Iron isotopes trace primordial magma ocean cumulates melting in Earth’s upper mantle

Williams, Matthews, Rizo, Shorttle, Science Advances 7(11), eabc7394 (2021). doi: 10.1126/sciadv.abc7394

Evidence for an ancient magma ocean on Earth is preserved in the geochemistry of 3.7 billion year old metabasalts from Isua, Greenland. Previously work suggested these rocks are derived from melting a mantle source formed by Bridgmanite crystallisation and accumulation in the lower mantle. Bridgmanite crystallisation has previously been proposed to result in oxidation of the mantle, as it incorporates Fe3+ into its crystal structure, even when the magma it crystallises from contains only Fe2+. To balance the reaction Fe-metal is produced, which could be extracted efficiently to the core.

This reaction is thought to be associated with a fractionation in Fe-isotopes, such that the remaining bridgmanite enriched mantle should have an excess of 57Fe and 56Fe over 54Fe. In this study we demonstrated that such a fractionation is present within the Isua rocks, providing further confirmation for these magma ocean processes having taken place on the early Earth. However, the concentration of trace elements in the lavas suggest a more complex process, involving remelting and recrystallisation in the lower mantle.

To explore the consequences of these processes further, we performed THERMOCALC phase-equilibria modelling to determine how these magma-ocean derived mantle lithologies will melt in the upper mantle. We found that our hypothesised source for the Isua metabasalts melts at an anomalously low temperature. This would likely mean that such heterogeneities were rapidly melted out of the mantle, their evidence being largely erased.

The image is Figure 5 in the manuscript. It shows one such iteration of the phase-equilibria calculations. See the manuscript for further explanation.

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