[meteorite-list] Re: Mercury Meteorite Puzzle

From: E.P. Grondine <epgrondine_at_meteoritecentral.com>
Date: Thu Apr 22 10:04:50 2004
Message-ID: <20020521151701.86480.qmail_at_web11607.mail.yahoo.com>

Hi Ron, all -

Most fascinating to read the comments on the formation
of olivine, particularly as it relates to parent body
size(s).

Pressure seems to be the key. It strikes me that
there would have been at least 2 sources of pressure -
one, gravity; and two, the cooled outer crust pressing
against the still molten core. I imagine one could
write up a nifty computer program relating the two,
dump in the olivine formation pressure, et voila.

Another question crosses my mind on this, which is
whether anyone has ever added together the estimated
masses of each of the known LPBE impactors.

all the best -
ep

--- Ron Baalke <baalke_at_zagami.jpl.nasa.gov> wrote:
>
> 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
>
>
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Received on Tue 21 May 2002 11:17:01 AM PDT