Fluids are important media for elemental cycling in subduction zones, but the compositions of such fluids remain obscure, especially those derived from dehydration of serpentinite. To better understand the role of subduction-zone fluids in arc magmatism of the Chinese Eastern Tianshan (CET), we present Mg and B isotopic data for the arc diorites (e.g. Kalatage and Nanshankou). Our results show that the Kalatage diorites have higher δ26Mg (−0.23 to −0.13‰) and δ11B (−0.04 to +1.08‰) values than the corresponding mantle values, whereas the Nanshankou diorites display relatively lower δ26Mg (−0.26 to −0.19‰) and δ11B (−10.7 to −2.97‰) values. Because high-temperature magmatic and post-magmatic processes have limited effects on Mg and B isotope fractionation of our samples, their distinct Mg–B isotopic signatures inherited from the mantle sources should be caused by subduction-related metasomatism. Because neither altered oceanic crust (AOC) directly melting nor the incorporation of subducted sediments could induce significant Mg–B isotope fractionation in these rocks, we infer that their variable δ26Mg and δ11B values were probably associated with subduction fluids that were derived from different sources. Specifically, dehydrated fluids from serpentinites possess high δ26Mg and δ11B values, which resulted in the heavy Mg–B isotopic compositions of the Kalatage diorites, whereas mantle metasomatized by fluids from AOC and subducted sediments were responsible for the Nanshankou rocks with relatively low δ26Mg and δ11B values. Taking into account the different effects of serpentinite-derived and slab-derived fluids in the formation of arc magmatism, subduction-zone fluids can be probably an important factor in changing the Mg–B isotope composition of mantle sources.

Supplementary material: Supplementary data tables and parameters used in models are available at https://doi.org/10.6084/m9.figshare.c.6702686

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