertaken at 9°N on the EPR (Tong et al., 2004). Together with observations from Expedition 312, these cores will enable significant progress toward understanding the interdependency of magmatic and hydrothermal processes in crust formed at fast spreading rates.

Establishment of the contribution of different layers of the oceanic crust to marine magnetic anomalies is a primary objective of Expedition 309/312. Unfortunately, all cores recovered to date from Hole 1256D suffer from very strong magnetic overprints and measurement of true paleomagnetic vectors and intensities remains extremely difficult. A nonmagnetic BHA (bit and bit sub, for example) may reduce magnetic overprinting during drilling, and that concept should be investigated. Also essential is a functioning, gyroscopically oriented, three-component wireline magnetometer with a temperature endurance (≥100°C) that allows it to be deployed in deep basement drill holes. Such a tool would enable the magnetic properties of the ocean crust to be measured in situ.

The Expedition 309 wireline logging program generally returned good data, although only preliminary results were available onboard ship. Drilling-induced hole enlargement due to the transit of the drill string has led to the erosion of the borehole walls in places, resulting in inferior data for tools that require eccentralization and good contact with the borehole wall (Accelerator Porosity Sonde [APS], Hostile Environment Litho-Density Sonde [HLDS], UBI, and FMS). The WST failed to enter Hole 1256D past the casing, and the VSP experiment could not be conducted. Deployment of this short, light tool should probably not have been risked in this deep basement hole, particularly when superior wireline VSP tools are available.

Preliminary scientific assessment of Expedition 312

The first major objective of Expedition 312 was to test the prediction, from the correlation of spreading rate with decreasing depth to the axial melt lens, that gabbros representing the crystallized melt lens would be encountered at 900–1300 m subbasement (msb) at Site 1256. As the final scheduled expedition of this multiexpedition mission, Expedition 312 successfully achieved this scientific objective. Despite loss of nearly one-third of the scheduled coring time to remedial hole operations (reaming, fishing, and milling), drilling during Expedition 312 penetrated 252.0 m to 1507.1 mbsf (1257.1 msb), successfully achieving the main goal of the Superfast Spreading Crust mission. The hole now extends through the 345.7 m thick sheeted dike complex and 100.5 m into gabbroic rocks. The latter were first encountered at 1406.6 mbsf, near the middle of the depth range predicted from geophysical observations. This confirms the prediction from the inverse correlation of spreading rate with depth to axial melt lenses, in particular for crust generated at an ultrafast spreading rate, significantly greater than any observed on the Earth today. A complete suite of wireline logging, including a VSP, was carried out, and the hole remains clear and open for future drilling deeper into the plutonic foundation of the crust.

Two other major objectives of Expedition 312 were (1) to determine the lithology and structure of the upper oceanic crust for the superfast-spreading end-member and (2) to investigate the interactions between magmatic and alteration processes, including the relationships between extrusive volcanic rocks, feeder sheeted dikes, and underlying gabbroic rocks. Cores recovered from the sheeted dike complex during Expedition 312 include chilled dike margins grading to microcrystalline and fine-grained doleritic material, confirming the sheeted dike lithology. Chilled dike margins were recovered during Expedition 309, but gradations in grain size were not, leading to some question as to whether the common massive basalts were flows, sills, or dikes. Penetration through the sheeted dikes during Expedition 312 reveals the effects of a steep temperature gradient. Generally similar hydrothermal alteration is present in the dikes of Hole 1256D and Hole 504B, indicating similar hydrothermal conditions at the tops and bases of their sheeted dike complexes. However, because the dike section in Hole 1256D is only ~300 m thick, compared to the 1 km of dikes in Hole 504B, the thermal gradient at Site 1256 was much steeper (~50°C/100 m versus 15°C/100 m).

An important new finding in Hole 1256D is the presence of granoblastic textures resulting from contact metamorphism of the lowermost ~60 m of dikes by intrusion of underlying gabbros. This type of metamorphism has been described locally in ophiolites but never before from the seafloor. This process must be important as mid-ocean ridges go through the waxing stage of magmatic–tectonic cycles.

An important scientific question to be addressed was the nature of the melt lens (e.g., does it have the composition of a basaltic liquid or is it a cumulate?). Gabbroic rocks in Hole 1256D are diverse and range from gabbro to oxide gabbro and gabbronorite and include differentiated rocks (trondjhemite and quartz-rich oxide diorite). Bulk compositions of the two gabbroic bodies fall at the primitive end of the range of compositions for the lavas and dikes but are evolved compared to primitive melts in equilibrium with olivine in the mantle. This means that cumulates must form elsewhere, within the lower crust or at the crust/​mantle boundary, and the lower crust cannot form by subsidence of such high-level evolved melt lenses as penetrated in Hole 1256D. Although Expedition 312 penetrated gabbro bodies that would be imaged seismically as melt lenses, only the uppermost 100 m of plutonic rocks was cored. Drilling the underlying few hundred meters of rock would further test whether a crystallized melt lens representing the “gabbro glacier” is present or whether crystallized melt lenses similar to those already drilled exist, underlain by subvertically foliated gabbros, as observed in some ophiolites. This would provide additional and more conclusive tests to models for magma chambers and the formation of lower oceanic crust.

Coring in the dikes was extremely difficult, with generally slow ROP (<1 m/h) and low recovery (~15%). Penetration and recovery rates were higher in the gabbro (~1.5 m/h and ~30%). Although core recovery in the dikes was low, this did not significantly affect our understanding of igneous petrology/​geochemistry or alteration effects. Different dike units could be established based on changes in grain size and chemistry, and alteration effects generally vary over larger distances than individual hand specimens or individual cores. Losses of vein material or of brecciated dike margins may have occurred, however, possibly leading to these being underrepresented in the recovered cores. Study of the wireline logs and core scanning, however, should be able to provide an indication of what material was not recovered. Future drilling in dike rocks may benefit from use of coring bits constructed to penetrate harder rock formations (these are currently available as drill bits but not coring bits). Expedition 312 ran short of mud because of the extensive remedial hole cleaning operations and because it was difficult to clear cuttings from the deep hole. Thus, it is essential to have an ample supply of drilling mud for any future deep drilling expedition.

The final major objective of Expedition 312 was to correlate and calibrate remote geophysical seismic and magnetic imaging of the structure of the crust with basic geological observations. Unfortunately, all cores recovered to date from Hole 1256D suffer from very strong magnetic overprints, and measurement of true paleomagnetic vectors and intensities remains extremely difficult. Shore-based demagnetization analyses should be able to remove the drilling overprint, making it possible to address this problem. A nonmagnetic BHA (bit and bit sub, for example) may reduce magnetic overprinting during drilling. Also essential is a functioning, gyroscopically oriented, three-component wireline magnetometer with a temperature endurance (≥100°C) that allows it to be deployed in deep basement drill holes. Such a tool would enable the magnetic properties of the ocean crust to be measured in situ.

The wireline logging programs generally returned good data, although only preliminary results were available onboard ship and for this report. Results from the VSP experiment and core measurements will provide a basis for correlation of rock properties with the geophysical structure of the upper oceanic crust. Expedition 312 results show that the transition from Layer 2 to Layer 3 at Site 1256 does not correlate with the transition from dikes to gabbro. Because the Layer 2–3 transition occurs deeper than the bottom of Hole 1256D, however, we cannot draw any conclusions about what this seismic transition corresponds to in the crust. Understanding this seismic transition at Site 1256 could be accomplished with further drilling (~100–400 m). Drilling-induced hole enlargement due to the transit of the drill string has led to the erosion of the upper borehole walls in places, resulting in inferior data for tools that require eccentralization and good contact with the borehole wall (APS, HLDS, UBI, VSI, and FMS). The VSI otherwise performed well during the VSP experiment.