Chemistry of Hot Waters Sampled from Basaltic Basement in Hole 504B, Deep Sea Drilling Project Leg 83, Costa Rica Rift

Abstract

Seawater altered by reaction with basaltic basement was sampled from DSDP Hole 504B, penetrating into 5.9 Myr old crust on the southern flank of the Costa Rica Rift (East Pacific). Both 80° and 115° waters from the drillhole have the chlorinity of seawater but relative to seawater have lost Mg, Na, K, sulfate and O2 and have gained Ca, alky., Si, NH3, and H2S. The loss of sulfate is due to anhydrite pptn. The 87Sr/86Sr ratio has decreased to 0.7086 for the 80° water and 0.7078 for the 115° water. Based on chem. trends relative to seawater, the compositional changes in the 80 and 115° waters do not complement the changes inferred for the altered rocks at Hole 504B. This suggests that the bulk compn. of the altered rocks, like their mineralogy, is largely unrelated to the present thermal and alteration regimes in the hole. During its 2 yr residence in the drillhole, surface seawater could have reacted with the wall rocks and undergone exchange with interstitial formation waters by diffusion and possibly convection. Calcns. suggest that diffusion alone could have exchanged the surface seawater for interstitial water. However, the δ18O values of the water samples are indistinguishable from seawater and Δ14C of the 80° sample is similar to that of ocean bottom water. [on SciFinder(R)]