In-situ sulfur and lead isotope evidence for Eoarchean crustal recycling in southern West Greenland peridotites

Previous bulk sulfur isotope measurements of Eoarchean peridotites from south of the Isua Supracrustal Belt in southern West Greenland have revealed evidence for the presence of recycled surface material in the form of nonzero Δ33S values [1]. In order to confirm these results, identify potential heterogeneity related to multiple sulfur sources, and confirm the Eoarchean origins of the sulfides in these rocks, we combined petrography and mineral chemistry with in-situ multiple sulfur and lead isotope analyses of sulfide minerals via SIMS. Sulfides measured by SIMS were found to be composed primarily of pentlandite and pyrrhotite, with some pentlandite-like compositions consistent with horomanite, which is crystallographically distinct from pentlandite. Our in-situ isotope results confirm the existence of predominantly sediment-derived (positive Δ33S) material recycled from Earth’s surface in these rocks. Average SIMS Δ33S results for measured samples were generally in good agreement with previously measured bulk rock data. The lead isotope compositions confirm an Eoarchean age of the sulfides and show the typical high-µ signature that is observed in other Eoarchean rocks from the area [2]. We observe that highly unradiogenic 207Pb/206Pb isotope ratios in some sulfides plot close to the least radiogenic Pb isotope compositions measured in galenas from Isua.

[1] Lewis et al. (2022) EGU 22-5226

[2] Kamber et al. (2003) CMP 145, 25-46.