Sci. Rep. Kanazawa Univ.
Vol. 55, pp.17–24, 2011

Raman spectroscopy of synthetic zircon:
Effects of chemical composition
Noriko H ASEBE1 , Koji A RATAKE2 , Akihiro TAMURA3 ,Masayuki O KUNO2 , Shoji A RAI2 , Isamu S HINNO4
1 Institute of Nature and Environmental Technology, Kanazawa University

Kanazawa 920-1192, Japan
hasebe@staff.kanazawa-u.ac.jp
2 Department of Earth Sciences, Kanazawa University
Kanazawa 920-1192, Japan
3 Frontier Science Organization, Kanazawa University
Kanazawa 920-1192, Japan
4 Emeritus Professor, Kyushu University
Fukuoka 812-8581, Japan(Received December 19, 2011 and accepted in revised form January 11, 2012)

Abstract Raman spectra of zircon show a distinctive pattern according to the amountof stored radiation damage. Nasdala et al. (2001) demonstrated that a linear relationship exists between the full width at half-maximum (FWHM) of the ν 3(SiO4 )band, observed at ∼1000 cm−1 , and the alpha fluence calculated from the uranium
content, thorium content and the zircon age. Their finding could lead to a new microspectroscopic method to determine zircon ages. This study examines the effect ofchemical composition on Raman spectra by the analysis of synthetic zircons that weredoped with elements using Li-Mo flux. Zircons were found to be able to incorporatehigh concentrations of group 3 elements (including lanthanides and actinides) while
elements belonging to other groups were less incorporated. The FWHM of the group
3 doped zircon ν 3(SiO4 ) band showed a linear relationship with concentration, clearly
indicating that the FWHM is a function of chemical composition as well as of radiation damage. However, a very high concentration of group 3 elements is required toinfluence these Raman spectra (e.g., more than ∼5,000ppm).

1Introduction

Zircon is an important accessory mineral in Earth Sciences because of its high radionuclide concentration. To understand histories of terrestrial materials, zircons have been
analyzed using various dating techniques such as U-Pb, Pb/Pb, fission-track, U-Th andU,Th-He methods. A geochronological method that still needs to be developed will

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Raman spectroscopy of synthetic zircon: Effects of chemical composition

utilize alpha-damage (including alpha-recoil damage) of the zircon crystal through thedecay chain of U or Th (Gogan and Wagner, 2000). Spectroscopic methods such as
X-ray diffraction (e.g., Murakami et al., 1991) and infrared spectroscopy (Deliens etal., 1977, Zhang et al., 2000a) are successful in qualitatively estimating the degreeof metamictization, which is caused by the accumulation of radiometric damage andthe resulting crystal disorder. Raman spectroscopy has also been used to estimate thedegree of metamictization (e.g., Pidgeon et al., 2007) and these authors’ methodology
showed potential for quantitative determinations (Zhang, et al., 2000ab, Nasdala et al.,1995, 2001). Nasdala et al. (2001) found a linear relationship between the full widthat half-maximum (FWHM) of the ν 3(SiO4 ) band, observed at ∼1000 cm−1 , and thealpha fluence calculated from uranium and thorium concentrations as well as estimatedages. Raman spectra, however, reflect the nearest environment of vibrating bonds andthus incorporation of chemical impurities might affect Raman signals (Geisler et al.,2005). Here, we examine effects of chemistry by investigating synthetic zircons thatwere doped with various elements.

2 Samples and analyses
Zircons were synthesized using Li-Mo flux (Chase and Osmer, 1966). A mixture ofZrO2 (3 mol%), Li2 SiO3 (3 mol%), Li2 MoO4 (10 mol%), MoO3 (83.2 mol%) and thedoping element (oxide, 0.8 mol%) was heated at 1250◦ C for 2.5 hours and afterwards
cooled at a rate of 10◦ C/h to synthesize zircon crystals. Doping elements used for eachsynthesis experiment are listed in Table 1. Experimental details and resultant products
have been described in Shinno (1987). Synthetic zircon crystals >∼1 mm in diameter
were used for Raman spectroscopy and chemical analyses.
Raman spectra were measured with a FT-Raman SPECTRUM 2000R (PerkinElmerInc.) at the Wakasa Wan Energy Research Center, Fukui, Japan. Crystals were attachedto the target area with adhesive tape. The laser energy was 70 mW. The slit width(an instrumental resolution) was 2.00 cm−1 and data were acquired every 0.5 cm−1 .
Spectra between 950 and 1050 cm−1 were used to calculate the FWHM and are shownin Fig. 1. Resultant FWHM values are listed in Table 1.
Chemical compositions were measured by Laser Ablation - Inductively Coupled
Plasma - Mass Spectrometry (LA-ICP-MS) using an Agilent 7500 instrument equippedwith a Microlas Excimer laser ablation system at Kanazawa University, Japan (Ishidaet al. 2004; Morishita et al. 2005). After obtaining Raman spectra, crystals weremounted in epoxy resin and ground to expose their internal surface. Doped elements
and hafnium were measured along with the 29 Si internal standard and calibrated with
the NIST610 external standard (Pearce et al., 1997, Longerich et al., 1996). Becauseof expected chemical heterogeneity, three or four spots were analyzed for each grain

N. Hasebe et al.

Figure 1: Raman spectra of synthetic zircons after doping with all studied elements.

19

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Raman spectroscopy of synthetic zircon: Effects of chemical composition

doped element

Group ofelement

atomicnumber

FWHM (cm−1 )concentrationof dopedelement (ppm)

Hfconcentration
(ppm)

Sc
concentration
(ppm)

B

135

4.1

407±958400±400

210±7Sc

3

21

2.8

6360

8530

6360

Mn

7252.80.70±0.12

6190±90350±4Co927

2.80.38±0.025960±30

320±1As15332.1640

24.39

210

Sr2382.80.07±0.01

8180±100320±4Ba2562.80.016750

200

Ce

3582.1ND

5900±30340±3Nd360

2.822±2.56880±350

320±5Ho

3676.49600

6820

190

Yb

370

9.6

33200±2200

6830±460

280±20

Lu

371

5.9

17000±2400

7.6±2.0190±33U392

3.3

680±150

5700±190

300±20

FWHM: A full width at half-maximum (FWHM) of ν 3(SiO4 ) band (Nasdala et al., 2001). Error represents ±σ .

Table 1: Results of Raman FWHM measurements and chemical analyses.

except for the few grains that had a small exposed area. The detailed measurement procedure is described elsewhere (Jarvis and Williams,1993; Hasebe et al., 2004; 2009).
The error range was estimated based on variations in spot analyses or estimated to be
as large as ∼5 % for grains that underwent a single spot analysis through repeatedanalyses of the NIST612 glass (Hasebe et al., 2009). Results are shown in Table 1.

3 Results
Concentrations of doped elements are plotted against their group numbers from theperiodic table (Table 1 and Fig. 2). Even though the reagent was prepared to include
the same amount (mol%) of target element for each synthesizing experiment, Group 3elements show high concentrations of up to ∼3.5 wt% while other groups show low
concentrations of up to 640 ppm (µ g/g). Concentrations of hafnium and scandium arehigh for most samples probably because of incorporation from the original reagent (Table 1). Figure 3 shows no systematic relationship between the amount of hafnium andscandium and the doped elements. This may suggest that incorporated hafnium andscandium do not originate from doping elements reagents but from other reagents suchas ZrO2 . The resultant degree of incorporation of elements during synthesizing process
is similar to the situation found in natural zircon, namely the HREE (Ho, Yb, and Lu in
this study) is enriched relative to LREE (Ce and Nd). Therefore, substitution process

N. Hasebe et al.

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Figure 2: Concentrations of doped elements plotted against their periodic table group.

proposed for natural zircons (Hoskinand Shaltegger, 2003) should control the process and resultant chemical concentration.
The elementswith ionic radii matched by the Zr4+(smaller-radii HREE) are incorporated more than elements with thelarger LREE. In natural environment
where a variety of elements are available, substitution of multiple elements
to retain charge balance is also an important process to explain the resultant
chemistry, though it is not the case forsynthetic zircons analyzed here.
When concentrations of doped elements are plotted against the FWHM
(Fig. 4a), a linear relationship is foundwhile the concentration of hafnium
shows no significant relationship withthe FWHM (Fig. 4b). This result indicates that the FWHM is a function of
the chemical composition as well as ofFigure 3: Hafnium and scandium concentrations plottedagainst concentrations of doped elements. Error bars
represent ±σ . Solid symbols represent hafnium andopen symbols represent scandium data. Circles are forgroup 3 elements and triangles for others.

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Raman spectroscopy of synthetic zircon: Effects of chemical composition

radiation damage or the amorphous state (Zhang, et al., 2000ab, Nasdala et al., 1995,
2001). A very high concentration of group 3 elements influences Raman spectra (e.g.,more than ∼5,000ppm). However, such high concentrations are rarely observed innatural zircons so that as far as we deal with natural zircons we still have a possibility
to develop radiation damage dating using Raman spectroscopy.

 (a)

  
   (b)












 Figure 4: (a) Doped element concentrations that show a linear relationship with the FWHM. (b) The
hafnium concentration plotted against the FWHM with no significant relationship. Error bars represent±σ . Circles are for group 3 elements and triangles for others.

N. Hasebe et al.

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Acknowledgments
We are grateful to the editor and an anonymous reviewer for their comments to improvethe manuscript. This study was supported by a Japanese Grant-in-Aid for Scientific
Research.

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