The 3He, 4He, Ne, and Ar in sedimentary pore fluids, extracted in situ, were measured on samples from Deep Sea Drilling Project (DSDP) sites 398, 410, 419, 420, 424, 436, and 438. Earlier results from DSDP leg 15 are also discussed. Where regional sediment cover prevents direct penetration of seawater into basement rocks at sites 149, 436, and 438, He concentration profiles indicate slow vertical and/or horizontal advection of sedimentary pore fluids, suggesting that fluids can ventilate through the sediment cover either in diffuse, bulk flow or in “aquifers” confined to regions of relatively high permeability such as ash beds, sand/silt layers, or fracture channels in lithified sediments. At site 410, where basement outcrops are prevalent, no advection was detectable in the sediments; however, the basement was well flushed by seawater. On <2 m.y. crust near the East Pacific Rise and the Galapagos Spreading Center, the collection of only one sample per hole precluded Calculation of sedimentary fluid velocities. However, most juvenile helium is degassed from the crust in <6 × 105 years, except for the deep crust or upper mantle, where juvenile helium may be released locally to fluids advecting in deep fissures or faults. Fluid residence times in upper basement rocks increased from ∼5 × 103 years in 10 m.y. crust open to direct seawater penetration to ∼105–106 years in Cretaceous basement sealed by hundreds of meters of sediment, to >1–5 × 108 years (apparent age) in sediments on the active continental margin off NE Japan.
