A comparative study of the Rare Earth Element (REE) distributions within the Lower Cretaceous dolomites and limestones of Central Tunisia

Abstract

ABSTRACTREE distribution, strontium content and oxygen isotopic composition have been studied in the Lower Cretaceous dolomites and their interbedded and/or parental limestones of Central Tunisia. Dolomites, as indicated by previous sedimentologic, geochemical and isotopic study, are different in origin. Environments of dolomite formation include: evaporitic sabkha, deep phreatic, karst and lacustrine. With the exception of the sabkha environment, dolomitizing fluids presumably have been essentially meteoric. The comparison between REE distributions in dolomites and limestones clearly suggests that the general shapes of the REE patterns are preserved during dolomitization. Nevertheless, the total REE amounts are somewhat lowered in dolomites comparatively to parental limestones. Moreover, within the dolomites, the total REE contents are positively correlated to the Sr and δ18O contents; thus indicating that the decrease in REE, Sr, and δ18O contents is related to a decrease in the salinity of dolomitizing fluids. In addition, dolomites associated with emergence surfaces (karst) are the most impoverished in REE. Furthermore, they are relatively enriched in light REE, indicating a more or less important fractionation with the REE distributions. This enrichment has been related to the influence of the inorganic complexes which favours the greatest solubility of the heavy REE in basic solution with respect to the lightest members of the REE group.