Anna Brooks, BSc
A. Brooks1, C. Santos1, C. Partin1
1Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
The Earth has undergone fundamental changes in oxygenation throughout its history, with the first significant accumulation of atmospheric oxygen occurring in the Great Oxidation Event (GOE) at ca. 2.4 Ga. A major source of evidence for the GOE is the appearance of red beds deposited in fluvial-alluvial environments which contain oxidized minerals such as hematite. An important component of evidence that is often overlooked is whether the mineral constituents of red beds represent primary oxygenation from the atmosphere or were inherited from a source rock. To draw conclusions on the atmospheric state, hematite or its precursors must precipitate at the time of deposition in the form of detrital ferric hydroxides, or shortly after during shallow burial and infiltration of sediment by groundwater containing ferromagnesian minerals.
The Belcher Group of Nunavut, Canada, was deposited ca 2.0 to 1.8 Ga and contains alluvial-fluvial red beds that have not been studied in detail. The main hypothesis is that these rocks will show evidence of oxidative weathering, but a competing alternative hypothesis is that these rocks will not contain evidence of oxidative weathering because of a Deoxidation Event that began around 2 billion years ago. Red bed samples from the Belcher Group are compared with samples from the similarly aged red beds of the Missi Group and Martin Group, both in northern Saskatchewan, to determine the atmospheric conditions of the Archean cratons of Canada ca. 2.0 - 1.8 Ga. Samples were examined under transmitted and reflected light using a standard optical microscope. Identification of bulk iron oxide composition was performed using powder X-ray diffraction. Iron oxide phases showing promising habits were further distinguished using Raman microscopy. Backscatter imaging and microprobe analysis were carried out to highlight representative textures for each sample and obtain additional geochemical data.
Evidence of oxidizing conditions on the Archean cratons of Canada during the deposition and formation of the red beds of the Belcher, Missi, and Martin groups is present in the form of authigenic hematite as black rims on quartz, as alteration of phylosilicates such as biotite and muscovite, and as martitization of detrital magnetite grains. However, red beds provide only qualitative information on atmospheric oxygenation; more quantitative evidence is required to determine the magnitude of a deoxidation event.