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Earth Evolution Sciences, Vol. 14, pp. 3-8, March 1, 2020
In-situ EPMA dating of monazites in granulites from collisional orogens in
southern India and southern Africa
Hikaru Kadowaki a and Toshiaki Tsunogae b,c,*
a Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan
b Faculty of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan
c Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa
* Corresponding author
Abstract
The EPMA U-Th-Pb dating technique of monazite has
been newly developed using a JEOL JXA-8530F micro-
probe at the Chemical Analysis Division of the Research
Facility Center for Science and Technology, the Univer-
sity of Tsukuba. A natural cheralite from southern India
was adopted for the standard material of Th, together with
other oxide, phosphate, and metal standards. The applica-
tion of the technique on a composite monazite grain in a
metasediment from the Trivandrum Block, southern India,
gave a metamorphic age of 544 ± 17 Ma, which is nearly
consistent with the published near-peak metamorphic
age of monazites (555.1 ± 8.1 Ma) from the same sample
analyzed by LA-ICP-MS. Application of the analytical
technique on monazites from the Limpopo Complex in
southern Africa yielded a Paleoproterozoic metamorphic
age (1923 ± 19 Ma), also consistent with published re-
sults. The dating technique can be thus applicable to vari-
ous high-grade metamorphic rocks for unraveling thermal
history of orogenic belts.
Keywords: monazite geochronology, CHIME, Trivandrum
Block, Limpopo Complex
Introduction
Monazite often contains significant quantity of thori-
um and uranium, while the amount of initial Pb is little
compared to radiogenic Pb (Williams et al., 1983; Corfu,
1988; Suzuki and Kato, 2008). Therefore, EPMA (electron
microprobe) U-Th-Pb dating (or CHIME dating; Suzuki
and Adachi, 1991a, 1991b, 1994) of monazite has attract-
ed many geologists because of its advantages of quick-
ness and relatively high reliability (Montel et al., 1996;
Williams et al., 2006; Cocherie and Albarede, 2001). Par-
ticularly, the technique allows us to perform in-situ analy-
sis of complex monazite grains in thin sections with high
spatial resolution. The dating technique has been there-
fore widely applied to various metamorphic rocks for
unraveling texture-based thermal history of orogenic belts
(e.g., Santosh et al., 2003). In this study, we newly devel-
oped in-situ EPMA dating technique of monazite using a
JEOL JXA-8530F microprobe installed at the Chemical
Analysis Division of the Research Facility Center for Sci-
ence and Technology, the University of Tsukuba. The age
results are compared with those in published papers for
the evaluation of the technique.
Analytical procedure
The monazite dating was carried out using JEOL JXA-
8530F microprobe under an accelerating voltage of 20
kV, beam currents of 100 nA for monazites and 10 nA for
standard materials, and a beam diameter of 5 micron. The
data were regressed using an oxide-ZAF correction pro-
gram supplied by JEOL.
The standard materials adopted for quantitative anal-
ysis of monazite are galena (PbS) for Pb, metal uranium
for U, natural cheralite for Th, end-member synthetic
phosphates (XPO4) for rare-earth elements (La, Ce, Pr,
Nd, and Sm) and Y, CePO4 for P, and wollastonite for
Ca and Si. The composition of natural cheralite from
southern India, which was adopted as our standard, was
analyzed at JEOL laboratory at Akishima (Tokyo) on 16th
of December 2019 using JEOL JXA-8230 microprobe.
The cheralite composition is shown in Table 1. Peak and
background counting times and the X-ray lines adopted
in this study are summarized in Table 2. The X-ray lines
used are UMβ, PbMα, ThMα, PKα, SiKα, CaKα, NdLβ,
SmLα, CeLα, LaLα, PrLβ, and YLα. They were carefully
evaluated to avoid interferences. The counting time (peak
+ background) was 200 + 100 for U and Pb, 60 + 30 for
Th, and 20 + 10 for each of the other elements.
The procedure of CHIME age calculation is summa-
4
rized in Suzuki and Adachi (1991a, 1991b, 1994). For the
calculation, we employed the software developed by Kato
et al. (1999) with York (1966) method and decay con-
stants from Steiger and Jäger (1977). The concentrations
(in wt.%) of ThO2, UO2, and PbO obtained by EPMA
analysis as well as relative analytical errors (in %) of the
oxides were adopted. The relative analytical errors (E in
%) of the EPMA analysis are determined by calculation
using upper- and lower-background intensities (BG+ and
BG-, respectively, in cps/µA), net intensities (Net in cps/
µA), current value (C in nA), and measurement times (τ
in second) for Th, U, and Pb as follow;
BG BG
C
Ibackground
+
()
+−
()
×=
2
1000 ()
(1)
()
()
NetC
Ipeak
×
=
1000
(2)
II I
peak background
()
()
+=
(3)
E
II
II
background
background
(%) =+
()
×
−
()
××
()
()
τ
τ
100
(4)
where I is intensity in cps. The E values are used for age
calculations based on least squares method of York (1966).
Below, we briey summarize the procedure of CHIME
dating method based on Suzuki and Adachi (1991a,
1991b, 1994). In the rst step, an apparent age ‘t’ of each
spot is determined by the following calculation;
()
λ λ
PbO
W
ThO
Wt
UO
W
t
Pb Th
U
=−
()
{}
+
+
2
232
2235 2
1
137 88
exp
exp exp
λ
.338
138 88 1
t
()
−
. (5)
where WPb is molecular weight of Pb, and WTh and WU are
molecular weight of Th (= 264) and U (= 270), respec-
tively. WPb = 224 is adapted for ThO2-rich minerals (e.g.,
monazite), whereas WPb = 222 is adapted for UO2-rich
minerals (e.g., zircon). In this equation, PbO, ThO2, and
UO2 are concentration of PbO, ThO2, and UO2 (wt.%), re-
spectively. λ232 (= 4.9475×10-11/year), λ235 (= 9.8485×10-10/
year), and λ238 (= 1.55125×10-10/year) are decay constants
which are mentioned in Steiger and Jäger (1977). Current
isotopic ratio of U (238U/235U = 137.88; Steiger and Jäger,
1977) is used in this equation.
Using the apparent age (t), apparent amount of total
ThO2 (ThO2*) and UO2 (UO2*) are computed for ThO2-
rich and UO2-rich minerals as follow;
Table 1. Composition of cheralite adopted as
the standard material for thorium.
average error
SiO25.02 ± 0.10
CaO 1.40 ± 0.01
ThO226.96 ± 0.28
UO20.61 ± 0.01
P2O522.72 ± 0.17
Ce2O319.96 ± 0.21
La2O37.78 ± 0.09
Pr2O32.69 ± 0.04
Nd2O311.88 ± 0.09
Sm2O31.29 ± 0.03
Total 100.31
Table 2. List of analytical conditions of monazite.
Element Channel Crystal X-ray line Peak position
(nm)
Background
(+)
Background (-) Peak counting
time
Background
counting time
Si 2 TAP Kα77.285 5 5 20 10
Ca 4 PETH Kα107.061 5 5 20 10
U 4 PETH Mα118.49 0.7 0.5 200 100
Th 4 PETH Mα132.018 5 4 60 30
Pb 1 PETH Mα169.269 5 4 200 100
P 2 TAP Kα66.679 5 5 10 5
La 1 LIFH Lα185.313 5 5 20 10
Ce 1 LIFH Lα178.975 5 5 20 10
Pr 1 LIFH Lβ157.023 2 2 20 10
Nd 1 LIFH Lβ150.65 1.2 5 20 10
Sm 1 LIFH Lβ138.893 1 1 20 10
Y 4 PETH Lα206.241 5 5 20 10
5
()
λ λ
ThOThO UO W
Wt
t
Th
U
22
2
232
232 23
1
137 88
∗=+ ⋅
()
−
{}
⋅
+
exp
exp exp
λ
.88
138 88 1
t
()
−
. (6)
(for ThO2-rich mineral)
()
{}
UO UO
ThOW t
Wt
U
Th
22
2 232
235
138 88 1
137 88
∗=+
⋅
()
−
{}
()
+
.
.
exp
exp ex
λ
λpp λ238 138 88t−. (7)
(for UO2-rich mineral)
If all the portions in a single mineral grain is monoge-
netic, contain the same amount of initial Pb but dierent
amounts of Th and U, and have remained in a closed
system, all the analytical data will lie on a straight line
(isochron) with a slope (m) and an intercept (b) in a
ThO2* or UO2* versus PbO plot;
PbOmThOb
2 (8)
(for ThO2-rich mineral)
PbOmUO b
2 (9)
(for UO2-rich mineral)
The best-t regression line is calculated based on the pro-
cedure of York (1966), taking account of uncertainties (E)
in the microprobe analyses. Then, the first approximate
value of age (T) from the slope (m) of equations will be
calculated by the following equations;
TInm
W
W
Th
Pb
=+
11
232
λ (10)
(for ThO2-rich mineral)
mW
W
TT
U
Pb
=
()
+
()
−
exp expλ λ
235 238
137 88
138 88 1
.
. (11)
(for UO2-rich mineral)
The intercept (b) of the regression line is inferred to rep-
resent the concentration (in wt.%) of the initial PbO. Re-
calculation of ThO2* or UO2* with estimated T instead of
t provides reliable estimate of age and initial PbO.
Results and discussion
Khondalite from the Trivandrum Block
The Southern Granulite Terrane in India is composed
of several Mesoarchean to Neoproterozoic granulite
blocks intersected by Neoproterozoic to Cambrian suture/
shear zones (e.g., Drury et al., 1984). The Trivandrum
Block in the southern part of the terrane is composed
dominantly of metasediments (khondalite and leptynite)
and charnockite (Geological Survey of India, 1995a,
1995b). The block underwent high- to ultrahigh-temper-
ature metamorphism (e.g., Chacko et al., 1987; Cenki et
al., 2004; Morimoto et al, 2004; Tadokoro et al., 2008)
during Neoproterozoic (e.g., Santosh et al., 2005, 2006).
Kadowaki et al. (2019) obtained LA-ICP-MS monazite
age of an ultrahigh-temperature (920–1030°C and 6.0–7.6
kbar) khondalite from the block, and reported a broad age
range of 612 ± 11 Ma to 461 ± 7 Ma with two age peaks
at 555.1 ± 8.1 Ma and 501.9 ± 8.5 Ma. Their U-Pb anal-
ysis was done using LA-ICP-MS instrument at the Uni-
versity of Adelaide. In this study, we adopted the same
khondalite (sample KP5H) examined by Kadowaki et al.
(2019) to compare the results of CHIME and LA-ICP-MS
dating methods and evaluated the reliability of age results
obtained in this study.
A thin section was used for the in-situ dating. The
monazite examined in this study occurs as rounded
to partly irregular mineral included in coarse-grained
cordierite. A back-scattered electron (BSE) image of the
monazite grain is shown in Figure 1a together with the
positions of analyzed spots and ages. The grain is compo-
sitionally heterogeneous with thorium-rich (ThO2 = 26.9
wt.%, bright BSE) and thorium-poor (ThO2 = 7.5 wt.%,
dark BSE) portions. The analyzed spot ages vary from
559 Ma to 517 Ma. The ages obtained from the Th-rich
portion (537-531 Ma) are nearly consistent with the Th-
poor portion (559-534 Ma). Figure 2a shows a ThO2*-
PbO plot with a calculated age and error in one sigma for
all the analyzed nine spots. All the data are plotted on an
isochron showing the age of 541 ± 17 Ma, which is near-
ly consistent with the older age peak (555.1 ± 8.1 Ma) of
Kadowaki et al. (2019). As the analyzed monazite occurs
as an inclusion in cordierite rather than in the matrix, the
grain was probably not aected by the later ca. 502 Ma
thermal event.
Pelitic granulite from the Limpopo Complex
The Limpopo Complex has been regarded as a classic
example of collisional orogens formed by amalgamation
of the Kaapvaal and Zimbabwe Cratons during Neopro-
terozoic (e.g., Roering et al., 1992), although some recent
studies pointed out multiple collisional events during Ne-
oarchean to Paleoproterozoic (e.g., Kröner et al., 2018).
Particularly, the Central Zone of the Limpopo Complex is
dierent from other portions of the complex because the
zone underwent regional thermal event during Paleopro-
terozoic (ca. 2.0 Ga; Jaeckel et al.,1997). For example,
Buick et al. (2006) obtained SHRIMP weighted mean
6
207Pb-206Pb monazite age of 2028 ± 3 Ma from garnet–
cordierite–orthoamphibole gneisses from the western
Central Zone, South Africa. Chudy et al. (2008) also ob-
tained similar monazite age of 2015 ± 8 Ma from pelitic
rocks of the Krone Metamorphic terrane, South Africa,
by LA-ICP-MS. They regarded the age as the timing of
Paleoproterozoic metamorphic overprint under amphib-
olite-facies condition. Slightly younger monazite age of
2002 ± 10 Ma was obtained for monazites from partially
melted Fe-rich metagreywacke from Lose quarry in Bot-
swana as the timing of incongruent biotite melting related
to high-grade metamorphism (Chavagnac et al., 2001).
Samples C54D and C25A were collected from the
Zimbabwean side of the Central Zone. The dominant
lithologies of the area are pelitic gneiss, quartzite, mac
granulite, banded iron-formation, granitoid gneiss, and
calc-silicate, which occur as banded gneisses with abun-
dant N-S-trending foliation (e.g., van Biljon and Legg,
1983). Sample C54D is a garnet-bearing leucocratic rock
comprising coarse-grained plagioclase, quartz, K-feld-
spar, garnet, and biotite. Its eld occurrence as a leuco-
cratic layer in pelitic gneiss suggest partial-melting origin
of the rock. Sample C25A is a typical pelitic granulite that
consists of garnet, biotite, quartz, plagioclase, K-feldspar,
cordierite, and sillimanite, with accessory spinel, ilmenite,
zircon, and monazite. Foliation of the rock is well dened
by elongated biotite akes. In this study we analyzed one
representative monazite grain from each sample.
The analyzed monazite from sample C54D is subhe-
dral and medium grained (~100 micron), and occurs as
an inclusion in coarse-grained plagioclase. A BSE image
of the monazite grain is shown in Figure 1b together with
the positions of analyzed spots and ages. Similar to the
monazite from southern India, the grain is chemically
heterogeneous with Th-rich portions (bright BSE) and
Th-poor (dark BSE) portions, although the variation of
Th content is less (ThO2 = 21.2 to 24.8 wt.%) than sample
KP5H from southern India. The analyzed spot ages vary
from 2021 Ma to 1787 Ma, but mostly 2010-1970 Ma. In
contrast, the analyzed monazite from sample C25A, oc-
curring as an inclusion in rim of garnet, is compositional-
ly nearly homogeneous with no obvious heterogeneity in
BSE image (Figure 1c). Its ThO2 content shows a narrow
range of 4.1 to 5.6 wt.%. The analyzed spot ages vary
from 2108 Ma to 1981 Ma with an average age of 2052
Ma.
Figure 2b shows a ThO2*-PbO plot with a calculated
age and error in one sigma for all the analyzed spots from
the two samples. All the data are plotted along an isoch-
ron showing the age of 1928 ± 19 Ma. The age is slightly
younger than the obtained monazite ages (2020-2003 Ma)
from the Central Zone, although similar ca. 1.9 Ga age of
1982 ± 38 Ma has been obtained based on garnet Sm-Nd
age of garnet-bearing leucosome from Lose quarry (Bot-
swana) as the timing of high-grade metamorphism (Cha-
vagnac et al., 2001). It has to be noted that previous ages
have been reported from South African and Botswanan
side of the Central Zone, while this is the rst report of
monazite age from the Zimbabwean side. Therefore, the
timing of high-grade metamorphism in the Zimbabwean
side might be slightly younger.
Conclusion
The EPMA U-Th-Pb dating technique of monazite has
been developed using a JEOL JXA-8530F microprobe at
the Chemical Analysis Division of the Research Facility
Center for Science and Technology. We applied the meth-
od to monazites from the Trivandrum Block (southern
Fig. 1. Back-scattered electron images of monazites discussed in this study. (a) Monazite in sample KP5H from the Trivandrum Block.
(b) Monazite in sample C54D from the Limpopo Complex. (c) Monazite in sample C25A from the Limpopo Complex. Circles
indicate analytical points. Scale bars indicate 10 microns. Numbers indicate analytical numbers with ages in Ma (in parenthesis).
7
India) and the Limpopo Complex (Zimbabwe), and ob-
tained isochron ages of 541 ± 17 Ma and 1928 ± 19 Ma,
respectively, which are nearly consistent with available
ages from the regions. The results of this study conrmed
that the dating technique can be applicable to Paleoprote-
rozoic to Neoproterozoic high-grade metamorphic rocks
for unraveling texture-based thermal history of orogenic
belts.
Acknowledgement
Partial funding for this project was produced by a
Grant-in-Aid for Scientic Research (B) from Japan Soci-
ety for the Promotion of Science (JSPS) (No. 18H01300
and No. 19H19020) to Tsunogae. We thank Prof. Y. Ar-
akawa and Mr. Y. Takamura for their constructive review
comments. Mr. Y. Takamura is also acknowledged for his
support on the standard analysis.
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