[meteorite-list] Type 7 chondrites

From: Alan Rubin <aerubin_at_meteoritecentral.com>
Date: Mon, 4 Mar 2013 23:01:13 -0800
Message-ID: <EBDAAB85762C4027ADCFEFEAA9BE67BA_at_igpp.ucla.edu>

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: aerubin at ucla.edu
website: http://cosmochemists.igpp.ucla.edu/Rubin.html


----- Original Message -----
From: "Mendy Ouzillou" <ouzillou at yahoo.com>
To: "Richard Montgomery" <rickmont at earthlink.net>; "Alan Rubin"
<aerubin at ucla.edu>; "Peter Scherff" <PeterScherff at rcn.com>; "'Adam'"
<meteorite-list at 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 <rickmont at earthlink.net>
> To: Alan Rubin <aerubin at ucla.edu>; Peter Scherff <PeterScherff at rcn.com>;
> 'Adam' <meteorite-list at 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"
> <aerubin at ucla.edu>
> To: "Peter Scherff" <PeterScherff at rcn.com>;
> "'Adam'" <meteorite-list at 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: aerubin at ucla.edu
>> website: http://cosmochemists.igpp.ucla.edu/Rubin.html
>>
>>
>> ----- Original Message ----- From: "Peter Scherff"
> <PeterScherff at rcn.com>
>> To: "'Adam'" <meteorite-list at 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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Received on Tue 05 Mar 2013 02:01:13 AM PST