Liu, Hai-Quan, Tian, Feng, Hoernle, Kaj , Li, Jie, Huang, Xiao-Long, Zhang, Le, Bindeman, Ilya and Xu, Yi-Gang (2024) Molybdenum isotope insights into recycling of wedge serpentinite and fore-arc crust in subduction zones. Chemical Geology, 665 . Art.Nr. 122302. DOI 10.1016/j.chemgeo.2024.122302.

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Abstract

Highlights

• High δ98/95Mo in high-Mg andesites reflect eroded fore-arc serpentinite.
• Low δ98/95Mo in adakites represent the slab melt component.
• High δ98/95Mo in adakites represent eroded fore-arc crust.
• Mo isotopic data support a liquid-dominated mass transfer mode in subduction zones.

Abstract

Fore-arc crust and mantle wedge serpentinite, dragged down by the subducting slab, play a critical role in enriching the sub-arc mantle source of arc magmas. However, discriminating between these two components in arc magmas remains challenging because the fore-arc crust may contain serpentinite components inherited from previous subduction event(s). Here we present molybdenum concentration and isotope data, coupled with reanalyzed and published geochemical data, for common representatives of circum-Pacific high-Mg andesites and adakites. Elevated δ98/95Mo values (−0.13‰ to 0.00‰) in the Kamchatka and Aleutian high-Mg andesites are associated with elevated Mo/Ce (0.026 to 0.075), Ba/Th (138 to 808), and Sb/Ce (0.0026 to 0.0192) ratios, as well as Sr-Nd-Hf-Pb isotope compositions close to the depleted mantle and moderated δ18O values (+6.6‰ to +7.8‰). These characteristics are consistent with compositional contributions from slab-derived aqueous fluids via fore-arc serpentinites. In contrast, decreasing δ98/95Mo values (−0.07‰ to −0.48‰) in the Cascadia and Setouchi high-Mg andesites as well as four adakites from Fiji and the Austral Volcanic Zone, are generally accompanied by decreasing Mo/Ce (0.036 to 0.002) and Sb/Ce (0.0066 to 0.0002) ratios, increasing Sr/Y ratios (15 to 207), and Sr-Nd-Hf-Pb-O isotopic variations ranging from values reflecting altered oceanic crust to those resembling sediment input. These features can be accounted for by an increasing melt contribution from the subducted oceanic crust. Nine adakites from the Aleutians, Fiji, Panama, and the Austral Volcanic Zone exhibit δ98/95Mo values (−0.19‰ to −0.04‰) higher than the depleted mantle but lower Mo/Ce ratios (0.007 to 0.018) and low to moderate Sb/Ce ratios (0.0002 to 0.0025), indicating a potential serpentinite contribution. However, elevated Sr/Y ratios (57 to 295), unradiogenic Sr-Pb isotopes, radiogenic Nd-Hf isotopes, and moderate δ18O values (+6.3‰ to +6.5‰) suggest that these adakites are derived from fore-arc crust dragged down by the descending oceanic crust. The circum-Pacific adakites in this study show a decrease in δ98/95Mo with decreasing Mo/Ce, consistent with partial melting of oceanic crust and 98Mo pre-enriched fore-arc crust, respectively, having lost 98Mo during subduction dehydration. Thus, our study not only distinguishes the components between eroded fore-arc crust and wedge serpentinite but also indicates a major role of both dehydration and melting in facilitating material transfer in subduction zones.

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