B/Nb and N 11 B systematics in the Phlegrean Volcanic District, Italy S. Tonarini a; Ã , W.P. Leeman b , L. Civetta c , M. D’Antonio d , G. Ferrara a , A. Necco e a Istituto di Geoscienze e Georisorse, CNR Via Giuseppe Maruzzi 1, 56124 Pisa, Italy b Department of Earth Science, Rice University, Houston, TX, USA c Dipartimento di Scienze Fisiche, Universita ' ‘Federico II’, Napoli, Italy d Dipartimento di Scienze della Terra, Universita ' ‘Federico II’, Napoli, Italy e Dipartimento di Scienze della Terra, La Sapienza, Roma, Italy Received 18 May 2002; accepted 27 January 2003 Abstract Boron concentration and isotopic compositions were determined in representative products from the Phlegrean Volcanic District (PVD), which includes Campi Flegrei (CF), and the islands of Procida and Ischia. The most primitive products (from Procida and Ischia islands) are moderately enriched in B (4.6^12 ppm), whereas more evolved products of CF and Ischia Island are more enriched (21^118 ppm). The content of B is positively correlated with Nb, Zr, Th, La and, generally, with all incompatible elements. N 11 B values are generally lower and more uniform in samples from CF (36.8 to 3 10.6x) compared with those from Procida (33.6 to 38.5x) and Ischia (32.8 to 38.4x) islands. Overall, B-enrichments relative to fluid-immobile elements of PVD are only slightly higher than those observed in mid-ocean ridge basalts and there is no significant correlation between N 11 B and B/immobile element ratios. For CF samples including the Campanian Ignimbrite (CI) and most post-CI products, N 11 B is negatively correlated with 87 Sr/ 86 Sr, with extreme compositions represented by evolved products of the CI (higher N 11 B, lower 87 Sr/ 86 Sr) and less evolved shoshonite (lower N 11 B, higher 87 Sr/ 86 Sr). In contrast, samples from Procida, Ischia, and pre-CI products and one young shoshonite from CF define a distinct correlation with primitive trachybasalt as the high N 11 B, low 87 Sr/ 86 Sr end-member, and young shoshonites as the low N 11 B, high 87 Sr/ 86 Sr end- member. The overall interpretation of the geochemical and isotopic data suggests three conclusions: (1) Geochemical and isotopic variations of post-CI products from CF can be explained by mixing/mingling between at least two distinct magmas, i.e. the CI and the young shoshonite (Minopoli 1). (2) The isotopic trend described by Procida trachybasalts, Ischia samples, pre-CI samples from CF and Pigna St. Nicola shoshonite is a mantle trend suggesting the involvement in their genesis of a distinct component depleted in B, with low N 11 B and high 87 Sr/ 86 Sr. A similar component has been invoked to explain the differences between Stromboli potassic lavas and more typical calc- alkaline rocks of the Aeolian Islands. This component is consistent with ‘sediments or melts thereof’ introduced into the mantle by subduction, but depleted in B due to metamorphic reactions accompanying subduction. (3) The displacement from the mantle trend of CI and Minopoli 1 shoshonite magmas is probably due to crustal contamination processes. ß 2003 Elsevier B.V. All rights reserved. 0377-0273 / 03 / $ ^ see front matter ß 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0377-0273(03)00394-9 * Corresponding author. Fax: +39-050-3152360. E-mail address: s.tonarini@igg.cnr.it (S. Tonarini). VOLGEO 3022 6-4-04 Journal of Volcanology and Geothermal Research 133 (2004) 123^139 R Available online at www.sciencedirect.com www.elsevier.com/locate/jvolgeores