Paleoarchean seawater and seafloor hydrothermal processes: insights from 3.5 to 3.3 Ga carbonate geochemistry

Carbonates (3.5 to 3.3 Ga) in the East Pilbara Terrane (EPT), Western Australia, including interstitial carbonate between pillow basalts, fracture-filling calcite, sedimentary carbonates and carbonate associated with stromatolites, provide valuable geochemical archives for reconstructing Early Earth environments. This study highlights three key findings: (1) Fracture-filling calcite D-2-W from the 3.48 Ga Dresser Formation acts as a proxy for Paleoarchean shallow seawater geochemistry. It exhibits high Sr (1789 μg/g), δ¹³C (+2.20‰), δ¹⁸O (-13.03‰), and age-corrected ⁸⁷Sr/⁸⁶Sr (0.700596), alongside low concentrations of rare earth elements (REE) and Y (briefly REE+Y), near-chondritic Y/Ho ratios, and shale-normalized REE+Y patterns characterized by heavy REE enrichment, positive La and Y anomalies, and absence of Ce and Eu anomalies. These features reflect a shallow marine setting likely influenced by anoxygenic photosynthetic processes and low-intensity volcanic-hydrothermal interactions. (2) EPT interstitial carbonates reliably trace Paleoarchean seafloor hydrothermal systems driven by basalt-seawater interactions. During hydrothermal alteration, water-rock reactions enriched carbonates in basalt-derived trace elements (Mg, Fe, Mn, light REE), lowered Sr and δ¹³C, and elevated ⁸⁷Sr/⁸⁶Sr and Eu/Eu*. (3) Post-depositional alterations of the interstitial carbonates exhibit distinct multi-element behaviors: recrystallization, often driven by low-temperature hydrothermal fluids, subtly shifts original geochemical compositions while preserving REE+Y patterns and δ¹³C values. In contrast, ankeritization, induced by high-temperature fluids, resulting in significantly elevated abundances of basalt-derived elements, altered REE+Y patterns with middle REE enrichment and pronounced positive Eu anomalies, and markedly higher ⁸⁷Sr/⁸⁶Sr ratios. Consequently, this study challenges the classical paradigm of using singular isotopic proxies (e.g., ⁸⁷Sr/⁸⁶Sr) to trace continental emergence, by establishing key geochemical fingerprints to discriminate between seawater and hydrothermal signatures in Archean carbonates. These insights provide critical calibration points for modeling early Earth ocean-tectonic evolution.