When other age spectra show steps deviating markedly from each other value would be ~ 0.41, making the plateau age (100.00 ± 0.45 Ma) acceptable as a crystallization value.Step age errors must be estimated/calculated properly. above forms the main focus of this webpage and is illustrated below using examples taken from the literature.Note that the same value of MSWD (= 2.5), makes an isochron statistically acceptable for up to 5 points (nu = 3), but unacceptable when seven or more points are used (nu I begin by looking at a hypothetrical data set.Consider a case of five steps carrying equal quantities of gas giving rise to step ages of 99.0, 101.0, 100.0, 101.0 and 99.0 Ma, each with an error of ± 0.5 m.y.Any two points in the universe lie on a straight line!In evaluating isochron “goodness of fit parameters”, the number of degrees of freedom for Chi Square Tables is N-2, and for two steps, this is zero.Thus low-temperature steps normally exhibit ages lower than, or equal to, those of the higher temerature steps.“Descending staircase” type age spectra may result from two totally different phenomena.
This approach is designed to look at the gas released from sites of increasing argon retentivity.
The first is due to the presence of excess (1999) showed that almost all the ages were untenable as crystallization values.
In this web page, I take a similar approach (but one that is somewhat easier to visualize) to evaluate these ages based on their age spectra. Elsewhere it will be shown that this conclusion is fully supported by critical examination of the individual age spectra.
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.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”).