U-Th-Pb isotope data indicate phanerozoic age for oxidation of the 3.4 Ga Apex Basalt

The occurrence of ferric oxides in Archean rocks has played an important role in discussions on the amount of free oxygen in the atmosphere of the ancient Earth. Recognizing that post-Archean weathering may also produce oxide minerals, drill cores have been used to obtain samples beneath the depth of Phanerozoic weathering. The first core of the Archean Biosphere Drilling Project (ABDP-1) documented hematite as alteration products in 3.4 Ga basalts from the Marble Bar area of the Pilbara Craton, NW Australia, and this has been used to infer the presence of an O-2-bearing atmosphere in the Archean. It is possible, however, that despite recovery of samples from >100 m depth, oxidation of the basalts occurred much younger than the depositional age. In this study, the age of oxidation of the Apex Basalt from the ABDP-1 drill core at Marble Bar is constrained by U-Th-Pb geochronology. Lead and U concentrations of the basalts from the ABDP-1 drill core vary greatly, between <1-58 ppm and 0.08-1.04 ppm, respectively, whereas Th contents are more restricted (0.24-0.71 ppm). Pb-206/Pb-204 ratios are non-radiogenic and vary from 12.44 to 14.69. The linear array in terms of Pb-206/Pb-204-Pb-207/Pb-204 variations does not reflect an age but reflects two-component mixing between a non-radiogenic "ore lead" end member and a radiogenic "basalt lead" end member. The samples do not form isochrons on U-238/Pb-204-Pb-206/Pb-204, U-235/Pb-204-Pb-207/Pb-204, or Th-232/Pb-204-Pb-208/Pb-204 diagrams, indicating post-formation U and Pb addition. Comparison of measured U/Th ratios with "model" U/Th ratios calculated based on Pb-208/Pb-204-Pb-206/Pb-204 variations indicates that U enrichment most likely occurred in the last 200 Ma. The degree of U enrichment in the samples is correlated with Fe((III))/Fe-Total ratios, indicating that U addition and oxidation were related, most likely reflecting penetration of oxygenated surface waters in the Phanerozoic along bedding planes and shear zones. These results, therefore, indicate that oxidation of the Apex Basalt did not occur in the Archean, and therefore cannot be used to infer an oxygenated atmosphere at that time. (C) 2011 Elsevier B.V. All rights reserved.