Major, trace element and isotope geochemistry (Sr-Nd-Pb) of interplinian magmas from Mt. Somma-Vesuvius (Southern Italy)

Major, trace element and isotopic (Sr, Nd, Pb) data are reported for repres entative samples of interplinian (Protohistoric, Ancient Historic and Medie val Formations) activity of Mt. Somma-Vesuvius volcano during the last 3500 years. Tephra and lavas exhibit significant major, trace element and isoto pic variations. Integration of these data with those obtained by previous s tudies on the older Somma suites and on the latest activity, allows to bett er trace a complete petrological and geochemical evolution of the Mt. Somma -Vesuvius magmatism. Three main groups of rocks are recognized. A first gro up is older than 12,000 yrs, and includes effusive-explosive activity of Mt . Somma. The second group (8000-2700yrs B.P.) includes the products emitted by the Ottaviano (8000yrs. B.P.) and Avellino (3550yrs B.P.) plinian erupt ions and the interplinian activity associated with the Protohistoric Format ion. Ancient Historic Formation (79-472 A.D.), Medieval Formation (472-1139 A.D.) and Recent interplinian activity (1631-1944 A.D.) belong to the thir d group of activity (79-1944 A.D.). The three groups of rocks display disti nct positive trends of alkalis vs. silica, which become increasingly steepe r with age. In the first group there is an increase in silica and alkalis w ith time. whereas an opposite tendency is observed in the two younger group s. Systematic variations are also evident among the incompatible (Pb, Zr, Hf, Ta, Th, U, Nb, Rb, Cs, Ba) and compatible elements (Sr, Co, Cr). REE docume nt variable degrees of fractionation, with recent activity displaying highe r La/Yb ratios than Medieval and Ancient Historic products with the same de gree of evolution. N-MORB normalized multi-element diagrams for interplinia n rocks show enrichment in Rb, Th, Nb, Zr and Sm (>*10 N-MORB). Sr isotope ratios are variable, with Protohistoric rocks displaying Sr-87/S r-86=0.70711-0.70810, Ancient Historic Sr-97/Sr-116=0.70665-0.70729, and Me dieval Sr-117/Sr-86=0.70685-0.70803. Neodymium isotopic compositions in the interplinian rocks show a tendency to become slightly more radiogenic with age, from the Protohistoric (Nd-143/Nd-144=0.51240-0.51247) to Ancient His toric (Nd-143/Nd-144=0.51245-0.51251). Medieval interplinian activity (Nd-1 43/Nd-144:0.51250-0.51241)lacks meaningful internal trends. All the interpl inian rocks have virtually homogeneous compositions of Pb-207/Pb-204 and Pb -208/Pb-204 in acid-leached residues (Pb-207/Pb-204 similar to 15.633 to 15 .687, (208)pb/Pb-204 similar to 38.947 to 39.181). Values of Pb-206/Pb-204 are very distinctive, however, and discriminate among the three interplinia n cycles of activity (Protohistoric: 18.929-18.971, Ancient Historic: 19.01 8-19.088, Medieval: 18.964-19.053). Compositional trends of major, trace element and isotopic compositions clea rly demonstrate strong temporal variations of the magma types feeding the S omma-Vesuvius activity. These different trends are unlikely to be related o nly to low pressure evolutionary processes, and reveal variations of parent al melt composition. Geochemical data suggest a three component mixing scheme for the interplini an activity. These involve HIMU-type and DMM-type mantle and Calabrian-type lower crust. Interaction between these components has taken place in the s ource; however, additional quantitative constraints must be acquired in ord er to better discriminate between magma characteristics inherited from the sources and those acquired during shallow level evolution.