Evidence for Eoarchean crustal recycling from in-situ sulfur and lead isotope analyses of peridotite enclaves (southern West Greenland)

The timing of the onset of volatile recycling on Earth remains controversial. Eoarchean peridotite enclaves located south of the Isua Supracrustal Belt in southern West Greenland have been interpreted by some studies to represent Eoarchean mantle with bulk sulfur isotope compositions indicating incorporation of surface-derived, mass-independently fractionated sulfur (MIF-S), pointing to Eoarchean volatile recycling. However, the age of sulfide growth in these rocks has not been constrained. If MIF-S in the peridotites is significantly younger than the bulk rocks, it would not constitute evidence for Eoarchean volatile recycling. Here, we seek to confirm the Eoarchean origin of the MIF-S in the peridotites through electron microprobe and secondary electron microscopy based observations of sulfides, accompanied by in-situ lead and sulfur isotope analysis via secondary ion mass spectrometry. One group of sulfides shows evidence for metasomatism delivering sediment-derived sulfur with weighted average Δ 33 S compositions of 0.20‰ ± 0.02‰. A second group shows evidence for subsequent overprint and has weighted average Δ 33 S compositions of 0.09‰ ± 0.03‰. The sulfides yield unradiogenic Pb isotope compositions, including 206 Pb/ 204 Pb as low as 11.85 and 207 Pb/ 204 Pb as low as 13.08. Pb isotope compositions range in 206 Pb/ 204 Pb up to 19.08 and in 207 Pb/ 204 Pb up to 16.43, indicating partial disturbance during ca. 2.7 Ga metamorphism. Textural observations indicate an overgrowth of secondary amphibole over sulfides. Sulfide minerals observed are consistent with an origin in Earth’s mantle. These results support interpretations that the peridotites preserve Eoarchean sulfide grains hosting MIF-S, pointing to Eoarchean volatile recycling.