Abstract
Abstract The rare earth elements (REE) were analysed in the interstitial fluids, in the basalts and in the alteration products (clays and zeolites) of Mururoa atoll volcanic rocks (French Polynesia) in order to investigate REE behaviour during seawater/basalt interactions at low temperature. The REE distribution coefficients between fluids and secondary products were calculated using the EQ3/6 geochemical code. The reversible/irreversible character of the REE entrapment in secondary products was tested. The REE concentrations in the clays are close to that of basalt. The concentrations in zeolites are 10 times lower. Comparison of measured and modeled concentrations in solutions favours the irreversible incorporation assumption. The affinity of the REE for the secondary products is shown to be high. The calculated distribution coefficients range from 3×105 for Lu and Eu to 107 for Ce in clays and from 104 to 7×105 in zeolites. The distribution coefficients exhibit a progressive decrease from La to Lu. This behaviour is related to the greater stability of HREE complexes relative to LREE complexes in solution. Under reducing conditions in the interstitial fluids (Eh down to −500 mV), Eu undergoes reduction to the mobile Eu(II). This enhanced mobility is reflected by Eu distribution coefficient being lower than those of adjacent REE. The distribution coefficient of Ce is higher. As Ce is trivalent in the fluids, fractionation from the other trivalent REE was unexpected. The calculated distribution coefficients of REE are lower in zeolites than in clays and this can be related to the spatial distribution and paragenesis of the secondary phases: the dissolution of the basalt promotes the precipitation of clays at the basalt–fluid interface and of zeolites in the bulk solution. The REE released by dissolution are incorporated into the clays and further uptake occurs in zeolites for the REE left in solution.
Published Version
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