The Systematics of Boron Isotopes in Izu Arc Front Volcanic Rocks

We studied the processes of fluid release from the subducting slab beneath the Izu arc volcanic front (Izu VF) by measuring B concentrations and B isotope ratios of the Neogene fallout tephra (ODP Site 782A). The B isotopes were measured by secondary ion mass spectrometry (SIMS) of matrix glasses and plagioclase-hosted melt inclusions (=glasses) on samples that have been previously analysed for major and trace elements as well as radiogenic isotopes. The tephra glasses have high B abundances ( ~10-60 ppm) and high δ ¹¹B values, ranging from +4.5 ‰ to +12.0 ‰ , extending the previously reported range for Izu arc front volcanic rocks (δ ¹¹B =+7.0 to 7.3 ‰ ). The glasses show strikingly negative correlations of δ ¹¹B with Nb-normalized large ion lithophile elements (LILE). These correlations cannot be explained by mixing of two separate slab fluids that originate from the subducting sediment and the subducting basaltic crust, respectively (model A). Two alternative models (model B and model C) are presented. Model B suggests that the inverse correlations are inherited from the altered oceanic crust since the crust shows a systematic decrease of B and LILE with increasing depth (from layer 2A to layer 3) that is paralleled by a increase in δ ¹¹B (from ~ 1 ‰ to >10 to +24 ‰ ). Model C explains the correlations by mixing of variable amounts of a low-δ ¹¹B ( ~ +1 ‰ ) slab-derived fluid with a high-δ ¹¹B ( ~ +14 ‰ ), B-rich (1-2 ppm) mantle wedge. The infiltration of the wedge with heavy ¹¹B mostly likely occurred during slab dehydration beneath the forearc, and reflects the preferred partitioning of ¹¹B into fluids. A decreasing flux of high-δ ¹¹B, wedge-derived B with increasing depth could explain the decreasing B and δ ¹¹B observed in across-arc transects, without requiring a general decrease of fluid flux from the subducting slab. Cyclic, short-term fluctuations on the scale of a few million years predominate at the Izu VF since 15 million years. This suggests that fluctuations of the slab fluids are a natural background 'noise' of slab dehydration rather than a driving force for the well-known global periodicity of arc volcanism during the Neogene.