In-situ analysis of lead and multiple sulfur isotopes in southern West Greenland peridotite sulfide grains reveal evidence for Eoarchean crustal recycling

Eoarchean peridotites from the area south of the Isua Supracrustal Belt (SOISB) in southern West Greenland have been found by previous studies to contain sulfur subject to mass independent fractionation (MIF-S), with positive Δ³³S values indicating that these rocks have incorporated sedimentary sulfur recycled from Earth’s surface [1]. New in-situ secondary ion mass spectrometry measurements of sulfide grains found within these peridotites reveal that this MIF-S is hosted within the sulfide grains. Electron microprobe analyses of the sulfide grains reveal that they are predominantly composed of pentlandite and pyrrhotite, consistent with the typical sulfide mineralogy of mantle rocks. The peridotites displaying the least petrographic evidence for melt overprint (Group 1) were found to contain the highest average in-situ Δ³³S values, +0.20±0.02‰, while those bearing more evidence for melt overprint (Group 2) had lower average in-situ Δ³³S values, +0.09±0.03‰. These findings are in good agreement with previous bulk rock S isotope results [1]. In-situ Pb isotope measurements of the sulfide grains reveal unradiogenic compositions consistent with an Archean origin. Furthermore, petrographic observations reveal that the sulfide grains are crosscut by amphiboles, indicating that the sulfide grains predate Neoarchean amphibolite-facies metamorphism. Our data support previous interpretations that these rocks are the oldest known mantle peridotites. These findings reinforce previous interpretations that SOISB peridotites preserve evidence of crustal recycling in the Eoarchean.

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