Errors in isotopic ratio measurements must be used correctly (see The significance of i. If the MSWD value is not quoted, it can easily be calculated and used in conjunction with Chi Square Tables to assess reliability. I briefly consider age spectra where low temperature steps yield older ages than those at higher temperatures.
The former and latter derive gas from low and high argon rentention sites respectively.
Evaluation of the corresponding isochron plot gave MSWD = 3.9 (Both the plateau and isochron approach statistics are unequivocal – a large amount of excess scatter is present in the data sets (“geological error”).
This mineral separate fails to yield either a plateau or an isochron age.
When a reasonable number of consecutive steps, carrying a substantial amount of the total argon released, give the same age, the resulting average value carries geological significance.
For unmetamorphosed igneous rocks, the latter would normally represent the crystallization age.
I look primarily at age spectra and note that plateau sections represent steps that “overlap” using ( All errors herein are given at the 1-sigma level.
This approach is designed to look at the gas released from sites of increasing argon retentivity.
An easier method is to look at the data on age-spectrum plots and assess the reliability of “plateau” sections.
Though definitions of what constitutes a plateau vary, a general guideline is that such sections of the age spectra should contain released, whose ages “overlap”. Two steps can never define a plateau, and such data cannot be evaluated on an isochron diagram.
Their MSWD value for these 12 steps is 3.05, for nu (degrees of freedom = N-1) = 11.
The corresponding Chi Square value is 33.55 (MSWD x nu), and Chi Square Tables tell us that the probability of this occurrence is p ~ 0.0004.