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

Helen M Williams; Simon Matthews; Hanika Rizo; Oliver Shorttle

doi:10.1126/sciadv.abc7394

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

Sci Adv. 2021 Mar 12;7(11):eabc7394. doi: 10.1126/sciadv.abc7394. Print 2021 Mar.

Authors

Helen M Williams¹, Simon Matthews², Hanika Rizo³, Oliver Shorttle^{2

4}

Affiliations

¹ Department of Earth Sciences, The University of Cambridge, Cambridge, UK. hmw20@cam.ac.uk.
² Department of Earth Sciences, The University of Cambridge, Cambridge, UK.
³ Department of Earth Sciences, Carleton University, Ottawa, ON, Canada.
⁴ Institute of Astronomy, The University of Cambridge, Cambridge, UK.

Abstract

The differentiation of Earth ~4.5 billion years (Ga) ago is believed to have culminated in magma ocean crystallization, crystal-liquid separation, and the formation of mineralogically distinct mantle reservoirs. However, the magma ocean model remains difficult to validate because of the scarcity of geochemical tracers of lower mantle mineralogy. The Fe isotope compositions (δ⁵⁷Fe) of ancient mafic rocks can be used to reconstruct the mineralogy of their mantle source regions. We present Fe isotope data for 3.7-Ga metabasalts from the Isua Supracrustal Belt (Greenland). The δ⁵⁷Fe signatures of these samples extend to values elevated relative to modern equivalents and define strong correlations with fluid-immobile trace elements and tungsten isotope anomalies (μ¹⁸²W). Phase equilibria models demonstrate that these features can be explained by melting of a magma ocean cumulate component in the upper mantle. Similar processes may operate today, as evidenced by the δ⁵⁷Fe and μ¹⁸²W heterogeneity of modern oceanic basalts.

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