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Methodology The basic principle involved is to evaluate critically how much the step ages on the plateau differ from one another.
This is done by calculating the relevant Mean Square Weighted Deviate (MSWD = – 2) = 1, 3, 5, 10 the corresponding probability values are 0.11, 0.06, 0.03 and 0.005, respectively.
For BI (Lower lava) a descending age spectrum was used to arrive at a plateau age of 60.56 ± 0.29 Ma.
The MSWD value of ~ 15 (p (2001) used 12 steps to arrive at a “plateau age” of 58.48 ± 0.18 Ma.
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.
“Descending staircase” type age spectra may result from two totally different phenomena.
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.
However, in many instances, the published “ages” have no merit, as they fail the simple statistical tests that should be applied to all such data. In these experiments, a sample is heated in steps of increasing laboratory extraction temperature, until all the argon is released.
The argon released in each step is measured to calculate a “step age” with an associated analytical error.
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At the end of the series of experiments, the step ages (± one sigma errors are quoted herein) are plotted against the cumulative amount of ).