Sr-He-O isotopes in mafic phenocrysts from products of the Neapolitan volcanoes (southern Italy): constraints on the geochemical features of the mantle sources

An investigation through Sr-He-O isotopes and major oxides was carried out on Mg-rich olivine and clinopyroxene phenocrysts from a selection of volcanic rocks of the Neapolitan Volcanoes (southern Italy), which include Somma- Vesuvius, Phlegraean Fields, and the islands of Ischia and Procida. The data produced in the present work extend the Sr-He-O dataset available for the Neapolitan volcanic districts (e.g., Graham et al., 1993; Martelli et al., 2004; Di Renzo et al., 2011; D’Antonio et al., 2013; Iovine et al., 2017), improving the knowledge especially on helium isotopic compositions of the Mt. Somma rocks, never analysed before. 3He/4He ratios (2.60-5.12 R/RA) of the investigated Mgrich phenocrysts are markedly lower than those measured at most volcanic arcs worldwide, and are among the lowest measured in subduction zones. This range of 3He/4He is coupled with quite large ranges of 87Sr/86Sr (0.704882 – 0.708480) and δ18O (4.85 – 6.70‰) measured in the same mafic minerals. The 3He/4He values, and the lowest measured Sr- and O-isotope ratios, do not suggest significant crustal or fluid contamination occurred during magmatic evolution, and they should reflect the composition of the metasomatized mantle beneath the Neapolitan region. Moreover, these values display a clear geographic trend, with radiogenic 4He-87Sr, and 18O generally increasing in on-lands districts. The results of these investigations are expected to allow us to better understand the original characteristics of the mantle source(s), and to define those of other components, likely originated in the continental crust, that were involved in the genesis of the primitive magmas feeding the Neapolitan volcanoes. D’Antonio, M., Tonarini, S., Arienzo, I., Civetta, L., Dallai, L., Moretti, R., Trecalli, A. (2013): Mantle and crustal processes in the magmatism of the Campania region: inferences from mineralogy, geochemistry, and Sr–Nd–O isotopes of young hybrid volcanics of the Ischia island (South Italy). Contrib. Mineral. Petrol., 165, 1173-1194. Di Renzo, V., Arienzo, I., Civetta, L., D'Antonio, M., Tonarini, S., Di Vito, M.A., Orsi, G. (2011): The magmatic feeding system of the Campi Flegrei caldera: architecture and temporal evolution. Chem. Geol., 281, 227-241. Graham, D.W., Allard, P., Kilburn, C.R.J., Spera, F.J., Lupton, J.E. (1993): Helium isotopes in some historical lavas from Mount Vesuvius. J. Volcanol. Geotherm. Res., 58, 359-366. Iovine, R.S., Mazzeo, F.C., Arienzo, I., D’Antonio, M., Wörner, G., Civetta, L., Orsi, G. (2017): Source and magmatic evolution inferred from geochemical and Sr-O-isotope data on hybrid lavas of Arso, the last eruption at Ischia island (Italy; 1302AD). J. Volcanol. Geotherm. Res., 331, 1-15. Martelli, M., Nuccio, P.M., Stuart, F.M., Burgess, R., Ellam, R.M., Italiano, F. (2004): Helium-strontium isotope constraints on mantle evolution beneath the Roman Comagmatic Province, Italy. Earth Planet. Sci. Letters, 224, 295-308.