Magnesium isotopic fractionation during post-serpentinization alteration: Implications for arc and oceanic Mg cycles

In recent years, numerous geodynamic studies have concentrated on serpentinites and the serpentinization processes that occur within subduction zones. However, as an important host phase for Mg, whether Mg isotopic fractionation occurs during peridotite serpentinization is still unclear. In our study, we conducted high-precision Mg isotope analyses on serpentinized peridotites and forearc serpentinites from the Mariana subduction zone. Except for one sample affected by strong late weathering with clay formation, which had an anomalously high δ26Mg value (0.01 ± 0.04‰), the moderately serpentinized peridotites (−0.24 ± 0.04‰, 2SD, n = 3) and deep (>40 mbsf) serpentinites (−0.24 ± 0.02‰, 2SD, n = 3) have Mg isotopic compositions consistent with mantle peridotites (−0.25 ± 0.04‰). Shallow serpentinites display slightly higher δ26Mg values (−0.19 ± 0.04‰, 2SD, n = 15) than mantle peridotites, and these values tend to increase toward the seafloor. These high δ26Mg values of forearc serpentinites cannot be attributed to serpentinizing fluid metasomatism and mantle wedge serpentinization processes, while post-serpentinization alteration involving brucite dissolution played a dominant role. Rayleigh fractionation simulations suggest that the heavy Mg isotopes are preferentially allocated to serpentinites during the post-serpentinization alteration process, where the calculated ∆26Mgbrucite-fluid varies from 0.1‰ to 0.8‰. This study deduces that the process of serpentinite dehydration in the subarc mantle cannot significantly change the Mg isotopic composition of arc lavas because serpentinite itself has a mantle-like Mg isotopic composition. Moreover, the fluids released by the brucite dissolution process have a low δ26Mg value, making an important contribution to the Mg budget of modern ocean seawater.