Magnesium isotopic composition of the Mariana forearc serpentinite: Implications for Mg isotopic composition of the mantle wedge and Mg isotopic fractionation during mantle wedge serpentinization

The serpentinized mantle wedge is critical for the geochemical cycling of water, volatiles, and fluid-mobile el-ements in the subduction zone. It is also a major reservoir of magnesium (Mg) in subduction zones, but its Mg isotopic composition is still not well constrained. To investigate Mg isotopic fractionation during mantle wedge serpentinization, and better understand Mg isotopic composition of the mantle wedge, we studied Mg isotopes of mantle wedge serpentinites/serpentinized peridotites exhumed by Mariana forearc serpentinite mud volcanoes. Whole-rock 826Mg values of these samples vary from-0.29 to -0.03%0. Some serpentinite/serpentinized peri-dotite samples have significantly elevated 826Mg values up to -0.03%0, which are caused by seafloor weathering after their exhumation by the mud volcanoes. In contrast, the unweathered serpentinized peridotites have ho-mogeneous 826Mg values of-0.29 to -0.27%0 (mean 826Mg =-0.28 +/- 0.01%0, 2SD, n = 3), which represent the primary Mg isotopic composition of the Mariana forearc mantle wedge peridotite. However, the unweathered mature serpentinites (i.e., completely serpentinized) have slightly heavier Mg isotopic compositions (826Mg =-0.29 to -0.21%0, mean 826Mg =-0.24 +/- 0.05%0, 2SD, n = 16) than the serpentinized peridotites, indicating that Mg isotopes are fractionated during the late-stage mantle wedge serpentinization. It is probably due to the leach of isotopically light Mg by the infiltrating slab fluids after the complete consumption of olivine. By compiling Mg isotopic data of mantle wedge and oceanic mantle peridotites, we find that mantle wedge peridotites have Mg isotopic compositions (826Mg =-0.27 +/- 0.04%0) that are identical to subcontinental lith-ospheric mantle peridotites (-0.25 +/- 0.04%0), which are more uniform and lighter than those of oceanic mantle peridotites (-0.21 +/- 0.12%0). The Mg isotope difference between them is most probably caused by mantle heterogeneity, not by different degrees of early partial melting as indicated by the incremental batch melting modeling.

Automated summary

The serpentinized mantle wedge undergoes magnesium isotopic fractionation during late-stage serpentinization, resulting in slightly heavier magnesium isotopic compositions. This is likely due to the removal of isotopically light magnesium by infiltrating slab fluids. Mantle wedge peridotites have magnesium isotopic compositions that are similar to subcontinental lithospheric mantle peridotites and lighter than oceanic mantle peridotites, suggesting mantle heterogeneity.