Molybdenum isotope insights into mass recycling in subduction zones

Discriminating between fore-arc crust [1] and wedge serpentinite [2] contributions in arc magmas is critical for understanding mass recycling mechanisms in subduction zones but remains challenging because fore-arc crust may include serpentinite signatures from prior subduction events. Here we present molybdenum (Mo) isotope and concentration data, along with reanalyzed and published geochemical data, for common representatives of circum-Pacific high-Mg andesites and adakites. Elevated δ^98/95Mo values (-0.13‰ to 0.00‰) in Kamchatka and Aleutian high-Mg andesites, accompanied by high Mo/Ce (0.026 to 0.075), Ba/Th (138 to 808), and Sb/Ce (0.0026 to 0.0192) ratios, as well as depleted mantle-like Sr-Nd-Hf-Pb isotopes and moderate δ¹⁸O values (+6.6‰ to +7.8‰), indicate slab-derived aqueous fluids via fore-arc serpentinites. In contrast, Cascadia and Setouchi high-Mg andesites, along with adakites from Fiji and the Austral Volcanic Zone, show decreasing δ^98/95Mo (-0.07‰ to -0.48‰), Mo/Ce, and Sb/Ce ratios, coupled with higher Sr/Y (15 to 207) and altered oceanic crust-like Sr-Nd-Hf-Pb-O isotopic compositions, reflecting melts from subducted oceanic crust. Nine adakites from the Aleutians, Fiji, Panama, and the Austral Volcanic Zone exhibit intermediate δ^98/95Mo (-0.19‰ to -0.04‰) with low Mo/Ce and Sb/Ce ratios, but high Sr/Y (57 to 295), radiogenic Nd-Hf isotopes, and low δ¹⁸O (+6.3‰ to +6.5‰), suggesting origins from fore-arc crust dragged by subducting slabs. These results link δ^98/95Mo variations to partial melting of oceanic and fore-arc crust, highlighting dehydration and melting [3, 4] as key processes in subduction zones.

[1] Liu et al. (2023) Geology; [2] Li et al. (2021), Nature Communications; [3] Elliott (2003) Inside the Subduction Factory; [4] Liu et al. (2024) Chemical Geology.