Hello Rob and List,
I'd like to chime in on this angrite subject, one I find very exciting
as attested to by my continued purchases of different angrite specimens.
I have been following the ongoing reasoned discussions by some very
smart investigators about a possible angrite-Mercury relationship. The
newest angrite members have certainly opened some new avenues which lend
some credibility to such a relationship. The decompression event which
was proposed after studies on NWA 2999 was shortly thereafter explained
by a different mechanism -- cooling under low pressure and oxidizing
conditions. However, studies of this new angrite NWA 4590 reveal glass
along mineral grain boundaries that incorporates re-precipitated primary
minerals, which is thought to have formed by a rapid melting and cooling
event consistent with decompression, as in the collisional stripping of
the lithosphere of a large planet.
One of the biggest hurdles for a Mercury connection was the
significantly higher FeO content for the angrites versus what is
observed on the "surface" of Mercury. The latest hypothesis out of UWS
suggests the angrite material may represent an ancient, higher-FeO
lithosphere from Mercury that has long since been removed through
impact-related dissemination. As you point out, this material would have
to enter a stable orbit around the Sun until relatively recently. In
return for accepting this probability, we get a great many APB
characteristics answered, such as its great age, planetary size,
chemistry (e.g., the reversal of the Fe/Mn ratios for olivine and
pyroxene as compared to those measured for other planetary bodies), the
large exogenous meteoritical component appropriate for Mercury's
location, and other characteristics. If not Mercury, the solution to
this group's origin is still an exciting story and I'm keeping up.
David
______________________________________________
Meteorite-list mailing list
[EMAIL PROTECTED]
http://six.pairlist.net/mailman/listinfo/meteorite-list
