Chemical variations of the A.D. 79 pumice deposits of Vesuvius

Abstract

The pumice-fall deposits of the A.D. 79 Vesuvius eruption are subdivided into a lower white and an upper grey pumice. Major and trace element variations show significant differences between them. The more evolved phonolitic white pumice exhibits well-developed, stratigraphically controlled chemical gradients in all major and trace elements whereas the tephriphonolitic grey pumice is compositionally less variable and lacks such gradients. Incompatible elements are mostly enriched in the white pumice, whereas the grey pumice shows higher contents of compatible elements such as Ti and Mg. The difference in chemical composition between white and grey pumice is interpreted in terms of fractional crystallization of clinopyroxene, sanidine and leucite. The well-developed gradients of the white pumice indicate a stable layering within the upper phonolitic magma body, which is thought to originate from diffusion processes of highly incompatible elements. The lack of a similar stratigraphy-related zonation within the grey pumice indicates a more homogeneous tephriphonolitic magma resulting from continuous convective mixing within the lower part of the magma chamber. The transition from white to grey pumice is characterized by a boundary zone containing, besides white and grey pumice clasts, a third type of pumice, «the boundary pumice», which is light grey in color, this pumice is internally homogeneous, without evidence of physical mingling between «white» and «grey» glass, and is typically characterized by intermediate MgO, K 2 O and Ba contents. It is interpreted to represent a third magma component in the magma chamber, the interface magma layer, resulting from mixing and diffusion processes across the phonolitic/tephriphonolitic boundary