GEOCHEMICAL AND ISOTOPIC (ND-PB-SR-O) VARIATIONS BEARING ON THE GENESIS OF VOLCANIC-ROCKS FROM VESUVIUS, ITALY

Alkaline volcanism produced by Monte Somma-Vesuvius volcano includes e xplosive plinian and subplinian activity in addition to effusive lava flows. Pumice, scoria, and lava (150 samples) exhibit major- and trace -element gradients as a function of SiO2 (58.9-47.2 wt%) and MgO (0-7. 8 wt%); Mg# values are <50. Internally gradational chemical groups or cycles are distinguished by age: (1) 25,000 to 14,000 yr B.P.; (2) 800 0 yr B.P. to A.D. 79; and (3) A.D. 79 to 1944. A small number of lavas , dikes and scoria were also analyzed from the Somma Formation (simila r to 35,000 to 25,000 yr E.P.). Within each group, contents of Na2O K2O increase with decreasing MgO along distinct arrays. Nb/Y values ar e variable from 0.66 to 3.14 (at SiO2 < 50 wt%) generally in the range of alkaline and ultra-alkaline rocks. Variations in contents of some major elements (e.g., P and Ti), and trace elements (e.g., Th, Nb, Ta, Zr, Hf, Pb, La, and Sc), as well as contrasting trends in ratios of v arious elements (e.g., Ta/Yb, Hf/U, Th/Ta, Th/Hf, Th/Yb, etc.) are als o generally consistent with the group subdivisions. For example, Th/Hf increases from similar to 5 to > 10 with decreasing age for the Vesuv ius system as a whole, yielding similar compositions. in the least evo lved rocks (low-silica, high-MgO, incompatible element-poor) erupted a t the end of each cycle. Internal variations within individual eruptio ns also systematically changed generally toward a common mafic composi tion at the end of each cycle, thus reflecting the dominant volume in the magma chamber. At the start of a new eruptive cycle, the rocks are relatively enriched in incompatible elements; younger groups also con tain higher abundances than older groups;. N-MORB-normalized multielem ent diagrams exhibit selective enrichments of Sr, K, Rb, Tn, and the l ight rare-earth elements; deep Nb and Ta negative anomalies commonly s een in rocks generated at orogenic margins are absent in our samples. Sr isotopic compositions are known to be variable within some of the u nits, in agreement with our data(Sr-87/Sr-86 similar to 0.70699 to 0.7 0803) and with contributions from several isotopic components. Isotopi c compositions for delta(18)O (7.3 to 10.2 parts per thousand), Pb for mineral separates and whole rocks (Pb-206/Pb-204 similar to 18.947 to 19.178, Pb-207/Pb-204 similar to 15.617 to 15.769, Pb-208/Pb-204 simi lar to 38.915 to 39.435), and Nd (Nd-143/Nd-144 similar to 0.51228 to 0.51251) also show variability. Oxygen isotope data show that pumices have higher delta(18)O values than cogenetic lavas, and that delta(18) O values and SiO2 are correlated. Radiogenic and stable isotope data p lot within range of isotopic compositions for the Roman comagmatic pro vince. Fractional crystallization cannot account for the radiogenic is otopic compositions of the Vesuvius magmas. We favor instead the combi ned effects of heterogeneous magma sources, together with isotopic exc hange near the roof of the magma chamber. We suggest that metasomatize d continental mantle lithosphere is the principal source of the magmas . This kind of enriched mantle was melted and reactivated in an area o f continental extension (incipient rift setting) without direct relian ce on contemporaneous subduction processes but possibly with input fro m mantle sources that resemble those that produce ocean island basalts . (C) 1998 Elsevier Science B.V. All rights reserved.