Ar ar dating minerals
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.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.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.The age of have a crystallization age in this range, but it contains excess argon (higher step ages on either side of the “plateau”).There is no justification for quoting an age of 58.48 Ma with a very small error margin of ±0.18
For unmetamorphosed igneous rocks, the latter would normally represent the crystallization age.
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.
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.