[meteorite-list] Portales Valley Classification Clarification

From: David Weir <dgweir_at_meteoritecentral.com>
Date: Tue May 17 16:57:15 2005
Message-ID: <428A5A88.3030808_at_earthlink.net>

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)."

------------------------------------------

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
Received on Tue 17 May 2005 04:56:40 PM PDT