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       #Post#: 93--------------------------------------------------
       GB/K-Ar DATING
       By: Admin Date: February 2, 2017, 6:37 pm
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       Guy Berthault
       Addendum
  HTML http://sedimentology.fr
       ....................... Ar (ppm)
       dacite 350,000 years 
       feldspar 340,000 years 
       amphibole 900,000 years 
       pyroxene 1,700,000 to 2,800,000 years 
       according to the model age equation.
       The report says “these ages, of course, are preposterous. The
       fundamental dating  assumption (no radiogenic argon was present
       when the rock formed) is questioned by  these data. Instead data
       from the Mt. St. Helens dacite, argue that significant  “excess
       argon” was present when the lava solidified in 1986. Phenocrysts
       of  orthopyroxene, hornblende and plagioclase, are interpreted
       to have occluded argon  within their mineral structure deep in
       the magma chamber and to have retained the  argon after
       emplacement and solidification of the dacite. Orthopyroxene
       retains the  most argon, followed by hornblende, and finally
       plagioclase”.
       The presence of abundant argon deep in the rocks produced at the
       time of the  eruption (recent or ancient) and which rises
       towards the surface of the magma,  gives the impression the
       rocks are older than they are when dated by the  potassium/argon
       method.
       The fact that the method has been used to date the
       Australopithecines raises the  question: what then is their real
       age?
       This fundamental dating assumption, as regards other radiometric
       dating is also  questioned, because every sample rock contains a
       quantity of daughter resulting  from the decay of the parent in
       the lava, before crystallisation, which makes the  rock appear
       older. The model age equation requires that the initial number
       of  daughter atoms be known. No analytical equipment, however,
       can give this value.
       The isochron age equation depends on several assumptions, the
       principal being that  rocks of a same formation, when they
       formed, had the same abundance of daughter, in  this case argon.
       This is not so for the dacite and its components mentioned above
       which only ten years after the eruption showed different
       respective quantities of  argon.
       The model age so determined corresponds to magma and not
       crystallisation (as for  the dacite). Moreover, gravitational
       settling between minerals exists in the  cooling magma. For
       example, strontium, which has the same valence and very similar
       ionic radius substitutes for calcium. So in fact, since
       plagioclase which carries  strontium is less dense than olivine,
       then due to gravitational settling in an  intrusion the greatest
       quantity of plagioclase and therefore strontium can  sometimes
       remain in the higher levels of that intrusion. So strontium can
       be more  abundant at a higher level in a magma intrusion which
       gives an apparently older  age.
       In conclusion the radioactive age does not necessarily refer to
       the crystallisation  of rocks and consequently not to geological
       dating.
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