K ar dating method steps
Orthopyroxene retains the most argon, followed by hornblende, and finally, plagioclase. Helens dates very much older than its true age because phenocryst minerals inherit argon from the magma. Helens dacite causes the more fundamental question to be asked—how accurate are K-Ar 'ages' from the many other phenocryst-containing lava flows world-wide? Helens in Washington State expressed itself directly during six explosive magmatic eruptions in 1980 (May 18, May 25, June 12, July 22, August 7 and October 17, 1980).
This magma produced the distinctive plinian, explosive eruptions for which the volcano is famous.
At the end of the series of experiments, the step ages (± one sigma errors are quoted herein) are plotted against the cumulative amount of ).
This approach is designed to look at the gas released from sites of increasing argon retentivity.
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.
K-Ar radioisotope analyses yielded a wide range of model ages, even for adjacent samples from the same outcrop of the same original lava flow.
No statistically viable K-Ar isochron age could be obtained because of so much scatter in the data, which is most likely due to Ar* mobility within these rocks.
For unmetamorphosed igneous rocks, the latter would normally represent the crystallization age.
For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years.
By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time.
Phenocrysts of orthopyroxene, hornblende and plagioclase are interpreted to have occluded argon within their mineral structures deep in the magma chamber and to have retained this argon after emplacement and solidification of the dacite.
The amount of argon occluded is probably a function of the argon pressure when mineral crystallization occurred at depth and/or the tightness of the mineral structure.
Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4.6 billion years.