ExpresSed

Quantitative high resolution cathodoluminescence spectroscopyof diagenetic and hydrothermal dolomites

Axel Gillhausa, Detlev K. Richtera,*, Jan Meijerb,Rolf D. Neusera, Andreas Stephanb

aDepartment of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, D-44780, GermanybDepartment of Experimental Physics III, Ruhr-University Bochum, D-44780, Germany

Received 7 November 2000; accepted 13 December 2000

Abstract

A combination of high resolution cathodoluminescence-spectroscopy (HRS-CL� high resolution spectroscopy of cathodo-

luminescence emission) with spatial resolving trace element analyses (PIXE� proton induced X-ray emission) is used to

establish a method for the quantitative determination of the Mn-content of diagenetic and hydrothermal dolomites using the

measurements of peak areas of Mn-activated CL emission bands. This method takes into account the overlap of the CL broad

bands of Mn21 in the Mg- and the Ca-lattice-position of dolomite. There is a linear correlation between the peak areas and Mn

concentration up to approx. 1000±1500 ppm. Thus, CL spectroscopy allows a determination of Mn concentrations below the

abilities of PIXE (10±15 ppm) to less than 1 ppm by extrapolation of this linear relation.

Up to an Fe-content of approx. 2000 ppm, no quenching effect of Fe on this linear relationship occurs. However, Fe-contents

above 2000 ppm result in a decrease of Mn-induced luminescence. Even at Fe-concentration .25,000 ppm spectroscopy

reveals that Mn-activated CL of dolomite is not entirely extinguished. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Dolomites; Cathodoluminescence; Spectral analysis; Manganese concentration; PIXE

1. Introduction

In carbonate minerals, Mn21 and trivalent REE are

substantial activators of extrinsic cathodolumines-

cence (CL), while Fe21 is regarded as the main

quencher element (Marshall, 1988; Pagel et al.,

2000). Similar to calcite (see Habermann et al.,

1998) the occurrence of a lower detection limit of

Mn-activation with regard to a visible yellow-orange

CL of dolomite has been controversially discussed.

Long and Agrell (1965); Martin and Zeeghers

(1969) indicate 0.1 wt% Mn as a lower luminescence

limit. Others claim a detectable Mn-activated CL of

dolomite even at lower concentrations: Pierson (1981)

80 ppm; Richter and Zinkernagel (1981) 20±40 ppm;

Fairchild (1983) 150 ppm; and Ten Have and Heijnen

(1985) 30±35 ppm. On the one hand, these discrepan-

cies may be due to the CL devices used (e.g. cold

versus hot cathode). On the other hand, investigations

based on visual observations can be considered as

subjective, which can be easily demonstrated when

assessing a dimly luminescing sample by two persons.

Habermann et al. (1998, 1999a,b) have established

an exact quanti®cation of the Mn-induced lumines-

cence of calcite using HRS-CL (high resolution

spectral analysis of CL emission) and PIXE (proton

Sedimentary Geology 140 (2001) 191±199

0037-0738/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.

PII: S0037-0738(01)00062-8

www.elsevier.nl/locate/sedgeo

* Corresponding author. Tel.: 149-234-32-23-253; fax: 149-

234-32-14-571.

E-mail address: rolf.neuser@ruhr-uni-bochum.de (D.K. Richter).

induced X-ray emission). Here, the peak heights of CL

emissions were used to evaluate the concentrations.

The method was calibrated by an Iceland spar and

covers concentrations between 10 and 3000 ppm Mn

in Fe-poor calcites. Mn-contents below 10 ppm Mn

(detection limit of PIXE) are obtained by extrapola-

tion, allowing the detection of Mn concentrations of

less than 1 ppm by evaluating the Mn-peak in the

CL-spectrum. Thus, it is shown that there is no low

Mn-activation limit for calcite, supporting the assump-

tion of Walker et al. (1989) based on theoretical

considerations.

However, an immediate application of Mn quanti-

®cation in calcite by HRS-CL to the mineral dolomite

is dif®cult, because Mn21 can occur in both the Ca-

and Mg-position of the lattice. This results in an over-

lap of the ¯anks of wide spectral bands in the

CL-spectra (Sommer, 1972; El±Ali et al., 1993;

Habermann et al., 1996). Gillhaus et al. (2000) have

studied dolomites luminescing red (Mn21 in Mg-

position: l � 656 nm) and yellow (Mn21 in Ca-

position: l � 575 nm) and predominantly used red

luminescing dolomites to examine the correlation

between Mn concentration (PIXE) and CL intensity

(HRS-CL). A linear correlation was found for samples

between 20 and 100 ppm Mn by peak height measure-

ments, which matches approximately the correspond-

ing correlation developed by Habermann (1997) for

Fe21-free to -poor calcite.

In this study, we present a quanti®cation of Mn-

induced CL of dolomites using a combination of

PIXE-analyses and peak area measurements in CL

spectra. This project promises a faster Mn-determina-

tion by CL spectra compared to combined height

measurements at ®ltered red- and/or yellow emission

bands.

2. Samples

The sample material is predominantly taken from

the more than 2500 m thick Permian/Jurassic succes-

sion of Hydra Island, Greece (see Richter, 1999;

Gillhaus, 1999). Here, fault-related coarse crystalline

dolostones with irregular shape in the range of some

tens of metres show white colour in the outcrop and

less CL zonation and intensity in thin section because

they are poor in trace elements. Fracture-related

single crystals of (saddle) dolomite show broad CL

zones of different luminescence intensities and occur

in all carbonate facies of the Permian/Jurassic succes-

sion. Early to late diagenetic dolomite phases in

Upper Permian and Upper Triassic sabkha sequences

often show narrow CL zonation due to changes in

growth conditions. Furthermore, there are pseudo-

morphic pores after gypsum and prism/sheet cracks

which often exhibit larger dolomite crystals ideal for

spectroscopic investigations.

Additionally, three extraordinarily Fe-rich dolo-

mites (dark brown CL, no zonation) from salt

domes in the Triassic salinar of the Maghreb

(Algeria) and an especially Mn-rich dolomite (bright

orange-yellow CL) in the anhydrite of the Leine

carbonate (Ca3, Lehrbach/Germany) as well as

three Upper Triassic(?)±Jurassic dolomite breccias

from the ¯ysch of Karpathos, Greece (as a compar-

ison to the fault-related dolomites from Hydra) were

investigated.

3. Methods

For our combined HRS-CL and PIXE analyses we

used thin sections polished on both sides (thickness

70±100 mm) of diagenetical and hydrothermal

(,2008C) dolomites, mounted on highly pure quartz

glass slides (Suprasil).

CL investigations were carried out on the hot cathode

CL-device at Bochum (HC1-LM, Neuser, 1995) in

combination with the high resolution spectral analysis

equipment (Neuser et al., 1996; Habermann et al., 1996,

1998). The spectrograph and detector technology with

digital data processing (EG&G spectrograph with N2-

cooled CCD camera) using special light guide optics

allows a spatial resolution of approx. 30 mm in

diameter. The maximum spectral resolution within the

visible region of the spectrum totals 0.06 nm at a repro-

duction precision of the peak intensity of $99.8%. All

CL spectra were recorded at 14 keV.

Trace element analyses were accomplished using

the proton microprobe (PIXE) at the dynamitron-

tandem laboratory in Bochum (method in Meijer et

al., 1994). Here, at a proton energy of 3 MeV a beam

diameter of nominally 10 mm is reached. The evalua-

tion of the point measurements was carried out with

the software-program gupix (Maxwell et al., 1995).

A. Gillhaus et al. / Sedimentary Geology 140 (2001) 191±199192

The detection limit for Mn totals 10±15 ppm with a

maximum indicated error of 10% from the current

measurement.

The standardization and calculation of the CL peak

areas can be done by any calculation program and is

shown in Fig. 1. Following the example of intrinsic

calcite spectra with broad bands between 400 and

660 nm of different intensity (see Habermann et al.,

1999b) the intrinsic part of the dolomite spectrum is

subtracted along with the background, before the

remaining Mn21 peak area is integrated. As intrinsic

CL spectra of pure, Mn-free dolomite were

previously unknown, the intrinsic portion of the CL

spectra of calcite is used for standardization of the

Mn-activated peak. This is following the example of

intrinsic calcite spectra (see Fig. 1 and Plate 1)

considering a linearly sloping function to higher

wavelengths.

Fig. 2 documents the equivalence of CL spectra

evaluation of different calcites by linear peak height

measurements (after Habermann, 1997) and peak area

measurements (this work). A comparison of the linear

correlations shows the equivalent applicability of the

peak area method for calcite.

4. Results

4.1. Correlation of Mn21-activated CL (HRS-CL) and

Mn concentrations (PIXE)

As dolomites are usually inhomogeneous (see

Reeder, 1983; compare plate 1), it is not meaningful

to calibrate the method by measurement of a single

dolomite sample (as Iceland Spar for calcite, see

above). Instead, all measured points of the combined

analyses (PIXE versus HRS-CL) were used for the

investigation of a supposed correlation.

Using combined analyses of 94 homogeneously

luminescing dolomite crystals and dolomite crystal

zones with Mn concentrations between 10 and

2671 ppm, we determined a linear correlation

(R� 0.98) for Fe-poor dolomite (,2000 ppm Fe)

with the equation Mn [ppm]� 1.3129ACL [normalized

area units] with 82 samples having Mn concentrations

above the detection limit of PIXE (10±15 ppm) (see

Fig. 3 and Table 1). The intensity of luminescence of

dolomites containing .2000, and up to 27,000 ppm

Fe deviates increasingly from the correlation line

due to the quenching effect of Fe21, making a

A. Gillhaus et al. / Sedimentary Geology 140 (2001) 191±199 193re

l.in

tens

ity

wavelength [nm]

sample 5a1:dominant

red CL

residual spectrumafter subtraction

sample 1b25:yellow-reddish CL-

mixing colors

300 500400 600 700 800

rel.

inte

nsity

sample 5a1:normalized peak areaof Mn -activated CL

(14keV, 5s)

2+

sample Zoo2 (calcite):intrinsic spectrum

wavelength [nm]

300 500400 600 700 800

~~

A CL

Fig. 1. Left side: cathodoluminescence spectra of two dolomite samples from Hydra Island, Greece. Sample 5a1 with dominant red CL, Mn21

predominantly in the Mg-position of the crystal lattice, ratio 8.9:1, compare Gillhaus et al. (2000) and sample 1b25 with yellowish-red mixing

colours of CL, Mn21 in Mg- and Ca-positions of the crystal lattice, are revealed by an obvious asymmetry of the broad emission bands to lower

wavelengths, compare Plate 1/1 and 1/2. Subtracting both spectra from each other reveals the additional part of Mn21-activated CL in the Ca-

position of sample 1b25 as a residual broad brand. Right side: normalization of the peak areas of Mn21-activated CL of dolomite is shown using

the example of sample 5a1. The intrinsic part of the dolomite spectrum is subtracted from a linear function in accordance with the evaluation of

intrinsic calcite spectra. Additionally, for the clari®cation of the procedure, the intrinsic spectrum of a Mn-free calcite is shown, cave sinter,

sample Zoo2, kindly made available by G. Wurth, RUB, 14 keV, 120 s, intensity adjusted.

A. Gillhaus et al. / Sedimentary Geology 140 (2001) 191±199194

Plate 1. CL-photomicrographs of spectroscopically studied dolomites. Spectra shown in Figs. 1 and 5 are taken in the areas marked with circles.

Scale bar 200 mm. 1: Fault-related dolomite sample 5a1 from Hydra Island, Greece. Dark luminescent dolomite crystal cores (compare Fig. 1).

Blotchy red luminescing crystal rims are partly dedolomitized. Cc� calcite cement. 2: Zoned dolomite crystals, `Weizenkorndolomit', sample

Lea, with visually strong yellow luminescent crystal zones. S� sulphate. 3: Peloidal loferite, sample T30b, Upper Triassic, Peloponnesus,

Greece. Dolomite rhombs with low intensity of CL emission (compare Fig. 5) on blotchy red luminescent dolomicrite, M. Cc� calcite cement.

4: Fe-rich dolomite crystal, sample Magh 1, with visually homogeneous dark brown CL emission (compare Fig. 5) Triassic salt domes, Algeria.

G� glass slide.

0.1

0.1 1

Mn [ppm] = 1.3053*A CL

Mn

[pp

m]=

I*0

.25

38

(1

99

7)

KH

AB

ER

MA

NN

normalized peak area (A ) of Mn -activated CLCL2+

1

10

R=0.999

10

100

100

1000

1000

calcite

Fig. 2. Comparison of CL-spectroscopically determined Mn concentrations of different calcites using peak heights after Habermann (1997) and

normalized peak areas, ACL, after Fig. 22, Habermann, (1997). The results agree very well, correlation coef®cient: R� 0.999, showing that Mn

concentration of calcites can also be determined CL-spectroscopically using peak areas, Mn [ppm]� 1.3053ACL.

quanti®cation of the Mn concentration in the Fe-rich

dolomites impossible with this methodology. Fe-rich

dolomites need additional Fe-analyses.

4.2. Lower activation limit of Mn21-activated CL of

dolomite

All of the studied dolomite crystals and crystal

zones spectroscopically show Mn-activated CL and

12 of them have Mn concentrations below the

detection limit of PIXE. For the CL spectra of the

latter, an extrapolation of the correlation line Mn

[ppm]� 1.3129ACL was done. Theoretically and prac-

tically, Mn concentrations below 1 ppm (see Figs. 4

and 5) are detectable by CL spectroscopy using the

peak areas. This shows that for dolomite, as for calcite

(see Habermann et al., 1998), there is no lower activa-

tion limit for Mn, which was already assumed by

Walker et al. (1989) due to theoretical considerations.

4.3. Quenching (suppression) of Mn21-activated CL

by increasing Fe concentrations

The bivalent Fe ion is well known as the most

effective quencher of CL of carbonates (Machel et

al., 1991). Pierson (1981) visually observed Fe

quenching of Mn21-activated CL starting from

10,000 ppm Fe as well as entire deletion of CL

above 15,000 ppm. However, Richter and Zinkerna-

gel (1975) still observed dark reddish-brown CL of

dolomites at .10 wt% FeCO3 using a CL microscope

with a `hot' cathode. The samples investigated in this

work spectroscopically show the beginning of

suppression of CL intensity from approx. 2000 ppm

by Fe by a slight deviation from the linear correlation

(Mn [ppm]� 1.3129ACL), clearly increasing with

rising Fe concentrations (see Fig. 3). At Fe concentra-

tions above 20,000 ppm visual CL is hardly detected,

even if a high performance device such as the CL

microscope at Bochum is employed. However,

A. Gillhaus et al. / Sedimentary Geology 140 (2001) 191±199 195

0.1

0.1 1

Mn

-co

nce

ntr

ati o

n[p

pm

]fr

om

PIX

E

normalized peak area (A ) of Mn -activated CLCL2+

1

10

10

100

100

1000 10000

1000

10000

Mn [ppm] = 1.3129*A CL

R=0.98

dolomite

n = 82

detection limitof PIXE

< 2000ppm Fe

2000 bis < 5000ppm Fe

5000 bis < 15000ppm Fe

> 20000ppm Fe

Fig. 3. Combined PIXE and HRS-CL-analyses of 82 homogeneously luminescing dolomite crystals and crystal zones. For Fe-poor dolomites,

,2000 ppm Fe, a good correlation exists, correlation coef®cient: R� 0.98. Thus, the Mn concentration of Fe-poor dolomites can be determined

CL-spectroscopically using peak areas, Mn [ppm]� 1.3129ACL. Dolomites.with .2000 ppm Fe deviate increasingly from the correlation line.

Twelve dolomites show Mn concentrations below the detection limit of PIXE, 10±15 ppm, and could not be used to prove the correlation.

A. Gillhaus et al. / Sedimentary Geology 140 (2001) 191±199196

Table 1

Peak areas of Mn21-activated CL emission spectra. Trace element

content Mn and Fe determined by PIXE. Twelve analyses of Mn

(marked with *) are extrapolated as described in Fig. 4; n.d., not

detected

Sample Peak area (area units) Mn (ppm) Fe (ppm)

1c3-1 27.5 50 669

1c3-2 42 47 n.d.

1b17-1* 8 11 200

1b17-2 27.4 35 42

1b17-3 23.3 30 43

1b17-4 17.1 22 n.d.

1b17-5 123.8 156 n.d.

1b25-1 21.2 100 1368

1b25-2 33 70 1257

1b25-3 93.6 115 499

1b25-4 69 103 284

1b25-5 118.2 122 468

1b25-6* 9.6 13 n.d.

1b25-7* 6.4 8 6

T30b-1* 0.7 1 112

T30b-2* 0.8 1 136

T9a-1* 4.6 6 156

T9a-2* 4.2 6 154

4c10-1 37 91 3322

4c10-2 64.1 134 2842

4c10-3 25.6 74 3505

4c10-4 41.3 104 2078

4c13.1-1 45.2 33 274

4c13.1-2 74.3 61 361

5a1-1 20 29 110

5a1-2 20.8 35 71

5a1-3 56.7 96 18

5a1-4 55.9 91 9

5a1-5 20.4 34 n.d.

5e3-1* 11.4 15 293

5e3-2* 9.8 13 52

5e3-3 20.9 27 63

5e3-4* 8.6 11 56

5e3-5* 12 16 102

5f1-1 433.9 383 n.d.

5f1-2 234.4 301 n.d.

GS215-1 24.4 54 220

GS215-2 18 24 153

GS215-3* 12.2 16 323

GS215-4 21.2 46 441

1b6.1-1 107.5 227 4066

1b6.2-1 45.9 415 27,476

1b6.2-2 31.9 289 8950

1b6.2-3 16.1 222 8813

1b3-1 35.7 53 126

1b3-2 100.7 105 698

1b3-3 70.1 96 777

Table 1 (continued)

Sample Peak area (area units) Mn (ppm) Fe (ppm)

1b3-4 92.7 103 485

6a7.2-1 211.4 702 7765

6a7.2-2 50.9 35 52

6a7.2-3 380.4 844 139

6a7.2-4 143.5 334 2497

6a11.2-1 947 1151 906

6a11.2-2 884.3 1125 764

6a11.2-3 554.6 1171 3006

6a11.2-4 478.6 2671 3655

1a6-1 18.9 28 136

1a6-2 18.8 41 173

1a6-3 54.5 74 1036

1a6-4 43.4 47 565

1a6-5 14.7 19 209

1a6-6 12.3 16 207

1a6-7 33.1 44 526

1a6-8 20 28 464

1c3S-1 56.5 73 183

1c3S-2 66.8 80 54

1c3S-3 13.4 17 33

1b171-1 67 62 231

1b171-2 123.9 86 107

1b171-3 60 90 82

RSD115-

1

13.2 258 9624

RSD115-

2

12 233 9118

Ku-1 6.1 216 11,245

Ku-2 5.9 146 11,185

Magh1-1 4.2 1496 25,046

Magh1-2 3.6 1551 24,967

Magh2-2 3.3 1658 24,931

Magh3-1 8.9 1234 24,888

Magh3-2 8.7 1257 23,705

Lea-1 864.9 778 10

Lea-2 799.5 905 22

Lea-3 696.6 705 24

Lea-4 678.4 1445 22

Leb-1 831 1197 n.d.

Leb-2 557.2 1540 n.d.

Leb-3 488 816 n.d.

Leb-4 644.9 1186 n.d.

GW18-1 20.4 16 n.d.

GW18-2 40 41 n.d.

GW18-3 45.4 47 n.d.

GW22-1 18.8 25 n.d.

GW22-2 16.2 18 n.d.

GW22-3 17.2 21 n.d.

VE52 534 929 548

A. Gillhaus et al. / Sedimentary Geology 140 (2001) 191±199 197

0.1

0.1 1

1

10

10

100

100

1000 10000

1000

10000

Mn [ppm] = 1,3129*A CL

dolomite

detection limitof PIXE PIXE

QHRS-CL

Mn

-co

nce

ntr

atio

n[p

pm

]fr

om

PIX

E

normalized peak area (A ) of Mn -activated CLCL2+

Fig. 4. Twelve of the 94 combined PIXE and HRS-CL-analyses (see also Fig. 3) show Mn concentrations below the detection limit of PIXE

(10±15 ppm). By extrapolation of the linear correlation determined in Fig. 3 concentrations of dolomites to less than 1 ppm can be determined

in Fe-poor dolomites using the equation Mn [ppm]� 1.3129ACL.

850

950

1050

900

1000

1100

300 500400 600 700 800

[cts

.]re

l.in

tens

ity

wavelength [nm]

Mn: 1ppm (QHRS-CL)(Mn + Fe not detectable by PIXE)

Mn2+

intrinsic broad bands

background

sample: T30b sample: Magh

Mn2+

850

950

1050

900

1000

1100

300 500400 600 700 800

[cts

.]re

l.in

tens

ity

wavelength [nm]

Mn: 1551ppm (PIXE)Fe: 24967ppm (PIXE)

Fig. 5. Left side: CL spectrum of a trace-element-poor dolomite from a Loferite of the Peloponnesos, Greece. Mn- and Fe-concentrations are

below the detection limit of PIXE (10±15 ppm). By extrapolation of the linear correlation, Mn [ppm]� 1.3129ACL, a Mn concentration of

1 ppm can be determined by evaluating the area of the Mn-activated broad band peak. Take notice of the high intensity and resolution of the

intrinsic part of the spectrum due to low Mn- and Fe-concentrations, compare Plate 1/3. Right side: CL-spectrum of the dolomite with highest

trace element content in this study from the Triassic salinar of Algeria. The broad emission band with an asymmetry to lower wavelengths,

Mn21 in Mg- and Ca-position, proves spectroscopically that, even at such high Mn and Fe concentrations, absolute extinction of CL emission

by self quenching or concentration quenching, 14 keV, 60 s; compare Plate 1/4, does not occur.

spectroscopic proof is successful (Fig. 5) even in this

case. The Fe concentration resulting in entire deletion

of Mn-activated CL of dolomite (depending on the

respective Mn concentration) could not be determined

due to the lack of appropriate sample material.

4.4. Detection limit and measuring precision of

Mn21-activated CL

Mn concentrations of dolomite ,1 ppm (below the

detection limit of PIXE at 10±15 ppm Mn) can be

determined by extrapolation of CL-spectroscopically

obtained peak areas (see Fig. 5). The studied sample

material contains Mn down to 0.9 ppm. Mn-free

dolomite with exclusively intrinsic CL was never

observed.

The lower detection limit for Mn in calcite and

dolomite is given at 0.1±5 ppm by Habermann

(1997) due to theoretical considerations (Johansson

and Campbell, 1988) according to measuring time

of CL spectra with the CL device at Bochum.

However, at the high measuring precision required,

such low detection limits are only available if the

measured spot of 30 mm is positioned in a relatively

large homogeneously luminescing area.

On the glass rim of the thin section, the CL emis-

sion of a dolomite is measurable even if the carbonate

is more than 100 mm away. This is due to scattered

light from irritated sample areas around the measuring

spot which cannot be eliminated by an aperture posi-

tioned far above the sample plane for technical

reasons. To avoid such errors on the measurement

of Mn-poor dolomite spectra (,20 ppm Mn), a

100 mm sized aperture in an aluminium foil was posi-

tioned in the beam of light above the thin section.

Thus, crystal zones of dolomite with an intensely

luminescing `neighbourhood' at a distance of

,100 mm were not used for quantitative spectral

analyses.

5. Discussion

The presence of a lower detection limit of Mn21-

induced activation of dolomite resulting in a visible

yellow-orange-red CL is discussed as controversially

as with calcite (Pierson, 1981; Richter and Zinker-

nagel, 1981; see compilation in Habermann et al.,

1998).

To assess this Mn-dependent luminescence of dolo-

mite, a quanti®cation using peak area measurements

of CL spectra was employed by combining HRS-CL

and PIXE. Here, the low detection limit of PIXE for

Mn (10±15 ppm) allowed a continuous processing of

sample material with representative Mn contents.

Our combined analyses of dolomites with Mn

concentrations between 10 and 2671 ppm show a

linear correlation (R� 0.98) for Fe-poor dolomite

(,2000 ppm Fe) with the equation: Mn

[ppm]� 1.3129ACL [normalized area units]. A

comparison of CL-spectroscopically determined Mn

concentrations of different calcites obtained by peak

height measurements after Habermann (1997) with

normalized peak area measurements reveals an

equivalent application for calcite with Mn

[ppm]� 1.3053ACL (see Fig. 2). By extrapolation of

CL-spectroscopically determined peak areas, Mn

concentrations as low as 1 ppm can be determined

in dolomites, which is below the detection limit of

PIXE (10±15 ppm Mn). From this it can be inferred

that for dolomite there is also no Mn21-activation

limit, coinciding with the previous assumption of

Walker et al. (1989) due to theoretical considerations.

The luminescence intensity of dolomites with

.2000±25,000 ppm Fe increasingly deviates from

the correlation line (see Fig. 3) due to the quenching

effect of Fe21. An entire extinction of CL by high Fe

concentrations could not be observed using the

present sample material. According to Richter and

Zinkernagel (1975) even dolomites with 15 wt%

FeCO3 show a weak brownish-red luminescence

using an older CL device with a `hot' cathode.

In summary. theoretical considerations on lumines-

cence are also supported by using the example of

dolomite. For Fe-poor dolomites, (,2000 ppm Fe)

there exists a linear relationship of luminescence

intensity and Mn content (between 10 and at least

1000 ppm Mn). There is no activation limit of dolo-

mite, but only a visual detection limit for Mn-acti-

vated CL depending on the CL-device in use.

Furthermore, the CL-device at Bochum allows the

detection of dolomite CL with less than 1 ppm Mn,

as well as dolomite with more than 25,000 ppm Fe,

using the existing sample material. These limits could

still be expanded in future by means of further sample

material and by minor technical improvements of the

device.

A. Gillhaus et al. / Sedimentary Geology 140 (2001) 191±199198

Acknowledgements

We thank Thomas GoÈtte (Ruhr-University

Bochum, Germany) and Dirk Habermann (University

of Freyberg, Germany) for discussions concerning

methodical aspects.

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