Ruthenium isotope vestige of Earth’s pre-late-veneer mantle preserved in Archaean rocks

Mario Fischer-Gödde (), Bo-Magnus Elfers, Carsten Münker, Kristoffer Szilas, Wolfgang D. Maier, Nils Messling, Tomoaki Morishita, Martin Kranendonk and Hugh Smithies
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Mario Fischer-Gödde: University of Cologne
Bo-Magnus Elfers: University of Cologne
Carsten Münker: University of Cologne
Kristoffer Szilas: University of Copenhagen
Wolfgang D. Maier: Cardiff University
Nils Messling: University of Cologne
Tomoaki Morishita: Kanazawa University
Martin Kranendonk: University of New South Wales
Hugh Smithies: Geological Survey of Western Australia

Nature, 2020, vol. 579, issue 7798, 240-244

Abstract: Abstract The accretion of volatile-rich material from the outer Solar System represents a crucial prerequisite for Earth to develop oceans and become a habitable planet1–4. However, the timing of this accretion remains controversial5–8. It has been proposed that volatile elements were added to Earth by the late accretion of a late veneer consisting of carbonaceous-chondrite-like material after core formation had ceased6,9,10. This view could not be reconciled with the ruthenium (Ru) isotope composition of carbonaceous chondrites5,11, which is distinct from that of the modern mantle12, or of any known meteorite group5. As a possible solution, Earth’s pre-late-veneer mantle could already have contained a fraction of Ru that was not fully extracted by core formation13. The presence of such pre-late-veneer Ru can only be established if its isotope composition is distinct from that of the modern mantle. Here we report the first high-precision, mass-independent Ru isotope compositions for Eoarchaean ultramafic rocks from southwest Greenland, which display a relative 100Ru excess of 22 parts per million compared with the modern mantle value. This 100Ru excess indicates that the source of the Eoarchaean rocks already contained a substantial fraction of Ru before the accretion of the late veneer. By 3.7 billion years ago, the mantle beneath southwest Greenland had not yet fully equilibrated with late accreted material. Otherwise, no Ru isotopic difference relative to the modern mantle would be observed. If constraints from other highly siderophile elements besides Ru are also considered14, the composition of the modern mantle can only be reconciled if the late veneer contained substantial amounts of carbonaceous-chondrite-like materials with their characteristic 100Ru deficits. These data therefore relax previous constraints on the late veneer and are consistent with volatile-rich material from the outer Solar System being delivered to Earth during late accretion.

Date: 2020
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DOI: 10.1038/s41586-020-2069-3

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