[meteorite-list] PAC vs Type-7 vs IMB? (Was: Portales Valley Classification Info)

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Tue May 17 07:33:39 2005
Message-ID: <5.1.0.14.2.20050517064317.02b29b50_at_gsvaresm02.er.usgs.gov>

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

At 06:29 AM 5/17/2005, Martin Altmann wrote:
>Hi Mr.Woolard, list
>
>Thanks for the oportunity, because I have a question:
>What are the hard criteria for to distinghuish from ot foe classifying a
>stone as
>
>Impact Melt Breccia
>versus
>pertological Type 7
>versus
>Primitive Achondrite
>
>Until now I couldn't find nor anybody could give me an exact answer,
>so that I almost was tempted to call the recent occurence of several L7s and
>H7s as a fad.
>
>May someone enlighten us?
>Curiously Bucklebooing
>Martin
>
>
>
>----- Original Message -----
>From: "Robert Woolard" <meteoritefinder_at_yahoo.com>
>To: <meteorite-list_at_meteoritecentral.com>
>Sent: Tuesday, May 17, 2005 4:21 AM
>Subject: [meteorite-list] Portales Valley Classification Info
>
>
> > Hello List,
> >
> > Well for years now, I know a lot of us were puzzled
> > by the classification of Portales Valley as an " H6
> > ordinary chondrite". (See my article in the May 2001
> > issue of Meteorite, titled " Portales Valley - A Not
> > So Ordinary (Ordinary Chondrite??)! " In the recent
> > past, the classification was modified a bit, being
> > changed to read as an " H6 Impact Melt Breccia ".
> >
> > I am excited to be able to say that there is a
> > distinct chance the true uniqueness of PV may soon be
> > reflected in a possible new moniker for this
> > intriguing meteorite. David Weir was kind enough to
> > make me aware of a new and comprehensive paper by Alex
> > Ruzicka, Marvin Killgore, David Mittlefehldt, and Marc
> > Fries in the current MAPS. In this detailed work, we
> > now have the proposed reclassification of PV as an "
> > H7, metallic-melt breccia (primitive achondrite)",
> > with the case made for a new meteorite type
> > designation of "Portalesite" due to this metallic-melt
> > breccia characteristic.
> >
> > You can read David's updated description of PV on
> > his excellent website here:
> >
> > http://www.meteoritestudies.com
> >
> > Many thanks to David for news of this exciting
> > paper, and to the authors of the paper as well.
> >
> > Sincerely,
> > Robert Woolard
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
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Received on Tue 17 May 2005 07:33:29 AM PDT