What problems are encountered in dating metamorphic rocks

Even a claimed near-concordant U-Pb age of 862Ma on one uraninite grain is identical to a false Pb-Pb isochron age, but neither can be connected to any geological event.Open system behavior of the U-Th-Pb system is clearly the norm, as is the resultant mixing of radiogenic Pb with common or background Pb, even in soils in the surrounding region.The Koongarra area is 25Okm east of Darwin (Northern Territory, Australia) at latitude 12°52'S and longitude 132°50'E.The regional geology has been described in detail by Needham and Stuart-Smith [19] and by Needham [17, 18], while Snelling [25] describes the Koongarra uranium deposit and the area's local geology.The Koongarra uranium deposit occurs in a metamorphic terrain that has an Archean basement consisting of domes of granitoids and granitic gneisses (the Nanambu Complex), the nearest outcrop being 5km to the north.Some of the lowermost overlying Lower Proterozoic metasediments were accreted to these domes during amphibolite grade regional metamorphism (estimated to represent conditions of 5-8kb and 550-630°C) at 1800-1870Ma.A thick sequence of essentially flat-lying sandstones (the Middle Proterozoic Kombolgie Formation) was then deposited unconformably on the Archean-Lower Proterozoic basement and metasediments.

foliation of the brecciated and hydrothermally altered quartz-chlorite schist host.

Of the various methods, uranium-thorium-lead (U-Th-Pb) was the first used and it is still widely employed today, particularly when zircons are present in the rocks to be dated.

But the method does not always give the expected results, leading to fundamental questions about its validity. some of the basic assumptions of the conventional Rb-Sr isochron method have to be modified and an observed isochron does not certainly define a valid age information for a geological system, even if a goodness of fit of the experimental data points is obtained in plotting Sr ratios, even an excellent line of best fit between ratios obtained from good cogenetic samples, and yet the resultant isochron and derived age have no distinct geological meaning.

Radiometric dating has now been used for almost 50 years to establish beyond doubt the earth's multi-billion year geological column.

Although this column and its age was firmly settled well before the advent of radiometric dating, the latter has been successfully used to help quantify the ages of the strata and the fossils in the column, so that in many people's minds today radiometric dating has proved the presumed antiquity of the earth.

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Groups of uraninite veinlets are intimately intergrown with chlorite, which forms the matrix to the host breccias.

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