[meteorite-list] NEW Plutonic Angrite - NWA 4590 "Tamassint"

[David Weir]

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