Serpentinite dehydration-driven redox heterogeneity in cold subduction zone magmas

Serpentinite-derived slab fluids likely control the oxidation state of the sub-arc mantle, yet their oxidation state remains controversial. To address this issue, we report the first set of redox-sensitive chromium isotopic data for arc and back-arc basin lavas. Samples from the Mariana Arc and eastern Manus Basin have heavier Cr isotopic compositions (δ⁵³Cr = −0.13 to −0.18 ‰) than the MORB-like δ⁵³Cr values in the Mariana back-arc basin (−0.21 to −0.31 ‰). The δ⁵³Cr values correlate with Ba/La and V/Yb ratios, suggesting the control of slab-derived fluids on across-arc Cr isotope and oxygen fugacity variations. The Cr⁶⁺-bearing oxidized alkaline fluids released by serpentinite dehydration at sub-arc depths are enriched in heavy Cr isotopes. Their infiltration and reactions with the overlying mantle wedge facilitate the sub-arc oxidation of Fe²⁺ to Fe³⁺ and Cr²⁺ to Cr³⁺, resulting in heavy Cr isotopes and high oxygen fugacity in arc magmas. Oxidized fluids diminish beyond sub-arc depths due to the complete breakdown of antigorite, thus back-arc basin basalts have MORB-like δ⁵³Cr values and low oxygen fugacity. Our study suggests that Cr isotopes can provide critical insights into the role of serpentinite-derived fluids in redox heterogeneity in subduction-related magmas.