The systematics of chlorine, fluorine, and water in Izu arc front volcanic rocks: Implications for volatile recycling in subduction zones

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Geochimica Et Cosmochimica Acta
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We studied the systematics of Cl, F and H2O in Izu arc front volcanic rocks using basaltic through rhyolitic glass shards and melt inclusions (Izu glasses) from Oligocene to Quaternary distal fallout tephra. These glasses are low-K basalts to rhyolites that are equivalent to the Quaternary lavas of the Izu arc front (Izu VF). Most of the Izu glasses have Cl similar to400-4000 ppm and F similar to70-400 ppm (normal-group glasses). Rare andesitic melt inclusions (halogen-rich andesites; HRA) have very high abundances of Cl (similar to6600-8600 ppm) and F (similar to780-910 ppm), but their contents of incompatible large ion lithophile elements (LILE) are similar to the normal-group glasses. The preeruptive H2O of basalt to andesite melt inclusions in plagioclase is estimated to range from similar to2 to similar to10 wt% H2O. The Izu magmas should be undersaturated in H2O and the halogens at their preferred levels of crystallization in the middle to lower crust (similar to3 to similar to11 kbar, similar to820degrees to similar to1200degreesC). A substantial portion of the original H2O is lost due to degassing during the final ascent to surface. By contrast, halogen loss is minor, except for loss of Cl from siliceous dacitic and rhyolitic compositions. The behavior of Cl, F and H2O in undegassed melts resembles the fluid mobile LILE (e.g.; K, Rb, Cs, Ba, U, Pb, Li). Most of the Cl (>99%), H2O (>95%) and F (>53%) in the Izu VF melts appear to originate from the subducting slab. At arc front depths, the slab fluid contains Cl = 0.94 +/- 0.25 wt%, F = 990 +/- 270 ppm and H2O = 25 +/- 7 wt%. If the subducting sediment and the altered basaltic crust were the only slab sources, then the subducted Cl appears to be almost entirely recycled at the Izu arc (similar to77-129%). Conversely, H2O (similar to13-22% recycled at arc) and F (similar to4-6% recycled) must be either lost during shallow subduction or retained in the slab to greater depths. If a seawater-impregnated serpentinite layer below the basaltic crust were an additional source of Cl and H2O, the calculated percentage of Cl and H2O recycled at are would be lower. Extrapolating the Izu data to the total length of global arcs (similar to37,000 km), the global arc outflux of fluid-recycled Cl and H2O at subduction zones amounts to Cl similar to2.9-3.8 x 10(12) g/yr and H2O similar to0.7-1.0 x 10(14) g/yr, respectively-comparable to previous estimates. Further, we obtain a first estimate of global arc outflux of fluid-recycled F of similar to0.3-0.4 x 10(12) g/yr. Despite the inherent uncertainties, our results support models suggesting that the slab becomes strongly depleted in Cl and H2O in subduction zones. In contrast, much of the subducted F appears to be returned to the deep mantle, implying efficient fractionation of Cl and H2O from F during the subduction process. However, if slab devolatilization produces slab fluids with high Cl/F (similar to9.5), slab melting will still produce components with low Cl/F ratios (similar to0.9), similar to those characteristic of the upper continental crust (Cl/F similar to0.3-0.9). Copyright (C) 2003 Elsevier Ltd.


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Doi 10.1016/S0016-7037(00)00307-7