Radiometric dating of sedimentary rocks: the application of diagenetic xenotime geochronology

• Published in: Earth-science reviews 2005, Vol.68 (3), p.197-243
• Main Author: Rasmussen, Birger
• Format: Article
• Language: English
• Subjects: Diagenesis; Geochronology; Geology; Precambrian; Radiometric dating; Radiometry; Rocks; Sedimentary rocks; Sediments; Xenotime
• ISSN: 0012-8252; 1872-6828
• DOI: 10.1016/j.earscirev.2004.05.004

Recent advances in the field of geochronology have led to a greater understanding of the scale and duration of geological processes. It is currently possible to date igneous and metamorphic rocks by a variety of radiometric methods to within a million years, but establishing the depositional age of sedimentary rocks has remained exceedingly difficult. The problem is most pronounced for Precambrian rocks, where the low diversity and abundance of organisms have prevented the establishment of any meaningful biostratigraphic framework for correlating strata. Also, most Precambrian successions have been metamorphosed, rendering original minerals and textures difficult to interpret, and resetting diagenetic minerals. Xenotime (YPO 4) is an isotopically robust chronometer, which is increasingly being recognized as a trace constituent in siliciclastic sedimentary rocks. It may start to grow during early diagenesis, typically forming syntaxial outgrowths on detrital zircon grains. Diagenetic xenotime occurs in a wide variety of rock types, including conglomerate, sandstone, siltstone, shale, phosphorite and volcaniclastic rocks, varying from early Archaean to Mesozoic in age. The formation of diagenetic xenotime is principally related to redox cycling of Fe-oxyhydroxides and microbial decomposition of organic matter, leading to elevated concentrations of dissolved phosphate and rare earth elements (REE) in sediment pore-waters. Xenotime has the properties of an ideal U–Pb chronometer, containing elevated levels of U (generally >1000 ppm) and very low concentrations of initial common Pb. In addition, it has an exceptional ability to remain closed to element mobility during later thermal events, and commonly yields concordant and precise dates. Because of the small size of diagenetic xenotime crystals and common textural complexities, an in situ isotopic technique with a spatial resolution of