M.A. Jirsa M. Ren S.L. Brantley N.D. Sheldon Dana C. Parker M. Schmitz S.G. Driese 2011 <p><span>Field and laboratory investigations of a 2690.83</span><span>&nbsp;</span><span>Ma (</span>²⁰⁷<span>Pb/</span>²⁰⁶<span>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 5–50</span><span>&nbsp;</span><span>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 TiO</span>₂<span>&nbsp;during weathering (</span><i>τ</i>_Ti,<i>j</i><span>), show depletion of SiO</span>₂<span>, Al</span>₂<span>O</span>₃<span>, Na</span>₂<span>O, CaO, MgO, and MnO, and to a lesser degree of K</span>₂<span>O, relative to least-weathered parent materials. Significant Fe was lost from the tonalite. A paleoatmospheric pCO</span>₂<span>&nbsp;of 10–50 times PAL is estimated based on geochemical mass-balance of the tonalite profile and assuming a formation time of 50–500</span><span>&nbsp;</span><span>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 pO</span>₂<span>&nbsp;than present day at the surface. Comparison with previous studies of weathered tonalite and basalt (Denison, 2.45–2.22</span><span>&nbsp;</span><span>Ga) in Ontario, Canada, reveal general similarities in paleoweathering with our study, as well as important differences related to lower paleoatmospheric pO</span>₂<span>&nbsp;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.</span></p> application/pdf 10.1016/j.precamres.2011.04.003 en Elsevier Neoarchean paleoweathering of tonalite and metabasalt: Implications for reconstructions of 2.69Ga early terrestrial ecosystems and paleoatmospheric chemistry article