Barium isotope fractionation during dehydration melting of the subducting oceanic crust: Geochemical evidence from OIB-like continental basalts

Continental basalts commonly show ocean island basalts (OIB)-like geochemical compositions, and their origin is geochemically linked to metasomatism of the big mantle wedge by the deeply subducting oceanic slab at postarc depths of >200 km. This linkage is tested by a combined study of Ba isotopes with major-trace elements and Sr-Nd isotopes in Cenozoic continental basalts from eastern China. These basalts show OIB-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions, suggesting incorporation of the dehydrated slab component into their mantle sources. Their delta Ba-138 values vary from -0.05 parts per thousand to +0.06 parts per thousand, which are lower than the depleted mid-ocean ridge basalts (MORB) mantle but similar to island arc basalts, OIB and OIB-like continental flood basalts. The variation of delta Ba-138 values cannot be ascribed to fractional crystallization and crustal contamination of mantle-derived magmas, but it results from mixing of the depleted mantle with the recycled crustal material of low delta Ba-138 values. In other words, the geochemical deviations from the depleted MORB mantle are attributable to the source mixing at mantle depths rather than the crustal contamination at crustal depths. Although the delta Ba-138 values of the dehydrated oceanic crust would be lower than the initially subducting crust based on the lower coordination number of Ba in liquid phases relative to solid phases in the crust, the low delta Ba-138 recycled crustal material including igneous oceanic crust (IOC) and seafloor sediment is still required to produce the low delta Ba-138 values for the studied basalts. The similarity in delta Ba-138 to island arc basalts not only indicates the similarly low delta Ba-138 values for the recycled crustal components in these basalts, but also suggests the limited Ba isotope fractionation during dehydration melting of the subducting oceanic crust at postarc depths.