[meteorite-list] Subtypes

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:11 2004
Message-ID: <3B253A84.2AF79C31_at_lehrer1.rz.uni-karlsruhe.de>

entropydave wrote:

> I wonder if someone can explain the
> decimal pet grades such as H3.4 or H3.7

Elton quoted Frank Cressy's remarks:

> Petrographic grades 3.0 through 3.9 are obtained by measuring changes
> of thermoluminescence (TL) of chondrules. TL measures the luminesence
> of the sample after it is irradiated.

> During mild metamorphism, tiny grains of feldspar start to grow in a
> glassy chondrule. The higher the metamorphism, the more feldspar is
> present and feldspar causes a marked increase in thermoluminence.
> This allows one to subdivide the petrographic grade 3 meteorites.


Hi All,

As the List has quieted down considerably, I'd like to share some
additional information on petrologic subtypes with those who are
still out there:

SCOTT E.R.D. (1984) Classification, Metamorphism, and Brecciation
of Type 3 Chondrites from Antarctica (Smithsonsian Contributions
to the Earth Sciences, Nr. 26, Smithsonian Institution Press, 1984,
pp. 81-84, excerpts):

Petrologic Subtypes

Type 3 ordinary chondrites have recently been subdivided according to
the degree of metamorphism that they have experienced (Sears et al.,
1980; Huss et al., 1981). The decimal subdivision into types 3.0 to 3.9,
which was proposed by Sears and coworkers, has been adopted by most
people and is based on measurements of thermoluminescence sensitivity.
This parameter appears to be the most useful for classifying type 3
ordinary chondrites ...

... extensive weathering, ... may lower the thermoluminescence
sensitivity and the subtype. Shock may also affect the thermo-
luminescence sensitivity (Sears, 1980).

Dodd (1969, 1981) finds that CaO concentrations in olivine vary
according to petrologic type in ordinary chondrites: type 3 olivines
have >= 0.1 wt.% CaO, type 4 have 0.06 wt.%, and type 5-6
0.02-0.05 wt.% CaO ... even though CaO concentrations in olivine
may vary widely within a single type 3 chondrite, the mean value
can also be used to classify type 3 chondrites according to subtype
with an accuracy better than ±0.2.

Unfortunately, the thermoluminescence sensitivities of type
4 chondrites are largely indistinguishable from type 3.8-3.9
chondrites (Sears et al., 1980). It appears that the boundary
between type 3 and 4 chondrites must remain indistinct, like
the boundary between type 4 and 5 chondrites.


Bedtime here,
Good Night,

Bernd
Received on Mon 11 Jun 2001 05:39:16 PM PDT