Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69Ga early terrestrial ecosystems and paleoatmospheric chemistry

摘要

Field and laboratory investigations of a 2690.83Ma (~(207)Pb/~(206)Pb 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 5-50m 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 TiO_2 during weathering (τ_(Ti,j)), show depletion of SiO_2, Al_2O_3, Na_2O, CaO, MgO, and MnO, and to a lesser degree of K_2O, relative to least-weathered parent materials. Significant Fe was lost from the tonalite. A paleoatmospheric pCO_2 of 10-50 times PAL is estimated based on geochemical mass-balance of the tonalite profile and assuming a formation time of 50-500Kyr. 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 pO_2 than present day at the surface. Comparison with previous studies of weathered tonalite and basalt (Denison, 2.45-2.22Ga) in Ontario, Canada, reveal general similarities in paleoweathering with our study, as well as important differences related to lower paleoatmospheric pO_2 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.