Fractional crystallization causes the iron isotope contrast between mid-ocean ridge basalts and abyssal peridotites

The iron isotope contrast between mid-ocean ridge basalts and abyssal peridotites is far greater than can be explained by mantle melting alone. Here we investigate a suite of mid-ocean ridge magma chamber rocks sampled by the Ocean Drilling Project Hole 735B in the Atlantis Bank of the Indian Ocean. We report major and trace element geochemistry from these rocks and measure their iron isotope compositions to investigate the potential role of fractional crystallization during melt evolution. We observe a large range of delta Fe-56 that defines a significant inverse curvilinear correlation with bulk rock MgO/FeOT. These data confirm that delta Fe-56 in the melt increases as fractional crystallization proceeds but, contrary to expectation, delta Fe-56 continues to increase even when oxides begin to crystallize. We conclude that iron isotope fractionation through fractional crystallization during the evolution of mid-ocean ridge basalts from abyssal peridotites reconciles the disparity in isotopic compositions between these two lithologies. Fractional crystallization during the evolution of mid-ocean ridge basalts can explain why they are so different in iron isotopic composition from abyssal peridotites, according to chemical analyses of drill core samples from the Atlantis Bank.