Electron microprobe chemical dating of uraninite as a reconnaissance tool for leucogranite geochronology

We suggest that electron microprobe techniques may be employed to date Tertiary samples of uraninite (UO2), which can contain very high concentrations of radiogenic Pb after only a few million of years of U and Th decay. Although uraninite is regarded as a rare accessory mineral, it is relatively abundant in leucogranitic rocks such as those found in the Himalayan orogen. We apply the U-Th-total Pb electron microprobe chemical dating method to a uraninite crystal from a ca. 18.3 Ma dike of the Mugu granite from the Upper Mustang region of central Nepal. With this technique, we calculate a mean chemical date that is consistent with isotope-dilution thermal ionization N at ur e P re ce di ng s : h dl :1 01 01 /n pr e. 20 07 .6 55 .1 : P os te d 8 A ug 2 00 7 mass spectrometry (ID-TIMS) U-Pb dates obtained from seven other uraninite grains and a monazite crystal from the same sample. Electron microprobe chemical dating yields results that typically will be an order of magnitude less precise than conventional dates: in the specific case of the Mugu granite, single point chemical dates each have ca. 1.5 Ma 2σ (95%) confidence level uncertainties. However, the mean chemical date of 15 point analyses of the crystal we study has a 2SE (2 standard error) uncertainty of ca. 400 ka, comparable to uncertainties obtained with ID-TIMS. These results show that electron microprobe chemical dating of uraninite has substantial promise as a reconnaissance tool for the geochronology of young granitic rocks. The electron microprobe work also reveals substantial chemical complexity within uraninite that must be taken into account. The analyzed crystal displays a texturally and chemically distinctive core and rim that suggests episodic growth. Concentration gradients in U, Th, and Y across the boundary imply diffusive modification with