Chemistry and boron isotope ratios of Izu arc front volcanic rocks
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| Format: | Dataset Open Access |
| Langue: | en |
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2002
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| _version_ | 1867168552926052352 |
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| author | Straub, Susanne M Layne, Graham D |
| author_facet | Straub, Susanne M Layne, Graham D |
| collection | Datos científicos de ciencias marinas y ambientales |
| contents | 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 delta11B values (+4.5‰ to +12.0‰), extending the previously reported range for Izu VF rocks (delta11B, +7.0‰ to +7.3‰). The glasses show striking negative correlations of delta11B 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 delta11B (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-delta11B (~ +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 delta11B (~ +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 delta11B 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 delta11B (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 delta11B variations of volcanic arc rocks. |
| format | Dataset Open Access |
| id | pangaea_https___doi_org_10_1594_PANGAEA_713681 |
| institution | PANGAEA |
| language | en |
| publishDate | 2002 |
| publisher | PANGAEA |
| record_format | pangaea |
| spellingShingle | Chemistry and boron isotope ratios of Izu arc front volcanic rocks Straub, Susanne M Layne, Graham D 125-782A; DRILL; Drilling/drill rig; Joides Resolution; Leg125; North Pacific Ocean; Ocean Drilling Program; ODP 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 delta11B values (+4.5‰ to +12.0‰), extending the previously reported range for Izu VF rocks (delta11B, +7.0‰ to +7.3‰). The glasses show striking negative correlations of delta11B 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 delta11B (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-delta11B (~ +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 delta11B (~ +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 delta11B 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 delta11B (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 delta11B variations of volcanic arc rocks. |
| title | Chemistry and boron isotope ratios of Izu arc front volcanic rocks |
| topic | 125-782A; DRILL; Drilling/drill rig; Joides Resolution; Leg125; North Pacific Ocean; Ocean Drilling Program; ODP |
| url | https://doi.org/10.1594/PANGAEA.713681 |