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

Abstract

Eleven Icelandic hyaloclastites altered in freshwater have been studied. The weathering of basaltic glass, which is their primary constituent, 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, ξ, can be calculated according to: , where Iw, IG and ISP are the isotopic strontium ratios of initial water, of pristine glass, and secondary products, respectively, and SrW and SrG 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.