Trace element evolution in the Phlegrean Fields (Central Italy): fractional crystallization and selective enrichment

Abstract

Trace element analysis of Phlegrean Fields volcanic products shows that they belong to a homogeneous series whose evolution is due mainly to fractional crystallization. However quantitative modelling of crystal fractionation using measured mineral/groundmass and computed bulk distribution coefficients singles out K, Sb, Cl and F as elements which are selectively enriched in the differentiation process with respect to proven hygromagmaphile (HYG) elements. This “anomalous” enrichment involving elements that are easily transported in a magmatic fluid phase is thought to be due to percolation of such a fluid through a stored magma body. This enrichment is strongest in products resulting from protracted differentiation (trachybasalts-latites). Isotopic data from the literature are reinterpreted in terms of this model and show good agreement. Trace elements concentrations in clinopyroxenes provide evidence that complex differentiation paths were followed to produce latitic magmas. Their origin by mixing of magmas from different parentages is not indicated by the data. However, mixing of batches of the same parentage but of different degree of evolution seems likely. Because HYG elements ratios are not modified in the evolution of the magmas, they record these ratios in the partial melting zone. For example, the Th/Ta ratio in the source areas of the magmas has apparently been quite heterogeneous.