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Re: A Look At The Age of The Earth.

>Hi Frank,
>
>Welcome back! I thought one means of measurement was that of Al26, turning
>into
>Mg26 its daughter isotope. This dating is based on the time it takes Mg26 to
>fully turn from one form to the other. The cause of the alteration is cosmic
>rays bombarding asteroid material and passing through it. Although I tend to
>accept the old age theory, what troubles me is how do we know that cosmic
>bombardment has always been the same frequency. Could it have been double,
>triple or even hundreds of folds more in the past. If so could there be error
>in the generally excepted age we now except? Of course the question is the
>same
>for all isotopes used in dating now a days. Could a super nova or nova event
>gives us the wrong sense of time?
>
>This is a pretty rough question that was asked without digging into my
>library
>so would appreciate everyone excepting my drift even though I might be off on
>the particulars.
>
>By the way, would enjoy hearing of your adventures if you would care to share
>them with us here. Did you do any meteorite hunting? Did you go and see any
>collections.?
>
>--AL

hello al; actually, the method you are talking about is the decay of Al26
into its daughter product Mg 26. it is a radiometric clock with a half life
of 0.7 Myrs. so it does not depend on the flux of cosmic rays.it is similir
to other standard geochronometers like the U series decay to Pb, Rb to Sr ,
K to argon and Th to Pb.the reason why Al is so impt to meteoritics is that
it has   a short half life  so it can be used to date the very earliest
solar system events. Also, the decay of Al 26 produces quite a bit of heat
and so if Al26 was abundant (in fact if Al26/Al25>1x10^-5 or so, then Al 26
may have been a very important source of heat...maybe it even played a role
in the heating and melting of chondrite sources to form the differentiated
asteroids.
the jury is still out on this but a recent paper by wasserburg et al at
caltech points in that direction.
regards
frank spera

Frank J. Spera ~
Department of Geological Sciences       750 Carlo Drive
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