Crustal contamination versus enriched mantle: 143Nd/144Nd and 87Sr/86Sr evidence from the Italian volcanics

Abstract

143Nd/144Nd, 87Sr/86Sr, and REE analyses are presented on a wide variety of Pliocene-Recent volcanic rocks from central Italy. 143Nd/144Nd varies from 0.51214–0.51289 and 87Sr/86Sr from 0.7255-0.7036; while the rare earth elements are characterised by light RE enrichment and a significant negative Eu anomaly. These Italian volcanics are tentatively subdivided into three zones: (1) N. Tuscany where the magmas are believed to reflect crustal anatexis. (2) A central zone in which hybrid (crust/ mantle) rocks have been recognised. (3) A southern zone, south of Rome, where mantle-derived magmas are identified which have been largely unaffected by interaction with continental crust. At Roccamonfina, in zone 3, Rb/Sr and Sm/Nd pseudo isochrons are observed but since the calculated ages are 0.5 and 2.0 b.y. respectively it is argued that a simple isochron model is not applicable and that the data are most easily explained by a recent mixing event within the upper mantle. It is envisaged that this occurred during metasomatism of the upper mantle source region by a fluid that had high 87Sr/86Sr and low 143Nd/144Nd and was enriched in K, Rb, and LREE's but relatively depleted in Sr2+ and Eu2+.