Low-56Fe fluid released from serpentinite as recorded by iron isotopes of Myanmar jadeitites

Melt extraction and subduction zone fluid infiltration can produce light Fe isotope compositions of arc lavas relative to the MORBs. However, the Fe isotope signature of subduction zone fluid is poorly constrained, which has hampered our understanding of Fe cycling in the subduction zone. Jadeitite veins in forearc serpentinite provide a unique record for the characteristics of subduction zone fluids. Here we report Fe isotope data for Myanmar jadeitites and associated rocks to constrain the Fe isotope signature of subduction zone fluids. Two types of jadeitites from Myanmar display distinct 656Fe much lower than typical igneous rocks. White jadeitites, directly precipitated from fluids, have variable 656Fe from-0.60%o to-0.04%o, while green jadeitites, formed by fluid-chromite interaction, have 656Fe (-0.44%o to-0.19%o) similar to the associated amphibole-rich blackwalls (-0.47%o to-0.23%o). The light Fe isotope signature of the jadeitites is not induced by weathering, kinetic fractionation, dissolution of Fe-rich carbonate, or mineral dissolution-reprecipitation process. Instead, it most likely reflects the Fe isotope variation of serpentinite-derived fluids. Correlations of 656Fe with REEs, Sb, and U in white jadeitites suggest that the jadeitite-forming fluid is a mixture of serpentinite-derived and sediment-derived fluids. In contrast, correlations of 656Fe with Si, Al, and Cr in green jadeitites record the interaction of low-656Fe fluids with chromite. Considering the abundance of the serpentinite-derived fluid, it can effectively influence the Fe isotope compositions of the ambient mantle and arc lavas.