Chemical composition and isotope ratios of basalts from DSDP Sites 417 and 418, West Atlantic Ocean

In the least altered samples, K, Rb, Cs, and Sr and Ba concentrations are lower than in average mid-ocean ridge basalt (MORB). Thus, the source rock for these basalts must have been highly depleted in incompatible elements.

However, 87/86Sr ratios in these basalts, even in microscopically fresh glass, are high (> 0.7030). These ratios, compared to normal MORB, are largely an effect of low-temperature alteration. Sr isotope exchange with sea water is postulated for basaltic glass (87/86Sr = 0.70301) with low incompatible-element abundances, but we cannot eliminate the possibility that these relatively high 87/86Sr ratios are primary.