Re: % homogeneous
- From: Tracy Hamilton <lastname@xxxxxxx>
- Date: Tue, 02 Aug 2005 05:58:00 -0500
allenroyboy wrote:
> While learning more about the isochron dating method, I read in Faure
> that the initial ratios at time zero of 87Sr/86Sr are expected to be
> the same, because the magma is expected to be homogeneous.
>
> Since real measurements are rarely exactly the same, what percent of
> homogenity is acceptable, or what precision is allowed?
>
> Any one know?
>
> I've been searching the literature and not having much luck a locating
> anything that discusses the measure of homogenity. Perhaps I'm not
> using the best search words.
Basically, in a melt, all atoms of the same element will be randomly
distributed. In other words, 87Sr and 86Sr are *chemically* identical,
so anything that discriminates by chemistry will not change the
87Sr/86Sr ratio.
There are processes that depend on physical properties, the
relevant one here being mass. Essentially, in a mixture at the
same temperature, the average kinetic energy is 1.5 RT (R is gas
law constant, T is temperature in Kelvin). The RMS speed can be
obtained from setting 0.5 m <v^2> equal to average kinetic energy.
Since m is different, <v^2> will be different. Slightly different.
However, these effects are measurable.
There is certainly not much difficulty in accurately *measuring*
isotope ratios nowadays.
In chemistry these are called kinetic isotope effects. In
crystallization it is called fractionation. The magnitude of the
effects is determined by mechanism, and by the ratio of the masses
(1.00 being no mass difference)).
Large istope effects are seen for hydrogen/deuterium/tritium since
the mass ratio is 2 or 3. For strontium it is 1.01 .
Tracy P. Hamilton
.
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- % homogeneous
- From: allenroyboy
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