[meteorite-list] Portales Valley Classification Clarification

[David Weir]

The authors of the MAPS paper wrote:

"Based on our work, it seems clear that the metal-sulfide and even the 
silicate portion of the meteorite was partly melted, suggesting that the 
petrographic grade of Portales Valley is higher than six. Considering 
this likely partial melt origin for PV, the H chondrite-like mineral 
compositions for most phases, and our inference of a mainly endogenic 
heat source, Portales Valley can be properly regarded as a primitive 
achondrite related to H chondrites.  In other words, it is an H7 
achondrite."

Jeff wrote:
"If I had to publish the announcement again today as editor, knowing 
what we do now, I'd probably go with "H melt breccia".

Jeff also wrote:
"Some people believe that melting in PACs was caused by impact 
processing, while others (I'd say the majority) think the heat source is 
internal.  If impacts played a role in their formation, then the line 
between IMB and PAC gets fuzzy at some point.  If they didn't play a 
role, then I suppose type 7 would transition into PAC once partial 
melting begins.  But I don't see any way to confuse type 7 (no melt) 
with IMB (contains melt)."

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That leaves me only a little bit wondering. So you can't have both a PAC 
and a type 7, they are mutually exclusive? As soon as melt is formed it 
ceases to be thought of as a petrologic grade 7 (i.e., petrologic grade 
becomes obsolete) and it is then either a PAC or an IMB, depending on 
the source of heat which produced the melt (PAC if endogenic and IMB if 
from impact event)? That would be pretty clear.

I would hazard a guess that there might be other lithologies somewhat 
distant from the PV rock (crater floor?) which would exhibit metamorphic 
effects only to the degree of an H7 type, without experiencing the 
degree of heating, endogenic or impact generated, necessary to cause 
partial melting.

David