Evolution of arc magmas and their volatiles

Of the volumetrically significant magma types, those emplaced in arcs at plate convergence zones are typically richest in dissolved volatiles (H₂O, CO₂, and S species). These volatiles are mostly products of large-scale recycling, derived though multistage processes involving devolatilization of variably hydrated and carbonated, sediment-bearing, subducted lithosphere, and transported towards the surface by magmas generated in the mantle overlying the subducted plate. Volatile contents of parental arc basalts are globally variable, but mostly range from ~ 0.5 to 10 wt% H₂O, <1000 ppm CO₂, and ≤3500 ppm S (as H₂S and/or SO₂). Arc magmas are also generally more oxidized than those of ridges and hot-spots. These characteristics lead to distinctive differences in the course of magmatic crystallization compared with dry, reduced types: plagioclase saturation is delayed, and Ca-rich upon appearance; olivine persists in the crystallization sequence to higher SiO₂ contents; a spinel phase appears early and persists throughout crystallization. Resultant relatively voluminous Na-K-feldspar- and SiO₂-rich residual magmas dominate the bulk continental crust. The complementary SiO₂-poor olivine-clinopyroxene-dominated fraction is probably recycled into the upper mantle. Volatile fluxes through the subduction cycle are not straightforwardly determined; total magma volume flux estimates range from ~ 1.2 to 7 km³/year, but are not well constrained for the full diversity of global arc systems. Even at the high end of this range, more C is subducted than returned via arcs, presumably with long-term effects on the global C cycle. At the lower end, deficiencies may exist in return fluxes of H₂O and S species.