G.D. Karner B. Taylor N.W. Driscoll D.L. Kohlstedt Rheology and deformation of the lithosphere at continental margins 2004 Columbia University Press New York 384 pp., ISBN 0231-12738-1, US $89.50 (cloth), US $49.50 (paper)

Abstract

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 (YPO4) 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 <10 μm is required to successfully date xenotime; to date, this has only been achieved by ion microprobe.In metamorphosed sedimentary rocks, diagenetic xenotime retains its age information up to lower amphibolite facies in sandstone, and up to mid-upper greenschist facies in pelitic rocks. In many Precambrian basins (e.g., Witwatersrand Basin, South Africa), diagenetic xenotime is overgrown by chemically distinct and texturally younger xenotime related to burial diagenesis, contact metamorphism, hydrothermal alteration or regional metamorphism. With the aid of petrography, geochemical microanalysis and the use of isotopic techniques with fine spatial resolution, it may be possible to use xenotime to date early diagenesis, and potentially every major fluid and thermal event to have affected a depositional basin.