[meteorite-list] NWA 2736 - Aubrite or not??

[David Weir]

Hey Adam,

If different types of material from a single PB forms a metamorphic 
continuum as has been published recently, what are the exact parameters 
for these different groupings; e.g., chondrite, metachondrite, primitive 
achondrite, achondrite, stony-iron, iron. I have not yet seen the 
particulars for where one stops and the other begins, but a continuum is 
not given to such deterministic lines in the true nature of things.

An aubrite by most definitions would be associated with the achondrite 
(igneously derived) portion of this continuum, but a primitive 
achondrite and even a metachondrite could be close enough that it would 
take in-depth analyses to distinguish between them, and some labs don't 
even recognize all of these metamorphic divisions to begin with. At 
least one aubrite is classified as a stony-iron (Mt. Egerton), and one 
enstatite achondrite is more like an iron (Horse Creek).

I suppose the classification of NWA 2736 as an aubrite may be acceptable 
given the fact that an aubrite is not a term referring to any one 
particular PB; e.g. Shallowater is from a unique aubritic PB, and there 
are several other ungrouped enstatite-related meteorites such as Itqiy, 
Zaklodzie, NWA 1235, and NWA 1839 just to name a few. I think O-isotopic 
composition data is needed for all of these enstatite-related rocks, and 
of course more samples of such meteorites will hopefully be found in 
order to answer some of these PB source questions and to better 
distinguish their petrologic history for an accurate classification 
assignment.

David