| Summary: | A 40-80 m thick pyrophyllite (AlSi2O5(OH))-rich alteration zone is developed beneath the oldest (~3.43 Ga) unconformity surface at Trendall Ridge in the North Pole Dome region of the East Pilbara Craton, Western Australia. The purpose of this work is to determine: 1) when this alteration occurred (>3.5 Ga, ~3.43 Ga, or <3.4 Ga); and 2) whether it was produced by submarine/subaerial hydrothermal activity or subaerial weathering (e.g., paleosol, modern soil). To answer these questions a combination of field and laboratory techniques were utilized including mapping, bulk chemical analysis, powder XRD, thin section analysis, and geochronology. The study has revealed that the alteration is divisible into 4 distinct zones by changes in chemistry and mineralogy: IV) the least altered parent (submarine andesite to basaltic andesite) at the base of the profile (>40-80m below the unconformity) which is characterized by chlorite and muscovite replacement of the original mineralogy, as well as probable losses in Ca and Na; III) a lower Fe-poor zone (17 - <40-80m) which contains poorly crystalline silica and muscovite with minor pyrophyllite and shows increased concentrations of Si, K, Na and depletions of other major elements and REEs; II) a horizon of Fe-enriched (up to 81 wt% Fe2O3) pillowed volcanics (0-17 m below the unconformity and 1 6 m thick) that contain pyrophyllite, muscovite, minor quartz, and enrichments of LREEs; and I) an upper Fe-poor zone (0 17m) which is enriched in pyrophyllite, poorly crystalline silica and muscovite but depleted in all major elements except Si and K. Other notable features of the alteration zone include: a loss of original volcanic mineral texture with increasing proximity to the unconformity; a significant loss of volume (40 60%) in Zones I and II; the confinement of the pyrophyllite-rich zone to below the unconformity; a lack hydrothermal features (i.e. veins); possible soil textures; and the presence of possible rip-up clasts of the alteration in conglomerates overlying the unconformity surface. Such features are consistent with other Archean paleosols. Sm-Nd and Rb-Sr dating has revealed that the alteration zone underwent a loss of all major elements at 3.47±0.46 Ga, contemporaneously with the development of the erosional unconformity surface, as well as an addition of Si, K, and Na by groundwater at 2.76±0.15 Ga. The geological, geochemical, geochronological, and mineralogical features of the alteration zone strongly suggest that it represents a paleosol that developed at ~3.4 Ga. Paleosols are physical records of the condition of the surface environments, composition of the atmosphere, and the nature of the biosphere present during their formation. The Warrawoona paleosol at Trendall Ridge is at least 250 Myr older than any other previously studied paleosol, and would provide insight into a poorly understood period of Earths early history when life was emerging.
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