GEOCHEMICAL EVOLUTION OF BASALTIC ROCKS SUBJECTED TO WEATHERING - FATE OF THE MAJOR ELEMENTS, RARE-EARTH ELEMENTS, AND THORIUM

Eleven Icelandic hyaloclastites altered in freshwater have been studied. The weathering of basaltic glass, which is their primary constituents, leads to precipitation of clayey and possibly zeolitic phases. The dissolution reaction progress (mass of dissolved glass per liter of solution) governs the chemistry of the secondary phases, which control the residence time of the solution through their influence on rock permeability. They contribute to the regulation of reaction advancement. The reaction progress, xi, can be calculated according to: xi = [Sr(w).(I(w) - I(SP)).(I(G) + 9.375)]/[Sr(G).(I(SP) - I(G)). (I(W) + 9.375)], where I(W), I(G), and I(SP) are the isotopic strontium ratios of initial water, of pristine glass, and secondary products, respectively, and Sr(W) and Sr(G) the strontium contents of initial water and glass. The amount of dissolved glass per liter of solution is estimated to be in the order of 0.01-0.1 g for samples 90000-100000 years old. A global agreement is found between this result and estimations that can be made on the basis of thermodynamic modelling. Expressing the geochemical budget as a function of the reaction progress allows a look at the evolving aspect of the rock/water interaction: (1) According to the budget calculated for the major elements, the rock undergoes a global loss of matter which decreases with increasing reaction progress (from -45 to about 0% losses). For the most evolved samples, the transformation of the pristine basaltic glass to the alteration products is a nearly conservative process. (2) There is increasing evidence that rare earth elements, particularly LREEs, can be mobilized during alteration processes. Our study corroborates this view as we demonstrate that REEs can be significantly mobilized during basaltic glass weathering. Nevertheless, no fractionation among the suite of REEs was observed. We show that Th and REE behaviours are similar. Mass balance calculations account for Th and REE losses up to 40%. These losses are not related to the reaction progress but could be due to a colloidal or particulate transport. The REE contents of the secondary clayey phases is showed to be linked to their degree of crystallinity. The need to take into account the latter parameter to modelize the long-term behaviour of the elements adsorbed onto clay minerals is outlined.