The systematics of boron isotopes in Izu arc front volcanic rocks

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Earth and Planetary Science Letters
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Apr 30
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The process of fluid release from the subducting Slab beneath the Izu are volcanic front (Izu VF) was examined by measuring B concentrations and B isotope ratios 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, Pb: bulk tephra). These tephra glasses have high B abundances ( similar to 10-60 ppm) and heavy delta(11)B values (+4.5parts per thousand to +12.0parts per thousand). extending the previously reported range for Izu VF rocks (delta(11)B. +7.0parts per thousand to +7.3parts per thousand). The glasses show striking negative correlations of delta(11)B with large ion lithophile element (LILE)/Nb 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 delta(11)B (from similar to +1parts per thousand to +10parts per thousand to +24parts per thousand). In this model. the contribution of sedimentary B is insignificant ( < 4&PTSTHOUSND; of B in the Izu VF rocks). Model C explains the correlation as a mixture of a low-&delta;B-11 (&SIM;+1&PTSTHOUSND;) 'composite' slab fluid (a Mixture of metasediment- and metabasalt-derived fluids) with a metasomatized mantle wedge containing elevated B (&SIM; 1-2 ppm) and heavy &delta;B-11 (&SIM;+14&PTSTHOUSND;). The mantle wedge as likely metasomatized by B-11-rich fluids beneath the outer forearc, and subsequently down drugged to arc front depths by the descending slab. Pb B isotope systematics indicate that. at are front depths. &SIM; 53% of the B in the Izu VF is derived from the wedge. This implies that the heavy &delta;B-11 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 (&SIM; 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 &delta;B-11 (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 &delta;B-11 variations of volcanic arc rocks. (C) 2002 Elsevier Science B.V. All rights reserved.


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