Abstract
The crystal structure of goethite, FeO(OH), has two distinct oxygen sites, one with exclusively Fe-O bonds, the other with bonds to both iron and hydrogen. We developed a method to assess the oxygen isotope contrast between these sites by measuring both the bulk goethite and the oxygen released in the conversion of goethite to hematite. The method involves collecting the water released by dehydroxylation, fluorinating that population of extracted atoms, and measuring the resulting oxygen isotope composition (extracted δO18). Then, on a separate aliquot, all structural oxygen is fluorinated and measured (bulk δO18). Using synthetic goethite precipitates grown under controlled environmental conditions, we found significant temperature-dependent fractionation, εbulk-extracted=(5.51±0.26)×(106/T2)−(44.5±2.8); T in Kelvin). This intracrystalline fractionation forms the basis of a single-phase paleothermometer with an estimated uncertainty of ∼2-3°C. The temperature dependence appears to be independent of the isotopic composition of the parent fluid from which the goethite formed and the pH of that fluid. This intracrystalline thermometer can be used to simultaneously determine the formation temperature of a goethite and the isotopic composition of the water from which it formed. Natural goethites analyzed with this technique yield geologically reasonable formation temperatures of between 15 and 41°C.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
