The systematics of boron isotopes in Izu arc front volcanic rocks

Straub, S. M. and Layne, G. D. (2002) The systematics of boron isotopes in Izu arc front volcanic rocks Earth and Planetary Science Letters, 198 (1-2). pp. 25-39. DOI 10.1016/S0012-821X(02)00517-4.

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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 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 (∼10–60 ppm) and heavy δ11B values (+4.5‰ to +12.0‰), extending the previously reported range for Izu VF rocks (δ11B, +7.0‰ to +7.3‰). The glasses show striking negative correlations of δ11B 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 δ11B (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-δ11B (∼+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 δ11B (∼+14‰). The mantle wedge was likely metasomatized by 11B-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 δ11B 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 δ11B (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 δ11B variations of volcanic arc rocks.

Document Type: Article
Keywords: Izu-Bonin Arc volcanic centers; boron; isotope ratios; slabs dehydration
Research affiliation: OceanRep > SFB 574 > C4
OceanRep > Geomar Research Center for Marine Geosciences
OceanRep > SFB 574
Refereed: Yes
DOI etc.: 10.1016/S0012-821X(02)00517-4
ISSN: 0012-821X
Projects: SFB574, ODP
Contribution Number:
SFB 5749
Date Deposited: 07 Apr 2010 11:42
Last Modified: 15 Nov 2016 10:25

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