[meteorite-list] Elemental Abundances

From: E. L. Jones <jonee_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:24 2004
Message-ID: <40358A00.7040300_at_epix.net>

Hello List.

This is from a discussion on another list I am on, however, because it
deals with meteorite composition, I thought I'd pass it along. It is
the foundation for understanding why we use isotope ratios to determine
where in our solar system a given meteorite may have originated.

In the discussion of dwarf stars having giant diamond crystals at their
cores, the question was raised, "Are there any statistics on
distribution of elements in our solar system"? Yes, There is a concept
called Cosmic(Universal) Abundance of Elements
<http://www.doane.edu/crete/academic/science/chem/Ast/Astro.html> This
illustration is interactive and overlain on the Andromeda Galaxy for
effect but it pertains to all mass in the universe. Using spectral band
measurements astronomers were able to sum up the elemental ratios of
the cosmos

It was correctly mentioned that elements heavier than iron are formed in
supernovae and all in a very short time. However, the ratios of light
elements to heavy ones are also biased by the number of hydrogen
particles combined to form a heavy element. (e.g. Roughly it takes 92
Tritium atoms to form one Uranium atom.) Working backwards, adding up
all the atomic masses of all the material in our solar system, would
approximate the number of hydrogen/helium atoms in our solar precesseor.
 
  For a bar graph representation of abundance (standardized as ratios to
silicon atoms), see
<http://jchemed.chem.wisc.edu/JCESoft/Programs/PTL/Sample/Plots/univabnd.html>
.

Below our feet, there is an terrestrial abundance
<http://www.daviddarling.info/encyclopedia/E/elterr.html>
which is actually an estimate of the abundance in the lithosphere(crust
and upper mantle). This does not include the deep mantle or core.

There was a sorting of the elements in our proto-solar system with the
heavier elements dominating the inner system and the volatiles the outer
edge. There are two mechanisims theorized as to how this occured(They
were probably co-dynamic). One concept is that the proto-solar system,
left over from the supernova of our sun's precesseor , scattered the
elements evenly over an approximately 100AU wide disk. The heavier
elements precipitated before the more volatiles due to the faster
cooling rate on the edge of the protodisk and fell back toward the
gravitational center of our solar system to be incorporated into the
inner planets. This illustration correlates formation temperature of
minerals and planet distances from the sun.
<http://www.geology.uiuc.edu/~hsui/classes/geo116/lectures/wk2folder/compodist-demo.html>


The other mechanism contends that the mixture of elements was sorted by
pressure waves -- an allegory to the solar wind, by tending to sweep
lighter elements toward the outer rim. This theory is supported by such
evidence as ratios of heavy to light isotopes decreasing with distance
from the center of the solar system.

Someone, I recall, rasied a question about the the amount of iron in
meteorites perhaps being higher than expected given that free iron is a
deep core occurrence. Irony meteorites represent the cores of asteroids
large enough gravationally and thermally to have allowed to nickel-iron
to sort to the center/core of the asteroid. We find these meteorites
because cosmic collisions opened up the cores of these asteroids and
flung the iron centers far and wide. Stoeny meteorites--specifically
Common Chondrites, have flakes of free nickel-iron metal because the
iron blebs which were incorporated into pyroxene/olivine matrix, come
from bodies which were to small to 1) keep warm enough long enough for
the nickel-iron to melt and 2) the internal gravitational pull on these
small bodies was insufficient to draw the nickel-iron to the core.

Regards,
Elton
Received on Thu 19 Feb 2004 11:16:00 PM PST