Heavy iron isotopes in arc rocks reveal anoxic sediment recycling in subduction zones

The formation mechanism of arc mafic rocks with heavy iron isotope signature remains enigmatic. Here we report Fe-Mo-Sr-Nd-Hf isotope data of arc rocks from Northeast China. The studied arc rocks have δ56Fe values (0.08‰ to 0.23‰) comparable to or higher than those of mid-oceanic-ridge basalts, and enriched Sr-Nd-Hf isotope compositions. Combined with the published data of global arc mafic rocks, we find there is a correlation between the δ56Fe values and radiogenic Sr-Nd isotopes, indicating that sediment recycling may have affected the iron isotope of arc rocks. Notably, the positive correlation between the δ56Fe and δ98Mo values further suggests that the metasomatism of mantle wedge peridotite by anoxic sediment-derived melts may be the first-order control for the heavy iron isotopes of arc mafic rocks. Therefore, the arc rocks with heavy iron isotopes can be explained by the addition of anoxic sediments to their mantle sources. The recycled anoxic sediments play an important role in determining the heavy iron isotope composition of arc magmas, according to Fe-Mo-Sr-Nd-Hf isotope analyses of Northeast China arc rocks and global data correlations.