Chlorine from seawater is key to the generation of calc-alkaline lavas

Most primary magmas evolve compositionally as either tholeiitic or calc-alkaline lava series. The most voluminous magmas are generated through decompression melting of the upper mantle beneath diverging tectonic plates and differentiate as tholeiitic lavas. Calc-alkaline lavas, on the other hand, consist mainly of the bulk of differentiating arc magmas generated through H2O-fluxed melting of the upper mantle beneath converging plates. The exact origin of the calc-alkaline lava series is one of the earliest but still unresolved controversies in petrology. Here I propose that the calc-alkaline lava series acquires its distinct compositional features mainly because of the Cl content of subducted seawater. The long-held view that the low FeO* differentiation trend of calc-alkaline lava series is primarily due to the early fractionation of its Fe-Ti oxides may be inaccurate. Instead, the calc-alkaline lava series is characteristically depleted in FeO* and oxidized because some of its Fe are sequestered early from its mantle source and parental arc magma by Cl-rich slab-derived fluids as Fe-chlorides, transit upward as part of a separate fluid phase in the upwelling magma and, then, precipitate eventually in the crust as Fe, ore metal-sulphides.