RELATIONSHIP BETWEEN THE ISOTOPIC COMPOSITION OF STRONTIUM IN NEWLY FORMED CONTINENTAL CLAY-MINERALS AND THEIR SOURCE MATERIAL

The 87Sr 86Sr ratios of recent montmorillonites and kaolinites newly formed in weathering profiles of western and central Africa and of Nosy Bé and La Réunion islands near Madagascar are directly related to the composition and age of the parent rocks or minerals. They may, therefore, be used as a genetic tracer. The 87Sr 86Sr ratios are about 0.704 when these clays crystallize from recent basalts and they are higher than 0.715 when the parent rocks are of sialic composition and old in age. Kaolinites newly formed in situ from feldspars contain small amounts of Sr with abnormally high 87Sr 86Sr ratios: in this study they are higher than 1.094. When these minerals crystallize from biotites, their 87Sr 86Sr ratios are much lower and can be close to the value of the primary Sr trapped in the biotites during their crystallization. On the other hand, the 87Sr 86Sr of continental montmorillonites are less scattered: they range, in this study, between 0.704 and 0.722. These low values, as well as the high adsorption capacities of these minerals in the sedimentary environment, allow the assumption that they frequently have 87Sr 86Sr ratios close to that of marine Sr during sedimentation. Therefore, montmorillonites are able to form homogeneous authigenic minerals by synsedimentary alterations. © 1979.