The Stable Isotope and Minor Element Geochemistry of Iron (III) Oxide Rocks and Minerals

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Recent work in this laboratory has shown that the common mineral goethite is an important new low temperature oxygen isotope geothermometer in combination with silica, carbonate or phosphate. A potentially important development in these studies has been the discovery of a CO2 component in natural goethites that might serve as a quantitative indicator of CO2 partial pressures. The proposed studies include experiments to calibrate the temperature dependence of the hematite-water oxygen isotope fractionation, substantiation of the goethite-water oxygen isotope curve, and further investigation of the CO2 component in goethite as a possible indicator of ancient P(CO2) values. Additional efforts will include infrared work to study both the amounts and chemical speciation of the CO2 in goethite. This knowledge will be applied to Phanerozoic oolitic hematitic and goethitic ironstones representing ore-grade concentrations of chemically precipitated, oxidized iron which formed during two extended "pulses" in the Phanerozoic. These pulses generally coincided with periods of high eustatic sea level, flooded continents, and rapid rates of sea-floor spreading. The study should yield information on surface or near-surface temperatures, water-rock interactions and possibly CO2 partial pressures in the local environment. More generally, such data could provide insight into plaeoclimate and possible secular variations in seawater 0-18/0-16 ratios at a time when there was continental glaciation in the southern hemisphere, thus increasing our knowledge of conditions during an interval of considerable interest in the Phanerozoic.