Alkali and Sr Isotope Geochemistry of Waters Collected from Basaltic Basement, Deep Sea Drilling Project Hole 504B, Costa Rica Rift

Abstract

Basalt formation waters collected from Hole 504B at sub-basement depths of 194,201, 365, and 440 meters show inverse linear relationships between Sr/Sr and Ca, Sr/Sr and Sr, and K and Ca. If the Ca content of a fully reacted formation water end-member is assumed to be 1340 ppm, the K, Sr, and Sr/Sr values for the end-member are 334 ppm, 7.67 ppm, and 0.70836, respectively. With respect to contemporary seawater at Hole 504B, K is depleted by 13%, Sr is enriched by 2.7%, and Sr/Sr is depleted by 0.8%. The Sr/Ca ratio of the formation water (0.0057) is much lower than that of seawater (0.018) but is similar to the submarine hot spring waters from the Galapagos Rift and East Pacific Rise and to geothermal brines from Iceland. At the intermediate temperatures represented by the Hole 504B formation waters (70°-105°C), the interaction between seawater and the ocean crust produces large solution enrichments in Ca, the addition of a significant basalt Sr isotope component accompanied by only a minor elemental Sr component, and the removal from solution of seawater K. The Rb, Cs, and Ba contents of the formation waters appear to be affected by contamination, possibly from drilling muds.