Volatile chemistry, and nitrogen sources and fluxes, in subduction zones : insights from the Izu-Bonin-Mariana Arc
A systematic study of the geochemistry of volatiles being emitted from the active volcanic front of the Izu-Bonin-Mariana (IBM) arc was undertaken in order to investigate the sources, flux, and mass balance of nitrogen across this 'cool' convergent margin. The IBM arc is an ideal location to study volatile recycling in subduction zones as it is an intra-oceanic convergent margin (IOCM), where the entire sedimentary sequence is subducted and where a number of parameters, including sediment composition, slab dip and slab age, vary systematically along strike of the arc. Volcanic emissions are typical of convergent margin volcanoes, and are dominated by H2O, CO2 and S species. Most samples have high N2/He and low CO2/N2,exc., due to addition of sedimentary nitrogen from the subducting slab. This is confirmed by δ15N values, which are generally positive – up to +5.5°. Quantitative mixing calculations indicate that, after correction for atmospheric contamination, an average of 75% of nitrogen is sediment-derived. Estimates of the volcanic front flux of sediment-derived nitrogen range from 0.12x108 mol a-1 N2 to 1.11x108 mol a-1 N2, representing 2-17% of the total nitrogen input flux, or 5-51% of the sedimentary nitrogen input flux. These results suggest a large fraction of the subducted nitrogen is delivered to the mantle, in contrast to studies of the relatively 'warm' Central American arc, where the majority of nitrogen appears to be recycled to the atmosphere. A major conclusion of the current study is that convergent margin thermal regime is likely the dominant control on the efficiency of nitrogen recycling in subduction zones. A secondary finding from this study, based on correlations between δ15N values and various trace element and radiogenic isotope ratios, is that slab-derived nitrogen is sourced from subducting sediments but transported into the mantle wedge by an aqueous fluid derived from dehydration of the altered oceanic crust. It is suggested here for the first time that both an aqueous fluid and a sediment melt are involved in the genesis of Izu arc magmas.
Volcanic Gases, Nitrogen Isotopes, Subduction Zones, Izu-Bonin-Mariana Arc
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