­AN INTERLABORATORY COMPARISON OF THE GOETHITE (SITE-SPECIFIC) OXYGEN ISOTOPE THERMOMETER

Goethite[FeO(OH)] is an iron-oxyhydroxide with two oxygen sites: 1) oxide as Fe-O and 2) hydroxide as Fe-OH, and commonly forms in soils when meteoric water dissolves Fe-bearing minerals and subsequently precipitates goethite. The oxygen isotope composition (δ¹⁸O) of goethite primarily reflects the meteoric water, making goethite useful for reconstructing paleoclimate; however, the stable isotope composition of goethite is influenced by the temperature-dependent isotopic fractionation between goethite and water. Hydroxyl-bearing minerals such as goethite are often composed of structurally non-equivalent oxygen sites that record distinct δ¹⁸O values. Recent work has calibrated a single mineral oxygen isotope thermometer by using a suite of synthetic goethites that were precipitated at known temperatures and determining the intracrystalline isotope fractionation between oxygen sites (Miller et al., 2020).

Here, we re-evaluate the goethite oxygen isotope thermometer by studying the samples used in Miller et al. (2020) but utilize a different method to measure the isotopic composition of oxygen that is bound as OH^- to iron and hydrogen. We dehydroxlated goethite in a thermogravimetric analyzer (TGA) that is coupled to a Cavity Ring Down Spectrometer (CRDS) to measure OH^- δ¹⁸O values. We subsequently use fluorination and isotope ratio mass spectrometry (IRMS) to measure δ¹⁸O values of the TGA-CRDS breakdown product (hematite, Fe₂O₃). δ¹⁸O values of OH-bound oxygen record a narrow range (δ¹⁸O = -8 to -10 ‰). Miller et al. (2020) measured OH-bound oxygen by dehydroxylating goethite in a vacuum line and reacting evolved water with BrF₅, and found a larger range of δ¹⁸O values (δ¹⁸O ≈ -5 to -25 ‰). The difference in measured δ¹⁸O values of OH-bound oxygen between the two methods suggests one or more steps in goethite precipitation, preservation, dehydroxylation, or subsequent δ¹⁸O measurements is poorly understood. In addition, measurements of goethite breakdown products will be compared with bulk goethite δ¹⁸O values; both measurements will be done with fluorination and IRMS. These results will help understand the observed isotopic difference between the two dehydroxylation methods and will improve our understanding of how oxygen stable isotope measurements can be optimized for routine goethite analyses.