Controls on uranium and thorium behaviour in ocean-floor hydrothermal systems: Examples from the Pindos ophiolite, Greece

A suite of hydrothermally altered basaltic rocks from the Jurassic Pindos ophiolite of Greece was analysed for its U and Th content. Samples taken from different levels within the ophiolite display variable degrees of hydrothermal alteration, ranging from low water/rock ratio recharge zone alteration to high water/rock ratio discharge zone alteration. Most samples analysed exhibit constant U/Th ratios, regardless of the degree of alteration. However, neither of these two elements correlate with any other immobile element (Zr, Y, Ti, REE). These results suggest that both U and Th have been mobile during high-temperature ocean-floor hydrothermal alteration, and indicate a common, probably mineralogical, control for both elements. The chemical composition of hydrothermal fluids from modern tectonic settings equivalent to that of the Pindos ophiolite (East Pacific Rise, Guaymas Basin) was used to predict theoretical aqueous speciation and solubility of U and Th during water-rock interaction in these environments. The results of this treatment agree with the above conclusions drawn from rock studies, and indicate that oxide mineral phases (uraninite, thorianite) control U and Th solubility during hydrothermal alteration at oceanic spreading environments.