IODP Proceedings    Volume contents     Search

Previous   |    Next

doi:10.2204/iodp.proc.309312.101.2006

Conclusions

Expeditions 309 and 312 were the second and third scientific ocean drilling cruises 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. During Expedition 309, Superfast Spreading Rate Crust 2, Hole 1256D (6.736°N, 91.934°W) was successfully deepened 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. 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.

Expedition 309 (July–August 2005) was followed closely by Expedition 312, Superfast Spreading Rate Crust 3 (November–December 2005). During Expedition 312, Hole 1256D was deepened by 252.0 m to 1507.1 mbsf (1257.1 msb), successfully achieving the main goal of the Superfast Spreading Crust mission, penetration through lavas and dikes into gabbros. 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. 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.

The principal achievements of Expedition 309/312 are as follows (Table T1):

  • Hole 1256D was deepened by 753.3 m to a total depth of 1507.1 mbsf or 1257.1 msb, penetrating 810.9 m thick extrusives, a 345.7 m sheeted dike complex, and 100.5 m into gabbroic rocks. The gabbros were encountered at 1406.6 mbsf, near the middle of the depth range predicted from geophysical observations.
  • The upper oceanic crust drilled in Hole 1256D is subdivided into the lava pond (250–350 mbsf), sheet and massive flows (534–1004 mbsf), transition zone (1004–1061 mbsf), sheeted dikes (1061–1406.6 mbsf), and plutonic section (1406.6–1507.1 mbsf). The basalt lavas and dikes show evidence of variable fractionation and replenishment downhole. Trace element concentrations are within one standard deviation of the average EPR MORB, albeit on the relatively trace element–depleted side. Gabbroic rocks are fine to coarse grained (mostly medium grained), range from gabbro to oxide gabbro and gabbronorite, and include differentiated rocks (trondjhemite and quartz-rich oxide diorite). The base of the section contains a gabbronorite of uncertain origin (intrusive gabbronorite or metamorphosed dike) and is cut by a late dike. 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.
  • 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 lavas 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. The upper dikes (<1255 mbsf) contain greenschist facies minerals, actinolite becomes abundant below ~1300 mbsf, and hornblende and secondary plagioclase are present below ~1350 mbsf, reflecting a steeper thermal gradient in the dikes of Hole 1256D than in Hole 504B. Superimposed on this is recrystallization of the lowermost 50 m of dikes in Hole 1256D as the result of intrusion by underlying gabbros.
  • Dike intrusion, brecciation, and hydrothermal alteration are intimately associated, and these features become more common downhole below 1000 msb. Dips of structures generally become steeper from lavas into the dikes. Subvertical dike margins imaged by FMS and UBI in the sheeted dikes 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 transition from lavas to dikes. The porosity of massive lavas decreases from 4% to 2% across the top of the sheeted dikes, and P-wave velocities increase from <5.5 to >6 km/s at 1240 mbsf. Average thermal conductivity in the sheet and massive flows is 1.8 ± 0.2 W/(m·K), but there is a significant increase in thermal conductivity starting in the transition zone and a distinct steplike increase to 2.1 ± 0.1 W/(m·K) at the top of the sheeted dikes. Physical properties also change downward across the dike/​gabbro contact, exhibiting increased porosity and decreased velocity and density in the uppermost gabbros.
  • 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 10 and 14 inches, with the smaller diameters in the deepest part of the hole (1300–1450 mbsf). However, comparison of the pre- and postdrilling hole caliper of the upper 500 m of basement does indicate local enlargement of the shallower portions of Hole 1256D due to drilling, with a number of intervals quite strongly eroded. Velocities from a VSP experiment run in the hole generally parallel trends in the sonic log and discrete sample measurements.

Top of page   |    Previous   |    Next