[meteorite-list] Correlation of Fa & Fs for ordinary chondrites

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Thu Apr 22 10:29:52 2004
Message-ID: <5.1.0.14.2.20030908072529.02f812c8_at_gsvaresm02.er.usgs.gov>

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The relationship that you found was documented in meteorites shortly after
the first electron microprobes became available to meteorite
researchers. The landmark paper was:

KEIL K. and FREDRIKSSON K. (1964) The iron, magnesium, and calcium
distribution in coexisting olivines and rhombic pyroxenes of chondrites. J.
Geophys. Res. 69, 3487-3515.

Here is part of the abstract:

"Iron, magnesium, and calcium in olivines and pyroxenes of 95 chondritic
meteorites were analyzed with an electron microprobe. Of these, 86 were
'ordinary' chondrites, each having constant iron-magnesium ratios in
olivine and pyroxene. Between different meteorites these ratios vary
within three narrow ranges which reflect the H- and L-group chondrites, as
well as a third group, designated the LL group. The ratio of iron in
olivine to that in coexisting pyroxene is nearly constant (~1.1),
particularly within the groups mentioned. A classification of stony
meteorites based on the amount of iron in silicates and metal is proposed..."

This relationship is due to equilibrium partitioning of Fe and Mg between
olivine and pyroxene. In partially equilibrated chondrites (e.g., high
type 3's), you will find departures from this relationship because olivine
equilibrates much more rapidly than pyroxene. In unequilibrated
chondrites, there is no such relationship, but it may be followed within
individual chondrules.

jeff

At 11:02 PM 9/7/2003, Matson, Robert wrote:
>Hi All,
>
>I decided it was still too hot to go to the desert this weekend, so
>instead I spent the better part of today on a problem I've been
>meaning to tackle for a few months. Hopefully my results will be
>useful to someone here. I don't know if the work is quite worthy
>of a paper -- perhaps.
>
>My interest in the problem primarily has to do with pairing of
>equilibrated ordinary chondrites. While some labs measure both
>olivine fayalite (Fa) and pyroxene ferrosilite (Fs) mol %, others
>measure only one or the other (usually Fa). Occasionally, you'll
>have two specimens that are potentially paired, but Fa was measured
>on one, and Fs on the other. As it turns out, Fa and Fs are somewhat
>correlated, and thus it is possible to derive expressions for
>converting one value to the other, within certain error bars.
>
>So I created a database containing only Antarctic equilibrated
>ordinary chondrites that had measured values for both Fa and Fs,
>subdivided by type and petrologic grade. As you might imagine,
>this took a while! I excluded meteorites that had ranges (rather
>than single values) listed for either Fa or Fs, and I tossed out
>five outliers that would have unduly skewed the statistics. That
>still left me with 3449 meteorites!
>
>I plotted these in Excel, with separate symbols and colors for
>H4, H5, H6, L4, L5, L6, LL4, LL5, LL6 and LL7. It turns out that
>there were no major correlation differences between petrologic
>grades within each type, so I lumped all the grades together and
>did linear regression fits for H, L and LL. Here are the results
>for converting a ferrosilite value into a fayalite value:
>
>H: Fa = .932*Fs + 3.20 rms residual = +/- 0.38% (2448 points)
>L: Fa = .958*Fs + 4.60 rms residual = +/- 0.57% (801 points)
>LL: Fa = 1.057*Fs + 3.76 rms residual = +/- 0.66% (200 points)
>
>(I have corresponding equations for going in the reverse direction.
>I also computed the linear coefficients by petrologic grade if
>that interests anyone).
>
>When you consider that most of the Antarctic Fa and Fs values were
>given to the nearest whole number percentage, I'd say the fit is
>quite good.
>
>Cheers,
>Rob
>
>
>______________________________________________
>Meteorite-list mailing list
>Meteorite-list_at_meteoritecentral.com
>http://www.pairlist.net/mailman/listinfo/meteorite-list

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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<html>
<font face="Courier New, Courier">The relationship that you found was
documented in meteorites shortly after the first electron microprobes
became available to meteorite researchers.&nbsp; The landmark paper
was:<br><br>
K</font><font face="Courier New, Courier" size=2>EIL</font><font face="Courier New, Courier">
K. and
F</font><font face="Courier New, Courier" size=2>REDRIKSSON</font><font face="Courier New, Courier">
K. (1964) The iron, magnesium, and calcium distribution in coexisting
olivines and rhombic pyroxenes of chondrites. <i>J. Geophys. Res.</i>
<b>69</b>, 3487-3515.<br><br>
</font>Here is part of the abstract:<br><br>
&quot;Iron, magnesium, and calcium in olivines and pyroxenes of 95
chondritic meteorites were analyzed with an electron microprobe.&nbsp; Of
these, 86 were 'ordinary' chondrites, each having constant iron-magnesium
ratios in olivine and pyroxene.&nbsp; Between different meteorites these
ratios vary within three narrow ranges which reflect the H- and L-group
chondrites, as well as a third group, designated the LL group.&nbsp; The
ratio of iron in olivine to that in coexisting pyroxene is nearly
constant (~1.1), particularly within the groups mentioned.&nbsp; A
classification of stony meteorites based on the amount of iron in
silicates and metal is proposed...&quot;<br><br>
This relationship is due to equilibrium partitioning of Fe and Mg between
olivine and pyroxene.&nbsp; In partially equilibrated chondrites (e.g.,
high type 3's), you will find departures from this relationship because
olivine equilibrates much more rapidly than pyroxene.&nbsp; In
unequilibrated chondrites, there is no such relationship, but it may be
followed within individual chondrules.<br><br>
jeff<br><br>
At 11:02 PM 9/7/2003, Matson, Robert wrote:<br>
<blockquote type=cite class=cite cite>Hi All,<br><br>
I decided it was still too hot to go to the desert this weekend, so<br>
instead I spent the better part of today on a problem I've been<br>
meaning to tackle for a few months.&nbsp; Hopefully my results will
be<br>
useful to someone here.&nbsp; I don't know if the work is quite
worthy<br>
of a paper -- perhaps.<br><br>
My interest in the problem primarily has to do with pairing of<br>
equilibrated ordinary chondrites.&nbsp; While some labs measure
both<br>
olivine fayalite (Fa) and pyroxene ferrosilite (Fs) mol %, others<br>
measure only one or the other (usually Fa).&nbsp; Occasionally,
you'll<br>
have two specimens that are potentially paired, but Fa was measured<br>
on one, and Fs on the other.&nbsp; As it turns out, Fa and Fs are
somewhat<br>
correlated, and thus it is possible to derive expressions for<br>
converting one value to the other, within certain error bars.<br><br>
So I created a database containing only Antarctic equilibrated<br>
ordinary chondrites that had measured values for both Fa and Fs,<br>
subdivided by type and petrologic grade.&nbsp; As you might
imagine,<br>
this took a while!&nbsp; I excluded meteorites that had ranges
(rather<br>
than single values) listed for either Fa or Fs, and I tossed out<br>
five outliers that would have unduly skewed the statistics.&nbsp;
That<br>
still left me with 3449 meteorites!<br><br>
I plotted these in Excel, with separate symbols and colors for<br>
H4, H5, H6, L4, L5, L6, LL4, LL5, LL6 and LL7.&nbsp; It turns out
that<br>
there were no major correlation differences between petrologic<br>
grades within each type, so I lumped all the grades together and<br>
did linear regression fits for H, L and LL.&nbsp; Here are the
results<br>
for converting a ferrosilite value into a fayalite value:<br><br>
H:&nbsp; Fa = .932*Fs + 3.20&nbsp;&nbsp; rms residual = +/- 0.38% (2448
points)<br>
L:&nbsp; Fa = .958*Fs + 4.60&nbsp;&nbsp; rms residual = +/- 0.57% (801
points)<br>
LL: Fa = 1.057*Fs + 3.76&nbsp; rms residual = +/- 0.66% (200
points)<br><br>
(I have corresponding equations for going in the reverse direction.<br>
I also computed the linear coefficients by petrologic grade if<br>
that interests anyone).<br><br>
When you consider that most of the Antarctic Fa and Fs values were<br>
given to the nearest whole number percentage, I'd say the fit is<br>
quite good.<br><br>
Cheers,<br>
Rob<br><br>
<br>
______________________________________________<br>
Meteorite-list mailing list<br>
Meteorite-list_at_meteoritecentral.com<br>
<a href="http://www.pairlist.net/mailman/listinfo/meteorite-list" eudora="autourl">http://www.pairlist.net/mailman/listinfo/meteorite-list</a>
</blockquote>
<x-sigsep><p></x-sigsep>
Dr. Jeffrey N. Grossman&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; phone: (703) 648-6184<br>
US Geological Survey&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; fax:&nbsp;&nbsp; (703) 648-6383<br>
954 National Center<br>
Reston, VA 20192, USA<br><br>
</html>

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Received on Mon 08 Sep 2003 08:10:10 AM PDT


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