Synthesis of U-Pb doped hematite using a hydrated ferric oxide approach

Abstract

Hematite (α-Fe2O3) has been successfully co-doped with uranium and lead using hydrated ferric oxide (HFO) as an intermediary, precipitated by ammonia from a ferric nitrate solution. A novel synthesis approach has been developed, involving a wet stage of doping colloidal HFO by absorption from nitrate solution containing U and Pb nitrates, followed by drying and heating to 700 °C to convert HFO to Fe2O3. The crystal phase present in the sample was confirmed as hematite by X-ray powder diffraction. Scanning electron microscopy was used to determine sample morphology and U/Pb isotope homogeneity was assessed using laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS). Although reasonably homogeneous at the scale of a LA-ICP-MS ablation analysis spot, conspicuous domains are recognized that appear ‘bright’ or ‘dark’ on backscatter electron images. These domains display distinctly different element concentrations and U/Pb ratios on LA-ICP-MS isotope maps. Nevertheless, this synthetic U-Pb-doped hematite represents a potential reference material for use in microbeam U-Pb geochronology and the results show that the preparation methods and doping conditions are effective. The observed chemical and isotopic heterogeneity between bright and dark domains may, however, inhibit widespread use of the synthesized hematite as a reference material. U/Pb heterogeneity may be controlled by the heating and cooling regimes of the method. Assuming the ‘bright and dark domains’ can be analytically or mechanically separated through refinement of the preparation method, then a suitable reference material may be produced. Subject to validation via high-precision chemical analysis, this reference material can be used to date uranium-bearing hematite from various types of deposits. The high speed and precision of LA-ICP-MS analyses will allow measurement of geological ages in a very common mineral, providing new insights for mineral exploration worldwide.