Leg 206 Preliminary Report (Text and T1-T11)

Drilling a complete section of oceanic crust has been an unfulfilled ambition since the inception of scientific ocean drilling. Recovery of in situ oceanic crust is imperative to understand igneous accretion and the complex interplay between magmatic, hydrothermal, and tectonic processes, as well as a means for calibrating remote geophysical observations, particularly seismic and magnetic data. Only by drilling a complete section of upper crust formed away from fracture zones can the processes operating at normal mid-ocean ridges be understood. There is an observed relationship between the depth to axial low-velocity zones imaged at active midocean ridges and spreading rate. Recent recognition of an episode of superfast spreading (200–220 mm/yr) on the East Pacific Rise ~11–20 m.y. ago presents an opportunity to drill through the upper oceanic crust into the gabbroic rocks in minimal time. Even allowing for significant burial by lavas that have flowed off axis (~300 m), the upper gabbros, thought to be the frozen axial melt lens, are predicted to occur at ~1100–1300 meters below seafloor (mbsf). Leg 206 completed the initial phase of a planned two-leg project to drill a complete in situ section of the upper oceanic crust that will eventually extend through the extrusive lavas and sheeted dikes and into gabbros. Drilling was conducted at Ocean Drilling Program (ODP) Site 1256 (6.736°N, 91.934°W), which resides on ~15-Ma oceanic lithosphere of the Cocos plate that was formed by superfast spreading (>200 mm/yr) at the East Pacific Rise. To fully characterize the sedimentary overburden and establish depths for the casing strings, three pilot holes were cored that recovered a nearly complete section of the 250.7 m of sediment overlying basement and penetrated 88.5 m into basement with very good recovery (61.3%). The sediments can be subdivided into two main lithologies. Unit I (0–40.6 mbsf) is clay rich with a few carbonate-rich intervals, whereas Unit II (40.6–250.7 mbsf) is predominantly biogenic carbonate. More than 500 m of young Pacific extrusive lavas was cored with moderate to high rates of recovery, following the installation of a reentry cone with a 16-in diameter casing string that extended 20 m into basement in Hole 1256D. Axial sheet flows with subordinate pillow lavas, hyaloclastites, and rare dikes are capped by a more evolved massive flow >75 m thick and other sheet flows that probably ponded in small faulted depressions several kilometers off axis. The lavas have normal mid-ocean-ridge (N-MORB) chemistries and display moderate fractionation upsection as well as heterogeneous incompatible element ratios. The lavas are only slightly affected by low-temperature hydrothermal alteration, and very little interaction with oxidizing seawater is apparent. The rocks are much less oxidized than those from Holes 504B and 896A in 6.9-Ma crust formed at an intermediate spreading rate and are more akin to the background alteration in Hole 801C (180 Ma), albeit with very little carbonate at Site 1256. The complete lava sequence formed over a sufficient time period to record the transition from a stable shallowly dipping magnetic field in the axial lavas to a more steeply dipping field (inclination > 70°) in the overlying ponded flow. If our interpretation is correct, ~20% of the extrusive sequence cored so far formed from lava flows that flowed significant distances from the axis. A complete suite of geophysical wireline logs, including the first deployment in a basement hole of the Ultrasonic Borehole Imager (UBI), confirmed that Hole 1256D is in excellent condition, with robust margins and within gauge for its complete depth. Formation MicroScanner and UBI imaging will be integrated with other geophysical logs and the recovered core to refine the igneous stratigraphy and structure. Hole 1256D was exited cleanly, leaving the hole clear of debris, open to its full depth, and primed for future deepening into the sheeted dikes and gabbros early in the next phase of ocean drilling. LEG 206 PRELIMINARY REPORT 7