(Table 1) K, Rb, Cs, Sr, and 87Sr/86Sr in vein materials from DSDP Hole 72-516F

A well-constrained Rb/Sr isochron of secondary celadonites and carbonate from DSDP Hole 516F yields an age of 70.1 ± 1.6 Ma. This age is younger than the age of crust formation (about 82 Ma) and may provide evidence for a shortlived hydrothermal event on the Rio Grande Rise caused by a phase of off-ridge volcanism. The initial 87Sr/86Sr ratios of the celandonites and carbonate are lower than the contemporaneous seawater and indicate that the dissolved load of the hydrous solutions contains approximately 2% of basaltic component. K, Rb, and Cs contents in Hole 516F celadonites are similar to corresponding phases from DSDP Hole 417A, indicating that the upper crust at this site may represent a significant sink for seawater alkalies. The alkali ratios K/Rb and K/Cs of Hole 516F celadonites are also similar to the ratios in celadonites from 108-Ma-old crust at DSDP Holes 417 and 418. This correspondence shows that the K/Rb and K/Cs ratios in seawater remained essentially constant during the Cretaceous, from 108 to 70 Ma.