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

[Martin Altmann]

Many thanks,

this was the most helpful answer I got until now.
(Ähem, can we place the ureilites somewhere there?)

Martin

----- Original Message ----- 
From: "Jeff Grossman" <jgrossman at usgs.gov>
To: <meteorite-list at meteoritecentral.com>
Sent: Tuesday, May 17, 2005 1:33 PM
Subject: Re: [meteorite-list] PAC vs Type-7 vs IMB? (Was: PortalesValley
Classification Info)


> 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
>
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