[meteorite-list] Mesosiderites

[Matson, Robert]

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