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03-Mar-2019 03:20

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.

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.

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.

For BI (Lower lava) a descending age spectrum was used to arrive at a plateau age of 60.56 ± 0.29 Ma.

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.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 adjunct method of evaluation is to present the relevant data on an isotopic ratio plot and look for straight-line segments through three or more consecutive heating steps. The resultant “goodness of fit” parameter, in tandem with the number of points on the line, must be evaluated for statistical reliability (95% confidence level generally) using Chi Square Tables (, 1992).If the data scatter badly around the best-fitting straight line, the resulting “age” should be rejected as being not reliable.

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.

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 adjunct method of evaluation is to present the relevant data on an isotopic ratio plot and look for straight-line segments through three or more consecutive heating steps. The resultant “goodness of fit” parameter, in tandem with the number of points on the line, must be evaluated for statistical reliability (95% confidence level generally) using Chi Square Tables (, 1992).

If the data scatter badly around the best-fitting straight line, the resulting “age” should be rejected as being not reliable.

An easier method is to look at the data on age-spectrum plots and assess the reliability of “plateau” sections.