Significant petrogenetic importance has recently been placed on the Nd isotopic composition and variable rare earth element (REE) content of basalts, although the effect of alteration on the REE has not been fully investigated. REE analyses of oceanic1-6 and ophiolitic basalts7,8 suggest that the light REE (La, Ce, Nd, Sm, Eu) are mobile during basalt-seawater interaction at relatively low temperatures. However, the behaviour of the trace and rare earth elements during greenstone formation at higher temperatures is less well understood9-15, primarily because of uncertainties regarding the composition of the unaltered precursor, temperature, pressure and effective water/rock ratio. In an attempt to overcome these uncertainties we have analysed basalt glass subjected to 10 individual basalt-seawater interaction experiments. In all these experiments the REE are found to be immobile at temperatures of 150-350 °C even when the basalt is totally altered to clay. Furthermore the data suggest that the minor fluctuations observed in the light REE content of some greenstones 9,10 may result from a low temperature overprint due to retrograding of the greenschist facies basalts.