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GEOLOGICA CARPATHICA

, DECEMBER 2018, 69, 6, 558–572

doi: 10.1515/geoca-2018-0033

www.geologicacarpathica.com

In-situ U–Th–Pb geochronometry with submicron-scale 

resolution: low-voltage electron-beam dating of complexly 

zoned polygenetic uraninite microcrystals 

MICHAEL WAITZINGER

 and FRITZ FINGER

Department of Chemistry and Physics of Materials, University of Salzburg, Jakob-Haringer-Strasse 2a, A-5020 Salzburg, Austria;  

 

Michael.Waitzinger@sbg.ac.at

(Manuscript received August 2, 2018; accepted in revised form November 5, 2018)

Abstract: Complexly zoned microcrystals of uraninite were encountered in orthogneiss from the central Tauern Window 

in Austria (K1 gneiss, Felbertal scheelite mine) and analysed in-situ for U, Th and Pb with state-of-the-art FE-SEM/EDX 

techniques. A three times finer spatial resolution was achieved using an acceleration voltage of 8 kV, compared to  

the classic 15–20 kV set-up of U–Th–total Pb electron microprobe dating. The lower voltage allows a spheroid of  material 

with a diameter of only 0.3 µm to be selectively analysed. Careful tests on three uraninite reference materials show that 

the low-voltage method yields sufficient precision and accuracy for U–Th–total Pb uraninite dating, with errors on 

 individual spot ages in the order of 10–30 Ma. By means of this innovative analysis technique, small-scale age zoning 

patterns could be resolved and dated in the uraninite microcrystals from the orthogneiss. Based on microstructures 

 observed in backscattered electron images we interpret that an older uraninite generation in the rock, with a late Permian 

formation age (~260 Ma), was recycled two times through a coupled dissolution–reprecipitation process at ~210 Ma and 

at ~30 Ma. The younger dissolution–reprecipitation phase at ~30 Ma coincides with the Alpine regional metamorphism 

(lower amphibolite facies). The two older ages (~210 Ma and ~260 Ma) have been previously recognized in rocks from 

the Tauern Window by uraninite dating, but it is the first time here that both are recorded in the same rock and even  

the same uraninite grain. The present study shows that recrystallized accessory uraninite can provide a sensitive  geological 

“hard disk” where several discrete thermal events of an area are stored. In addition, our work attests that the mineral 

uraninite has an unexpected geochronological robustness, even on the microcrystal scale. 

Keywords: uraninite, high-resolution in-situ U–Th–Pb dating, K1 gneiss, Tauern Window. 

Introduction

U–Th–Pb geochronometry is undoubtedly one of the most 

 important analytical tools that are used in the Earth Sciences. 

Since its beginnings (Holmes 1911), the field has continuously 

improved with regard to better precision, more sophisticated 

analytical techniques, lesser sample material, and in-situ 

 dating with high spatial resolution (e.g., Krogh 1982; Kober 

1986; Williams 1998; Davis et al. 2003; Foster et al. 2004; 

Mattinson 2005; Amelin & Davis 2006; Frei & Gerdes 2009; 

Schoene et al. 2010; Schaltegger et al. 2015). Apart from 

 zircon and monazite, which are the dominant targets for  

U–Th–Pb dating at present, minerals like titanite, rutile, xeno-

time, columbite, uraninite, thorite, and several others have 

been used. 

The common method for U–Th–Pb age dating is based  

on isotopic measurements with mass spectrometers (TIMS, 

SIMS, SHRIMP, Laser-ICP-MS). An alternative is the deter-

mination of total Pb contents in Th- and U-rich minerals using 

electron beam methods (Parslow et al. 1985; Bowles 1990; 

Suzuki et al. 1991; Montel et al. 1996, 2017; Williams & 

Jercinovic 2002). The latter method has the advantage of a high 

spatial analytical resolution (typically 1–2 µm in the tradi-

tional setups), thus enabling the targeting of small crystals as 

well as the detailed study of age-zoned crystals. Furthermore, 

chemical data is collected simultaneously with the age infor-

mation, which is helpful for many petrogenetic issues. Electron 

beam analyses can be performed on minerals in thin section, 

and, thus, the age dates can be directly related to mineral tex-

tures. Disadvantages of U–Th–total-Pb electron-beam dating 

include its poorer precision and accuracy in the measured 

ages, and poorer control on determining the possible effects of 

Pb loss or common Pb presence. Electron beam dating is cur-

rently mainly applied to monazite, although successful studies 

with other U–Th-bearing minerals have been made as well, for 

instance with zircon (Geisler & Schleicher 2000), xenotime 

(Bernhard et al. 1998), zirconolite (Tropper et al. 2007), 

thoria 

nite, thorite, huttonite and uraninite (Förster 1999; 

Santosh et al. 2003; Cocherie & Legendre 2007; Naemura et 

al. 2009; Yokoyama et al. 2010; Cross et al. 2011; Votyakov et 

al. 2013;  Allaz et al. 2015). 

This work is focused on two examples of complexly zoned 

microcrystals of uraninite that were encountered in an ortho-

gneiss from the Tauern Window in the Eastern Alps. In a pre-

vious study (Finger et al. 2017), a significant intra-crystal 

variation of U/Pb ratios was observed in these zoned grains, 

pointing to a polygenetic origin. Notably, other uraninite micro-

crystals in this gneiss showed a homogeneous intra-crystal  

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GEOLOGICA CARPATHICA

, 2018, 69, 6, 558–572

U/Pb distribution with an either Permian or Paleogene total  

Pb age. The origin and geological significance of the two 

 age-zoned grains thus constituted an interesting problem for 

further research.

The investigation of small-scale compositional heteroge-

neities in micron-sized uraninite grains requires a particularly 

high spatial analytical resolution. Using FE-SEM/EDX tech-

niques and a low-voltage (8 kV) electron beam, crystal volu-

mes with an approximately 0.3 µm diameter can be selectively 

analysed. This low-voltage method is applied here for the first 

time to the U–Th–Pb dating of minerals. Therefore, a signifi-

cant part of this paper will be devoted to methodology. 

Methodology 

General remarks

Using the standard electron microprobe setup with a 15–20 kV 

acceleration voltage, electron beam dating excites a spheroid- 

shaped volume with 1–3 µm diameter in the targeted minerals 

(Jercinovic et al. 2012). The exact size and geometry of  

the exci tation volume depends on mineral density and compo-

sition. This 1–3 µm resolution is too coarse for the study of 

micron-sized crystals. A significantly better spatial resolution 

of electron beam dating can be achieved by lowering the acce-

leration voltage. For instance, at an acceleration voltage of  

8 kV, and a beam width of 100 nm, the excitation volume in 

uraninite  should  theoretically  be  only  ~ 0.3  µm  in  diameter, 

compared  to  ~ 0.8  µm  at  15  kV  (Fig.  1).  Low-voltage  (i.e.,  

8 kV) electron beam measurements on uraninite thus permit 

U–Th–total Pb dating at a three times finer spatial resolution, 

but the intensity of the generated X-ray signal that is needed 

for chemical analysis is much weaker in such conditions. 

Fortunately, special large-area EDX detectors have become 

available over the past ten years that are more sensitive and 

with much better counting statistics than previous generation 

EDX detectors. In addition, new field emission (FE) cathodes 

are a great advantage for high-resolution analysis, as they can 

produce a small coherent beam with a high current density.

Analytical setup 

In this study, uraninite analyses were carried out on polished 

and carbon-coated rock thin-sections using a Zeiss ULTRA-

PLUS scanning electron microscope (SEM) equipped with  

a field emission (FE) cathode. Uniform electron beam condi-

tions were used for quantitative analysis involving an acce  le-

rating voltage of 8 kV, 2 nA beam current and 100 nm  

beam width. EDX analysis was performed with the large-area 

(50 mm

2

) silicon drift detector X-MAX 50 from Oxford 

Instruments. A full spectrum range from 0 to 10 keV was 

recorded with 2048 channels and a resolution of 5 eV per chan-

nel. Concentrations of U, Th and Pb were derived from count 

rates  obtained  on  the  Mα  lines  using  Oxford  INCAEnergy 

software. Background, matrix and fluorescence effects, as well 

as line overlaps, are automatically computed and corrected by 

this software. Quantification of concentrations is based on 

intensity values taken from the Oxford INCAEnergy internal 

standard data bank (Table 1) and a 180 s calibration measure-

ment on a Si wafer before every analytical session.

 For quantitative uraninite measurements, the following 

 element list was used: Al

2

O

3

, SiO

2

, P

2

O

5

, SO

3

, CaO, TiO

2

FeO, Y

2

O

3

, La

2

O

3

, Ce

2

O

3

, PbO, ThO

2

, and UO

2

. Some of  

these  elements are uncommon in uraninite but were included 

to identify contamination from inclusions or boundary 

 minerals. Oxygen was generally calculated using the stoi-

chiometry based on the cations and not from the measured 

oxygen signal, as the latter is difficult to calibrate accurately 

and is over sen sitive to surface contamination and oxidization. 

However, the oxygen (and carbon) peak intensities were 

 routinely checked for anomalies in order to recognize any 

local altera tion involving oxidization, hydration or carbo-

nation. Fur thermore, every spectrum was checked for the pre-

sence of any additional peak not included in the quantitative 

analysis.

The challenge for U–Th–total Pb dating is to analyse these 

three elements with such precision that permits sufficiently 

precise age dates to be calculated. When analysing uraninite 

under the aforementioned beam conditions, the Oxford 

X-MAX 50 acquires a total count rate of ~2*10

6

 counts within 

Fig. 1. Spatial resolution of electron beam dating of uraninite illustrated by Monte Carlo simulations of electron scattering at 8 kV (A) and  

15 kV (B). Assumed beam width is 100 nm. Red: backscattered electrons. Used software: CASINO, Drouin et al. (2007).

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GEOLOGICA CARPATHICA

, 2018, 69, 6, 558–572

c. 3 minutes (120 s live time). This results in an analytical 

precision of approximately ± 0.4 wt. % (1σ) for U, while Pb 

can be analysed at a detection limit of ~ 0.2 wt. %. Thorium and 

other minor elements, like Si, Ca, Ce, and Y, that are occasio-

nally reported in uraninite (Allaz et al. 2015; Alexandre et al. 

2016), can be analysed at detection limits of 0.1–1.3 wt. % 

(Table 1). We are aware that these detection limits are far 

beyond the sensitivity of WDX microprobe analysis, but 

believe that this creates no essential problem for uraninite 

 dating. For instance, a 90 Ma old uraninite crystal (see Table 2) 

contains ~1 wt. % radiogenic Pb, which is well within the mea-

surable range. Using an 8 kV/2 nA beam and a counting time 

of 3 minutes, the analytical uncertainty for Pb in uraninite is 

typically ~ 0.1 wt. %, which corresponds to an error in the order 

of 20 Ma (1 σ) for a single spot age. Even when the counting 

time per spot is reduced to one minute (30 s live time), reaso-

nable age errors in the order of 20–30 Ma can still be obtained. 

Increasing the counting time per analysis spot to five minutes 

or more improves detection limits and precision and could be 

favourable for very young, Pb-poor uraninite. However, we 

must be aware that this increases the risk for beam drift impe-

ding the desired spatial resolution, as well as the risk for beam 

damage effects.

A sufficient analytical precision is a prerequisite for U–Th–

total Pb dating. However, the successful application of any 

ana lytical method is also dependent on the accuracy of  

the element analysis, in particular the determination of Pb.  

As shown by Pyle et al. (2002) and Jercinovic et al. (2005, 

2012) for electron-microprobe-based U–Th–total Pb monazite 

dating, there are many possible pitfalls that can superimpose  

a systematic analytical error on the Pb determination; the most 

important of these refer to background and interference cor-

rection problems. 

For uraninite dating, these analytical perils are less severe 

because Pb contents are relatively high in uraninite; for exam-

ple, they are commonly an order of magnitude higher than in 

monazite and generally reach a major element concentration. 

Nevertheless, we have carried out several of the recommended 

tests (Jercinovic et al. 2012) in order to assess the reliability of 

our data (see next sections). 

Testing the calibration with synthetic uraninite and other 

mineral standards

A synthetic UO

2

 crystal with 99.9 % purity was measured to 

control and validate the so-called “standardless” calibration 

provided by the Oxford INCAEnergy software. The obtained 

UO

concentrations were between 98 and 102 wt. % (mean 

99.6 ± 0.8 wt. %, n = 8), confirming that UO

2

 concentrations can 

be sufficiently accurately determined at 8 kV conditions with 

the internal INCAEnergy calibration. For a synthetic ThO

2

 

crystal,  we  obtained  a  value  of  101.10 ± 0.8  wt.  %  ThO

,  

which is a little too high, but the effects of this ~1 wt. % 

standar  di zation inaccuracy on the age dating results are negli-

gible. Measurements on various Pb minerals also reproduced 

the recom mended values within 3 % deviation. Even for 

 dif ficult-to-analyse Pb minerals such as galena (line overlap  

of S Kα and Pb Mα) acceptable values were obtained (e.g., 

87.3 wt. % Pb vs. expected 86.6 wt. % for stoichiometric 

galena).

Repeated measurements on the nominally Pb-free UO

showed that

 

the computation of the Pb by INCAEnergy 

resulted  in  a  small  but  systematic  blank  value  of  − 0.16  ±  

0.06 wt. % PbO. This blank value was externally corrected for 

all uraninite analyses, weighted by the individually measured 

UO

2

. The vali 

dity of this procedure was independently 

assessed by measu rements on uraninite reference materials 

(see below). The un certainty of the blank value determination 

(± 0.06  wt.  %)  was  incorporated  in  the  total  Pb  error  and  

the age error.

Appreciable inaccuracies in the automatic INCAEnergy 

background and interference correction for uraninite were also 

encountered in the case of Th. A blank test on pure UO

2

 gave 

a slightly negative ThO

2

  content  of  − 0.83 ± 0.37  wt.  %  (see 

Table 1), probably because the peak interference of U Mα on 

Th Mα was slightly overcorrected. Consequently, an external 

Th correction was applied for every uraninite analysis, 

weighted according to the individual UO

content. This empi-

rical Th correction is certainly not ideal, but the problem  

with accurately determining the Th will generally have only  

a minor influence on the final uraninite age calculations. For 

instance, 1 wt. % more or less Th alters the calculated age by 

less than 1 Ma. Nevertheless, it could be argued that the impre-

cise Th data may perhaps have some effect on the peak 

 interference correction for Pb, as there is a small Th line  

close to the Pb Mα position. Indeed, Th and Y interference 

effects create a well-known analytical problem for electron- 

beam monazite dating (Scherrer et al. 2000). However, for 

uraninite dating, this will, in general, be negligible, owing to 

Element

Line

Calibration

standard

Detection

limit 120s*

Typical

error 120s*

Blank

correction

Al

K_SERIES

Al

2

O

3

0.06

0.02

Si

K_SERIES

SiO

2

0.15

0.05

P

K_SERIES

GaP

0.09

0.03

S

K_SERIES

FeS

2

0.06

0.02

Ca

K_SERIES Wollastonite

0.18

0.06

Ti

K_SERIES

Ti

0.36

0.12

Fe

L_SERIES

Fe

0.99

0.33

Y

L_SERIES

Y

0.36

0.12

Zr

L_SERIES

Zr

0.33

0.11

La

L_SERIES

LaB

6

1.29

0.43

Ce

L_SERIES

CeO

2

1.26

0.42

Pb

M_SERIES

PbF

2

0.21

0.12

− 0.16 ± 0.06

Th

M_SERIES

ThO

2

0.75

0.25

− 0.83 ± 0.37

U

M_SERIES

U

0.40

Table 1: SEM/EDX set-up used in this study for low-voltage (8 kV) 

uraninite analysis. In columns 4–5, detection limits and typical errors 

(1 σ) are listed for selected elements, at 3 min counting time (120 s 

live time). The given detection limits and errors have been calculated 

by the INCAEnergy software based on measurements on synthetic 

uraninite and the Mitterberg uraninite standard, respectively. Blank 

values refer to pure UO

2

.

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GEOLOGICA CARPATHICA

, 2018, 69, 6, 558–572

the commonly high radiogenic Pb concentrations. The excep-

tion would perhaps be when analysing young, Pb-poor ura-

ninite or very Th- and Y-rich uraninite.

Measurements on uraninite reference materials 

Three in-house uraninite reference materials were measured 

for control and to serve as sensitive secondary Pb standards for 

comparison with the obtained U–Th–total Pb ages. The first 

reference material is magmatic uraninite from the Kirchberg 

granite, Erzgebirge, Germany, previously dated at 326 ± 4 Ma 

(Förster  1999)  and  330 ± 5  Ma  (Kempe  2003)  by  EPMA.  

The second is hydrothermal uraninite from Mitterberg, 

Austria,  dated  at  90 ± 5  Ma  by  a  concordant  U

238

–Pb

206

  

TIMS age (Paar & Köppel 1978). The third reference material 

is magmatic uraninite from the Königshain granite, Erzgebirge, 

dated by EPMA at 328.6 ± 1.9 Ma (Förster et al. 2012).

Our low-voltage measurements reproduce the published 

geochronological dates in all three cases surprisingly well 

(Fig. 2), showing that the Pb analyses are sufficiently accurate. 

Individual spot ages for the Mitterberg uraninite ranged 

between  85 ± 11  Ma  and  102 ± 11  Ma,  with  a  calculated  

mean  age  of  90.7 ± 6.2  Ma  (95 %  confidence  level,  n =12).  

For the Kirchberg uraninite, individual ages ranged between 

320 ± 12 and 335 ± 12 Ma, with a mean of 327.3 ± 6.8 Ma (95 % 

confidence  level,  n =12).  For  the  Königshain  uraninite,  we 

obtained an age range of 320–331 Ma and a mean age of  

325.9 ± 8.7 Ma (95 % confidence level, n = 8). The low analy-

tical totals of the Königshain uraninite (Table 2) result from 

additional REE amounts, which can be qualitatively recog-

nized in the EDX spectra (Fig 3), but were 

unquantifiable within the used analytical 

setup. According to electron microprobe 

data (Förster 1999; Förster et al. 2012),  

the Königshain uraninite has a total REE 

content of 4–8 wt. %.

The reproduction of the published age 

for the Königshain reference material 

shows that our analytical setup also pro-

vides sufficiently accurate Pb values for 

Y-, REE-, and Th-rich uraninite, implying 

that the Pb background and interference 

corrections of the INCAEnergy software 

are good. The latter is independently sup-

ported by blank tests made for Pb-free syn-

thethic ThO

2

 and YPO

4

, which yield a zero 

concentration result for Pb. In addition,  

Pb blank testing was done on Pb-free 

quartz and pyrite, and the measured Pb 

values were also close to zero. 

Finally, a test for beam damage and 

charging effects was made using uraninite 

from Mitterberg. Repeated 30 s measure-

ments on the same standard point yielded 

constant U count rates over a five-minute 

observation time, with a standard devia-

tion not greater than the counting statistics 

of the instrument. The specimen current 

also remained constant within these five 

minutes of exposure time, suggesting that 

beam damage and charging effects play no 

or only a minor role at the given beam 

energy dose.

Testing the spatial resolution of low- 

voltage electron beam dating

The Mitterberg uraninite carries several 

small inclusions and veins of gold (Fig. 4). 

We recorded detailed chemical profiles 

across the uraninite-gold grain boundaries 

Sample

SiO

2

CaO

Y

2

O

3

PbO

ThO

2

UO

2

Total

Age

Error

Kirchberg

0.22

<0.11

<0.38

4.32

1.39

93.74

99.67

335

12

Kirchberg

<0.15

<0.11

<0.38

4.29

1.26

94.61

100.17

330

12

Kirchberg

0.21

<0.11

<0.38

4.21

1.21

94.43

100.05

325

12

Kirchberg

<0.15

<0.11

<0.38

4.28

1.42

94.32

100.01

330

12

Kirchberg

0.26

<0.11

<0.35

4.27

1.10

94.19

99.81

330

12

Kirchberg

0.33

<0.11

<0.38

4.11

1.02

92.20

97.66

325

12

Kirchberg

0.40

<0.11

<0.38

4.06

1.01

92.46

97.92

320

12

Kirchberg

0.42

<0.11

<0.35

4.24

1.05

94.54

100.25

327

12

Kirchberg

0.32

<0.11

<0.35

4.23

1.10

93.96

99.60

328

12

Kirchberg

0.37

<0.11

<0.35

4.25

0.95

93.95

99.51

329

12

Kirchberg

0.35

<0.11

<0.35

4.14

1.11

94.05

99.66

321

12

Kirchberg

0.31

<0.11

<0.35

4.16

1.00

92.69

98.17

327

12

Average age (95 % c. l.)

327.3

6.8

Mitterberg

0.41

0.41

0.77

1.29 <0.48

94.06

97.22

102

11

Mitterberg

0.37

0.47

0.81

1.12 <0.48

94.82

97.87

89

11

Mitterberg

0.34

0.53

0.83

1.16 <0.54

97.89

101.06

89

11

Mitterberg

0.32

0.58

1.49

1.08 <0.48

95.11

98.86

85

11

Mitterberg

0.36

0.44

1.42

1.16 <0.53

96.34

100.04

90

11

Mitterberg

0.33

0.48

1.31

1.09 <0.48

95.98

99.48

85

11

Mitterberg

0.20

0.67

1.11

1.12 <0.50

96.30

99.99

87

11

Mitterberg

0.26

0.65

0.89

1.18 <0.50

94.11

97.59

93

11

Mitterberg

0.25

0.66

0.79

1.24 <0.48

96.37

99.82

96

11

Mitterberg

0.30

0.44

0.85

1.17 <0.50

95.88

98.93

91

11

Mitterberg

0.27

0.63

0.64

1.18 <0.48

97.25

100.24

91

11

Mitterberg

0.31

0.49

0.61

1.16 <0.48

97.22

100.06

89

11

Average age (95 % c. l.)

90.7

6.2

Königshain

0.57

<0.14

4.23

3.56

7.09

76.90

92.34

329

13

Königshain

0.54

<0.14

4.31

3.56

6.58

76.51

91.49

331

13

Königshain

0.64

<0.14

4.66

3.54

6.90

77.25

92.99

326

13

Königshain

0.52

<0.14

4.36

3.48

6.57

75.99

90.91

326

13

Königshain

0.69

<0.12

3.48

3.58

7.49

79.47

94.72

320

12

Königshain

0.67

<0.12

3.77

3.61

6.81

79.49

94.35

324

12

Königshain

0.56

<0.12

3.20

3.63

7.61

80.08

95.08

322

12

Königshain

0.73

<0.12

3.55

3.60

7.60

77.42

92.90

330

13

Average age (95 % c. l.)

325.9

8.7

Table 2: Chemical data (wt. %) for the in-house uraninite reference materials Kirchberg, 

Mitterberg and Königshain. Single point ages and errors (1 σ) were calculated based on  

the measured U, Th and Pb concentrations, using the algorithm of Săbău (2012). Counting 

time per analysis is c. 5 minutes (180 s live time). The age errors always include the analy-

tical uncertainties in the Pb and U determination as well as the error for the performed Pb 

blank correction. Mean ages (95 % confidence level) were calculated using the program 

ISOPLOT of Ludwig (2003).

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at 8, 10, and 15 kV, using a step width of 0.1 µm, to empiri-

cally determine the lateral analytical resolution and the size of 

the excitation volume, respectively. Results are shown in 

Figure 5. At 15 kV, we note a ~1 µm wide transition zone, 

where the electron beam has excited both minerals, simulta-

neously producing signals for Au and U. The width of this 

transition zone is reduced from ~1 µm to ~ 0.6 µm at 10 kV 

and to ~ 0.3 µm at 8 kV, demonstrating that the spatial resolu-

tion of U–Th–Pb uraninite dating is about three times better at 

8 kV than at 15 kV.

Recommended analysis strategy for uraninite dating

The suitability of monazite for electron beam U–Th–Pb 

 dating has been highlighted many times (no significant Pb 

loss, little common Pb), but the potential of using uraninite as 

a geochronometer is still insufficiently investigated. There is 

agreement that the amount of common Pb in uraninite is 

 generally strongly subordinate compared to the vast amounts 

of radiogenically produced Pb (e.g., Kotzer & Kyser 1993; 

Fayek et al. 2002; Chipley et al. 2007). Thus, the common Pb 

effect on a total Pb uraninite age date will be negligibly low in 

most cases. 

However, problems with Pb loss can be serious. 

Investigations made on hydrothermally overprinted U depo-

sits show that primary uraninite can lose Pb and become 

 variably “rejuvenated” by interaction with fluids (Fayek et al. 

2002; Alexandre & Kyser 2005; Decrée et al. 2011). More-

over, U gain can also occur when uraninite undergoes hydro-

thermal alteration (Kempe 2003). The key point here is to find 

Fig. 2. Geochronological dates obtained for three uraninite reference materials using a low voltage electron beam (8 kV). Age data are sorted 

from oldest to youngest left to right. Recording time is c. 5 minutes (180 s live time) per analysis point. Mean ages, errors and statistical 

parameters were calculated with the program ISOPLOT (Ludwig 2003).

Fig. 3. Examples of EDX spectra (8 kV, 2 nA) for almost pure uraninite with ~1 wt. % PbO (Mitterberg), and Th, Y and REE bearing uraninite 

with ~ 3.5 % PbO (Königshain).

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and develop an effective analytical strategy that enables us to 

determine whether the Th–U–Pb system of a given crystal (or 

crystal domain) is chemically disturbed or not (thermodyna-

mically a closed or open system). Measuring the U–Th–Pb 

concentrations within single crystals at a high spatial reso-

lution, for example, along a profile, could provide such  

a strategy. A constancy of ages would strongly suggest that  

the U–Th–Pb system is undisturbed, whereas a disturbed sys-

tem will exhibit irregular U–Th–Pb distributions and inconsis-

tent ages (Kempe 2003). If several adjacent spot ages along  

a profile are iden tical within the analytical error, they can be 

statistically ave raged to produce a domain age. If all spot ana-

lyses in a grain are identical within error, they can be averaged 

to give a single grain age.

The case study: complexly zoned uraninite 

 microcrystals in orthogneiss 

Geological background and sample petrography

The Tauern Window exposes the deepest tectonic units  

of the Eastern Alps (Fig. 6). It contains crustal rocks of  

the European plate that were overridden by various nappes 

derived from the southern Adriatic plate during the Alpine 

orogeny (see Schmid et al. 2004, 2013 for review). As a con-

sequence of this tectonic burial, the Tauern Window experien-

ced upper greenschist to middle amphibolite facies regional 

metamorphism in the late Paleogene, at c. 30 Ma. This phase 

of regional metamorphism has been well constrained by 

 various geochronological studies, including K–Ar and Ar/Ar 

mica and hornblende dating, Sm–Nd garnet dating (data com-

pilation in Pestal et al. 2009) and U–Pb allanite dating (Cliff et 

al. 2015).

Approximately half of the Tauern Window complex com-

prises the so-called Penninic Units (Fig. 6) of Mesozoic 

 sedimentary and volcanic rocks, including obducted Jurassic– 

Cretaceous ophiolites (Frasl & Frank 1966). An older, pre- 

Mesozoic basement (Subpenninic Units in Fig. 6) of Early 

Paleozoic, island arc-type crust was variably metamorphosed 

during the Variscan period (Habach Complex and Altkristallin 

in Fig. 6), and intruded by large volumes of Variscan grani-

toids, now the so-called Central Gneisses. The pre-Alpine 

geo chronology of the Tauern Window is based mainly on 

Carboniferous to Permian zircon dates from the Variscan 

grani toids (Eichhorn et al. 2000; Vesela et al. 2012) and 

Cambro–Ordovician to Devonian zircon dates from the older 

arc-type crust (Eichhorn et al. 1995; Kebede et al. 2005). 

Carboniferous (Variscan) metamorphism is recorded through 

garnet and monazite ages (Von Quadt 1992; Finger et al. 

2016). 

Microcrystals of accessory uraninite have recently been 

found in several of the Central Gneisses of the Tauern Window. 

They have different formation ages (Finger et al. 2017).  

The youngest generation of uraninite microcrystals formed 

during  the  Alpine  orogeny  in  the  Paleogene  at  ~ 30  Ma.  

A Permian (~ 265 Ma) generation of uraninite microcrystals 

has been identified in the Central Gneiss types with a lower 

Carboniferous intrusion age (K1 gneiss and Felbertauern 

Augengneiss).  Triassic  (~ 215  Ma)  uraninite  microcrystals 

were found in the Central Gneisses with Permian intrusion 

ages (Granatspitz and Reichenspitz gneiss). The pre-regional 

metamorphic uraninite microcrystals are interpreted as dating 

Fig. 4. Intergrowths of uraninite (grey) and gold (bright) in 

 

the Mitterberg sample (backscattered electron image).

Fig. 5. Profiles across a uraninite-gold boundary (uraninite left, gold right) with a stepwidth of 0.1 µm, showing the spatial resolution of 

 electron beam excitation at 15, 10 and 8 kV acceleration voltage (see text for further explanation). The gold occurs in solid solution with  

~ 10 % silver.

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discrete low-T events related to increased heat and fluid 

 activity during the Permo–Triassic thinning of the European 

plate (Finger et al. 2017). 

The K1 Gneiss, the host rock of the two complexly zoned 

uraninite crystals of this study, is a small orthogneiss body  

in the realm of the prominent scheelite mine Felbertal 

(47°13’27.1” N, 12°29’18.2” E; Fig. 6). It is a fine grained, 

SiO

2

 rich, metagranitic rock (Kozlik & Raith 2016) with  

high concentrations of W (up to 3000 ppm), Nb (up to  

100 ppm), U (up to 50 ppm) and has been regarded as  

the source of the local tungsten mineralization. The quartz–

feldspar–mica fabric of the gneiss is entirely metamorphic 

(Alpidic). The magmatic protolith age of the rock has been 

constrained by zircon dating at 339.6 ± 1.2 Ma (Kozlik et al. 

2016). 

Microstructures, EDX analyses and U–Th–total Pb ages 

On average, 10–20 uraninite microcrystals are observed in  

a single thin section from the K1-gneiss. They are included in 

feldspars, epidote, titanite or zircon, only a few are interstitial. 

As mentioned earlier, most uraninite grains in the rock are 

unzoned, with a homogeneous intra-crystal U/Pb distribution 

that corresponds either to a Paleogene (~ 30 Ma) or a Permian 

(~ 265 Ma) total Pb age (Finger et al. 2017). Compositionally 

heterogeneous grains with internally variable U/Pb ratios and 

total Pb ages, as described in the following, are thus rather  

the exception than the rule.

Grain 1

This uraninite grain, although only some 6 µm wide, 

 presents a striking zonation in the BSE image with a darker 

core and a brighter rim (Fig. 7). The boundary between the core 

and the rim zone is sharp, but irregularly and multiply embayed 

indicating replacement of older uraninite core substance by 

younger rim substance via coupled dissolution–reprecipitation 

(Putnis 2002; Harlov et al. 2011). The grain is enclosed in 

titanite. Other uraninite micrograins occurring nearby (Fig. 7a) 

are unzoned and entirely Permian in age. A small elongated 

fluorite crystal, recognizable by an extremely bright CL  signal, 

adheres to grain 1 (Fig. 7c). 

In order to obtain some three dimensional information,  

the grain surface was reground in a second step of investiga-

tion with a layer of ~ 0.3 µm being removed. The bright rim 

zone became wider (Fig. 7c), implying that larger volumes of 

the bright substance reside at the bottom of the grain. A nar-

row, ~ 0.2 µm wide dark chip from the older uraninite conti-

nues into the bright rim zone, documenting that coupled 

dissolution–reprecipitation can produce very fine-scale intra-

crystal heterogeneities. Such fine intergrowths are difficult to 

resolve and to analyse even with FE-SEM methods and natu-

rally there is always a danger to obtain mix analyses and to 

calculate mix ages here.

A chemical traverse with ~ 0.3 µm resolution (Figs. 7b, 8a; 

Table 3) shows that the bright rim of grain 1 has a much lower 

Pb content than the core (< 1 vs. ~ 3 wt. %), a lower Y

2

O

3

 con-

tent (0–2 vs. 3–4 wt %), and a higher UO

2

 content (~ 90 vs.  

~ 80–85  wt.  %).  Notably,  there  is  no  significant  change  in  

the Th content between core and rim, but the profile reveals  

a Th-enriched inner core within the core zone with 

 

~10–13 wt. % ThO

2

. This Th rich inner core is not very visible 

in the BSE image but it could be approximatively delineated 

by x-ray mapping. Four shorter chemical profiles were addi-

tionally recorded in different domains of grain 1 after it was 

reground (Figs. 7c, 8b–e, Table 3). These profiles confirm  

the crystallo-chemical heterogeneities within grain 1 as 

described before.

Fig. 6. Tectonic overview map of the Eastern Alps after Schmid et al. (2013) and geological map of the Tauern Window after Pestal et al. 

(2009).

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Total Pb ages (Fig. 8, Table 3) imply that the bright uraninite 

rim formed during the regional metamorphic overprint of  

the  gneiss  at  ~ 30  Ma,  although  the  three  single  point  ages 

measured in the rim sector of profile a (analyses a1–a3 in  

Table 3) do not give fully consistent ages. This is probably  

due to the presence of fine undigested remains of the precursor 

uraninite, which became apparent in the BSE image after 

regrinding (Fig. 7b and c). Profile c seems to contain lesser 

impurities and indicates an age of 26 ± 11 Ma for the uraninite 

rim. 

Fig. 7. BSE images for uraninite grain 1, before (B) and after (C) regrinding (see text for further information). White arrows indicate the mea-

sured chemical profiles.

Fig. 8. Chemical profiles and total Pb ages (with 1 σ errors) recorded in grain 1. Positions of profiles are shown in Fig. 7. Given domain ages 

(bold) are weighted average ages (95 % confidence level).

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The total Pb ages measured in the dark 

core zone of grain 1 pivot around a mean 

of  ~ 262  Ma.  This  is  consistent  with  

the Permian ages of most other uraninite 

grains in the rock (Finger et al. 2017). 

The total Pb age of the inner high-Th 

core falls in the same range (261 ± 12 Ma). 

The core thus represents a part of the Per-

mian growth history of the grain. 

Grain 2

This relatively large subhedral urani-

nite grain with a size of approximately 

20×40 µm is completely enclosed in epi-

dote. Along its periphery several small 

fluorite crystals with bright CL signals 

are identified (Fig. 9a). High resolution 

BSE  imaging  (Fig.  9 b  and  c)  reveals  

a complex patchy zonation involving dark 

grey domains, medium grey domains and 

light grey domains. The light grey 

domains are typically located along 

 

the grain margin, while interior parts of 

the grain are middle-grey. The contacts 

between the middle grey central domains 

and the light-grey marginal domains 

appear rather sharp, but the BSE contrast 

is not very strong. Embayment textures 

suggest that the light-grey uraninite has 

replaced the middle grey uraninite via 

coupled dissolution–reprecipitation (Putnis 

2002; Harlov et al. 2011). A thin dark 

grey crystal domain with elongated shape 

is seen in the left half of grain 2 (Fig. 9). 

It is sharply bordered and almost com-

pletely embedded in the middle grey sub-

stance. In the upper part of the grain in 

Fig 9, two puzzling small bright lenses 

are observed (labelled 1–2 in Fig. 9). 

Several short chemical profiles were 

recorded in different parts of grain 2  

in order to determine domain ages 

 

(Table 4). Profiles b and g are positioned 

in the dark grey domain. Both show  

a relatively high Th content (~ 11–12 wt. % ThO

2

), while U  

is  relatively  low  (~ 80  wt.  %  UO

2

). The higher Th/U ratios 

explain the darker BSE signal. The two measured domain ages 

are  257 ± 7  Ma  and  254 ± 8  Ma,  respectively,  indicating  

that the dark grey zone has a Permian formation age, like  

most other uraninite crystals in the K1 gneiss (Finger et al. 

2017).

The middle-grey zone is represented by profiles c, h and i 

(Figs. 9, 10). It has lower ThO

2

 (~ 7 wt. %) and higher UO

2

 

(84–87 wt. %) content. The measured domain ages are  

213 ± 8,  214 ± 7  and  200 ± 7  Ma.  The  middle  grey  part  of  

the crystal thus formed during the Triassic being clearly 

younger than the remnant dark grey domain.

The light grey marginal zone of grain 2 (profiles d and f in 

Figs. 9, 10) differs from the middle grey zone mainly by a 

lower Y content (2–2.5 vs. 2.5–3 wt. % Y

2

O

3

). The measured 

total-Pb ages are 199 ± 9 and 208 ± 7 Ma (Fig. 10). These age 

dates do not differ from those measured in the middle grey 

zone within the given analytical errors. Nevertheless, textures 

do clearly suggest that the light grey uraninite is at least 

slightly younger, as it has seemingly replaced the middle grey 

uraninite via coupled dissolution–reprecipitation. 

Grain 1 

profile/point SiO

2

CaO

Y

2

O

3

PbO

ThO

2

UO

2

Total

Age 

[Ma]

Error 

(1 σ)

Live time 

[s]

a1

0.81

<0.27

<0.63

0.71

6.35

88.69

96.56

59

27

60

a2

0.61

<0.27

1.41

0.04

7.50

91.83

101.40

3

25

60

a3

0.73

<0.27

2.00

0.90

6.52

86.87

97.01

76

27

60

a4

0.75

<0.27

3.80

2.71

7.39

81.74

96.38

238

29

60

a5

0.52

0.46

3.83

2.99

6.99

79.99

94.78

268

30

60

a6

0.52

<0.27

4.07

2.76

6.98

80.97

95.30

245

30

60

a7

0.64

<0.27

4.30

2.60

7.16

80.95

95.65

231

30

60

a8

0.67

<0.27

4.24

3.06

6.90

79.70

94.57

275

30

60

a9

0.54

<0.27

3.89

2.96

7.41

77.44

92.23

273

31

60

a10

0.66

0.48

3.87

3.20

7.85

79.06

95.13

288

30

60

a11

0.80

0.64

3.58

3.11

8.15

79.06

95.35

280

30

60

a12

0.62

<0.27

3.21

2.87

8.43

80.18

95.31

255

30

60

a13

0.61

0.46

3.05

3.11

9.44

80.76

97.42

273

30

60

a14

0.70

<0.27

3.28

2.74

10.85

79.38

96.95

244

30

60

a15

0.51

<0.27

3.22

2.94

11.31

80.17

98.15

259

30

60

a16

0.63

0.46

3.52

2.82

11.42

78.12

96.98

254

30

60

a17

0.59

<0.27

3.50

3.05

11.76

77.75

96.65

275

30

60

a18

0.66

<0.27

3.62

2.91

10.35

78.81

96.35

261

30

60

a19

0.69

<0.27

4.43

3.45

7.82

80.29

96.68

306

30

60

a20

0.68

<0.27

4.20

3.08

7.70

81.91

97.57

269

29

60

a21

0.68

0.46

4.57

3.25

6.84

82.31

98.11

283

29

60

a22

0.70

0.55

5.14

2.92

6.36

81.27

96.94

258

30

60

Mean

A4-22

261

15 (2σ)

b1

0.61

<0.18

4.49

2.84

12.79

74.60

95.33

266

22

120

b2

0.58

0.31

4.74

2.76

12.80

75.81

97.00

255

23

120

b3

0.60

0.48

4.51

2.97

12.66

74.60

95.82

278

23

120

Mean

266

26 (2σ)

c1

0.46

0.46

0.66

0.30

6.91

88.71

97.50

25

18

120

c2

0.49

0.33

0.90

0.15

6.88

87.87

96.61

12

19

120

c3

0.52

0.36

0.67

0.48

7.13

88.15

97.31

40

19

120

Mean

26

21 (2σ)

d1

0.51

0.43

4.32

2.80

7.01

80.77

95.85

249

21

120

d2

0.59

0.47

3.87

3.08

8.95

80.09

97.05

273

22

120

d3

0.55

0.49

3.68

3.07

9.70

79.64

97.12

273

22

120

d4

0.60

0.39

3.29

2.93

10.45

77.80

95.46

266

22

120

d5

0.68

0.35

3.58

2.71

10.05

78.81

96.18

244

22

120

Mean

261

19 (2σ)

e1

0.59

0.49

4.05

3.09

6.86

81.61

96.69

271

21

120

e2

0.61

0.29

4.23

2.83

7.37

81.07

96.39

250

21

120

e3

0.62

0.41

3.75

2.78

10.18

79.19

96.94

249

22

120

e4

0.59

0.33

3.12

2.72

10.28

79.08

96.13

244

22

120

Mean

254

21 (2σ)

Table 3: Chemical (wt. %) and age data for uraninite grain 1 from the Felbertal K1-gneiss 

(profiles a – e; see Figs. 7, 8). Single point age errors are 1 σ. Note that a shorter counting time 

(60 s detector live time) was used for profile a.

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In contrast to grain 1 we find no Paleogene rim zone in  

grain 2. However, one of the small bright lenses (profile a in 

Fig. 10) is very poor in Pb (PbO < 0.5 wt. %) and thus likely 

represents a young regional metamorphic crystal domain. It is 

possible that the “lens” is, in reality, the protrusion of a larger 

Paleogene rim domain that is hidden underneath the crystal. 

Profile a shows that there was no substantial exchange of Pb 

between the lens and the surrounding older uraninite by solid 

state diffusion. 

The second bright lens (profile e) is, surprisingly, of dif-

ferent origin. Here, the PbO contents are in the order of  

2.7 wt. % corresponding to a domain age of 215 ± 9 Ma. This 

lens thus formed during the Triassic, like most other parts of 

grain 2. Lens 2 has an unusually high UO

2

 content of close to 

92 wt. %, while ThO

2

 (~3 wt. %), and Y

2

O

3

 contents (<1 wt. %) 

are significantly lower compared to the other parts of grain 2. 

The distinctive chemical composition of lens 2 and its sharp 

outlines would imply that practically no solid state diffusion 

has taken place when the whole crystal underwent reheating to 

~ 500 °C in the Paleogene, in connection with Alpine regional 

metamorphism. 

Discussion and conclusions

Uraninite recrystallization by coupled dissolution– 

reprecipitation 

The two complexly zoned uraninite crystals of this study 

exhibit polygenetic, distinctly embayed zonation patterns, 

which are indicative for recrystallization by coupled disso-

lution–reprecipitation (Putnis 2002; Harlov et al. 2011). 

 

We have the suspicion that F-bearing fluids have caused  

the dissolution of the uraninite, because both recrystallized 

uraninite grains are bordered by small fluorite crystals.  

The important role of F for the solubility of U in melts and 

fluids has been pointed out many times (Keppler & Whyllie 

1990; Peiffert et al. 1996).

We would argue that, in our case, the process of uraninite 

recrystallization by coupled dissolution–reprecipitation invol-

ved only small local fluid volumes, because in the presence of 

larger volumes of U-undersaturated fluids, the dissolved U 

would have been most likely transported away and not repre-

cipitated at the spot. 

The fact that dissolution–reprecipitation driven recrystalli-

zation of uraninite obviously can happen a couple of times 

during the geological evolution of a rock, is extremely fortu-

nate for the field of geochronology. Multiply recrystallized 

uraninite can be viewed as a sensitive “hard disk” where 

 several geological events of an area are stored. Most impor-

tantly, this “uraninite hard disc” may include information of 

discrete low-T events that are commonly not recorded by other 

geochronometers (Finger et al. 2017). 

Geochronological robustness of uraninite

In-situ chemical dating of uraninite by means of electron 

beam excitation and X-ray spectroscopy is not new (Parslow 

et al. 1985; Bowles 1990). It was successfully used before 

chemical U–Th–Pb dating of monazite started, but was never 

widely applied. The method remained restricted to studies of 

uraninite and pitchblende in U deposits (Alexandre & Kyser 

2005; Cross et al. 2011; Shahin 2014; Ge et al. 2014) and to 

accessory magmatic uraninite in granitic rocks (Förster 1999; 

Fig. 9. CL and BSE images of grain 2. The CL image (A) reveals tiny fluorite crystals along the uraninite margin. The position of the measured 

profiles and a tentative delineation of different dissolution–reprecipitation zones in grain 2 are given in B and C.

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, 2018, 69, 6, 558–572

Kempe 2003; Hurtado et al. 2007; Cocherie 

& Legende 2007; Votyakov et al. 2013). 

A more widespread application would 

result from the study of uraninite micro-

crystals in metamorphic rocks. The increa-

sing use of electron microscopy in 

petro graphic studies shows that uraninite 

microcrystals occur in many metamorphic 

rocks. The potential of uraninite dating in 

metamorphic terranes will greatly depend 

on the geochronological robustness of 

 

the U–Th–Pb system in the mineral. We 

present here unequivocal evidence that 

even very small uraninite crystals can sur-

vive an amphibolite facies overprint, pre-

serving domains with intact U–Th–Pb 

ratios. It must be taken into account, 

though, that different age domains in poly-

genetic uraninite can be so small and inti-

mately intergrown with each other that 

they are very difficult to resolve and to 

analyse. Dating results can be problematic 

in such cases. The risk of Pb loss through 

solid state diffusion is low for unaltered 

uraninite at low to medium temperatures. 

However, fine scale uraninite alteration to 

U silicate (coffinite) and other secondary 

U minerals may cause Pb loss (Kempe 

2003). It is therefore imperative that ura-

ninite age data is always collected in com-

bination with detailed backscatter electron 

imagery and high-resolution composi-

tional profiles across single grains. 

U–Th–Pb geochronometry with sub-

micron- scale resolution and the potential 

of the SEM/EDX method

It is obvious that the geochronological 

investigation of micron-sized zoning pat-

terns in uraninite microcrystals requires  

an analytical resolution much finer than 

what is presently possible with Laser-

ICP-MS methods (typically 15–30 µm 

spots), and even with the latest and best 

SHRIMPs (around 5–10 µm spot resolu-

tion). Among all the currently available 

isotopic analytical methods, only the SIMS 

technique possesses the required spatial 

Grain 2 

profile/point SiO

2

CaO

Y

2

O

3

PbO

ThO

2

UO

2

Total

Age 

[Ma]

Error  

(1 σ)

Live time 

[s]

a1

0.31

0.41

2.53

2.71

10.03

81.70

97.69

236

29  60

a2

0.49

0.47

2.66

2.72

8.47

83.41

98.22

234

30  60

a3

0.46

0.40

3.20

2.64

10.72

79.98

97.40

234

31  60

a4

0.40

<0.27

2.71

0.62

11.43

81.20

96.35

55

28  60

a5

0.47

<0.27

2.58

0.38

10.12

84.60

98.15

32

27  60

a6

0.51

<0.27

2.38

0.24

9.49

83.95

96.57

21

28  60

a7

0.43

<0.27

2.29

0.36

10.09

84.59

97.76

31

27  60

a8

0.38

<0.27

2.27

0.30

10.26

85.42

98.63

26

27  60

a9

0.43

<0.27

2.20

0.45

9.94

85.71

98.74

38

27  60

a10

0.34

<0.27

2.18

0.33

10.76

84.06

97.67

29

27  60

a11

0.55

<0.27

2.75

0.15

9.40

84.90

97.74

12

27  60

a12

0.34

<0.27

2.38

0.28

9.52

84.94

97.46

24

27  60

a13

0.53

<0.27

2.61

0.61

9.68

85.16

98.58

52

27  60

a14

0.45

<0.27

3.19

1.17

11.43

80.88

97.12

104

29  60

a15

0.48

0.48

3.25

2.57

11.17

79.39

97.33

229

31  60

a16

0.48

0.49

3.32

2.73

11.39

78.01

96.42

247

32  60

a17

0.42

0.50

3.36

3.01

11.60

78.50

97.40

269

32  60

b1

0.48

0.53

3.07

2.94

10.95

80.72

98.69

257

21

120

b2

0.47

0.51

2.97

2.95

11.36

80.26

98.51

259

22

120

b3

0.60

0.51

3.00

3.00

11.30

79.53

97.94

266

22

120

b4

0.76

0.53

3.33

2.92

11.31

78.13

96.98

263

23

120

b5

0.84

0.38

3.29

2.94

11.57

78.63

97.65

263

22

120

b6

0.86

0.32

3.44

2.74

11.48

78.64

97.48

246

22

120

b7

0.67

0.49

3.12

2.89

11.48

79.33

97.98

257

22

120

b8

0.77

0.53

3.38

2.82

11.56

78.04

97.11

255

23

120

b9

0.75

0.63

3.07

2.75

11.55

79.54

98.29

244

22

120

b10

0.75

0.57

3.43

2.89

11.76

78.16

97.57

260

23

120

Mean

257 14 (2σ)

c1

0.52

<0.18

3.18

2.72

8.40

84.39

99.21

231

21

120

c2

0.73

<0.18

3.20

2.44

8.27

82.91

97.56

211

21

120

c3

0.75

<0.18

3.37

2.36

7.87

83.41

97.76

204

21

120

c4

0.73

<0.18

2.37

2.65

6.86

85.76

98.36

222

20

120

c5

0.71

<0.18

2.32

2.50

6.50

86.33

98.37

210

20

120

c6

0.70

<0.18

2.29

2.41

6.37

86.62

98.39

201

20

120

Mean

213 16 (2σ)

d1

0.43

<0.18

2.32

2.30

8.47

85.05

98.57

194

19

120

d2

0.48

0.24

2.07

2.39

8.62

83.70

97.51

205

20

120

d3

0.47

<0.18

2.21

2.50

8.19

83.31

96.70

216

20

120

d4

0.55

<0.18

2.44

2.19

8.31

82.90

96.39

190

20

120

d5

0.44

0.25

2.32

2.22

8.14

84.44

97.80

189

20

120

Mean

199 18 (2σ)

e1

0.34

<0.18

<0.39

2.63

3.10

92.62

98.69

208

18

120

e2

0.36

<0.18

0.54

2.71

3.49

92.57

99.66

214

18

120

e3

0.41

0.34

0.59

2.81

2.85

91.78

98.77

224

19

120

Mean

215 21 (2σ)

f1

0.44

<0.18

2.32

2.34

7.22

85.46

97.79

198

19

120

f2

0.43

<0.18

2.18

2.61

7.47

84.75

97.44

221

20

120

f3

0.45

<0.18

2.46

2.72

7.53

85.27

98.42

229

20

120

f4

0.39

0.26

2.14

2.39

7.44

85.09

97.71

202

20

120

f5

0.42

<0.18

2.26

2.52

7.17

84.56

96.93

215

20

120

f6

0.41

0.30

2.13

2.29

7.74

85.18

98.05

194

20

120

f7

0.45

0.25

2.39

2.34

7.19

84.81

97.43

199

20

120

f8

0.49

<0.18

2.46

2.52

7.19

84.71

97.37

214

20

120

f9

0.40

<0.18

2.16

2.35

7.62

85.57

98.10

198

20

120

Mean

208 13 (2σ)

g1

0.47

0.52

3.20

2.93

11.76

79.85

98.73

258

21

120

g2

0.44

0.64

3.16

2.72

11.73

78.29

96.99

245

22

120

g3

0.56

0.50

3.25

2.67

11.62

77.42

96.01

243

22

120

g4

0.42

0.49

2.99

2.91

12.01

78.11

96.93

261

22

120

g5

0.51

0.56

3.22

3.12

11.72

77.56

96.69

282

22

120

g6

0.55

0.52

3.07

2.70

11.79

77.99

96.61

244

22

120

g7

0.53

0.55

3.11

2.70

11.95

78.65

97.49

242

23

120

g8

0.56

0.35

3.19

2.81

11.71

78.16

96.79

253

22

120

Mean

254 16 (2σ)

h1

0.44

0.55

2.62

2.56

7.12

84.07

97.37

219

20

120

h2

0.38

0.63

2.59

2.70

7.32

84.20

97.80

230

21

120

h3

0.46

0.43

2.77

2.56

7.04

85.67

98.92

216

20

120

h4

0.47

0.54

2.64

2.75

7.12

84.41

97.92

235

21

120

h5

0.46

0.66

2.55

2.44

7.47

84.63

98.21

208

21

120

h6

0.50

0.52

2.78

2.38

7.00

84.38

97.56

204

21

120

h7

0.50

0.44

2.57

2.44

7.11

84.59

97.65

208

21

120

h8

0.54

0.79

2.81

2.30

7.22

85.26

98.91

194

21

120

h9

0.44

0.55

2.62

2.56

7.12

84.07

97.37

219

20

120

Mean

214 14 (2σ)

i1

0.50

0.49

2.73

2.40

6.92

85.38

98.41

203

20

120

i2

0.41

0.57

2.67

2.24

7.38

85.83

99.11

188

20

120

i3

0.51

0.42

2.83

2.53

6.79

84.39

97.48

217

21

120

i4

0.50

0.47

2.70

2.59

7.22

85.75

99.24

218

20

120

i5

0.49

0.39

2.76

2.37

6.97

87.02

99.99

196

20

120

i6

0.53

0.39

2.79

2.26

6.91

85.45

98.34

191

21

120

i7

0.54

0.66

2.79

2.26

6.87

85.58

98.70

191

20

120

Mean

215 15 (2σ)

Table 4: Chemical (wt. %) and age data for 

uraninite grain 2 from the Felbertal K1 gneiss 

(profiles a – i; see Figs. 9 and 10). Single point 

age errors are 1 σ. Note that a shorter counting 

time (60 s detector live time) was used for 

 profile  a.

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resolution to analyse micron-sized crystal domains. The Cameca 

nano-SIMS, for instance, allegedly can achieve a spatial reso-

lution of 50 nm (Kilburn & Wacey 2014). However, practical 

applications of Th–U–Pb dating with nano-SIMS measure-

ments have not yet been made with spot sizes < 2–3 µm (Fayek 

et al. 2002; Stern et al. 2005; Koike et al. 2014) as it is metho-

dically difficult, even with the latest nano-SIMS devices. 

Thus, there appears to be currently no real alternative to elec-

tron beam dating on such small scales.

So far, the ~1 µm spot size obtained by electron micro-

probe-based dating in the classical 15 kV set-up was the lower 

limit for quantitative Th–U–Pb geochronometry. In this study, 

we have applied for the first time finer-scale total Pb dating on 

the submicron level by means of low-voltage SEM/EDX tech-

niques. We have used so-called standardless EDX analysis for 

uraninite dating. This method relies on factory-based intensity 

ratios (counts per wt. %) that were initially measured for all 

elements at 20 kV on appropriate element standards (see  

Table 1). Therefore, the term “standardless” may be somewhat 

misleading. These intensity values must be “updated” before 

every analytical session by means of a monitor standard.  

We did this by measuring Si count rates and background 

 signals on a silicon wafer. 

We know that EDX analysis, especially when run in stan-

dardless mode, is viewed by many workers in the field with 

scepticism with regard to whether it can provide sufficient 

precision and accuracy for U–Th–total Pb dating. The main 

problems are that the factory-based intensity values need to be 

extrapolated to other beam conditions (in our case, 8 kV) by 

using fundamental parameters, and that intensity ratios 

between elements may change when the instrument ages. 

However, based on systematic measurements on a number of 

reference materials, we propose here that standardless SEM/

EDX analyses with the Oxford INCAEnergy system can pro-

vide geologically meaningful uraninite ages with a reasonable 

precision and accuracy. The correctness of the primary stan-

dardization can easily be tested by means of control measure-

ments on synthetic UO

2

 and ThO

and other element standards. 

If these standards give systematically to high or too low 

 values, this could be reasonably considered with a later 

 external correction and recalibration. A certain analytical 

problem could result from the fact that Th Mα and U Mα lines 

overlap in the EDX spectrum (Fig. 2). This involves the risk of 

an inaccurate U and Th determination in Th rich uraninite. 

However, as mentioned earlier, slight errors in the U and Th 

contents will have little effect on the age dates.

Fig. 10. Chemical profiles and total Pb ages recorded in grain 2 (positions of profiles are shown in Fig. 9). Shown error bars are 1 σ. Given 

domain ages (bold) are weighted average ages (95 % confidence level).

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, 2018, 69, 6, 558–572

By far the most important factor for successful U–Th– 

total Pb uraninite dating is certainly the accuracy of the Pb 

determination. Fortunately, the latter can be sensitively con-

trolled with uraninite reference materials, but a variety of such 

“age standards” with different compositions is needed to mini-

mize the risk of systematic errors caused by inaccurate peak 

interference or the background correction. A minor systematic 

error in the age, on the order of a few million years, may still 

remain unrecognized though, due to the limited precision of 

EDX analysis. Either way, the EDX data quality appears to be 

definitely sufficient to study essential features of uraninite 

recrystallization and alteration. The special construction of 

scanning electron microscopes, directed towards a particularly 

high beam stability, is advantageous for high spatial resolution 

measurements, as is the ability of EDX detectors to analyse all 

elements simultaneously.

A severe limitation of our analytical setup and of the EDX 

method, in general, concerns the dating of very young ura-

ninite  (< 20  Ma),  where  radiogenic  Pb  is  close  to  or  even 

remains below the detection limit. WDX analysis would cer-

tainly have more potential here. Moreover, WDX analysis 

could provide superior precision for U–Th–total Pb uraninite 

dating. It seems possible that, with a good WDX setup, 

 uraninite spot analyses can be made with errors of less than  

1 million years. This would theoretically permit high-preci-

sion dating of uraninite. However, due to the danger of small-

scale intra-crystal age heterogeneities, such application would 

only make sense in combination with a high spatial analytical 

resolution and a very high beam stability, respectively. 

Whether electron microprobes can achieve equally good beam 

stability behaviour as the state-of-the-art FE-SEMs still needs 

to be tested.

Acknowledgements:  Noreen Vielreicher is thanked for 

 stylistic and linguistic improvements of the text. Hans-Jürgen 

Förster, Axel Renno and Olaf Tietz donated hand specimens of 

uraninite bearing granite types from the Erzgebirge, which 

served as valuable uraninite “age standards”. Reinhard 

 

 

Wagner and Werner Paar provided uraninite crystals from  

the Mitterberg ore deposit. Funding in the frame of the program 

“Forschungspartnerschaften Mineralrohstoffe” of the Austrian 

Geological Survey (GBA) and by the Austrian Science Fund 

(P 31901-N29) is gratefully acknowledged. The paper bene-

fited from competent reviews by Daniel Harlov, Bernhard 

Schulz and Armin Zeh.

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