Evolutionists believe that the oldest rocks on earth are
meteorites. These are supposed to be left over from the original
cloud (solar nebula) which condensed to form the sun, planets and
meteorites. All normal earth rocks are thought to have had an
igneous origin more than a billion years after meteorites formed.
Because of this assumption, evolutionists believe that isotopic
analyses of meteorites provide data from which to date the earth.
Lead-lead isochron plots are the current way that
evolutionists have estimated the age of the earth at 4.6 billion
years. A simple discussion is found in Gunter Faure's book
PRINCIPLES OF ISOTOPE GEOLOGY. The ratio of lead-207/lead-204 in
meteorites is plotted against lead-206/lead-204 as shown below.
+| * |
| * |
| * |
| * | LEAD-LEAD
| * | ISOCHRON
Pb-207/Pb-204 | * | PLOT
| * |
| * |
| * |
Each lead isotope ratio of each meteorite plots as a point
on the graph. Together the points for meteorites describe a line
as suggested on the above sketch. The line is called an
"isochron" because all points on the line should have the same
age. The slope of the line is supposed to be directly
proportional to the age. A simple mathematical calculation using
the slope of the line gives the age of the earth!
Lead-204 is the common lead not formed from radioactive
decay. It is supposed to have survived as the solar nebula
condensed to form the earth. Lead-207 in meteorites forms from
decay of uranium-235, and lead-206 forms from decay of
uranium-238. Different meteorites have different concentrations
of uranium. As time goes by, the ratios of radiogenic leads
increase relative to the common lead giving the sloping line.
The lead-lead isochron method assumes that the original
nebula had uniform lead isotopic composition, and that the
deviations from the initial composition have been caused by
radioactive decay of uranium-235 and uranium-238 over the ages.
There is also the assumption that the earth and meteorites formed
at the same time from the same nebula.
As a creationist I find these series of assumptions more
than I can accept. I am even offended. Is there any compelling
argument proving initial lead isotope uniformity? Could the
meteorites have formed having different lead isotope ratios? Are
we sure that there has been significant decay of uranium isotopes
in these rocks?
It seems that one has to buy an evolutionary cosmology
before one can determine the age of the earth to be 4.6 billion
years! I had hoped that the age of the earth would be a more
straightforward calculation. I don't like speculative cosmology.
Steven Austin, Ph.d
Institute for Creation Research
PO Box 2667
El Cajon, CA 92021
Note: the author failed to understand the distinction of isochrons
and psudo-isochrons, which yields his conclusions in error. Write
to him and ask him to look at isochrons, not psudo-isochrons.