Sulfur isotope evidence from peridotite enclaves in southern West Greenland for recycling of surface material into Eoarchean depleted mantle domains

Abstract

Eoarchean peridotite enclaves found in southern West Greenland's Itsaq Gneiss Complex (IGC) represent an important and valuable record of Earth's early geodynamic history. However, the origins of these rocks and the processes acting on them in the Eoarchean remain a subject of debate. Some researchers have proposed that these peridotites represent the oldest preserved pieces of Earth's mantle, while others have suggested that they represent ultramafic cumulates. Similarly, the geodynamic context in which they formed and were emplaced is subject to multiple interpretations. Some researchers argue that only vertical tectonic processes operated in the Eoarchean and others contend that these peridotites were embedded in the crust by horizontal tectonic processes. We present multiple sulfur isotope data from IGC peridotite enclaves, offering new insights into these rocks' origins and tectonic processes in the early Earth. Generally small but significant nonzero Δ33S values of 0.04 to 0.21‰ were detected in the studied peridotites. Peridotites with almost no petrographic signs of metasomatic alteration have on average slightly higher Δ33S values, whereas peridotites with clear petrographic and geochemical signatures of melt metasomatism were found to have slightly lower Δ33S values. These Δ33S values point to incorporation of surface-derived material of Archean age or older. Correlations between Δ33S values and previously published major and trace element data support the view that these peridotites were subject to hydrous melt depletion of incompatible elements, followed by variable melt re-enrichment. Notably, a distinct correlation also exists between previously published Hf isotope data and Δ33S values, indicating a depleted mantle source for melt percolating through the peridotites in the Eoarchean. The sulfur isotope data demonstrate that these processes included the introduction of surface-derived sulfur, pointing to an early onset of crustal recycling. These findings point to the existence of depleted mantle domains in the Hadean and suggest that crustal recycling processes operated during the Eoarchean or earlier.