Zinc Isotopes of the Mariana and Ryukyu Arc‐Related Lavas Reveal Recycling of Forearc Serpentinites Into the Subarc Mantle.

Mantle source heterogeneity of arc‐related magmas was traditionally thought to be predominantly affected by slab‐derived components. However, the role of forearc serpentinites in causing subarc mantle heterogeneity remains poorly constrained. Here, we present the first Zn isotope data for lavas from the Mariana and Ryukyu subduction zones. Notably, the δ66Zn values of basaltic lavas from the Mariana arc (0.16–0.18‰) and southern Okinawa Trough (0.15–0.17‰) are generally lower (∼0.10‰) than those of the mid‐ocean ridge basalts (MORB) (0.27 ± 0.05‰). Since mantle melting and magmatic differentiation respectively induce heavy Zn isotope enrichment in primary and evolved magmas, while melt extraction yields the limited Zn isotope fractionation in the mantle, lavas with low δ66Zn values thus potentially indicate the addition of subducted components. The negative correlation between the δ66Zn values and the Ba/La ratios of the basaltic lavas suggests the involvement of isotopically light fluids in their mantle sources. Forearc serpentinites are typically characterized by extremely light Zn isotope compositions. Such forearc materials were likely dragged downward to subarc depths and released isotopically light Zn in fluids to modify the overlying mantle wedge, thereby producing low δ66Zn values in arc‐related magmas. Beyond subarc depths, forearc serpentinites are broken down completely, so light Zn isotope fluids are absent. Accordingly, the basalts from the middle Mariana and Okinawa Trough display MORB‐like δ66Zn values. Collectively, Zn isotope provides new insights into the role of the recycling of forearc serpentinites in generating chemical heterogeneity in the mantle source of subduction‐related magmas. Plain Language Summary: Forearc serpentinites are hydrated by fluids released from the subducting slab, and they typically have negative δ66Zn values and strong enrichment in fluid‐mobile elements. To understand this forearc material recycling in subduction zones, we analyzed the Zn isotope composition of lavas from the Mariana and Ryukyu subduction zones. To our knowledge, this is the first study to find that the arc‐related lavas have lower δ66Zn values than those of the mid‐ocean ridge basalts. Notable negative correlations between δ66Zn and proxies for fluid addition (e.g., Ba/La and Sr isotope ratios) provide critical evidence for the importance of recycled forearc serpentinites in generating mantle Zn isotopic heterogeneity in subduction zones. Most importantly, this study proves the efficiency of using Zn isotopes as a powerful tool to distinguish the contributions of slab and mantle wedge serpentinite‐derived fluids to arc magmas. Key Points: Lavas from the Mariana and Ryukyu subduction zones have δ66Zn values lower (∼0.10‰) than those of mid‐ocean‐ridge basaltsLight Zn isotope enrichment in arc‐related lavas reflects the recycling of forearc serpentinites into subarc mantle sourcesZn isotopes can be used for discriminating the contributions of slab and mantle wedge serpentinite‐derived fluids to arc magmas [ABSTRACT FROM AUTHOR]