Cathodoluminescence spectra of diamonds in UHP rocks from the

Mineralogy
19
Cathodoluminescence spectra of diamonds in UHP
rocks from the Kokchetav Massif, Kazakhstan
O. G. Iancu·
b
C
',
R. Cossio', A. V. Korsakov R. Compagnon"', C. Popa
d
,
'Department ofMineralogical and Petrological Sciences, University of Torino, Via Valperga Caluso, 35 10125 Torino,
Italy
bUniversity ''A/. 1. Cuza" ofIasi, B-dul Carol I 20A, 700505 Iasi, Romania
clnstitute ofGeology and Mineralogy, SB RAS, Koptyug Pr. 3, 630090 Novosibirsk, Russia
dAstronomical Observatory ofCapodimonte, Salita Moiariel/o 16,80131 Napoli, Italy
*Corresponding author. Permanent address: University "Al. 1. Cuza" ofIasi, B-dul Carol I 20A, 700505 Iasi, Romania.
E-mail address:[email protected] (o.G. Iancu) .
Abstract
We propose ta investigate the diamonds from the Kokchetav Massif, northern Kazakhstan, which is the best example of
diamondeclogite facies metamorphism, using cathodoluminescence (CL) techniques. CL spectra measurements ofdiamonds fram
garnetpyroxenequartz rocks and dolomitic marbles, made at 80 K, revealed peaks at 2.156, 2.463, and 3.188 e Vand a broad band
at 2.722.80 e V. Panchromatic and monochromatic diamond images analysed by CL reveal concentric zones of variable
luminescence. This indicates that the diamond crystals grow during several metamorphic stages under ultra high pressure (UHP)
metamorphism conditions. The inhomogeneous broadening and lower 2.156 e V ZPL peak suggests the presence of a higher
concentration ofdefects and stresses in the rim compared ta the core. r 2008 Elsevier B. V. Al! rights reserved.
1. Introduction
The main purpose of this study is to investigate the characteristics of microdiamonds from a gametpyroxenequartz rock
and a dolomitic marble (Kokchetav Massif) using scanning electron microscope (SEM) with cathodoluminescence (CL)
spectrometer. The Kokchetav Massif of northem Kazakhstan is a typical example of diamondeclogite facies metamorphism. The
petrographic studies performed recently have proved that the diamondbearing rocks have experienced pressures in excess of 4.0
GPa, corresponding to mantle depths greater than 120 km.
In the Kokchetav Massif, microdiamonds are common inclusions in gamet, zircon, and, rarely, in clinopyroxene,
kyanite, zoisite, quartz, biotite, and phlogopite from garnetbiotite gneiss, garnetbiotiteclinozoisite gneiss,
gametkyanitephengitequartz schist, gametpyroxenequartz rock and dolomitic marble [18]. Microdiamonds are also commonly
found in the pseudomorphs after gamet, consisting of mica+chlorite+amphibole aggregates and tourmaline [2,3]. Diamonds were
rarely detected as intergranular phases in quartzfeldspar (QtzKfs) aggregates [9]. Diamonds usually coexist with graphite. In
dolomitic marbles, graphite is included in dolomite and occurs either as polycrystalline aggregate (530 mm) of extremely finegrained crystals, most likely pseudomorphous after diamond [7] or as euhedral platy single crystals (20 mm_75 mm).
2. Materials and method s
For this study, gametpyroxenequartz rocks and dolomitic marbles were used to make polished thin sections (30 mm
thick) at the Department ofMineralogical and Petrological Sciences, University ofTurin, Italy.
The thin sections were frrst studied in both transmitted and reflected light by using an Olympus BX 60 optical microscope
in order to identify the microdiamonds, zircons and the other minerals and microstructures of ultra high pressure (UHP)
metamorphism. After the carbon coating, the thin sections were investigated by means of a Cambridge S360 SEM equipped with
an Oxford Inca Energy 200 for quantitative electron microprobe analyses, an Oxford MONO-CL2 spectrometer, and an MMR
technologies sample holder cold stage (80400K) for CL studies at the Universita' di Torino, Departments ofMineralogical and
Petrological Sciences, and of Experimental Physics.
This CL system operates in both monochromatic (MC) and panchromatic (PC) modes. In the MC mode, we have used a
plane diffraction grating (1200 l/mm blazed at 500 nm) and a spectrometer wavelength resolution (setting entrance and exit slit
aperture) of 1 nm for whole·spectra (300800 nm) and 0.5 or 0.25nm to resolve fine zero phonon line (ZPL) structure. Outgoing light
is focused into a HAMAMATSU R376 photomultiplier tube. To compare photon emission from different diamond grains, spectra
were collected using the same exciting conditions (lprobe C 20 nA, EO C 30 keV).
The measured spectra were corrected forwavelength position using a calibratedArHg lamp (Ocean Optics Hg-l), and for
intensity response using a calibrated tungsten halogen light source (Ocean Optics LSI-CAL) [10]. All fitting procedures were
performed using Microcal Origin 6.0 Professional software.
20
Mineralogy
3. ResuIts
One diamond from a gametpyroxenequartz rock (Fig. 1(a» and three diamonds from a dolomitic marble (Fig. 1(bd» are
presented to illustrate the results: the diamond sizes vary from 10 to 100 rom.
For each grain, three monochromatic eL images were obtained at 650, 525 (Fig. 2) and 420 nm, in the red, green and blue
parts ofthe spectrum, respectively, with a resolution of 10nm fu1l width at halfmaximum (FWHM). False colour images ofthe
analysed diamonds (Figs. 3 and 5) were obtained by combining all three monochromatic images. This colour combination reveals
zones with different emis sion intensity at different wavelengths due to different eL features (Fig. 3). eL spectral measurements on
diamond ofFigs. 4 and 5 from sample GO, made at 80K, revealed ZPL at 2.156, 2.463, and 3.188 eV, respectively, and a broad band
at2.722.80eV (Fig. 6, Table 1).
Voigtian fitting on 2.156 eV ZPL gives the results of Fig. 7 (Sp03) and Table 1 (aH spectra), fitting on the relative vibronic
side-band give results of Table 1. Fitting on the 2.463 eV ZPL (Sp02) and on the 2.722.80 eV broadband are reported in Table 1. Tlie
peak at 3.188 eVis only observed in SpO 1 and Sp03 with very low intensity: it is impossible to have a fitting on this feature and the
relative phonon side-band.
Fig. 1.
Secondary electron (SE) images of diamonds from garnetpyroxenequartz rock (a) and from dolomitic marble (bd).
4. Discussion
Diamonds from the gametc1inopyroxene rocks have crystals, ranging in size from 15 to 150 rom, which appear as
yellowish cuboids in white light. In eL, most diamonds display up to four growth zones, parallel to their cuboid faces [11]. The
ZPL at 2.156 eV (the 57 5nm centre) occurs in many nitrogen-containing natural diamonds. The measured phonon energy (Zo:C 45
meV) is in agreement with the literature values [12]. The most developed atomic model for this centre assumes the association of a
vacancy and an isolated substitutional nitrogen atom in the neutral charge state (NV 1) [13]. The 2.463 eV ZPL (the H3 centre) is the
Mineralogy
21
Fig. 2.
Monochromatic eL
image (525 nm) of
diamonds in
Fig. Irad).
Fig. 3.
False colour image of
diamonds in Figs. i
and 2: red, 650 nm;
green, 525 nm; and
blue E 420 nm.
Mineralo
22
Fig. 4.
SE image of diamond from a garnetpyroxenequartz rock.
Fig. 5.
False colour image of diamond in Fig. 4: red, 650 nm;
green, 525 nm; and blue E 420 nm.
Table 1
Fitting results on ZPL peaks (with related vibronic side-band) and on broad-bands
Photon energy
(eV)
Type
Width FWHM (meV)
Pbonon energy, 1/(J) (rneV)
Model
2.156
ZPL
4S
Vacancy aod the nearest subJtitutional nitrogen
atom in tbe neutral charge latc (NV)0
2.463
3.188
2.80 (SpOI)
2.72 (Sp02)
2.77 (Sp03)
ZPL
ZPL
Broad
SpOI : wL .. 6.2, wG .. 4.9
Sp02: wL
10.4, wG = 9.7
Sp03: wL .. 8.2, wG 5.8
SP02: wG = II
SPOI : wG = I
400 (SPOI)
270 (S1>02)
430 (SP03)
39
Not measured
V- N complex
Radiation damag (Iow Ice electrons)
• Radiative recombination at disJocation.
• lntrncentre lran 'tioos at tbe BI(N9) centre
(pla teletA)
=
I
1
~
1:0=30 kV. 1.,.-:2OnA. Res=1nm
4x1OS
:j
~
3x105
spedr .Res=O.25 nm
I
Data: spo3
ModetVoI~
::i
R2"' 0.998
!
1.0x1OS
xc .. 2.156 eV
A .. 3317
wGaS.8meV
wL = 8.2 meV
..
!P
2x105
I -e- e : a t
1.5x1OS
~
ai
<:
~
E:3OkV. 1=2OnA,
2.0x1OS
- - sp02
-sp03
5x1OS
,.....
=
.J
...J
()
(J
5.0x1OC
1x1OS
1.6
1.8
2.0
2 .2
2.4
2.6
2.8
3.0
32
3.4
E(eV)
Fig. 6.
eL spectra acquired on points (SpOI, Sp02, Sp03) ofFig. 5.
0.0 +--.........,...'i--r~~.,.....~~,......~~,..........;:!!!1"""'
2.13
2.14
2.15
2.16
2.17
2.18
E(eV)
Fig. 7.
Voigtianfitting of2.156 eVZPLpeakon Sp03.
Mineralo
23
most common naturally occurring optical feature ofnitrogen-containing diamonds. The measured phonon energy (Zo ~ 39 meV)
is also in agreement with the literature values [12]. The two widely used atomic models consist of either a pair of substitutional
nitrogen atoms separated by a vacancy (NVN) or anA-aggregate ofnitrogen bounding two vacancies (VNNV) [12]. The narrow
weak peak at 3.188 eV is a ZPL characteristic for radiation-damaged diamonds. This centre is activated in pristine and irradiated
diamonds with an electron beam of a few ke V energy (in our case EO ~ 30 keV). The broad-band at 2.722.80 eV (theA-band) is one
of the most characteristic luminescence feature of diamond. Models are related to radiative recombination at dislocation or
intracentre transitions at the Bl(N9) centres (platelets) [12]. In surnmary, we have a core with an intense 2.156 eV centre (NVl )
(SpO 1), an intermediate portion (Sp03) with a little lowering ofthe 2.156 eV contribution and a constantA-band contribution, and a
rim portion (Sp02) with a more intense H3 peak, a weaker (NV 1) peak, and a lower energy (2 .72 eV) and sharper (0.27 eV FWHM)
A-band (cf. Table 1).
The Voigtian fitting of the 2,156 eV centre, after having taken into account the spectrometer distortion, gives a resulting
peak with a lower Gaussian width (06 meV) and a higher intensity for the core (SPO 1), and a higher Gaussian width (E 1Ome V) and
a weaker intensity for the rim (SP02).
5. Conclusions
Our CL results confum that the cubic growth zoning dominates among the Kokchetav microdiamonds [14,15]. Based on
differences in morphology, crystal orientation, FWHM of Raman bands and CL spectra between the core and the rim parts of
microdiamond from dolomitic marble of the Kokchetav Massif, Ishida et al. [16] and Ogasawara et al. [17] concluded that they
formed during two different growth stages. According to these authors, it is improbable that, unlike the core, the rim formed simply
by inversion from graphite, but most likely precipitated from a fluid phase.
The microdiamonds from the Kokchetav massifhere studied show the same type ofCL zoning described by Refs. [16,17]
which suggests a polyphase growth history. The Voightian fitting ofthe ZPL at 2.156 eV for the spectrum in the rim region shows a
significant inhomogeneous broadening and a lower peak intensity in comparison to the core region. These results suggest the
presence ofhigher concentration of defects and stresses [12] : this is in agreement with different growth conditions for the rim with
respects to the core. Furthermore, the same microdiamonds systematically show an intense vibronic sideband with ZPL at 2.463
eV (in the green part of the spectrum) similar to that observed in kimberlitic and lamproitic diamonds, preventing its use for
distinguishing the origin of diamond formed at different geodynamic conditions. On the contrary, it is noteworthy that in the
Kokchetav microdiamonds, formed in a subduction environment, is missing the N3 centre, which is a1ways a companion peak of
the H3 centre in othernatural diamonds [12, p. 262].
Acknowledgements
We are deeply grateful to Nikolai V. Sobolev for providing us some samples used in this study. A NATO advanced fellowship in the
field of Earth and Environmental Sciences from the Italian National Research Council (Consiglio Nazionale delle Ricerche)
supported this work. Careful and constructive comments and suggestions from two anonymous referees, that significantly
improved the article, are greatly appreciated.
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