Type 7 is considered by most of those who use it to represent the
highest degree of thermal metamorphism that a chondrite can experience
without melting. As implied in that first sentence, some petrologists
don't distinguish these from type 6. The term "primitive achondrite" is
widely taken to be the next stage: you make them when a chondrite
partially melts, and the process of crystal-melt separation begins. The
"primitive" part says that the bulk composition is still fairly close to
chondritic. But these definitions are not used by everybody, and you
will get arguments about them.
Clearly, the "LL" part of an LL7 classification for NWA 3100 is
unlikely. O isotopes are below the terrestrial fractionation line,
which basically rules it out. So it is not an LL7. Bunch has shown
that the O isotopes are closer to CR chondrites.
The hard part is the type 7 vs. primitive achondrite distinction. Bunch
et al.'s 2005 and 2008 LPSC abstracts do not report anything in NWA 3100
that I take as evidence of melting or differentiation. So I don't see
any reason to call these primitive achondrites, at least not based on
these findings. I think the Bunch et al.'s conclusion that NWA 3100 is
a CR6 is the best we have right now, but I think you still have to think
of this as preliminary. Ted can correct me, but I think it was actually
the nomcom that pushed for calling this a PAC, amid controversy on the
committee.
Jeff
On 12/5/2011 8:23 PM, Ruben Garcia wrote:
Hi all,
I just bought a smallish collection and several of the slices that
came with are NWA 3100. Mike Farmer's card was included and lists NWA
3100 as an LL7. The Met-Bul calls NWA 3100 a Primitive achondrite -
not an LL7.
My question is this,
Does LL7 denote a particular Primitive achondrite? If so which one? If
not then what type is this?
BTW - I think Ted Bunch did the classification
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