Page 272 - Microtectonics

10.2 · Techniques to Study Deformation Fabrics 265
point defects. These may include ions of 3d elements (tran- both types of analysis is monochromatic CL imaging. Here,
2+
3+
3+
sition group metals, such as Mn , Cr and Fe ), ions of distribution patterns are obtained separately for certain
3+
4f elements (rare earth elements, such as Dy ), other ions wavelengths. Most studies apply panchromatic (or integral)
–
[e.g., (S ) in sodalite and haüyne] and vacancies and re- imaging, which uses the entire detected CL signal independ-
2
lated defect centres in the crystal lattice (Richter and ent of its wavelength composition. One advantage of OM-
Zinkernagel 1981; ten Have and Heynen 1985; Solomon CL imaging is that colour variations within minerals are
1989; Barbin and Schvoerer 1997; Götze 2000; Pagel et al. easily observed, which provides a rough indication of the
2000; Nasdala et al. 2004a). wavelength. A combination of trace element analysis and
A large number of mineral species has been studied us- CL spectroscopy promises to become a powerful tool for
ing CL, including carbonate minerals (Spötl 1991; El Ali et al. mapping of trace element distribution in microstructures
1993; Yang et al. 1995; Habermann et al. 1996, 1998; Barbin (Yang et al. 1995; Barbin and Schvoerer 1997).
and Schvoerer 1997; Gillhaus et al. 2000), diamond
(Smirnova et al. 1999; Mineeva et al. 2000; Davies et al. 2003), 10.2.2
zircon (Ohnenstetter et al. 1991; Cesbron et al. 1993; Hanchar Raman Spectroscopy
and Miller 1993; Hanchar and Rudnick 1995; Nasdala et al.
2003), monazite (Seydoux-Guillaume et al. 2002), apatite Raman spectroscopy is based on the interaction of the (vi-
(Barbarand and Pagel 2001; Kempe and Götze 2002), quartz brating) electric field vector of light with vibrations in the
and silica (Fig. 10.1; Matter and Ramseyer 1985; Shimamoto sample (molecule, crystal lattice). Light can be absorbed by a
et al. 1991; Gorton et al. 1996; Götze et al. 2001), feldspar sample upon the excitation of a vibration, provided that its
group minerals (Mora and Ramseyer 1992; Mariano and photon energy corresponds to the phonon quantum energy
Roeder 1989; Finch and Klein 1999), and clay minerals of an allowed vibration in the sample (vibrational quanta
(Götze et al. 2002). Cathodoluminescence is a well-estab- are called phonons). This is the case for light in the infrared
lished technique in sedimentology to study mineral growth range, and such absorption is used in infrared absorption
processes and provenance of sediments (Marshall 1988; analysis. In contrast, the phonon energy of visible light is
Augustson and Bahlburg 2003) but is also useful in micro- too high to be transferred to the sample through the excita-
tectonic studies. The intensity distribution pattern and col- tion of a vibration. However, it is possible that a small frac-
our of CL may be independent of optically visible micro- tion of the photon energy of visible light is taken to excite
structures and can reveal mineral growth from solutions, the sample to vibrate. After such interaction, a scattered light
e.g. in microcracks (Narahara and Wiltschko 1986; Laubach photon will be lowered in energy and shifted in wavelength
et al. 2004), in veins and along grain boundaries and details towards the red end of the electromagnetic spectrum, a
of the microstructure of cataclasites (Stel 1981; Blenkinsop process known as Stokes-type Raman scattering. Alterna-
and Rutter 1986). In veins, CL can reveal growth surfaces tively, it is possible that the vibrational state of the sample
that are invisible in ordinary light (Dietrich and Grant 1985; be lowered in which case the scattered light has gained en-
Urai et al. 1991). Fracture patterns can be easily distin- ergy and is blue-shifted. This is called anti-Stokes Raman
guished (Kanaori 1986). Overgrowth structures along grain scattering. In the case of Rayleigh scattering no energy shift
boundaries, which are invisible in normal transmitted light, of the light results. Raman analyses are mostly done in the
can be used for strain analysis (Onasch and Davis 1988). Stokes region because of the higher intensity of Stokes-type
Dislocation networks can be made visible in some cases Raman scattering compared with anti-Stokes scattering.
(Grant and White 1978). Raman scattering is a weak effect. Only a small frac-
Cathodoluminescence analysis can be carried out using tion of the incident light interacts with vibrations in the
either “hot cathode” or “cold cathode” systems attached to sample as described above. The Raman shift is measured
an optical microscope (then called OM-CL) and in a SEM as the wave number difference between incident and scat-
or electron microprobe (called SEM-CL; (Fig. 10.9a, tered light. The wave number is defined as the reciprocal
–1
×Photo 10.9; Yacobi and Holt 1990; Shimamoto et al. 1991; wavelength and is given in cm (McMillan and Hof-
Lloyd 1994; Götze 2000; Nasdala et al. 2003). In both cases meister 1988; Marfunin 1995; Smith and Carabatos-
ordinary polished sections covered with a thin conductive Nédelec 2001; Loudon 2001; Nasdala et al. 2004b).
layer (carbon or gold) are used. The CL technique involves Raman spectroscopy is nowadays generally done with
the analysis of spectra or the generation of images. Catho- a laser beam, which is directed at a sample using high-
doluminescence spectroscopy provides information about resolution objective lenses. Powerful Raman microprobe
the spectral composition of the emitted light, which is systems have a depth resolution as good as 2 µm when
needed for the sound interpretation of causal CL-active the beam is focused at the sample surface and a real lat-
defects etc. Cathodoluminescence imaging, in contrast, is eral resolution of 1–1.5 µm, which may result in a real
3
predominantly used to reveal internal structures of miner- volume resolution better than 5 µm (Markwort et al.
als, for which it may not be absolutely necessary to know 1995). The scattered light is analysed in a spectrometer.
the causes for local variations of the CL. A combination of Apart from the analysis of single spectra, Raman is also