Secular Evolution of Mantle Sulfur Isotopes Since the Archean

Sulfur isotopes in mantle plume‐derived magmas show heterogeneities attributed to recycling of material from the Earth's surface. The sedimentary sulfur isotope record exhibits dramatic temporal variations over Earth history, raising the question of whether its secular evolution is echoed in mantle plume‐derived magmas. We present new secondary ion mass spectrometry and X‐ray absorption near‐edge structure measurements of δ 34 S and Fe 3+ /ΣFe in naturally glassy melt inclusions from the 2.7 Ga Belingwe komatiites. We find that δ 34 S of Belingwe melt inclusions is relatively homogeneous (+3.2 ± 0.9) and elevated relative to the depleted upper mantle. We evaluate explanations for elevated magmatic δ 34 S including degassing, sulfide fractionation, assimilation, subduction‐like processes, and mantle recycling. We find no evidence for significant sulfur isotope fractionation via degassing or sulfide saturation. The elevated δ 34 S in the Belingwe komatiites could reflect late‐stage assimilation of sediments or seawater. Alternatively, Belingwe komatiites may have formed in a subduction‐like setting, as modern arcs display a bias toward positive δ 34 S. However, we find these scenarios less favorable. Instead, our preferred interpretation is that elevated δ 34 S was supplied to the komatiite mantle source via recycled lithologies such as sediments, altered oceanic crust, and/or pyroxenite. Combined with δ 34 S data from mantle plume‐derived magmas spanning a wide age range, our results resemble the secular evolution of δ 34 S in surface reservoirs. We suggest that the δ 34 S record in mantle plume‐derived magmas may echo secular evolution in the surficial sulfur reservoir with a delay of several hundred million years, linking Earth's surface and interior sulfur cycles.