『Abstract
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 oxygenerated 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 intrabasional (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
(τTij), 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.
Key words: Paleoweathering; Neoarchean; Northeastern Minnesota;
Saprolite; Biosignatures』
1. Introduction
1.1. Important questions
2. Geologic setting
2.1. Geochronology
2.2. Field description and sampling strategy
2.3. Saganaga Tonalite profile
2.4. Paulson Lake metabasalt profile
3. Methods
4. Results
4.1. Petrographic analysis
4.1.1. Saganaga Tonalite profile
4.1.2. Metabasalt profile
4.1.3. Ogishkemuncie conglomerate
4.2. Electron microprobe analysis
4.2.1. sample G357A, least-weathered Saganaga Tonalite
4.2.2. sample G357F, most-weathered Saganaga Tonalite
4.2.3. Additional observations, Saganaga Tonalite
4.2.4. Sample G362A, least-weathered metabasalt
4.2.5. Sample G362C, most-weathered metabasalt
4.3. Whole-rock geochemistry
4.3.1. Geochemcal patterns
5. Interpretation and discussion
5.1. Paleoweathering evidence
5.2. Trace element “biosignatures”
5.3. Estimating 2.69 Ga paleoatmospheric pO2
5.4. Estimating 2.69 Ga paleoatmospheric pCO2
6. Summary
Acknowledgments
Appendix A. Supplementary data
References