Stable hydrogen isotopes in iron oxides—II. DH variations among natural goethites

Goethite samples analyzed for this study have a δD range from ?202 to ?98 per mil with a corresponding δD range of associated waters from about ?110 to +7 per mil. Goethites with the most positive δD values are from marine environments. Goethite-water equilibrium $\text{D}\text{H}$ fractionation factors measured in this laboratory at 100°C and 145°C and estimated for sedimentary temperatures from data on natural samples have values of about 0.900 at all three temperatures. These data suggest that goethite δD values may be a direct, temperature-independent measure of the δD values of the waters with which the goethite last equilibrated. Most of the goethite δD values in this study reflect the δD values of the modern waters in the locales of origin.Substitution of Mn and/or Al for Fe in the goethite structure may affect the mineral-water fractionation factor. Manganese appears to decrease the value of α. The effect of Al substitution on α has not yet been measured, but preliminary arguments suggest that increasing Al content could increase α values.