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. Plain Language Summary: Altered oceanic crust (AOC) is an important subduction component that contributes to the generation of arc magmas. However, identifying this contribution to arc magmas is challenging for the lack of effective tracers. Nonetheless, AOC has two characteristic features: (a) Sr‐Nd isotope decoupling (i.e., enriched in 87Sr/86Sr relative to 143Nd/144Nd in contrast to the "normal" mantle array); and (b) relatively heavy Ba isotope compositions. Here, we analyze the Ba‐Sr‐Nd isotopic compositions of lavas from the Mariana arc and back‐arc, which are combined with a statistical analysis of the Sr‐Nd isotopes for global arc volcanoes. Our results confirm that fluid (or hydrous melt) derived from subducted AOC, rather than sediment, is responsible for the Sr‐Nd isotope decoupling observed in island arc lavas. Strikingly, the magnitude of the Sr‐Nd isotope decoupling correlates well with the Ba isotope compositions of the arc lavas. Thus, our results establish an intimate link between subducted AOC, heavy Ba isotope ratios, and Sr‐Nd isotope decoupling signature in island arcs. This combination of Ba‐Sr‐Nd isotopes can therefore now be used as a powerful tracer to identify the contributions of subducted AOC to arc magmas. Our results also suggest that AOC and serpentinite show synergistic behavior during subduction recycling. Key Points: Global arc data confirm that the Sr‐Nd isotope decoupling of arc lavas is caused by AOC‐derived fluidDecoupled Sr‐Nd isotopes plus heavy Ba isotope is a good tracer for recycled altered oceanic crust (AOC) in arc magmasAOC and serpentinite show synergistic behaviors during subduction recycling [ABSTRACT FROM AUTHOR]