The influence of oceanic anoxic events on convergent margin magmas

Abstract In general, the arc magmas are oxidized, forming magnetite-series granitoids. However, there are also ilmenite-series granitoids at convergent margins. The mechanism that controls the formation of different types of granitoids remains obscure. Here we show that the temporal distributions of ilmenite-series granitoids in Japan have a good correlation with oceanic anoxic events (OAEs). The subduction of OAE sediments with high contents of organic matters releases reducing fluids/melts into the mantle wedge, leading to the formation of ilmenite-series granitoids. This is supported by sulfur isotope compositions and high concentrations of organic gas of the ilmenite-series granitoids in Japan. The temporal correlation between OAEs and Il-series granitoids suggests that the OAE sediments was predominantly distributed within short distances (