Étude géochimique et pétrogenèse des laves de l'Etna,Sicile, Italie |
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| Authors: | J L Joron M Treuil |
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| Institution: | 1. Groupe des Sciences de la Terre, Lab. Pierre Süe, CEN Saclay, 91191, Gif-sur-Yvette, France
2. Lab. de Géochimie Comparée et Systématique, Université P. et M. Curie, 75005, Paris, France
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| Abstract: | An extensive set of samples representative of the evolution of Etna since the beginning of its activity in the Quaternary has been analyzed for trace element contents by neutron activation on whole rock samples. Major element data were taken fromTanguy (1980). Several petrogenetic processes have played a role in the observed chemical variations: binary trace element plots form linear arrays extrapolating to the origin for most second row and higher transition elements and the rare alkali metals thus indicating their hygromagmaphile (HYG) character; 3d transition elements show two types of behavior when plotted against an HYG element. Concentrations remain relatively constant for the less evolved rocks in the range from tholeiite through transitional basalts to alkali basalts. On the contrary, concentrations drop rapidly in the series evolving from alkali basalts to trachytes; in addition to the latter two general trends, geochemical regularities specific to Etnean lavas are born out. Rare alkali metals, as well as potassium, for instance, show a trend of abnormal enrichment in the most recent historical eruptions. The recurrence in time of rather primary magmas indicates that the petrogenetic evolution of the lavas cannot be explained by the differentiation of a single batch of mantle derived magma. However, the chemical composition of the mantle source has remained relatively homogeneous with a nature very near the chondritic one. Mineralogically, the source must differ from that for oceanic basalts to account for Th/Ta differences. Early tholeiitic primary magma evolving into later alkali basalts is adequately modeled by variations of the degree of partial melting of a closed system source. Highly evolved products have episodically been emitted as a result of extensive crystal fractionation, during which no basalts are erupted. These episodes are abruptly interrupted by the eruption of basalts presumably indicating recharge of the magma plumbing system by pristine mantle derived material. Selective contamination in alkali elements from the sedimentary basement is also indicated by the data. The presence of partially digested sedimentary xenoliths is an additional argument for this process. |
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