Quartz CL spectroscopy and trace element distribution

The spectral response of the CL of quartz in the range of visible light is characterised by 9 emission bands between 1.7 – 2.2 eV (red) and 2.4 – 3.1 eV (blue) (Fig. 9.3, Table 9.1). The intensity of the emission bands changes with the exposure time of electron radiation, whereby the blue emission decreases and the red emission increases, indicating the decay and creation of luminescence centres,

Fig. 9.2 SEM-CL images of quartz phenocrysts from the Hub stock topaz-bearing granites (Slavkovský Les Mts.). (a) Zoned quartz phenocryst (qz1, Ju-20). The line A-B shows the position of the trace element profile in figure 9.4a, c, and e. (b) Scheme of the three quartz generations, which are distinguishable in the SEM-CL image (a): zoned phenocryst (qz1), zoned microphenocrysts (qz2), and the matrix quartz (qz3) developed as ongrowths on qz1 and qz2. The nucleation and growth of qz2 begins with the growth of the outer step zone of qz1. (c) Zoned quartz phenocryst (qz1, Ju-20). The stepped zones (1) are truncated by resorption surfaces (arrowed) and overlain by the sub-ordinate oscillatory zoning. Three large scale resorption events are recorded in the zoning pattern. (d) Grey scale profile through the quartz phenocryst in figure 3c. High grey scale corresponds to blue CL and low grey scale to red-brown CL. Within a stepped zone the grey scale (blue CL) tendentiously decreases in the growth direction. The grey scale correlates with the Ti distribution (see Fig. 9.5a). (e) Quartz phenocryst showing growth impediments. The zoning fits the shape of the impediment caused by immiscible melt, vapour phases or minerals which stick onto the crystal surface during growth. During further growth the impediment becomes enclosed. (f) Zoned quartz phenocryst from the Wachtelberg Rhyolite (Eastern Erzgebirge). The zoning pattern of rhyolitic phenocrysts is similar to the zoning occurring in the phenocrysts of the Hub stock granites.

Fig. 9.3 CL spectra of (a) violet luminescent phenocryst quartz of sample Ju-10, (b) violet luminescent phenocryst quartz of sample Ju-20, and (c) red-brown luminescent matrix quartz (Ju 10). The spectra were recorded after 30 s, 2 min, and 7 min electron radiation. The 7-min-spectra are fitted with Gaussian curves. Each Gaussian curve represents a single emission band.

Table 9.1 CL emission bands between 1.4 and 3.1 eV observed in the magmatic quartz of the topaz-bearing granites from the Hub stock and their identification (NBOHC - non-bridging oxygen hole centre; STE - self-trapped exciton)

CL band position

(eV)

Half width (eV)

Identification Reference

1.73±0.02 0.3±0.02 substitutional Fe³⁺ Pott and McNicol (1971)

1.84±0.01 0.22±0.01 Associated with NBOHC with Si--O and peroxy linkage precursor or ≡Si: centre

Stevens Kalceff and Phillips (1995)

1.96±0.02 0.22±0.02 Associated NBOHC with −OH precursor Stevens Kalceff and Phillips (1995)

2.15±0.02 0.38±0.01 STE associated with Ge Luff and Townsend (1990)

2.47±0.02 0.3±0.03 Impurity Itoh et al. (1988)

2.58±0.01 0.18±0.005 Associated with Al³⁺-defect structures STE

Nassau and Prescott (1975) Remond et al. (1992)

2.68±0.01 0.23±0.01 STE

Associated with Ti-defect structures

Stevens Kalceff and Phillips (1995) Marfunin (1979)

2.79±0.01 0.26±0.01 Oxygen related centre Hagni (1987)

2.96±0.02 0.3±0.02 Associated with Ti-defect structures This study

respectively. The CL colours are more unstable in the matrix quartz than in the phenocrysts.

The CL signal obtained from phenocrysts within both granitic varieties is similar, whereas the CL spectra of the matrix quartz show a dominance of the red emission (Fig. 9.3).

The comparison of CL emission band and trace elements revealed the following:

• Growth zones with blue CL show high Ti concentrations up to 70 ppm (Fig. 9.4a and b).

We found that the blue CL emission at 2.96 eV is associated with Ti (Fig. 9.5a). From this observation we conclude, that variations in Ti are mainly responsible for the magmatic zoning pattern within these quartz phenocrysts. The 2.96 eV band is beside the 2.47, 2.58, 2.68, and 2.79 eV one of five bands of the blue emission range. Generally, crystallisation

temperatures >500°C are necessary for the substitution of Si⁴⁺ through Ti⁴⁺ caused by the high field strength of Ti⁴⁺ (F = 1.04; Blankenburg et al., 1994). However, it is generally not clear whether Ti is a CL activator or sensitizer (Marshall 1988, Götze 2000).

• Phenocryst quartz (qz1 and 2) shows an Al content between 170 and 270 ppm, whereas matrix quartz (qz3) has higher concentrations between 250 and 370 ppm (Fig. 9.4c and d).

Al concentrations >400 ppm correlate with K concentrations >30 ppm indicating the contamination of the analysis by feldspar microinclusions. Al generally behaves in an opposite way to Ti with abundances being low in the blue and higher in the red-brown luminescent quartz especially in the matrix quartz (qz3; Fig. 9.5b). In contrast Ramseyer and Mullis (1990) and Perny et al. (1992) assume that the high Al and Li concentrations are the cause of the blue CL of hydrothermal quartz. According to Siegel and Marrone (1981), Griscom (1985), and Stevens Kalceff and Phillips (1995) the red CL emission around 1.96 eV is related to OH^- and/or adsorbed H2O. The increase of the emission during electron radiation is explained by radiolysis of hydroxyl groups and/or adsorbed H2O of the quartz lattice, which leads to the formation of non-bridging oxygen hole centres (NBOHC). Hydroxyl groups and adsorbed H2O acting as charge compensator of Al³⁺ forming [2SiO3-H2O-M⁺2AlO4] defects, where M⁺ is a combination of Li, K, and Na ions (Bambauer et al., 1963; Maschmeyer and Lehmann, 1983; Kronenberg et al., 1986;

Stenina, 1995). This association of Al and hydroxyl groups and/or adsorbed H2O may explain the weak correlation of Al with the red emission.

• The Fe content varies predominantly between 10 and 30 ppm (Fig. 9.4e and f). It was observed that Fe increases towards the grain boundary up to 260 ppm indicating a high diffusion rate of Fe in the quartz lattice. This grain rim shows no change in CL properties.

Pott and McNicol (1971) and Kempe et al. (1999) found that high Fe³⁺ causes the 1.73 eV CL emission. However, in our samples we found no correlation between the Fe concentration and the 1.73 eV band intensity. The lack of correlation may be explained by the fact that Fe occurs as divalent and trivalent ions.

• Weak to non-luminescent, post-magmatic (secondary) quartz is depleted in Ti, Al, Fe and K (Fig. 9.5a and b).

Fig. 9.4 Trace element profiles of quartz of the topaz granites from the Hub stock. Lower Ti and higher Al are characteristic for the matrix quartz. High Al (>400 ppm) correlates with K indicating analysed micro-inclusions of feldspar. The steep increase of Fe near the grain boundary to plagioclase demonstrates the high diffusion rate of Fe in the quartz lattice. Secondary (post-magmatic) quartz between 100 and 130 µm in (b), (d), and (f) is depleted in Ti, Al, Fe and K. The arrows at the axis of ordinates mark the detection limits (dl).