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Preliminary Operational Assessment Accomplished versus Planned Drilling

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 an rate of penetration 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 CC-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 Scientific Assessment

The primary operational objective of Expedition 309 and upcoming Expedition 312 (November 2005–January 2006) is 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 has now been deepened to a total depth of 1255 mbsf (1005 msb) and, following a comprehensive program of wireline logging, was exited cleanly. Hole 1256D is in good condition, clear of junk and ready for deepening during Expedition 312. The bottom of Hole 1256D is now in a region of sheeted intrusives (below 1061 mbsf), having sampled ~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 T1) 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 occur is between 1275 and 1550 mbsf (Table T1). At a total depth of 1255 mbsf, Hole 1256D is nearing a depth where gabbros are predicted to occur if our precruise predictions remain valid. 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 intrusive 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 DSDP/ODP 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 mid-ocean ridge basalts 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 intrusives may be open to debate. Subvertical chilled margins are common from ~1061 mbsf (Fig. F16), 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 due to 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 the forthcoming 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 Expeditions 309 and 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 wireline logging program generally returned good data, although only preliminary results were available onboard ship and for this report. 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, hostile environment lithodensity sonde, UBI, and FMS). The WST failed to enter Hole 1256D past the casing, and the vertical seismic profiling experiment could not be conducted. The deployment of this short, light tool should probably not have been risked in this deep basement hole, particularly when superior wireline vertical seismic profiling tools are available.

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