Iron isotopes of ocean island basalts as tracers of mantle melting pressure

Iron (Fe) isotope fractionation during high-pressure (P > 7 GPa; i.e., 200 km) mantle melting remains poorly constrained, limiting the use of Fe isotopes to probe the genesis of ocean island basalts (OIBs). Here, we experimentally constrain Fe isotope fractionation of almandine (Fe ²⁺ ₃ Al ₂ Si ₃ O ₁₂ ) and andradite (Ca ₃ Fe ³⁺ ₂ Si ₃ O ₁₂ ) relevant to upper mantle conditions by determining the Fe force constant ⟨F⟩ values using diamond anvil cell experiments combined with synchrotron-based nuclear resonant inelastic X-ray scattering. Based on the framework of Dauphas et al. (2014), we model Fe isotope fractionation during mantle melting over a range of upper-mantle pressures. We then calculate the Fe isotope compositions of parental melts produced by partial melting of two end-member mantle lithologies, garnet peridotite and eclogite, and compare these results with fractional crystallization–corrected δ ⁵⁷ Fe values of natural OIBs. Our results indicate that Fe isotopes provide a promising proxy for discriminating among relative mantle melting pressures and for identifying OIBs with deep-Earth origins.