Yes, Jim, and this is why arm-chair science is not a good idea! We
really have to wait for the publication to see what was done.
There is a vast and long literature on XRF analysis of geological
materials, including meteorites. The scientific community has accepted
these for decades. The classic XRF technique involving preparation of
fused disks and wavelength-dispersive analysis for major and minor
elements has provided some of the most beautiful datasets in meteoritics
(and earth science). In the 1960s, von Michaelis and co-workers
produced classic papers showing the narrow range of bulk composition in
chondrite groups using this kind of method. These and the wet-chemical
analyses of Jarosewich (a now-extinct method, as far as I'm aware)
provide some of the best, complete major-element data in bulk meteorites
that we have to this day. At the other extreme, there are many "quick
and dirty" energy-dispersive XRF methods these days that have much less
precision and accuracy, e.g. the use of hand-held XRF systems on
irregular bulk samples. And, there are many good and not-so-good
methods in between. XRF is a very broad term, and we don't know what was
done. So, I would not be so quick to dismiss XRF. It can be highly
quantitative using a variety of well-documented, time-proven methods...
and it can be virtually useless for the kind of interpretation that I
did in my previous email.
Jeff
On 1/1/2014 9:25 AM, Jim Wooddell wrote:
Hi Jeff and all!
I'd say XRF data can and does vary. Not enough info in the write up
on testing methods. What is the accepted procedure
agreed to using XRF to test? BIG QUESTION!
Read on!
A few years ago, XRF seemed to not be considered much in this
community. Only a few were using it pretty much only for determining
if a rock
had the attributes to be considered a meteorite. Somewhat like PIXE
tests. Some places have XRF, some have PIXE where they are looking
for key elements.
I know XRF technology has improved. I found it refreshing that the
XRF data was listed.
Correlations being standard methods of lab testing and XRF showed to
be 0.85 to 0.95 (or there abouts) by the EPA in a paper about testing
lead a
while back that I read. Calibration reference is key to accurate,
repeatable measurements with XRF.
In the gold and silver industry, they have been accepted widely but
generally on massed samples (by melt - Homogenous mixture).
My question about the XRF data is how was the measurement taken. It
stated whole rock and the mean of two shots??? So, does that mean
that the sample was massed and pressed into a disk then shot twice or
what? I'd love to know how this was performed.
Overall, with probe data, the XRF is somewhat redundant and without
what it was referenced to, eye candy, but very interesting.
Don't think XRF would take the place of probe data. Both can be
subjective to a point. It would be nice to read if the same
standards were used for
calibration for both the probe and XRF were used and the correlation.
I do think XRF can have it's place. Standard's should be developed on
how it might be used. Maybe they are out there. Point and shoot, if
you are looking
for a quantitative answer, is not the way IMHO.
Jim
On 12/31/2013 6:04 PM, Jeff Grossman wrote:
Can't resist doing some arm-chair science... usually a bad move, but
oh well... I'll probably end up retracting much of this speculation...
There IS something strange about this meteorite to me. I don't know
how good the XRF analysis is, but it is not what I would expect from
an L chondrite. These analyses show a 30-40% enrichment in Ca and Al
relative to Si over what an L chondrite should be, and siderophiles
are ~20% too high as well. If these are accurate, then there has
been fractionation, suggestive of enrichment in low-melting
components (which is odd). Sodium does not fit this story, but it's
a harder element to analyze by xrf. I also agree that coarse
poikilitic grains are hard to explain by solid-state metamorpism, but
they could also be derived from relict chondrules. If this rock was
melted to a large extent, I'd expect it to be depleted in metal and
sulfide. So I'm betting that the whole system has experienced
low-degree partial melting, and some of these melts have infiltrated
this particular chunk of high-metamorphic-grade L chondrite.
I agree with Carl that this has hallmarks of what many people call a
type 7 chondrite. But the whole issue of how to draw lines (or if
there ARE lines) between primitive achondrites, type 7 chondrites,
and products of shock heating/melting is very fuzzy and tends to be
highly interpretive. In a sense, this is the same discussion that
surrounds Portales Valley, an ordinary chondrite that has also "been
around the block."
Here is an article on Katol that Laurence Garvie pointed me to:
http://www.geosocindia.org/abstracts/2013/feb/p151-157.pdf
Jeff
______________________________________________
Visit the Archives at http://www.meteorite-list-archives.com
Meteorite-list mailing list
Meteorite-list@meteoritecentral.com
http://six.pairlist.net/mailman/listinfo/meteorite-list
