Your comment to Glenn Morton's statement about geological dating, "Thank you
for clarifying some of matters re geological dating. I'm sure the setting of
the 'radiometric clock' is indeed 'a long and involved question' - and,
undoubtedly, crucial to the whole enterprise." caught my attention. I
suspect that 'setting the radiometric clock' may convey an image of 'knob
twiddling' until you get the answer you are looking for. This is, of
course, not the case.
There are a number of radiometric clocks that can be used in addition to Rb
and Sr. One of these is the K/Ar couple. K(potassium) consists naturally
of a number of isotopes, one of which, K-40, is radioactive. It decays to
Ar-40 (argon-40) which is stable. Argon is a gas and not very soluble in
molten rock. Ana analogy is gases in water. At room temperature, water can
hold a fair bit of gas, but when you boil water, you expel most of the
dissolved gases. When lava or magma cools, it contains therefore very
little, if any Ar. However, it can (and will) form minerals that contain K
and with it, a little bit of K-40. As time goes by, the K-40 decays to
Ar-40 and the Ar-40, now trapped in the mineral crystal, cannot escape and
remains trapped in the crystal like a fly in a piece of amber. If we now
take the mineral crystal and put it in a quartz tube and heat it until it
melts, while passing a stream of inert gas over the sample, the Ar-40 is
released and we can measure the amount that has been formed since the
mineral crystal solidified. We know the half life of K-40 and the rate at
which Ar-40 is produced and, from that information, can calculate how long
it took for the Ar-40 to build up in the crystal.
Lead isotope data can also be used in geochronology. Uranium and thorium
decay via a long series of radioactive daughters to stable lead isotopes.
However, not all lead isotopes are formed as decays products of U and Th
and, by looking at the Pb isotope ratios in a sample of, for example,
pitchblende, one can calculate the amount of U and/or Th that has decayed
since the mineral was formed.
Hope this helps. If you already knew all this, I apologize for taking up
your time.
Shalom!
Chuck Vandergraaf
Pinawa, MB