The discovery of mass independent fraction (MIF) of the sulfur isotopes in pre-2.45 Ga sulfides and sulfates opened a new window on the chemical evolution of the atmosphere-ocean system. At present, the MIF data for sulfur in Archean sediments are few and contradictory. We propose to undertake a thorough study of the MIF of sulfur in pyrite for three pairs of highly carbonaceous sedimentary rock units deposited between 2.7 and 2.5 Ga in Western Australia, South Africa, and Canada. Pyrite will be selected for this study on the basis of textural, chemical, and isotopic evidence indicative of a syngenetic or diagenetic origin. Pyrite in each of the six rock units will be dated by the Re-Os method. Preliminary data show that it is possible to determine the age of 2.3 Ga sedimentary pyrites with a precision of 1 m.y. Initial 187Os/ 188Os ratios inferred from the pyrite isochrons will provide information on secular variations in the Os isotopic composition of seawater, which probably reflects changes in oxidative weathering and transport of Os from the continents. The dating of Archean sedimentary pyrite can therefore yield important information regarding Os cycling. The MIF measurements will be made on the MAT 253 mass spectrometer at the Geophysical Laboratory in Washington, D.C, using laser fluorination and continuous flow inlet technology. The Re-Os dating will be done at the AIRIE laboratory at Colorado State University (CSU) using Carius tube dissolution and negative thermal ionization mass spectrometry. Sample preparation, microscopy and trace element analyses will be done at Harvard University. We expect that our data will demonstrate to what extent local conditions determined the MIF of the sulfur isotopes in these rock units, to what extent global differences were important, and to what extent global differences reflected changes in the composition of the atmosphere during the last 200 Ma of the Archean. Broader Impacts Criterion The project will promote training and learning through involvement of students at all levels (postdoctoral, graduate, undergraduate, and high school) with at least the younger students drawn from underrepresented groups. All will be engaged in application of state-of-the-art techniques, and will be encouraged to present their findings at professional meetings. New collaborations are established through this project between Harvard University and CSU, among researchers on three continents, and across geologic sub-disciplines. Dissemination of results will reach beyond immediate research communities, as the Harvard group reports to NASA through participation on the Astrobiology Team, and the AIRIE Program at CSU is based on international collaboration, and transfers new techniques and concepts quickly to applications of direct use in mineral exploration.
