[meteorite-list] fundamental processes of meteorite formation

From: Steven Singletary <jumper_at_meteoritecentral.com>
Date: Thu Apr 22 09:49:01 2004
Message-ID: <200109172156.RAA09014_at_melbourne-city-street.mit.edu>

> First, why does the process of
>differentiation or presence on a differentiated body bring about
>fractionation in isotopes that is different than that found in the same
>elements on undifferentiated bodies? (I know this is an ignorant question,
>but I'm trying... I really am.)

I suppose the first thing we should ask is why do isotopes fractionate? To
paraphrase heavily from an isotope geology textbook - Fractionation occurs
because there are certain thermodynamic properties of molecules that depend
on the mass of the atoms that are involved (ie the bond strength between
atoms). As an example - think of water evaporating. That involves a
change of state from a liquid to a vapor - essentially you are just
speeding up the molecules so they vibrate more and are not as tightly
packed. It is easier to speed up lighter molecules. Hydrogen can be
either mass 1 or mass 2 (deuterium). The lighter isotope of hydrogen will
preferentially enter the vapor phase because it is easier to "speed up"
than the heavier isotope. So that evaporation fractionates the isotopes of
hydrogen. Thats just one example. So the upshot of all that is that we
expect to see different stable isotopes fractionated in different types of
environments, depending on what processes are taking place. When we look
at any rock from the Earth, Moon, Mars, or the HED's they all show oxygen
isotope fractionation, such that the isotopes plot along a slope 1/2 line.
That is due to the melting and rehomogenization of the material that makes
up the body.

Now what is puzzling here, is that ureilites have big crystals in them and
resemble igneous rocks - meaning they have been melted, precipitated, or
what have you. If we were to see this rock in any other setting we could
call it an igneous rock. SO the isotopes should show fractionation. But
they don't!!! That's one of the interesting puzzles that keeps me getting
up every morning :)

I'll stop there for now. I'll get to the rest of your message tomorrow if
that's okay. I've got labs to finish preparing. If there is something I
didn't explain very well or if I'm getting too basic, please let me know.

Steven Singletary
Dept. Earth, Atmospheric, and Planetary Sciences
Cambridge, Ma., 02139

Blue Skies!!
Received on Mon 17 Sep 2001 05:58:07 PM PDT

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