Deciphering contribution of recycled altered oceanic crust to arc magmas using Ba-Sr-Nd isotopes

Altered oceanic crust (AOC) plays a critical role in geochemical recycling in subduction zones. However, identifying contributions of subducted AOC to arc magmas remains a conundrum due to the lack of effective tracers. Here, we investigate the Ba-Sr-Nd isotopic compositions of lavas from the Mariana arc and back-arc. Based on a statistical analysis of the Sr-Nd isotopes for global arc volcanoes, we confirm that AOC-derived fluid (or hydrous melt), rather than sediment-derived melt or fluid, is responsible for the Sr-Nd isotope decoupling (i.e., ⁸⁷Sr/⁸⁶Sr is “excessively” enriched relative to ¹⁴³Nd/¹⁴⁴Nd when compared to the “normal” mantle derivates) observed in island arc lavas. We show that the arc lavas with increasingly decoupled Sr-Nd isotopes generally have heavier Ba isotope ratios, which is also a characteristic feature of AOC-derived fluids. Thus, these results establish an intimate link between subducted AOC, heavy Ba isotope compositions, and Sr-Nd isotope decoupling signature in island arcs, which provides a powerful tool to trace the AOC recycling in subduction zones. Furthermore, a similar correlation is observed between Sr-Nd isotope decoupling and heavy B isotope ratios for global arc lavas, implying that the recycling of AOC component is generally linked to serpentinite dehydration in subduction zones.