On-the-Fly Calibration for Rapid Raman Chondrite - USRA

46th Lunar and Planetary Science Conference (2015)
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ON-THE-FLY CALIBRATION FOR RAPID RAMAN CHONDRITE CLASSIFICATION.
L.C. Welzenbach, Galactic Analytics, 15918 Mesa Verde Dr, Houston TX 77059 ([email protected]).
Introduction:
Classification of ordinary chondrites is typically done through measurements of the
composition of olivine and pyroxenes, via electron
microprobe, oil immersion or other methods [1-4].
These methods can be time consuming and costly
through lost sample material during thin section preparation. Previously the author described the methods by
which Raman microscopy can perform the same measurements [5] but considerably faster and with much
less sample preparation. Raman spectroscopy as a classification tool is now being tested and corroborated by
other researchers [6] to speed the classification of large
amounts of chondrites such as those retrieved from
Antarctica.
The concept of using Raman spectroscopy to classify meteorites is scientifically robust as seen in previous uses of this technique for olivine composition
analyses [8-13] It has also been shown that the analysis
is insensitive to isotopic variation [14]. However, one
difficulty must be overcome before Raman analysis
can be used as a standard method for classifying meteorites – the need for robust, high fidelity spectral calibration in order to discern the relatively small peak
shifts that must be measured. While this is not a severe
technological limitation, it must be addressed.
Raman spectroscopy, as described in detail by
Kuebler et al (2006) (Figure 2)[9]. The composition of
pyroxenes has likewise been demonstrated in a semiquantitative fashion [10,11]. For the purpose of this
study, we will focus on olivine. The Raman spectrum
of olivine is dominated by two vibrational modes that
appear at ~820 and ~850 cm-1 . These peaks shift in
accordance with Fo [8-12]. The full range of Raman
peak shift over the 0-100 Fo range is ~10 cm-1 for the
~820 cm-1 Raman peak, and ~19 cm-1 for the ~850 cm1 peak. The span of Fo numbers exhibited by ordinary
chondrites covers ~60 to 90 Fo, for a span of about 30
Fo units. Therefore, the range of Raman peak positions
of interest run span a range of ~3 cm-1 for the ~820 cm1
peak and 5.7 cm-1 for the ~850 cm-1 peak. This means
that single-cm-1 spectral resolution is necessary, which
is a reasonable constraint for most Raman instruments.
More importantly, it means that the spectral calibration
must be stable and repeatable with a sub-cm-1 accuracy.
This is a challenge. The normal vibrational and thermal
environment can be sufficient to cause variation in
spectral calibration on the order of one or two cm-1 in
the course of the normal operation of a typical Raman
instrument.
Figure 1: Classification of chondrites by type and petrographic
grade, using composition vs. standard deviation of composition
values. T hese graphs are from the Meteoritical Society Database
website [7] with a detailed description provided on that site. Ordinary chondrites can be classified using a statistically relevant number
of olivine and/or pyroxene
The Challenge: Classification of ordinary chondrites is made by measurement of the chemical composition of olivine and pyroxene (Figure 1)[7,8]. Composition of olivine is a straightforward measurement by
Figure 2: Raman spectrum (inset) of olivine showing two strong
Raman peaks around ~820 and ~850 cm -1 (“DB1” and “DB2”, respectively). T he positions of these peaks are an indicator of olivine
Fo number. T he graph shows the Fo number calculated from individual spectra collected from EETA79001 as a function of positions
of the Raman peaks. Figure adapted from [9].
46th Lunar and Planetary Science Conference (2015)
This small shift can affect calculated Fo values and
lead to erroneous classifications.
The Solution: One way to solve this problem is
with spectral calibration cycles before and after every
spectrum, but this is a time-consuming operation. To
solve this problem we devised a second option; realtime spectral calibration. We have modified a Raman
instrument to collect Raman spectra and neon emission
lamp spectra concurrently. Every spectrum, regardless
of the actual physical alignment of the instrument,
comes with a reference spectrum of sufficient fidelity
to calibrate the spectrum to sub-cm-1 accuracy. The
reference spectrum comes from the NIST Atomic
Spectra Database [15]. This is a standard spectroscopic
technique, but consistent measurements of individual
wavenumber cm-1 -accuracy may also allow us to accurately identify the classes of un-equilibrated chondrites
as well.
Figure 3: Raman spectrum from the Fukang pallasite showing the
typical olivine doublet and Ne spectra that will be be used for realtime calibration of every spectra. After spectra has been calibrated,
the Ne peaks will be subtracted from the spectra for calculation of Fo
values.
References: [1] Van Schmus W. and Wood J.
(1967) GCA 31 447-465. [2] Dodd R. et al (1967)
GCA 31 p.921-951 [3] Krot S. et al (2004) Treatise on
Geochemistry, Ch. 1.05 p. 84-128. [4] Lunning N. et
al 2012 Abstract #1566. [5] Fries M.D. and Welzenbach L. (2014) LPSC Abstract #2519. [6]Pitarello L. et
al (2014) 77th Meteoritical Society Meeting, Abstract
#2087. [7] Meteoritical Society Database website accessed
06
Jan
2014
and
references
there, http://www.lpi.usra.edu/meteor/drawplot.php?x=
24.9&y=0.4&z=&plot=2&label=Katol%20(L6)&meth
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od=linear&slope=0.52
[8]
Kuebler
K. et
al 36 th LPSC (2005) Abstract #2086. [9] Kuebler K. et
al Geo. et Cosmo. Acta 70 (2006) pp. 6201-6222. [10]
Foster N.F. et al Geo. Et Cosmo. Acta 121 (2013) 1-14.
[11] Mouri T. and Masaki E. J. Miner. Petrolog.
Sci. 103 (2008) 100-104. [12] Guyot F. et al Phys.
Chem. Minerals 13, 2 (1986) 91-95. [13] Kolesov B.A.
and Tanskaya J.V. Mat’ls. Res, Bull. 31, 8 (1996)
1035-1044. [14] Kolesov B.A. and Geiger C.A. Phys.
Chem. Minerals 31, 3 (2004) 142-154. [15] Kramida A
et al and NIST ASD Team (2014). NIST Atomic Spectra Database (ver. 5.2), http://physics.nist.gov/asd National Institute of Standards and Technology.