Boninite petrogenesis: Chemical and Nd-Sr isotopic constraints

Abstract

Major and trace element abundances and Nd and Sr isotopic compositions have been determined for boninites from the Bonin Islands, Cape Vogel (Papua New Guinea) and New Caledonia and for similar lavas from Cyprus and New Zealand. SiO 2 , CaO, incompatible element abundances and ε Nd values are shown to vary systematically with the degree of light rare earth element (LREE) enrichment. La N /Yb N varies from 5.4 to 0.3. The range of ε Nd values is from +8.0 to +1.9. These features are interpreted in terms of mixing of two components: a LREE-depleted magma, derived by partial melting of an already-depleted peridotite source, and a hydrous, LREE-enriched fluid. If La for the fluid is taken as 100 × chondrites, REE abundances observed in boninites can be reproduced by 1–15% addition of the LREE-enriched component. Using a LREE-depleted component inferred from the Cyprus samples (La N /Yb N =0.10), the LREE-enriched component for Tertiary boninites has been calculated to have a La N /Yb N ∼20. Th9+-e LREE-depleted component contains most of the CaO, Sc, HREE and TiO 2 , and has an ε Nd of ⩾ +8. The enriched component contains H 2 O and most of the Zr, Nb and LREE, and has an ε Nd ranging from +6 to at least −5. Origins for the LREE-enriched fluid include derivation from mantle sources similar to those proposed for ocean islands or, more likely, from subducted oceanic or continental crust in island-arc settings.