Oxygen isotopes in iron (III) oxides: 2. Possible constraints on the depositional environment of a Precambrian quartz-hematite banded iron formation

Abstract

Quartz (SiO 2) and hematite (Fe 2O 3) possess considerably different gram formula weights and different stoichiometric coefficients for their oxygen. These differences mean that the numerical values of SiO 2 and Fe 2O 3 compositions when reported as weight percent are not the same as the numerical values of X(O), i.e. oxygen in the mineral as a mole percent of the total oxygen in a binary quartz-hematite system. Consequently, end-member quartz and hematite δ 18O-values which are determined from linear extrapolations of δ 18O vs. weight percent Fe 2O 3 data from binary mixtures can be in error by as much as several per mil. Extrapolation to end-member δ 18O-values in binary quartz-hematite systems should always be performed using mole fraction of oxygen as the compositional variable. The hematite-water fractionation curve of Yapp and the quartz-water curve of Knauth and Epstein were used together with the kinetic isotope exchange model of Criss et al. to constrain the depositional environment of the Late Proterozoic quartz-hematite banded iron formation deposits of Urucum, Brazil. The depositional temperatures permitted by the model assumptions employed here range from 0° to ∼ 35°C (or 54°C). The permitted range of depositional water δ 18O-values is from −6.6‰ (or −8.9‰) to 0.0‰. While the range of permitted depositional conditions is rather large, it does encompass the possible cool temperatures and brackish or fresh waters which are indicated by sedimentologic evidence in these deposits.