ASSESSING SEAWATER BASALT EXCHANGE OF STRONTIUM ISOTOPES IN HYDROTHERMAL PROCESSES ON THE FLANKS OF MIDOCEAN RIDGES

We have measured the Sr-87/Sr-86 of discharging geothermal waters from three ridge flank hydrothermal systems: (1) the Galapagos Mounds Hydrothermal Field, on 0.7 Myr old crust; (2) a region with crustal age of 4.6 Myr on the west flank of the East Pacific Rise at 19-degrees S; and (3) an area having a crustal age of approximately 3 Myr in the Mariana Trough, a back arc spreading center. Half spreading rates at our study sites range from 1 to 8 cm/yr, and our sites include both normal spreading centers and a back arc basin. Our sites thus span the range of ridge flank tectonic environments. At each of our study sites, the hydrothermal nature of our study samples is confirmed by supporting heat flow and chemical evidence. Our results show that the Sr-87/Sr-86 of these ridge flank hydrothermal solutions is virtually identical to that of modern seawater, indicating that Sr isotope exchange on the ridge flanks occurs too slowly to be quantitatively important in the oceanic mass balance of the Sr isotopes.