B/Nb and δ 11B systematics in the Phlegrean Volcanic District, Italy

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. δ 11B values are generally lower and more uniform in samples from CF (−6.8 to − 10.6‰) compared with those from Procida (−3.6 to −8.5‰) and Ischia (−2.8 to −8.4‰) 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 δ 11B and B/immobile element ratios. For CF samples including the Campanian Ignimbrite (CI) and most post-CI products, δ 11B is negatively correlated with 87Sr/ 86Sr, with extreme compositions represented by evolved products of the CI (higher δ 11B, lower 87Sr/ 86Sr) and less evolved shoshonite (lower δ 11B, higher 87Sr/ 86Sr). 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 δ 11B, low 87Sr/ 86Sr end-member, and young shoshonites as the low δ 11B, high 87Sr/ 86Sr 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 δ 11B and high 87Sr/ 86Sr. 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.