[meteorite-list] Re: Mercury Meteorite Puzzle

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:04:50 2004
Message-ID: <200205202051.NAA19924_at_zagami.jpl.nasa.gov>

>>While it is still possible the parent body for the E chondrites and NWA 011
>> may be Mercury or Venus, you can't rule out asteroids just yet either. >
>
> Both of these statements are incorrect. Both Mercury and Venus are
> differentiated bodies. Neither can be the source of "chondrites" of any
> kind. They could however the the source of various "achondrites".

Since the planets are formed from chondritic asteroids, I don't think you can
rule them out just because the meteorite is a chondrite.

> If people are interested in a mystery here is a big one. Pallasites are
> generally
> assumed to be from the core / mantle boundry.

That is correct.

>There are quite a few
> pallasites
> so their formation doesn't appear to be an unusual occurance.

Actually, pallasites are on the rare side, particularly when you compare them to the
stony chondrites.

>There are
> also a
> lot of irons from the core side of the boundry.

In terms of number of falls, irons are also somewhate rare (but not
as rare as pallasites), but they are common in terms of total known weight.

>There are however no, as in
> zero, meteorites with pallasitic type olivine crystals with out an iron
> matrix.

Since the olivine crystals are only found in an iron matrix, that should
be your first big hint. Olivine is a common mineral in stony chondrites, though
not in crystalline form, as you noted. Since olivine is common in the stony chondrites,
then you would assume that the olivine was somehow converted to crystalline form
when it became a pallasite.

> The mantle by volume would be larger than the core of most differentiated
> bodies. So where are the olivine meteorites? One would think they would be
> at least as abundent as the pallasites. Is the pallasite theory incorrect?

My guess would be the olivine crytals came about when the chondrite layer interacted
with the molten iron core. Since the olivine is intermixed with the nickel-iron, that
implies that they were mixed together when the metal portion was in a hot molten form,
which futher implies the chondritic material was exposed to high temperatures and
probably some pressure as well. Due to the exposure to the high temperatures,
the olivine was converted to a crystalline form. That would explain why you only
see the olivine crystals only in pallasites, the boundary layer between the
metal core and the chondritic mantle.

Ron Baalke
Received on Mon 20 May 2002 04:51:00 PM PDT