[meteorite-list] H7 vs. PAC

From: David Weir <dgweir_at_meteoritecentral.com>
Date: Tue May 24 18:14:04 2005
Message-ID: <4293A6FD.4090509_at_earthlink.net>

Hello Adam (and Team LunarRock),

I noticed that you have an ebay auction with the following
classification information:

"NWA 3145 Rare H7 Primitive Achondrite Meteorite"

Just one week ago Jeff Grossman addressed the issue of H7 vs. PAC on the
List. Here is his conclusion regarding this issue:

"Type 7 chondrites (if you want to call highly metamorphosed type 6
chondrites by this name) are NOT primitive achondrites, never having
been partially melted."

I inferred from his descriptions of type 7, PAC, and IMB, that the use
of the Van Schmus-Wood classification scheme was not valid when a
partial melt occurred, but instead, the meteorite would be either an
impact melt breccia (or just impact melt), or a primitive achondrite if
the heating was endogenous. This meteorite NWA 3145 is listed in the
MetBull Preview as a PAC, with no reference to H7. According to the
MetBull, it's likely paired to NWA 2353 and 2635, which are both
ungrouped PACs with O-isotopes that plot slightly outside the range of
the H chondrite PB. Granted, Ted initially considered a classification
of H7 for these paired meteorites, but since the O-isotopes were
completed, it has been modified. Therefore, having nothing better to do,
I find it curious why you call this an H7 PAC - can't we all get on the
same page with this terminology?

The full text of Jeff's post is pasted below if anyone didn't read it.

David

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Petrological type 7 is generally taken to be an extension of the
solid-state metamorphic sequence defined by Van Schmus and Wood (1967).
  Mittlefehldt and Lindstrom (2001, Meteoritics & Planetary Science,
vol. 36, no. 3, p. 439-457) endorsed this concept and specifically
excluded from type 7 impact melt breccias and other meteorites where
there was partial melting caused by impact heating.

Primitive achondrites are meteorites that have near-chondritic
compositions and nonchondritic textures (work of Prinz, McCoy, and
others). They have experienced partial melting and, usually, melt
segregation, resulting in the deviations in composition from those of
their parent chondrites. Type 7 chondrites (if you want to call highly
metamorphosed type 6 chondrites by this name) are NOT primitive
achondrites, never having been partially melted.

Impact melt breccias, of course, are meteorites in which shock causes
partial melting and mixing of chondritic debris with the melt.

Ruzicka et al. conclude that PV was essentially a type 6 chondrite near
its peak metamorphic temperature, when a light shock event raised the
temperature just enough to cause partial melting and mobilize the metal.
  Thus PV is an IMB and NOT a type 7.

Why did Ruzicka reach the conclusion he did? Probably because there is
lots of gray area caused by model-dependency of some of these terms.
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).

Science plods on.

Jeff
Received on Tue 24 May 2005 06:13:17 PM PDT