[meteorite-list] Re: Mercury Meteorite Puzzle

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:04:50 2004
Message-ID: <200205210507.WAA22858_at_zagami.jpl.nasa.gov>

The evidence of linking E chrondites to Mercury or Venus is that
the pyroxene in these meteorites contains no iron, only magnesium.
The pyroxene in most meteorites would be expected to have a ratio of
magnesium and iron depending upon the availability of oxygen. The lack
of iron in the pyroxene in the E chrondites is due to a low oxygen
content, and one way to explain this if the meteorite had formed somewhere
close to the Sun. Hence, if these E chrondites had originated from Mercury
or Venus, then that would explain their lack of iron. This would also apply
if the parent body was an asteroid that formed inside of Earth's orbit.

> Could be, except that olivine is found as massive formations of large olivine
> crystals without an iron matrix, just not in meteorites. One notable
> location
> is the San Carlos Apache reservation where unaltered mantle material
> was erupted to the surface. It is now mined for gem quality olivine,
> commonly
> known as peridot. So the question remains if there are olivine crystals 60
> to
> 100 kilometers deep on earth (the depth of the peridot source) why haven't
> we found meteorites composed of just olivine crystals?

99% of meteorites come from asteroids, so your Earth analogy is not very
applicable. There are large deposits of diamonds and gold in Earth, too, but
I would not expect to find diamond meteorites or gold meteorites. The
mineralogy formed inside a low-gravity body like an asteroid is not exactly the
same as from the Earth.

>There is no currently
> accepted answer.

I just suggested a plausible scenario with the iron molten core for asteroid,
which you haven't countered, but I'll repeat it for you again.

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 Tue 21 May 2002 01:07:47 AM PDT