[meteorite-list] Re: Petrologic types and point values

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Thu Apr 22 10:23:48 2004
Message-ID: <5.1.0.14.2.20030313145549.02905e70_at_gsvaresm02.er.usgs.gov>

The divisions are mostly based on measurements of induced
thermoluminescence, which changes in response to crystallization of
feldspar in chondrule glass. Induced TL changes by a factor of 1000 across
the type 3's, so a logarithmic scale was created, and divided into 10
slices. I don't know if any parameter is linearly correlated with these
divisions (e.g., feldspar content, peak metamorphic temperature, volatile
element contents)... in fact, I doubt it.

Also, TL is not a perfect measurement, and other parameters are factored
into the assignments of subtypes. These include homogeneity of silicate
compositions, volatile contents, degree of matrix recrystallization, and
other things. The properties of the actual meteorites are quite
complex. Ultimately, the main usefulness of the subtype assigned to each
meteorite is that you can compare it to other meteorites and say which is
more "primitive."

jeff

At 02:48 PM 3/13/2003, Lars Pedersen wrote:
>Hi Jeff and list
> >You are correct that the scale
> >should not be thought of as linear.
>
>Ok ...
>But when it is not linear, there must be specific definitions for each
>subdivision ?.
>
>Best
>Lars
>
>----- Original Message -----
>From: "Jeff Grossman" <jgrossman_at_usgs.gov>
>To: <meteorite-list_at_meteoritecentral.com>
>Sent: Thursday, March 13, 2003 8:31 PM
>Subject: Re: [meteorite-list] Re: Petrologic types and point values
>
>
> > No, it's simple. The decimals divide petrologic type 3 into ten finer
> > divisions. They are meant to convey a sense of relative metamorphic
>grade,
> > just as the original 3-4-5-6 numbers did. The reason this was done for
> > type 3 and not for 4-5-6 is that the differences between a low-type-3 (now
> > 3.0) and a high-type-3 (now 3.9) are as important as the differences
> > between a type 4 and a type 6. So it was decided that subdivision was
> > needed to make these distinctions (I was a coauthor on that original
>Nature
> > paper proposing this scheme back in 1980). You are correct that the scale
> > should not be thought of as linear.
> >
> > jeff
> >
> > At 02:18 PM 3/13/2003, E.L. Jones wrote:
> > >Hello Gents, List
> > >
> > >To my best recollection, the decimal value is not a scale from 3 to 4,
> > >such that 3.5 is "half way" would be between the two grades. It is an
> > >index for another characteristic peculiar to feldspathic minerals , I
>believe.
> > >
> > >The value after the decimal (e.g. 3."X", 3.5, 3.7 etc.) is a index of
>the
> > >degree to which some feldspar content fluoresces, indicating another,
> > >specific type of metamorphic process beyond the equilibration(
> > >comingling/co-melting) of all types of chondrites as they move from
> > >3-4-5-6 etc. I infer that the process is lost after stage 3 and may be
> > >insignificant(???)
> > >
> > >Perhaps someone else would offer the specifics as to who designed the
> > >index and what its significance is. We have discussed this before perhaps
> > >it is in the archives.
> > >
> > >Regards,
> > >Elton
> > >
> > >Lars Pedersen wrote:
> > >
> > >><snip>?
> > >>But what is the "tecknical" background for 3.2 - 3.8 ?
> > >>I would like to dig a litle bit deeper.
> > >>
> > >>Thanks
> > >>Lars
> > >>
> > >>
> > >>
> > >>>Hei Lars,
> > >>>Even finer grading, lowest value of 3.x means the chondrules
> > >>>are the most distinct, higher values that they become
> > >>>more 'blured'. But even 3.8 are quite distinct since
> > >>>it's 3, not 4,5,6 in the first place.
> > >>>
> > >>>
> > >>>
> > >
> > >
> > >
> > >______________________________________________
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> >
> > Dr. Jeffrey N. Grossman phone: (703) 648-6184
> > US Geological Survey fax: (703) 648-6383
> > 954 National Center
> > Reston, VA 20192, USA
> >
> >
> >
> > ______________________________________________
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Dr. Jeffrey N. Grossman phone: (703) 648-6184
US Geological Survey fax: (703) 648-6383
954 National Center
Reston, VA 20192, USA
Received on Thu 13 Mar 2003 03:16:26 PM PST