Oxygen and hydrogen isotope variations among goethites (α-FeOOH) and the determination of paleotemperatures

Abstract

A plot of δD vs. δ 18 O values from a diverse group of natural goethites defines a data array which is approximately parallel to the meteoric water line. The δ 18 O values range from −9.5 to +4.4 per mil, while the corresponding δD values vary from −202 to −94 per mil. There is considerable scatter in the δD vs. δ 18 O array, which can be attributed primarily to different temperatures of mineral formation. As determined previously (YAPP and Pedley, 1985), and confirmed in this study, the goethite-water D H fractionation factor is essentially independent of temperature over the range from 25°C to 145°C. However, the goethitewater 18O 16O fractionation factor does vary with temperature, as indicated by initial goethite synthesis experiments conducted at 25°C, 44°C and 62°C. The synthetic goethite 18 α G– w values suggest that the natural goethites in this study formed at temperatures that may range from about 8°C to 66°C. Support for the validity of these approximate, calculated temperatures is provided by a natural goethite sample from the Atlantis II Deep in the Red Sea. These results suggest that goethite can be used as a single-mineral, low temperature geothermometer. Perhaps more importantly, goethite can be used in low temperature, oxygen isotope mineral-pair geothermometers such as chert-goethite, calcite-goethite, phosphate-goethite, etc.