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

Basalts from Sites 417 and 418 are relatively depleted in light REE compared to a chondritic REE distribution. 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. By studying closely associated glass-palagonite-crystalline rock samples, we can identify two types of alteration processes: (1) palagonite formation which causes loss of REE elements, without changing their distribution, and decreasing K/Rb and K/Cs with increasing K content; and (2) glass-sea water exchange which is apparently a diffusioncontrolled process, causing increasing K/Rb and K/Cs with increasing K content but not affecting REE.