No mantle residues in the Isua Supracrustal Belt

A critical component of our understanding of the evolution of Earth's mantle comes from rocks identified as direct mantle samples. Eoarchaean dunite lenses from the Isua Supracrustal Belt (ISB), North Atlantic Craton, Greenland, have been previously interpreted as depleted mantle wedge residues, complementary to arc-like volcanic rocks in the supracrustal sequence. This would place the ISB dunites among Earth's oldest mantle samples. We present new major element, platinum-group element (PGE) and Re-Os isotopic data for the ISB dunites, and critically assess the criteria previously used to invoke a mantle origin for the dunites. We find no evidence that uniquely supports a mantle origin. Instead, evidence of chromite and Os-Ir alloy fractionation, consistent Pt and Pd depletion, elevated Ni contents, and trace element systematics indicate that the dunites formed as olivine ± chromite cumulates with varying amounts of intercumulus melt. Their compositions indicate crystallisation from magmas represented by ISB volcanic rocks, and their Re-Os model ages overlap the ∼3720 Ma age of the volcanic sequence, consistent with the dunites representing magma chambers or conduits that fed the volcanic eruptions. Formation of the Isua dunites as cumulates removes an important line of evidence used to interpret the ISB as an ophiolite, and highlights the risks of using criteria that do not discriminate mantle residues from olivine-rich cumulates. Extending this reasoning to other Eoarchaean crustal peridotites previously identified as mantle rocks suggests there may be no mantle residues anywhere in the Itsaq Gneiss Complex, and that the oldest mantle samples may only be found as xenoliths in volcanic rocks.