In-situ quadruple sulfur isotopic compositions of pyrites in the ca. 3.2–2.72 Ga metasedimentary rocks from the Pilbara Craton, Western Australia

Abstract

The sulfur mass independent fractionation (S-MIF) of Mesoarchean is distinct from the other Archean periods by the dampened magnitudes of Δ33S and a steeper slope of Δ36S/Δ33S compared with the Archean Reference Array (ARA) according to previous studies. However, samples available of this interval are limited owing to poor or no preservation. Additionally, the majority of the data were obtained by analysis of bulk rocks, thus are essentially the sulfur isotopic compositions of mixed sulfur components due to multiple stages of post-diagenesis overprints and modifications. This study systematically measured the multiple sulfur isotopic compositions of pyrites (nodular, disseminated) in ca. 3.2–2.72 Ga metasedimentary rocks from the Pilbara Craton, Western Australia, where one of the best-preserved Archean records occur, using the upgraded SHRIMP-SI, after detailed pyrite generation and genesis investigation by BSE imaging and NaOCl etching. The results show that Δ33S values of the diagenetic pyrites in these metasedimentary rocks range from −2.4 to 0.8‰. Such a range is inconsistent with the other Archean intervals and any experiment producing S-MIF so far, where the magnitude of the maximum positive Δ33S value is considerably larger than the corresponding minimum negative Δ33S value. The dampened positive Δ33S is explained in several ways from the perspective of production and preservation, including original photochemical products with larger magnitudes of negative Δ33S than positive Δ33S in a different atmospheric composition, non-zero Δ33S of initial SO2, different mechanism producing S-MIF (chain formation), and destruction of positive Δ33S by subduction. The downward Δ36S/Δ33S deviations from ARA in the diagenetic pyrites of 2.72 Ga Tumbiana Formation can be attributed to modifications induced by microbial activities. This is based on the negative (0.9Δ33S + Δ36S) values similar to biological products and the large range of negative δ34S values. The downward Δ36S/Δ33S offsets in the hydrothermally altered diagenetic pyrites of 2.78 Ga Mt. Roe Basalt are most likely mixing of primary sulfur on ARA with later sulfur disobeying ARA. The upward Δ36S/Δ33S deviations in the diagenetic pyrites of 2.93 Ga Mosquito Creek Formation likely reflect different atmospheric composition.