Deep crustal geothermal measurements, hole 504B, Costa Rica Rift ( DSDP).

We report an extensive suite of geothermal measurements in the deepest borehole yet drilled into the oceanic crust, hole 504B of the DSDP. Located in 6.2Ma old crust of the Costa Rica Rift, hole 504B was cored during legs 69 and 70 in late 1979 and leg 83 in late 1981, to a total depth of 1350m beneath the seafloor, through 274.5m of sediment and 1075/5m of basalt. During the three drillings legs, downhole temperatures were logged 11 times, and the thermal conductivities of 239 sediment and basalt samples were measured. The results indicate a dominantly conductive mode of heat transfer through the complete section, at 190+ or -10mW m². This is consistent with the predicted plate heat transfer and the hypothesis that the thick sediment cover acts as a seal against hydrothermal circulation of seawater to basement.-Authors