[meteorite-list] Earth's Birth Date Turned Back

From: Starbits_at_aol.com <Starbits_at_meteoritecentral.com>
Date: Thu Apr 22 10:21:07 2004
Message-ID: <689DD2F4.557173CE.00848CE4_at_aol.com>

GT40dawg_at_aol.com wrote:

<Doesn't this (and most all radioactive dating
  techniques) assume that radioactive decay begins once
  the Earth is in a form (solid or semi-solid).  Why
  can't decay occur while the (insert radioactive isotope
  name here) is still floating freely in space?   Many of
  these studies act as if decay magically begins once all
  of the pieces have begun to coalesce.>

  Decay goes on continuously.  The reason that age dating
gives the age of the formation of solids is because that
is when the parent and daughter products are locked
into the crystal structure.  It is the ratio of these,
and stable isotopes that are used for age determination.
  Obviously if decay is occuring before solidification
there will be daughter products in the solid from decays
prior to solidification.  This is dealt with by 3 isotope
plots.  For example rubidium (Rb)87 decays to strontium
(Sr) 87.  Umm, this is hard without a chalk board.  On the
"Y" (vertical) axis is plotted 87Sr the daughter product
divided by the stable isotope 86Sr.  An assumption is made
that the strontium is homogeniously mixed throughout
the magma, or solar nebula.  In other words the 87Sr/86Sr
ratio is the same or nearly the same everywhere.
   On the "x" (horizontal) axis is plotted 87Rb/86Sr,
the parent isotope divided by the same stable isotope.  
Different physical processes can change the ratios of
rubidium and strontium.  So at time "zero" samples can plot
at different points on the x axis but will form a
horizontal line because the 87Sr/86Sr ratio is the same
in each case.  As newly decayed daughter products start
building up in your solid the 87Sr/86Sr ratio changes
and a plot is no longer a horizontal line.
This is because samples with twice as much 87Rb will
produce twice as much daughter product.  If you analyze
and plot a bunch of samples you should get a straight
line with a slope that is related to the age.  Where the
line intercepts the "Y" axis is the point giving the
ratio of 87Rb/86Sr at the time of solidification. As it
turns out this point doesn't matter because it is the
slope of the line that is important for age determination.
And that is why, "magically", decay prior to solidification
doesn't affect age determination.  
   One of the better books around on this subject is
"Principles of Isotope Geology" by Gunter Faure.  

Eric Olson
http://www.star-bits.com
Received on Fri 18 Jul 2003 04:25:32 PM PDT