Slab and mantle controls on the Sr-Nd-Pb-Hf isotope evolution of the Post 42 Ma Izu-Bonin volcanic arc

Abstract

The arc outflux is a hybrid of material fluxes from slab, mantle and crustal sources, whose proportions and composition may vary through time. Here we investigate the causes of temporal variability in the post 42 Ma eruptive products of the Izu-Bonin arc in the NW Pacific Ocean basin using Sr-Nd-Pb-Hf isotopes in combination with major and trace element systematics in mafic to evolved arc magmas. Following arc initiation at ~50 Ma, Izu-Bonin arc formation was continuous except for a short period of quiescence (~20-23 Ma) during back-arc basin formation in the early Miocene.The Sr-Nd-Pb-Hf isotope trends do not co-vary through time. An increasing trend of Pb isotope ratios with time reflects a change in the trench sediment input as it becomes richer in radiogenic Pb. Sr isotopes show no unidirectional trend with time owing to the dominant contribution of unradiogenic Sr from the subducting igneous crust (~66% of arc Sr) and mantle (~31%) that buffer the minor amount of radiogenic Sr contributed from subducting sediment or altered oceanic crust. Nd and Hf isotope compositions are both controlled by the depleted sub-arc mantle. Nd isotope ratios increase significantly with time, whereas any temporal increase in Hf isotope ratios remains negligible relative to the inherent data variability. This temporal divergence is due to the much higher parent/daughter ratios of the sub-arc mantle source (147Sm/144Nd = 0·24; 176Lu/177Hf = 0·04), which result in significant ingrowth in 143Nd but a negligible ingrowth of 176Hf. Overall, the isotopic evolution is unaffected by along-strike arc rifting and back-arc basin formation, or by the evolution of the siliceous Izu-Bonin arc crust. Compared with the greater isotopic variability of the broader Izu-Bonin-Mariana arc system, the post 42 Ma Izu-Bonin arc exemplifies the long-term evolution of a simple arc system with comparatively uniform input from slab and mantle sources. Its isotopic evolution is hence not representative of that of other arcs, and this may reflect the different subduction geometry and components involved. © The Author 2010.