The designations of petrologic type is based on texture (pristine or
recrystallized) and degree of mineralogical equilibration. Those that
appear most recrystallized have rather uniform mineral compositions
indicative of a high degree of annealing or thermal metamorphism. Although
other properties also vary with petrologic type (e.g., the concentration of
some volatile elements), those other properties are not the main
classificatory criteria. The petrologic types are the phenomenon that
requires us to think of a heat source. In the past, some maintained that
chondrites accreted hot from the nebula and were "autometamorphosed." Few
believe that these days. As I said before, most researchers attribute the
apparent annealing to the decay of 26-Al, which was undoubtedly present. My
papers over the past 20 years have provided evidence that impact-heating was
a major heat source for chondi\ritic meteorites. Both mechanisms may have
been active. As far as achondrites are concerned, their designation is
based on having a chondrule-free, usually igneous, texture. They appear to
have formed from a melt. The particular isotopic compositions of different
achondrite groups (e.g., HEDs, aubrites, angrites) are not important for
distinguishing chondrites from achondrites. But we are again faced with the
question of what heat source or sources caused achondrites to melt. Most
researchers would maintain that it was mainly 26-Al.
Alan Rubin
Institute of Geophysics and Planetary Physics
University of California
3845 Slichter Hall
603 Charles Young Dr. E
Los Angeles, CA 90095-1567
phone: 310-825-3202
e-mail: aeru...@ucla.edu
website: http://cosmochemists.igpp.ucla.edu/Rubin.html
----- Original Message -----
From: "Mendy Ouzillou" <ouzil...@yahoo.com>
To: "Richard Montgomery" <rickm...@earthlink.net>; "Alan Rubin"
<aeru...@ucla.edu>; "Peter Scherff" <petersche...@rcn.com>; "'Adam'"
<meteorite-list@meteoritecentral.com>
Sent: Monday, March 04, 2013 5:35 PM
Subject: Re: [meteorite-list] Type 7 chondrites
Dr. Rubin,
If I read your response carefully, I believe you are saying that the
petrologic state should not depend on the type of metamorphic process which
makes sense. Seems to me that the the isotopic analysis should be used to
identify chondritic material from achondritic material.
Mendy Ouzillou
----- Original Message -----
From: Richard Montgomery <rickm...@earthlink.net>
To: Alan Rubin <aeru...@ucla.edu>; Peter Scherff <petersche...@rcn.com>;
'Adam' <meteorite-list@meteoritecentral.com>
Cc:
Sent: Monday, March 4, 2013 5:14 PM
Subject: Re: [meteorite-list] Type 7 chondrites
What thoughts about Taffessasset in this regard? Anyone wish to chime in?
Richard M
----- Original Message ----- From: "Alan Rubin"
<aeru...@ucla.edu>
To: "Peter Scherff" <petersche...@rcn.com>;
"'Adam'" <meteorite-list@meteoritecentral.com>
Sent: Monday, March 04, 2013 3:41 PM
Subject: Re: [meteorite-list] Type 7 chondrites
Most classifiers don't use the type-7 designation because many of the
chondrites that have been called type-7 seem to be impact-melt breccias.
Most
researchers believe that thermal metamorphism probably caused by
asteroidal
heating engendered by the decvay of short-lived radionuclides like 26-Al
heated
chondrites from type 3 to 4 to 5 to 6. If shock was responsible for
causing a
rock to be called type 7, then it seemed more prudent to just call it
shocked
and not use the type-7 designation. Most researchers believe that the
primitive
achondrites were also partly (or completely) melted by heating caused by
the
decay of 26-Al. I am not of these camps; it seems to me that heating of
chondrites from type 3 to type 6 also results from impact heating and that
the
primitive achondrites formed in an analogous way, but that is another
story.
Alan
Alan Rubin
Institute of Geophysics and Planetary Physics
University of California
3845 Slichter Hall
603 Charles Young Dr. E
Los Angeles, CA 90095-1567
phone: 310-825-3202
e-mail: aeru...@ucla.edu
website: http://cosmochemists.igpp.ucla.edu/Rubin.html
----- Original Message ----- From: "Peter Scherff"
<petersche...@rcn.com>
To: "'Adam'" <meteorite-list@meteoritecentral.com>
Sent: Monday, March 04, 2013 3:14 PM
Subject: [meteorite-list] Type 7 chondrites
Hi,
Is there any consensus about petrologic type 7 chondrites? Are they
better
classified as Primitive Achondrites? If type 7 is different from
primitive
achondtites what is the line between them?
Thanks,
Peter Scherff
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