SHRIMP U-Pb geochronology of authigenic xenotime and its potential for dating sedimentary basins

SHRIMP (Sensitive High-Resolution Ion MicroProbe) analytical procedures have been developed to enable dating of the small, early diagenetic xenotime overgrowths that commonly occur on zircons in siliciclastic sedimentary rocks. The method will be particularly useful in Precambrian terranes, where diagenetic xenotime dating could play a role equivalent to biostratigraphic dating in the Phanerozoic. Reliable Pb-207/Pb-206 data are more readily obtained than Pb-206/U-238, which also favours application to the Precambrian. However, it is demonstrated that Pb-206/U-238 dating of larger overgrowths (> 10 mu m) is also viable and applicable to Phanerozoic samples. SHRIMP Pb/Pb geochronology of authigenic xenotime in an unmetamorphosed Palaeoproterozoic sandstone in the Kimberley Basin has constrained diagenesis to a precision of +/- 7 Ma. In contrast, greenschist-facies metasediments of the Archaean Witwatersrand Basin, South Africa, contain both authigenic and alteration xenotime that record a complex history of growth from early diagenesis to the last major thermal event to affect the basin.