THE REDOX-SENSITIVE TRACE ELEMENTS, MO, U AND RE IN PRECAMBRIAN CARBONACEOUS SHALES: INDICATORS OF THE GREAT OXIDATION EVENT

104 samples of 2.7-1.6 Ga carbonaceous shales were analyzed for major, minor, and trace elements. In the ca. 2.7 Ga shales of the Ventersdorp Group and the 2.3 Ga Boshoek, Timeball Hill, and Rooihoogte Formations of the Transvaal Supergroup in South Africa the concentration of Mo, U, and Re is close to their crustal average and is not correlated with the organic carbon content of the shales. This is also true for the 2.1-2.2 Ga shales of the Francevillian in Gabon. There is a great deal of scatter in the Mo-C^o, U-C^o, and Re-C^o plots for the 2.0-2.1 Ga shales from Puolanka and Melalahti in Finland. A considerable number of the analyses of these shales fall on or close to the correlation lines in Devonian shales, but many of the high-C^o shales show little or no enrichment in Mo, U, or Re. In the 1.6 Ga shales of the McArthur and Glyde Basins of Australia the concentration of all three elements is well correlated with that of C^o. Their ratios are close to those in Devonian shales. These data are consistent with and support those from other lines of evidence for the evolution of the oxidation state of the atmosphere and oceans. The absence of an enrichment of Mo, U, and Re in > 2.3 Ga carbonaceous shales indicates that O₂ was absent from the atmosphere, that these elements were not oxidized significantly during weathering, and that they were transported to the oceans as detrial grains rather than in solution as oxyanions. The essentially modern relationship between Mo, U, Re and C^o in the 1.6 Ga shales indicates that oxidative weathering and the state of the oceans were similar to those of the present day. The rather scattered data for the 2.0-2.2 Ga shales probably reflects the transition of the oxidation state of the atmosphere and oceans from a pre- 2.3 Ga anoxic to a post- 2.0 Ga oxygenated state.