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

Abstract

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 < 100 bars to ∼ 7 kbar. This indicates that olivine crystallized from variably degassed melts over a wide range of depths extending from the lower crust (>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.