Initial Reports of the Deep Sea Drilling Project - Volume 83

This volume covers Leg 83 of the cruises of the Drilling Vessel Glomar Challenger. During Leg 83 of the Deep Sea Drilling Project (DSDP), the Glomar Challenger devoted nearly two months to the most successful basement hole in the oceanic crust, Hole 504B. Located in 5.9 m.y. old crust about 200 km south of the spreading axis of the Costa Rica Rift, Hole 504B had been cored to a depth of 836 m beneath the seafloor during Legs 69 and 70 in late 1979. Thus, Leg 83 scientists approached Hole 504B with the unique opportunity to pioneer in the direct sampling of and experimentation within the deeper oceanic basement. In a broad sense, the main reason for the return to Hole 504B on Leg 83 was to extend our knowledge of the nature of the oceanic crust to the zone underlying the interlayered basaltic pillow-lava flow formation previously encountered in every basement hole cored by DSDP. Hole 504B was doubly interesting, in that it was cored in relatively young crust, capped by a thick sediment layer (274.5 m). Thus it provided the opportunity to study the in situ state of young oceanic crust, with particular focus on the petrological, geochemical, and physical signature of hydrothermal circulation in the porous, igneous basement near a spreading center. Leg 83 was specifically planned to deepen Hole 504B to sample and conduct geophysical experiments in the lower levels of Layer 2. Specific objectives of the studies of recovered samples included: 1. Determination of the lithostratigraphy and structure of the lower portion of Layer 2. 2. Determination of the geochemical nature of the recovered basalts. 3. Study of the alteration mineralogy and geochemistry and its variation with depth. 4. Measurement of various physical properties of recovered basalts. Leg 83 commenced on 14 November 1981 at Balboa Harbor, Panama, and terminated on 5 January 1982, again at Balboa Harbor.