Wallace, Paul J.

[1] Subduction zone recycling of volatiles (H2O, Cl, S, F) is controlled by the nature of subducted materials and the temperature‐pressure profile of the downgoing slab. We investigate the variability in volatile and fluid‐mobile trace element enrichment in the Sunda arc using melt inclusion data from Kawah Ijen and Tambora volcanoes, together with published data from Galunggung, Indonesia. Combining our results with data from other arcs, we investigate the mobility of these elements during slab dehydration and melting. We observe correlations between Sr, H2O and Cl contents, indicating coupling of these elements during subduction zone recycling. Sulfur is more variable, and fluorine contents generally remain at background mantle values, suggesting decoupling of these elements from H2O and Cl. Partial melting and dehydration models constrain the source of Sr and the volatiles and suggest that the altered oceanic crust (AOC) is the main source of the hydrous component that fluxes into the mantle wedge, in agreement with thermo‐mechanical models. Sediment melt remains an important component for other elements such as Ba, Pb, Th and the LREE. The Indonesian volcanoes have variable concentrations of volatile and fluid‐mobile elements, with Kawah Ijen recording higher AOC‐derived fluid fluxes (Sr/Nd and H2O/Nd) compared to Galunggung and Tambora. Kawah Ijen has H2O/Ce ratios that are comparable to some of the most volatile‐rich magmas from other cold slab subduction zones worldwide, and the highest yet measured in the Sunda arc.

Volatiles in high-K magmas from the western Trans-Mexican Volcanic Belt: evidence for fluid fluxing and extreme enrichment of the mantle wedge by subduction processes

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Primitive, high-K minettes and basanites erupted during the Pleistocene from cinder cones on the flanks of the Colima Volcanic Complex in the western Trans-Mexican Volcanic Belt. Melt inclusions in olivine (Fo89–92) from tephra at these cones reveal that both magma types are oxidized and volatile rich, with high H2O (≤6·2 wt%), CO2 (≤5300 ppm), S (≤6700 ppm), Cl (≤2300 ppm), and F (≤8100 ppm) contents. A nearby calc-alkaline basaltic andesite cinder cone with more evolved composition (Fo78–80 olivine) has melt inclusions with similarly high H2O (≤5·5 wt%) but much lower CO2, S, and Cl compared with the potassic magmas. Melt inclusions from each cone have highly variable H2O and CO2, corresponding to crystallization pressures of 25 km depth) to very shallow levels. The H2O and CO2 variations cannot be explained by simple degassing models but instead requiring more complex, open-system processes or possibly reflect disequilibrium degassing. Trace element variations in the melt inclusions suggest that phlogopite and garnet were residual minerals during melting in the mantle source, and the presence of garnet suggests an origin in asthenospheric rather than lithospheric mantle. Decompression melting of phlogopite–garnet peridotite cannot produce the high H2O contents of the potassic magmas, and thus the presence of fluids during melting is required. Trace element modeling of a mantle source (intermediate in composition between enriched mid-ocean ridge basalt and ocean island basalt sources) that is fluxed with an H2O-rich fluid or hydrous melt from the subducting slab can reproduce most of the trace element characteristics of the potassic melts, demonstrating that they are clearly linked with subduction processes. Formation of the potassic magmas probably involved slab rollback, trenchward migration of the arc into the region above metasomatically enriched forearc mantle, and heating of this veined and fluid-fluxed mantle as a result of upwelling of hot mantle through a tear between the subducted Cocos and Rivera plates. [ABSTRACT FROM AUTHOR]

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