The systematics of boron isotopes in Izu arc front volcanic rocks

摘要

The process of fluid release from the subducting slab beneath the Izu arc volcanic front (Izu VF) was examined by measuring B concentrations and B isotope rations in the Neogene fallout tephra (ODP Site 782A). Both were measured by secondary ion mass spectrometry, in a subset of matrix glasses and glassy plagioclase-hosted melt inclusions selected from material previously analyzed for major and trace elements (glasses) and radiogenic isotopes (Sr, Nd, Pd; bulk tephra). These tephra glasses have high B abundances (～10-60 ppm) and heavy δ~(11)B values (+4.5‰ to +12.0‰), extending the previously reported range for Izu VF rocks (δ~(11)B, +7.0‰ to +7.3‰). The glasses show striking negative correlations of δ~(11)B with large ion lithophile element (LILE)/Nd ratios. These correlations cannot be explained by mixing two separate slab fluids. originating from the subducting sediment and the subducting basaltic crust, respectively (model A). Two alternative models (models B and C) are proposed. Model B proposes that the inverse correlations are inherited from altered oceanic crust (AOC), which shows a systematic decrease of B and LILE with increasing depth (from basaltic layer 2A to layer 3), paralleled by an increase in δ~(11)B (from ～+1‰ to +10‰ to +24‰). In this model, the contribution of sedimentary B is insignificant (<4% of B in the Izu VF rocks). Model C explains the correlation as a mixture of a low-δ~(11)B (～+1‰), 'composite' slab fluid (a mixture of metasediment- and metabasalt-derived fluids) with a metasomatized mantle wedge containing elevated B (～1-2 ppm) and heavy δ~(11)B (～+14‰). The mantle wedge was likely metasomatized by ~(11)B-rich fluids beneath the outer forearc, and subsequently down dragged to arc front depths by the descending slab. Pb-B isotope systematics indicate that, at arc front depths, ～53% of the B in the Izu VF is derived from the wedge. This implies that the heavy δ~(11)B values of Izu VF rocks are largely a result of fluid fractionation, and do not reflect variations in slab source provenance (i.e. subducting sediment vs. basaltic crust). Since the B content of the peridotite at the outer forearc (7-58 ppm B, mean 24 ± 16 ppm) is much higher than beneath the arc front (～1-2 ppm B), the hydrated mantle wedge must have released a B-rich fluid on its downward path. This 'wedge flux' can explain (1) the across-arc decrease in B and δ~(11)B (e.g. Izu, Kuriles), without requiring a progressive decrease in fluid flux from the subducting slab, and (2) the thermal structure of volcanic arcs, as reflected in the B and δ~(11)B variations of volcanic arc rocks.