Magnesium Isotope Composition of Subduction Zone Fluids as Constrained by Jadeitites From Myanmar

Subduction zone fluids are critical for transporting materials from subducted slabs to the mantle wedge. Jadeitites from Myanmar record fluid compositions and reactions in the forearc subduction channel. Here we present high-precision Mg isotope data of the Myanmar jadeitites and associated rocks to understand the Mg isotope composition of subduction zone fluids at forearc depths. Two types of jadeitites (white and green) exhibit distinct Mg isotope compositions. The white jadeitites precipitated from Na-Al-Si-rich fluids and have low Mg-26 values, varying from -1.55 to -0.92, whereas the green jadeitites have higher Mg-26 values (-0.91 to -0.74) due to metasomatic reactions between fluids and Cr spinel. The amphibole-rich blackwall in the contact boundaries between jadeitites and serpentinites also exhibits low Mg-26 values (-1.17 parts per thousand to -0.72 parts per thousand). Therefore, the jadeite-forming fluids have not only high concentrations of Na-Al-Si but also low Mg-26 values. The low Mg-26 signature of the fluids is explained by the dissolution of Ca-rich carbonate in subducted sediments or altered oceanic crust, which is supported by the negative correlation of Mg-26 with CaO/TiO2, CaO/Al2O3, and Sr in the white jadeitites. Given the common occurrence of Ca-rich carbonates in the subduction channel, the Mg isotope composition of low-Mg aqueous fluids would be significantly modified by dissolved carbonates. Metasomatism by such fluids along conduits has the potential to generate centimeter-scale Mg isotope heterogeneity in the forearc mantle wedge. Therefore, Mg isotopes could be a powerful tracer for recycled carbonates not only in the deep mantle but also in the shallow regions of subduction zones.