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Thus low-temperature steps normally exhibit ages lower than, or equal to, those of the higher temerature steps.
There is no justification for quoting an age of 58.48 Ma with a very small error margin of ±0.18 The plateau age of 100.00 Ma, with a standard error on the mean: of ± 0.22 Ma is found to be unacceptable, with p ~ 0.003 ( lie between ~ 99 and 101 However, its exact value, and equally important its error estimate, cannot be stated with statistical confidence.The plate motion model derived by (2001) reported a number of plateau/isochron ages for both rocks and mineral separates from the BTIP. BM67 and 64 (Ben More lavas) were said to yield plateau ages of 58.66 ± 0.25 Ma and 58.19 ± 0.26 Ma, respectively, although many step ages obviously do not overlap in each case.These authors used these “ages” for defining the time of initiation and duration of flood-basalt volcanism in this province and to refine details (cryptochrons) of the geomagnetic polarity time scale from ~ 60 – 55 Ma. The MSWD ~ 26 and ~ 18, yield ( 2001), that the statistically significant mean value is 58.19 ± 0.26 Ma?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.
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.
For plateau ages, I list the standard error on the mean (the SEM).
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.
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.