LEG 148

Hole 504B - Costa Rica Rift

Hole 504B, the deepest hole ever drilled into the oceanic crust, constitutes a reference hole for the structure and composition of "normal" oceanic crust and provides our best opportunity for sampling the sheeted dike/underlying gabbros transition. In the seven earlier DSDP and ODP programs, 275 m of sediment, 900 m of volcanic lavas, and nearly 1 km of sheeted dikes had been penetrated.

During Leg 148, Hole 504B was deepened 111 m to a total depth of 2111 mbsf, penetrating the seismic transition to layer 3, the deepest layer of crust. Actual penetration into the gabbros and examination of the relationships between lithologic and seismic transitions were prevented when the drill bit became stuck in a zone of soft material interpreted to be fault gouge. The recovered fine- to medium-grained massive diabases are a continuation of the sheeted dike complex drilled on earlier legs. The dikes, moderately evolved MORB with incompatible element ratios comparable to MORB but depleted in incompatible elements, may be primitive or the result of multistage melting of a normal MORB mantle source followed by moderate extents of crystal fractionation. Sonic velocities increase steadily to values typical of layer 3 at the lowermost depths, implying that the transition to seismic layer 3 may begin within the sheeted dikes and not correspond directly to the appearance of gabbros. Depth-related trends in alteration mineralogy indicate higher temperatures of hydrothermal alteration in the lowermost 500 m. The continued downward depletion of Zn below 1600 mbsf is similar to that observed near the base of sheeted dikes in ophiolites and suggests that the base of Hole 504B lies near or within the transition to underlying gabbros. In ophiolites, listric normal faults that flatten at depth form a detachment surface separating the sheeted dikes from the underlying gabbros; the faults encountered at Hole 504B may be deeper expressions of the normal faults bounding the tilted basement blocks, a further indication that gabbros may lie just beneath the hole-terminating fault.

Similar core was recovered from Hole 896A, located 1 km to the southeast, although pillows are less common, breccias, massive units, and veins of secondary minerals are more common, and the basement is characterized by oxidation zones around fractures, evidence of interaction with seawater. The textures of many secondary veins and some breccias indicate formation by the crack- seal mechanism, implying periods of relatively high pore pressures which cause hydraulic fracturing. Although still permeable, Hole 896A is more extensively sealed than Hole 504B as a result of higher heat flow and upwelling basement ground waters that precipitated secondary minerals and sealed cracks.