[meteorite-list] angrites unlikely to be Mercurian

[Larry Lebofsky]

Melinda:

I agree with you that the angrites cannot have come from the present surface 
of Mercury. However, something happened to the original surface of Mercury (in 
most models) and so what happened to that material?

Just a thought. If you stripped off the crust of Mercury (an impact similar to 
the formation of the Moon), could you have formed a few "aseroids?" Then the 
cosmic ray exposure ages date from the breakup of the "asteroids." If you can 
form the Moon by an impact, why not a few objects in orbit around the Sun? We 
need to look for asteroids inside the orbit of the Earth. 

Sorry, I sound like Immanuel Velikovsky who believed that Venus was a comet 
that came out of Jupiter and created the Great Red Spot. No, I am not 
supporting that idea!

Larry

Quoting mhutson at pdx.edu:

> 
> While I found Tony Irving's abstract ("Unique Angrite NWA 2999: The Case for
> Samples from Mercury") to be interesting, I find it unlikely that angrites
> could be samples of Mercury.  While it is remotely possible that they are
> remnants of a crust stripped off 4.5 billion years ago, they would have to
> have
> been floating in space for the last 4.5 billion years or so (inconsistent
> with
> Cosmic Ray Exposure Data).  Any samples derived from the current Mercurian
> surface should be consistent with the following:  crystallization age <4.55
> by
> (possibly much less).  Many places on Mercury have crater counts
> intermediate
> to the lunar highlands and the lunar maria, so I would suspect the surfaces
> to
> be in the 4.3-3.8 by old range.  The Mercurian albedo (way it reflects
> light)
> is similar to that of the lunar highlands or somewhat brighter, but redder
> (implying the possibility of a larger amount of glass or agglutinates).  One
> thing that is very clear is that rocks on the surface appear to be iron poor
> (pyroxenes < 5% FeO), inconsistent with the relatively iron rich olivines
> and
> pyroxenes in angrites.  There was also a poster/abstract by Ann Sprague and
> co-workers a couple of years back suggesting that the Mercurian spectrum was
> consistent with Fe-poor clinopyroxene and an intermediate composition
> feldspar
> (aproximately a labradorite containing both Na and Ca), also inconsistent
> with
> NWA 2999 where the feldspar is pure anorthite (Ca only). Another paper in
> JGR
> by Cooper and coworkers suggested that some areas on Mercury looked mafic,
> but
> again with very low Fe contents in minerals.   So if not from Mercury, where
> do
> angrites come from?  Given their old crystallization ages, I'd say probably
> from
> a differentiated asteroid.  We have plenty of evidence that many asteroids
> differentiated (look at all the different kinds of irons).  If it heated
> quickly, an asteroid wouldn't have to be that big to differentiate.  We
> could
> easily have overlooked a small asteroid.
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