This is by definition untenable

Ar-ar age dating

When other age spectra show steps deviating markedly from each other i. Based on evaluation of isochron plots for each specimen Baksi showed that almost all the ages were untenable as crystallization values.

When the relevant

The plate motion model derived by Muller et al. Age spectrum yielding an unacceptable age. None of these specimens yields an age plateau or isochron sufficiently accurate to be utilized for tracing hotspot tracks. It on this last issue that I shall focus. This work followed the first efforts Brooks et al.

These tests are outlined herein. This is by definition untenable. Thus low-temperature steps normally exhibit ages lower than, or equal to, those of the higher temerature steps.

Modern methods of analysis allow individual regions of crystals to be investigated. In these experiments, a sample is heated in steps of increasing laboratory extraction temperature, until all the argon is released. Age spectrum yielding an imprecise but usable age for evaluating the Tristan da Cunha hotspot track. Since it is not desirable to reinterpret all published data, I offer some guidelines for data sets that are worth closer inspection.

Age spectra yielding unacceptable

Clearly this is no plateau. Evaluation of their petrogenesis relative to the Deccan Traps, Proc. Evaluation of the timing of volcanism relative to the K-T boundary, Earth Planet. Age spectra showing older apparent ages for low-temperature steps than for high-temperature steps.

The first sample shows a descending-staircase type of age spectrum. Based on statistical evaluation of the isochron plots Baksi showed that each of these was untenable. Proper statistical evaluation confirms this. Thus, discretion and interpretation of age dating is essential. This will be considered in detail for two specimens below.

Thus, although a crystallization age is not recorded, the information is still useful in constructing the thermal history of the rock. Figures must be presented to permit quick but accurate visual inspection of results. The resulting figure is called an age spectrum e. Age spectra for disturbed samples from the Tristan da Cunha hotspot track the Walvis ridge. For unmetamorphosed igneous rocks, the latter would normally represent the crystallization age.

These ages were subsequently used by Muller et al. In my estimation, the best ages for this province are those of Mussett wherein the data were scrupulously evaluated by proper statistical criteria.

Age spectra that are suggestive of excess argon. For this reason see Introduction above the corresponding isochron plots need to be critically examined.

When the relevant Chi Square Tables e. Age spectra yielding unacceptable ages from the Tristan da Cunha hotspot track. This mineral has been used very successfully to date rocks and generally yields good plateau ages.