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Expedition 309 is the second scientific ocean drilling cruise in a multiphase mission to Site 1256 to recover, for the first time, a complete section of the upper oceanic crust from extrusive lavas down through the dikes and into the uppermost gabbros. Expedition 309, Superfast Spreading Rate Crust 2, successfully deepened Hole 1256D (6.736°N, 91.934°W) by 503 m to a total depth of 1255.1 mbsf (1005.1 msb). At the end of Expedition 309, Hole 1256D had penetrated a total of >800 m of extrusive lavas and entered a region dominated by intrusive rocks. Hole 1256D is now the fourth deepest hole drilled into oceanic basement since the inception of scientific ocean drilling in 1968 and the second deepest penetration into in situ ocean crust behind Hole 504B. Following the completion of a comprehensive wireline logging program, the hole was successfully exited and left clear of equipment with only minor unconsolidated fill at the bottom of the hole. Hole 1256D is in excellent condition and ready for deepening.
The principal achievements of Expedition 309 are as follows:
Hole 1256D was deepened by 503.1 m to a total depth of 1255.1 mbsf or 1005.1 msb, penetrating 810.9 m thick extrusives and 194.1 m into sheeted intrusives, with an average recovery of 36% and penetration rate of 15 m per coring day. At 1255 mbsf, Hole 1256D is tantalizingly close to the predicted minimum estimated depth to the frozen axial magma chambers.
The upper oceanic crust so far drilled in Hole 1256D is subdivided into the lava pond (250350 mbsf), sheet and massive flows (5341004 mbsf), transition zone (10041061 mbsf), and sheeted intrusives (>1060 mbsf). The basalts show evidence of fractionation and replenishment downhole. Trace element concentrations are within one standard deviation of the average EPR MORB, albeit on the relatively trace elementdepleted side.
Hole 1256D is only the second penetration of the transition from low-temperature alteration to high-temperature hydrothermal alteration in a continuous section of oceanic crust, and this occurs at ~1000 mbsf. Prior to Expedition 309, this transition had only been described in Hole 504B. The rocks at Site 1256 are less altered compared to most other basement sites (e.g., Sites 417 and 418 and Holes 504B and 896A), and there is not a steady decrease in the effects of seawater alteration with depth. Instead, alteration is most commonly associated with well-developed steeply dipping vein networks. Although pyrite is abundant in the Expedition 309 cores, stockwork mineralization, such as that present in Hole 504B, has not been penetrated in the transition from extrusive to intrusive rocks or the change from low-temperature to hydrothermal alteration.
Dike intrusion, brecciation, and hydrothermal alteration are intimately associated, and these features become more common downhole below 1000 msb. Fractures and veins are generally steeply dipping in the section of Hole 1256D drilled during Expedition 309, in contrast to both gentle and steep dips in the upper hole drilled during Leg 206. Subvertical dike margins imaged by FMS and UBI in the sheeted intrusives suggest that the steeply inclined (>75°) chilled margins observed in the cores have true dips toward the northeast, consistent with the paleoridge axis orientation and slight tilting toward the ridge axis.
Physical properties show marked changes across the lithologic transition zone. The porosity of massive lavas decreases from 4% to 2% at the base of the transition zone, and P-wave velocities increase from <5.5 to >6 km/s at 1240 mbsf. The average TC in the sheet and massive flows is 1.8 ± 0.2 W/(m·K), but there is a significant increase in TC starting in the transition zone and a distinct steplike increase to 2.1 ± 0.1 W/(m·K) at the top of the sheeted intrusives.
A full sequence of downhole logs, including pre- and postdrilling temperature profiles and multiple triple combo passes and FMS and UBI imaging runs were recorded. Wireline logs confirm that Hole 1256D is in very good condition. Calipers on the triple combo and FMS tool strings indicate hole diameters typically between 11 and 14 inches. However, comparison of the pre- and postdrilling hole caliper of the upper 500 m of basement does indicate enlargement of Hole 1256D due to drilling, with a number of intervals quite strongly eroded.
Expedition 309 (JulyAugust 2005) will be followed closely by Expedition 312 (NovemberDecember 2005). Despite our grueling pace of advance (15 m/day), progress with deepening Hole 1256D was steady. Optimistically anticipating the same benign drilling conditions and good fortune and assured of highly astute rig floor operations, Expedition 312, with 37 days of drilling operations, is set to deepen Hole 1256D by a further 500 m, well beyond the depths where geophysical interpretations predict gabbros to occur.
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