Rubidium,strontium content and strontium isotopic composition of strongly alkalic basaltic rocks from the Cape Verde islands |
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| Authors: | Jean Klerkx Sarah Deutsch Paul De Paepe |
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| Institution: | (1) Laboratoire de Géologie, Pétrologie, Géochimie, Université de Liège, Belgium;(2) Laboratoire de Minéralogie et Pétrologie, Université Libre de Bruxelles, Belgium;(3) Laboratorium voor Aardkunde, Rijksuniversiteit Gent, Belgium |
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| Abstract: | The Cape Verde islands are characterized by the presence of very strongly alkalic lavas. Cenozoic volcanics—covering the broadest compositional range present in the archipelago—and ranging from alkali-basalts to phonolites, associated with plutonic essexites and nepheline syenites, were analyzed for Sr isotopic compositions and concentrations in K, Rb and Sr. The close values of the Sr87/Sr86 ratios (ranging from 0.7029 to 0.7033) indicate a comagmatic origin for the different rock types; no correlation appears between the Sr isotopic composition and the K-content of the lavas, thus indicating that the lavas with high K2O/K2O + Na2O ratio are generated from a primary magma by differentiation at shallow depths. The values of the Sr isotopic composition are distinctly lower than most values obtained for lavas of other oceanic islands. The origin of the magma type is discussed on the basis of these isotopic compositions and the K/Rb and Rb/Sr ratios: it is suggested that the primary magma has a nephelinitic composition and was formed by partial melting of a small fraction of undepleted mantle peridotite, containing phlogopite; the deeper part of the mantle where this nephelinitic magma generates would have a strontium isotopic ratio of about 0.703 and a Rb/Sr ratio lower than that of the upper part. |
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