Pb pb isochron dating

At the Koongarra uranium deposit, Australia, there is ample evidence of open system behaviour, or repeated migration, of U and Pb — ore textures, mineral chemistry, supergene alteration, uranium/daughter disequilibrium, and groundwater and soil geochemistry.

Yet U-Th-Pb isotopic studies of the uranium ore, host rocks and soils have produced an array of false ‘isochrons’ that yield ‘ages’ which are geologically meaningless.

Of the various radiometric 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.

Instead, it is increasingly being applied in more sophisticated ways to geological ‘dating’ problems.

Zheng documented the copious reporting of this problem in the literature where various names had been given to these anomalous isochrons, such as apparent isochron, mantle isochron and pseudoisochron; secondary isochron, inherited isochron, source isochron, erupted isochron, mixing line, and mixing isochron.

Similar anomalous or false isochrons are commonly obtained from U- Th-Pb data, which is hardly surprising given the common open system behaviour of the U- Th-Pb system.

This alteration extends for up to 1.5 km from the ore in a direction perpendicular to the host quartz-chlorite schist unit, because the mineralisation is essentially stratabound.

The outer zone of the alteration halo is most extensively developed in the semi-pelitic schists, and is manifested by the pseudomorphous replacement of biotite by chlorite, rutile and quartz, and feldspar by sericite.

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In the conclusion to a recent paper exposing shortcomings and criticising the validity of the popular rubidium-strontium (Rb-Sr) isochron method, Zheng wrote: ‘…

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