Metamorphic devolatilization of subducted oceanic metabasalts: implications for seismicity, arc magmatism and volatile recycling

Subducted oceanic metabasalts are believed to be a primary source of volati les for are magmatism and fluid-induced seismicity. From phase equilibria c omputed for an average oceanic metabasalt we present a model for subduction zone devolatilization for pressures up to 6 GPa (similar to 180 km). Along high temperature geotherms complete dehydration occurs under forearcs, whe reas dehydration does not occur along low temperature geotherms. For interm ediate geotherms, major dehydration occurs under subarcs and provides a sub jacent H2O source for are volcanism. Decarbonation is negligible along cold and intermediate geotherms and limited along high temperature geotherms. B ecause decarbonation is limited for all subducted carbonate-bearing litholo gies, transfer of CO2 from subducted slabs to are magmas may be triggered b y aqueous fluid infiltration. Metabasalt devolatilization could induce seis micity in forearcs (high temperature geotherms) and subarcs (intermediate g eotherms); however, because of the lack of devolatilization, metabasalts wo uld not be a fluid source for seismicity with low temperature geotherms. Al ong low temperature geotherms, limited devolatilization of subducted oceani c metabasalts and marine sediments in forearcs and subarcs provides a mecha nism for return of volatiles to the deeper mantle. (C) 2001 Elsevier Scienc e B.V. All rights reserved.