Abstract EANA2026-254

The anoxic, ferruginous, and silica-saturated early Archean ocean hosted a unique geochemical regime. Theoretical models suggest highly reactive mixed-valent iron minerals, particularly green rust (GR), drove the early Earth iron cycle. However, GR's extreme thermodynamic instability causes irreversible dehydration and phase transitions during deep burial, creating a taphonomic paradox and leaving a void of direct physical evidence in the deep-time rock record. Here, we present the first nanoscale physical and crystallographic evidence of authigenic metastable sulfate GR preserved in 3.4 Ga cherts from the Strelley Pool Formation, Western Australia. To overcome billions of years of geological alteration, we employed a state-of-the-art multiscale in-situ correlative workflow combining Focused Ion Beam (FIB) milling, Transmission Electron Microscopy (TEM), and Atom Probe Tomography (APT). Targeted FIB/STEM-EDS mapping reveals a sharp spatial decoupling between the intact 3D reticulate iron framework and the amorphous silica matrix, confirming an independent authigenic iron phase. HRTEM identified ultra-large basal d-spacings (1.06–1.10 nm) characteristic of layered double hydroxides, definitively matching sulfate GR. Furthermore, APT atomic-scale 3D reconstructions identified a highly localized segregation of trace phosphorus strictly confined to a sharp 1–2 nm Fe-Si interface, lacking any thermodynamic diffusion halos. This nanometric P-enrichment acts as a "kinetic stopwatch," capturing the transient interface state during initial surface complexation. It quantitatively demonstrates that the massive infiltration and rapid flocculation of oversaturated amorphous silica occurred almost instantaneously. This ultra-early in-situ silicification entombment created a rigid 3D isochoric confinement, arresting the thermodynamic degradation of the metastable GR precursor. Ultimately, this unique taphonomic mechanism solves the paradox of metastable mineral preservation in deep time. By physically shielding delicate aqueous phases from subsequent diagenetic overprinting, this early silicification process provides an exceptional window into the pristine biogeochemical conditions of the Precambrian oceans, offering the oldest direct material baseline for the coupled iron-silica cycle.