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

Wanli Xiang, Dr., Jan-Peter Duda, Prof. Dr., Andreas Pack, Prof. Dr., Matthias Willbold, Joachim Reitner

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.