Ru isotope vestige of Earth’s pre-late veneer mantle preserved in Archean rocks

• Published in: Nature
• Main Authors: Fischer-Gödde, Mario, Elfers, Bo-Magnus, Münker, Carsten, Szilas, Kristoffer, Maier, Wolfgang D., Messling, Nils, Morishita, Tomoaki, Van Kranendonk, Martin, Smithies, Hugh
• Format: Text
• Language: English
• Published: 2020
• Subjects: Article; Greenland
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7212018/; http://www.ncbi.nlm.nih.gov/pubmed/32161386; https://doi.org/10.1038/s41586-020-2069-3

The accretion of volatile-rich material from the outer solar system represents a crucial prerequisite for Earth developing oceans and becoming a habitable planet(1–4). However, the timing of this accretion remains controversial(5–8). It was proposed that volatile elements were added to Earth by late accretion of a late veneer consisting of carbonaceous chondrite-like material after core formation had ceased(6,9,10). This view, however, could not be reconciled with the distinct ruthenium (Ru) isotope composition of carbonaceous chondrites(5,11) compared to the modern mantle(12), and in fact also not with any known meteorite group(5). As a possible solution, Earth’s pre-late veneer mantle could already have contained a significant amount of Ru that was not fully extracted by core formation(13). The presence of such pre-late veneer Ru could only be proven if its isotope composition would be distinct from that of the modern mantle. Here we report the first high-precision mass-independent Ru isotope compositions for Eoarchean ultramafic rocks from SW Greenland, which display a relative (100)Ru excess of +22 parts per million compared to the modern mantle value. This (100)Ru excess indicates that the source of the Eoarchean rocks already contained a significant fraction of Ru prior to the late veneer. By 3.7 Gyr the mantle beneath the SW Greenland rocks had not yet fully equilibrated with late accreted material. Otherwise, no Ru isotopic difference relative to the modern mantle would be observed. By considering constraints from other highly siderophile elements beyond Ru(14), the composition of the modern mantle can only be reconciled if the late veneer contained significant portions of carbonaceous chondrite-like materials with their characteristic (100)Ru deficits. These data therefore relax previous constraints on the late veneer and now permit that volatile-rich material from the outer solar system was delivered to Earth during late accretion.