Neoarchean Paleoweathering of Tonalite and Metabasalt: Implications for Reconstructions of 2.69 Ga Early Terrestrial Ecosystems and Paleoatmospheric Chemistry

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Field and laboratory investigations of a 2690.83 Ma (207Pb/206Pb age of Saganaga Tonalite) unconformity exposed in outcrop in northeastern Minnesota, USA, reveal evidence for development of a deep paleoweathering profile with geochemical biosignatures consistent with the presence of microbial communities and weakly oxygenated conditions. Weathering profiles are characterized by a 5–50 m thick regolith that consists of saprolitized Saganaga Tonalite and Paulson Lake succession basaltic metavolcanic rocks retaining rock structure, which is cross-cut by a major unconformity surface marking development of a successor basin infilled with alluvial deposits. The regolith and unconformity are overlain by thick conglomerate deposits that contain both intrabasinal (saprock) as well as extrabasinal detritus. Thin-section microscopy and electron microprobe analyses reveal extensive hydrolysis and sericitization of feldspars, exfoliation and chloritization of biotite, and weathering of Fe-Mg silicates and Cu-Fe sulfides; weathering of Fe-Ti oxides was relatively less intense than for other minerals and evidence was found for precipitation of Fe oxides. Geochemical analyses of the tonalite, assuming immobile TiO2 during weathering (τTi,j), show depletion of SiO2, Al2O3, Na2O, CaO, MgO, and MnO, and to a lesser degree of K2O, relative to least-weathered parent materials. Significant Fe was lost from the tonalite. A paleoatmospheric pCO2 of 10–50 times PAL is estimated based on geochemical mass-balance of the tonalite profile and assuming a formation time of 50–500 Kyr. Interpretations of metabasalt paleoweathering are complicated by additions of sediment to the profile and extensive diagenetic carbonate (dolomite) overprinting. Patterns of release of P and Fe and retention of Y and Cu in tonalite are consistent with recent laboratory experiments of granite weathering, and with the presence of acidic conditions in the presence of organic ligands (produced, for example, by a primitive microbial community) during weathering. Cu metal in the profile may document lower pO2 than present day at the surface. Comparison with previous studies of weathered tonalite and basalt (Denison, 2.45–2.22 Ga) in Ontario, Canada, reveal general similarities in paleoweathering with our study, as well as important differences related to lower paleoatmospheric pO2 and terrestrial biosignature for the older Minnesota profile. A falling water table in the Alpine Lake locality is presumed to have promoted formation of this gossan-like deep-weathering system that extends to 50-m depth.