[meteorite-list] Mesosiderites

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Wed May 5 16:14:36 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C86904EE5BFC_at_US-Torrance.mail.saic.com>

Hi All,

Tracy asked:

> We already assume that pallasites are from the core-mantle boundaries of
> asteroids large enough to have undergone differentiation at one point in
> their lives (before being turned into specimens in our collections!)
> What other theories for the formation of mesosiderites, besides the
> mashing together of a pallasite body and another rocky body, allow
> for the incorporation of the occasional rogue olivine crystal?

Alan Rubin devotes a chapter to mesosiderites in his very readable
_Disturbing the Solar System_. In particular, he outlines a biography of
the mesosiderite parent body, the central mystery of which is of course,
"What happened to the olivine?" For a typical differentiated asteroid
with a 200-km diameter, the iron-nickel core would be about 100-km in
diameter, and the basaltic crust would be about 5 km thick. Thus the
basalt and gabbro crustal portion of the asteroid is separated from
the metallic core by a ~45-km thick shell of olivine-rich mantle. Yet,
mesosiderites contain about a 50-50 split of silicates and Fe-Ni, with
only about 1-2% olivine. A bit difficult to mix the metal and the
silicates w/o also getting a lot more olivine.

Perhaps the most obvious solution is to have the iron and silicates
in mesosiderites come from different origins: start with a differentiated
asteroid as before, but then impact it with a relatively low-velocity
nickel-iron body. (A high-velocity impact won't work because the metal
then becomes vaporized and too dispersed.) Low-velocity impacts (< 2
km/sec)
between asteroids are rare today, but they would have been much more
common in the early solar system.

Alan has outlined a 5-stage process that ultimately results in mesosiderites
here on earth. I won't spoil the fun ... buy the book and learn all the
details. Less than $20 in paperback from Amazon, and about the same for
a good quality used hardcover there. --Rob
Received on Wed 05 May 2004 04:10:14 PM PDT