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Re: The Strange Mystery of the Albion Iron Meteorite

These are some rambling thoughts I had on this subject, it's a rather
abstract presentation and based on my current readings. As I am far from
an expert I welcome corrections to what I write, I don't want to spread
any misinformation on this complex subject.

The attempt to classify irons by Widmanstatten pattern and later by
chemical and isotope analysis has lead to the major divisions
we now see.

As I understand from my studies, the core of the earth (using this as a
model for small planetesimals), was thought to have formed
'catastrophically' - that is, as planetoids accreted in a molten mass,
suddenly the iron separated and sunk to the core along with heavier
elements. Later collisions added more material to the molten mass and
presumably these, too sunk to the core.

The formation of the planets proceeded very rapidly and the remaining
debris collided with the surviving larger bodies until the present
planets were formed. The bombardment frequency drastically reduced about
3700 million years ago. The planetary formation process was quite
efficient at sucking up all available matter - there is very little
matter between planets (except of course for the asteroids belt
nowadays.

Most of the iron meteorites, if not all, come from the asteroid belt and
would have to have come from differentiated bodies. It is not known, but
thought that a body would have to be a few hundred kilometers in
diameter to differentiate.

When differentiation occurs, will the core mix well before it
solidifies? If this is so, then radically different iron chemistries
would signify different parent bodies. If mixing isn't so thorough, one
core when fragmented might produce several distinct types of iron
fragments. How long would it take for a planetesimal's core to solidify?
Probably all small planetoids were solid 3800 million years ago, and
they started forming about 4600 million years go.

I think the best guess is that there were at least 50 or 60 large
differentiated asteroids that could account for the samples we have on
earth.

Finally, the meteorite samples on earth are just small samples of
objects from the asteroid belt, a rather skewed sampling, too perhaps.
The asteroid belt formed from a temperature gradient in the early solar
nebula - near the solar end the temperatures were enough to severely
reduce the matter and near the other end water and water-ice were
available. We have samples from the near end - enstatites, and
intermediate materials (H, L, LL) and farther out - CI. But we don't
seem to have samples from the farthest parts of the asteroid belt.

Possibly we only, or mostly, have samples from the material that falls
in the Kirkwood gaps - regions where the orbit of an object is a simple
fraction of the Jovian orbit.

In summary then, it is possible that all our meteorite evidence is
really just a sampling from a small part of the asteroid belt, and even
more so,  a very small sampling of the solar system.
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