Links between forearc decarbonation and generation of high δ26Mg fluids at subarc depths: constraints from carbonate-bearing forearc serpentinites

Subduction zones play a critical role in carbon exchange between Earth’s surface and interior. While devolatilization of forearc serpentinites has been proposed as a contributor to arc magma genesis, its role in subduction-zone carbon (C) and magnesium (Mg) cycling, as well as its influence on the Mg isotopic variability in global arc lavas, remains poorly constrained. This study investigates Mg isotope fractionation during interactions between forearc peridotites and CO2-rich fluids in the Mianlue tectonic mélange, focusing on carbonate-bearing serpentinites from the Jianchaling and Liangyazi regions. Five Jianchaling lizardite-only serpentinites exhibit a narrow range of δ26Mg values (−0.28 ± 0.02 ‰ to −0.13 ± 0.02 ‰), similar to those of four Jianchaling antigorite-lizardite serpentinites (−0.27 ± 0.03 ‰ to −0.17 ± 0.01 ‰). In contrast, seven Liangyazi antigorite-only serpentinites display a broader δ26Mg range (−0.27 ± 0.03 ‰ to −0.01 ± 0.02 ‰). Magnesium isotopic compositions of mineral separates reveal temperature-dependent inter-mineral Mg isotope fractionations between carbonate and co-precipitated serpentine minerals. Thermodynamic modeling and mass balance calculations indicate minimal Mg loss from peridotites to fluids during their interaction with CO2-rich fluids. Instead, subsequent fluid infiltration and carbonate dissolution are responsible for the observed Mg isotopic variations. The concurrent precipitation of isotopically heavy silicate minerals and light carbonates in the forearc provides a plausible explanation for the elevated δ26Mg values observed in some arc lavas. In subduction zones where the forearc slab-top temperatures can exceed the stability limit of antigorite (600 – 700 °C), significant forearc decarbonation produces CO2-rich fluids that interact with forearc peridotites at the bottom of the mantle wedge, forming abundant isotopically heavy silicate minerals. The subsequent breakdown of these silicates and limited release of slab-derived carbonates at subarc depths may be responsible for the high δ26Mg values and low CO2 outfluxes characteristic of these subduction zones. In contrast, subduction zones with cooler forearc slab-top temperatures, where subarc slab-top temperatures approach the stability limit of antigorite, experience less significant forearc decarbonation. In such settings, limited formation of isotopically heavy silicate phases and enhanced release of slab carbonates at subarc depths due to infiltrations of slab-derived fluids can potentially result in arc magmas with δ26Mg values close to or slightly below the normal mantle. These findings underscore the critical role of fluid-rock interactions in the forearc region in regulating deep carbon cycling and shaping the Mg isotopic signatures of arc magmas.