[meteorite-list] angrites unlikely to be Mercurian

From: mhutson_at_pdx.edu <mhutson_at_meteoritecentral.com>
Date: Fri Dec 23 22:52:12 2005
Message-ID: <1135396325.43acc5e514920_at_webmail.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.
Received on Fri 23 Dec 2005 10:52:05 PM PST