Holmes (1911) first established a method for dating minerals through normal wet chemical analyses on the basis of the assumption that in- herited Pb was negligible at the time of crystal- lization of minerals. Holmes (1931) later dealt exhaustively with the application of radioactivity to the measurement of geologic time. The method was used by many researchers and was referred to as the chemical age. After the isotopic method had been widely accepted, the inherited lead in uraninite was confirmed to be less than 0.2 % (cf. Holmes, 1960). Although in his 1931 paper Holmes presented a correct calculation method which was very complex, a simpler and more approximate age calculation method was applied for the minerals. Mineral age was simply obtained by Approximate age (Ma)a*Pb/(U+bTh) (1) where elemental concentrations are in wt%. The numerical values, a and b, were given as 7600 and 0.36, respectively, in Holmes (1931). Since then, many researchers have tried to modify the values of a and b. Bowles (1990) and Suzuki and Adachi (1990) have developed accurate age cal- culations for uraninite and monazite, respective- ly. Bowles (1990) assumed that the initial Pb concentration is negligible as did Holmes, where- as Suzuki and Adachi (1990) insisted that the ini- tial Pb is obtained by an isochron line. Both the calculations yield essentially the same age re- sults. From mineral composition analyzed by an electron microprobe analyser which is now a ubiquitous machine, many researchers have ob- tained mineral ages using the above calculation methods. In recent calculations of mineral age, the initial Pb has been treated as negligible in amount as was first done by Holmes (1911 & 1931). U-Th-total Pb ages of Uraninite and Thorite from Granititc Rocks in the Japanese Islands Kazumi Yokoyama 1 , Masako Shigeoka 1 , Atsushi Goto 2 , Kentaro Terada 2 , Hiroshi Hidaka 2 and Yukiyasu Tsutsumi 1 1 Department of Geology and Paleontology, National Museum of Nature and Science, Shinjuku-ku, Tokyo 169–0073, Japan [E-mail: [email protected]] 2 Petrology Course, School of Science, University of Hyogo (Branch of Himeji Institute of Technology), 2167, Shosha Himeji, Hyogo 671–2201, Japan 3 Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima, 739–8526, Japan * Author for correspondence: [email protected]Absract Dating based on total UO 2 , ThO 2 and PbO contents of uraninite and anhydrous thorite from granitic rocks was performed using an electron microprobe analyser (EPMA) at the National Museum of Nature and Science, Tokyo. After careful comparison with the ages obtained by iso- topic methods (SHRIMP and K-Ar), this study confirmed that the EPMA-derived ages of uraninites and thorites are consistent with ages obtained by isotopic methods. The ages of the uraninite sam- ples in this study ranged from 1.5 to 240 Ma. Standard deviation of uraninite age was 0.6 at 2.4 Ma, 0.7 at 5.2 Ma, 1.1 at 66.5 Ma and around 4 at over 180 Ma. As the standard deviation of urani- nite is expected to be less than 1 Ma from count statistics, the high deviation for the old uraninite may be due to partly metamicted conditions. Thorite is the most common radiogenic mineral in granitic rocks. It was totally hydrated in the granitic samples older than 150 Ma and highly hydrat- ed even in Tertiary granite. The age obtained from thorite was determined to be up to 120 Ma. Key words : uraninite, thorite, age, EPMA, granite Introduction Bull. Natl. Mus. Nat. Sci., Ser. C, 36, pp. 7–18, December 22, 2010
Transcript
Holmes (1911) first established a method fordating minerals through normal wet chemicalanalyses on the basis of the assumption that in-herited Pb was negligible at the time of crystal-lization of minerals. Holmes (1931) later dealtexhaustively with the application of radioactivityto the measurement of geologic time. Themethod was used by many researchers and wasreferred to as the chemical age. After the isotopicmethod had been widely accepted, the inheritedlead in uraninite was confirmed to be less than0.2 % (cf. Holmes, 1960). Although in his 1931paper Holmes presented a correct calculationmethod which was very complex, a simpler andmore approximate age calculation method wasapplied for the minerals. Mineral age was simplyobtained by
Approximate age (Ma)�a*Pb/(U+bTh) (1)where elemental concentrations are in wt%. The
numerical values, a and b, were given as 7600and 0.36, respectively, in Holmes (1931). Sincethen, many researchers have tried to modify thevalues of a and b. Bowles (1990) and Suzuki andAdachi (1990) have developed accurate age cal-culations for uraninite and monazite, respective-ly. Bowles (1990) assumed that the initial Pbconcentration is negligible as did Holmes, where-as Suzuki and Adachi (1990) insisted that the ini-tial Pb is obtained by an isochron line. Both thecalculations yield essentially the same age re-sults. From mineral composition analyzed by anelectron microprobe analyser which is now aubiquitous machine, many researchers have ob-tained mineral ages using the above calculationmethods. In recent calculations of mineral age,the initial Pb has been treated as negligible inamount as was first done by Holmes (1911 &1931).
U-Th-total Pb ages of Uraninite and Thorite from Granititc Rocks in the Japanese Islands
Absract Dating based on total UO2, ThO2 and PbO contents of uraninite and anhydrous thoritefrom granitic rocks was performed using an electron microprobe analyser (EPMA) at the NationalMuseum of Nature and Science, Tokyo. After careful comparison with the ages obtained by iso-topic methods (SHRIMP and K-Ar), this study confirmed that the EPMA-derived ages of uraninitesand thorites are consistent with ages obtained by isotopic methods. The ages of the uraninite sam-ples in this study ranged from 1.5 to 240 Ma. Standard deviation of uraninite age was 0.6 at 2.4Ma, 0.7 at 5.2 Ma, 1.1 at 66.5 Ma and around 4 at over 180 Ma. As the standard deviation of urani-nite is expected to be less than 1 Ma from count statistics, the high deviation for the old uraninitemay be due to partly metamicted conditions. Thorite is the most common radiogenic mineral ingranitic rocks. It was totally hydrated in the granitic samples older than 150 Ma and highly hydrat-ed even in Tertiary granite. The age obtained from thorite was determined to be up to 120 Ma.Key words : uraninite, thorite, age, EPMA, granite Introduction
Bull. Natl. Mus. Nat. Sci., Ser. C, 36, pp. 7–18, December 22, 2010
In electron microprobe analyses (EPMA),monazite and zircon in young igneous or meta-morphic rocks exhibit levels of radiogenic Pbthat are too low to calculate an accurate age, dueto low U and Th contents. For monazite, a num-ber of papers have treated sufficiently old rocks,i.e. Paleozoic or older (e.g. Montel et al., 1996;Williams et al., 1999; Foster et al., 2000a; San-tosh et al., 2006). On the other hand, uraninite,thorianite, thorite and huttonite contain high con-centrations of radiogenic Pb even in Tertiaryrocks. Uraninite ages were obtained by Bowles(1990) from various rocks ranging in age from19 to 1800 Ma, by Santosh et al. (2005) for 500Ma gneiss, and by Hurtado et al. (2007) for Ter-tiary granite. Yokoyama et al., (2010) obtainedages of thorite, uraninite and thorianite fromsamples of Vietnumese river sands. Naemura etal. (2008) obtained ages of thorianite in peri-dotite. Foster et al. (2000b) analyzed thorite andhuttonite in samples of beach sands from NewZealand, and obtained ages from 2.3 Ma to 210Ma. In most of these dating studies, the EPMAages were consistent with those obtained by iso-topic methods or the EPMA ages of monazite.
In the Japanese Islands, uraninites in graniticrocks have been analyzed using wet chemicalmethods and were calculated using the simpleequation of Holmes (Iimori, 1941; Nagashimaand Nagashima, 1960). The EPMA age method,i.e. U-Th-total Pb or chemical methods, has at-tracted attention for its quickness, high spatialresolution and relatively high precision as well asisotopic methods. Granitic rocks in the JapanseseIslands generally vary in age from 1.5 to 280 Ma.In this paper, we attempted to confirm the calcu-lated chemical ages of uraninite and thorite usingan EPMA in the National Museum of Nature andScience, and demonstrate the validity of themethod which yields age results comparable tothose obtained by the isotopic methods such asSHRIMP and K-Ar method. A study of monaziteages comparing the results obtained by the sameEPMA machine and isotopic ages has been pre-viously reported by Santosh et al. (2006).
Samples and methods
a: Sample preparation and chemical analysesMost of the granitic rocks in the Japansese Is-
lands have been analyzed using the K-Ar method(e.g. Kawano and Ueda, 1966: Shibata et al.,1984: Uchiumi et al., 1990). Cretaceous graniticrocks are the most ubiquitous, whereas Tertiarygranitic rocks occur mostly in Southwest Japanalong the Pacific Ocean. Permian to Triassicgranites occur only in the Hida Terrane. Ages offar older granitic rocks, Cambrian to Silurian,have been obtained using the SHRIMP method(Watanabe et al., 1995; Sakashima et al. 2003),but these samples were usually from small plu-tonic bodies. As such old granitic rocks do notpreserve uraninite and anhydrous thorite, weanalyse uraninites and thorites in eleven graniticrocks with ages from 1.5 Ma to 280 Ma (Fig. 1and Table 1). Four samples were from the HidaTerrane. Three samples were from Cretaceous re-gion, and the other three from Tertiary and Qua-ternary rocks.
All samples (except for the Quaternary aplite)have not been previously examined by isotopicmethods. Six samples are newly analyzed bySHRIMP in this study. The other four rocks are
8 Kazumi Yokoyama et al.
Fig. 1. Localities of the granitic rocks used forthe EPMA age measurements.
collected from the same outcrops or igneous plu-tons as those analyzed by isotopic methods. TheEPMA-derived ages of uraninites and thorites arecompared with the ages as given by SHRIMP orthe K-Ar methods (Table 1).
As uraninite and thorite are rare minerals ingranitic rocks, the rock samples were crushedinto fine fractions in a stainless-steel stamp mill.Powdered samples (�100 mm mesh size) werecleaned using water to remove the dust particles,and then dried in an oven. The magnetic mineralswere then removed using a hand magnet. Zircongrains were hand picked for SHRIMP analyses.Other heavy fractions were separated using meth-ylene iodide with specific gravity of 3.3, andmounted on glass slides using epoxy resin. Pol-ished grain mounts were used for the study ofgrain characteristics and EPMA analyses.
The chemical analyses were carried out usinga JEOL-8800 electron microprobe. Uraninite andthorite are at first fully analyzed to determine thepresence of major and trace elements (P, Si, Ce,Pr, Nd, Sm, Gd, Dy, Er, Tm, Yb, Lu, Y, U, Th, Pband Ca). The operating conditions of the micro-probe were 15 kV accelerating voltage and 0.02mA probe current. Counting time on each analyt-ical point was 15 seconds at peak and 5 secondsat backgrounds for all the elements. PRZ correc-tions (modified ZAF) were applied for the analy-ses. Standards used were wollastonite for Si and
Ca, synthetic (REE)P5O14 for REE and P. Thestandards for U, Th and Pb were synthesizedUO2, ThO2 and natural crocoite (PbCrO4), re-spectively. Element uranium was calculated asUO2. All the analytical points were selectedunder back-scattered images to avoid cracks andthe metamicted parts of the sample. The analyti-cal result is shown in Table 2.
In the age analyses, the elements were selectedto minimize the damage to the minerals. Theywere P, Si, Y, U, Th, Pb and Ca. The probe oper-ating conditions were 15 Kb and 0.1 mA. Count-ing times for Pb were 100 and 30 seconds at peakand backgrounds, respectively, whereas 50 sec-onds at peak and 20 seconds at backgroundswere used for U and Th analyses. Counting timesfor other elements were the same as those in thenormal analyses. The U Ma , Th Ma and Pb Malines were used in the U, Th and Pb analyses, re-spectively, and the spectral interferences of theTh and Y lines with the Pb Ma line, and the Thline with the U Ma line were corrected (cf. Pyleet al., 2002).
b: Uraninite and thoriteIn granitic rocks, uraninite (UO2) and thorite
(ThSiO4) are usually rare minerals. Uraninite oc-curs mostly as a euhedral grain in the heavy frac-tion (Fig. 2), rarely surrounded by pyrite. Thoriteis commonly altered into hydrous thorite. Anhy-
U-Th-total Pb ages of Uraninite and Thorite from Granititc Rocks in the Japanese Islands 9
Table 1. Summary of the analysed samples and ages obtained using EPMA and the other methods.
* same sample, ** sample is collected from the same outcrop, *** from the same granitoid body
10 Kazumi Yokoyama et al.
Tabl
e2.
Che
mic
al c
ompo
siti
ons
of u
rani
nite
and
tho
rite
in
gran
itic
roc
ks.
Ana
lyti
cal
cond
itio
ns a
re d
iffe
rent
fro
m a
ge a
naly
ses.
Cur
rent
: 2�
10�
8A
, M
easu
rem
ent
atpe
ak a
nd b
ack
grou
nd a
re 1
5 an
d 5
seco
nds,
res
pect
ivel
y.
sam
ple
9811
2501
9810
0304
SK53
SK53
IK
2S-
9S-
6S-
6T
D-2
TD
-2
No.
min
eral
uran
inite
n�27
*ur
anin
iten�
14*
uran
inite
n�12
*th
orite
n�12
*ur
anin
iten�
11*
thor
iten�
14*
uran
inite
n�7*
thor
iten�
18*
uran
inite
n�15
*th
orite
n�15
*
av.
1 sav
. 1 s
av.
1 sav
. 1 s
av.
1 sav
. 1 s
av.
1 sav
. 1 s
av.
1 sav
. 1 s
Com
men
t (wt%
)(w
t%)
(wt%
)(w
t%)
(wt%
)(w
t%)
(wt%
)(w
t%)
(wt%
)(w
t%)
P 2O
50.
01
0.01
0.
13
0.29
0.
01
0.02
1.
24
0.37
0.
01
0.01
0.
24
0.09
0.
02
0.01
0.
43
0.33
0.
02
0.01
1.
62
0.78
Si
O2
0.07
0.
13
0.11
0.
18
0.08
0.
06
17.9
8 0.
13
0.05
0.
04
18.3
2 0.
18
0.04
0.
02
18.2
1 0.
25
0.03
0.
02
17.6
7 0.
33
UO
285
.39
2.46
80
.56
3.46
84
.18
0.90
15
.60
4.40
93
.54
1.28
17
.33
7.80
81
.04
0.27
16
.01
5.26
79
.94
1.35
16
.40
1.61
T
hO2
8.09
1.
30
10.1
7 2.
42
8.35
0.
59
58.6
8 3.
52
2.24
0.
27
63.1
4 8.
18
8.94
0.
90
63.9
3 6.
39
7.89
1.
52
60.1
7 1.
90
PbO
3.
07
0.27
2.
42
0.35
1.
44
0.02
0.
54
0.06
1.
10
0.02
0.
05
0.01
0.
05
0.01
0.
01
0.01
0.
01
0.01
0.
00
0.01
C
aO
0.71
0.
12
0.78
0.
15
0.31
0.
03
0.02
0.
01
0.15
0.
02
0.02
0.
01
0.16
0.
04
0.02
0.
01
0.19
0.
14
0.01
0.
01
Y2O
30.
57
0.47
1.
89
0.65
2.
10
0.27
2.
49
1.02
0.
92
0.07
0.
38
0.12
5.
17
0.71
0.
58
0.44
6.
83
1.19
2.
46
1.17
C
e 2O
30.
30
0.10
0.
60
0.08
0.
45
0.09
0.
35
0.16
0.
20
0.05
0.
11
0.07
0.
74
0.06
0.
12
0.05
0.
75
0.27
0.
18
0.05
N
d 2O
30.
13
0.07
0.
54
0.13
0.
47
0.07
0.
42
0.17
0.
14
0.06
0.
08
0.05
0.
86
0.15
0.
12
0.05
0.
61
0.24
0.
14
0.06
Sm
2O3
0.07
0.
06
0.23
0.
08
0.27
0.
03
0.18
0.
09
0.11
0.
05
0.04
0.
04
0.51
0.
07
0.05
0.
05
0.41
0.
09
0.11
0.
05
Gd 2
O3
0.06
0.
05
0.20
0.
08
0.25
0.
05
0.29
0.
11
0.12
0.
06
0.08
0.
04
0.54
0.
08
0.07
0.
06
0.45
0.
08
0.16
0.
07
Dy 2
O3
0.09
0.
06
0.28
0.
09
0.37
0.
07
0.34
0.
15
0.18
0.
05
0.05
0.
05
0.89
0.
07
0.09
0.
08
1.10
0.
21
0.29
0.
14
Er 2
O3
0.04
0.
04
0.21
0.
10
0.24
0.
08
0.22
0.
09
0.10
0.
05
0.04
0.
05
0.59
0.
10
0.06
0.
05
0.86
0.
19
0.21
0.
13
Tm
2O3
0.01
0.
02
0.06
0.
06
0.05
0.
05
0.05
0.
04
0.02
0.
03
0.02
0.
03
0.13
0.
02
0.03
0.
04
0.13
0.
06
0.03
0.
03
Yb 2
O3
0.04
0.
05
0.19
0.
12
0.17
0.
05
0.13
0.
08
0.05
0.
06
0.03
0.
03
0.40
0.
12
0.04
0.
04
0.75
0.
29
0.15
0.
10
Lu 2
O3
0.07
0.
06
0.08
0.
05
0.07
0.
06
0.05
0.
04
0.04
0.
05
0.05
0.
04
0.13
0.
04
0.05
0.
05
0.23
0.
09
0.07
0.
06
Tota
l 98
.73
0.83
98
.46
1.16
98
.79
0.43
98
.57
0.51
98
.95
0.97
99
.95
0.34
10
0.20
0.
44
99.7
9 0.
49
100.
20
0.35
99
.66
0.44
*: n
umbe
r of
ana
lyse
d sp
ot
drous thorite is mostly preserved partly in hydrat-ed thorite grains (Fig. 2). In addition to the ob-servation under BSE image, hydrous thorite isrecognized by the presence of CaO content andlow overall concentration.
Natural uraninite is composed mainly of UO2
and ThO2 with subordinate amounts of Y and
heavy REEs. Anhydrous thorite is composedmainly of SiO2, ThO2 and UO2 with subordinateamounts of Y and lantanoids. In the routineanalyses for age determination, we analyzed onlyseven elements: P, Si, Y, U, Th, Pb and Ca. Underthese conditions, total amounts of uraninite andthorite were found to be in a range from 90 to
U-Th-total Pb ages of Uraninite and Thorite from Granititc Rocks in the Japanese Islands 11
Fig. 2. BSE images of the uraninites and thorites. A: fresh uraninite in TD-2, B: fresh uraninite in SK53, C:highly metamicted uraninite in a sample 98112501. D: fresh thorite in TD-2, E: partly hydrated thorite in S-9(dark parts are hydrous thorite), F: thorite surrounded by hydrous thorite in SK53.
100%. The lower values were usually Y-richuraninites and thorite. The total amount of unan-alyzed REE elements was roughly equivalent toY2O3 contents (Fig. 3). The analyses of U, Thand Pb that yielded low totals were checkedagainst Y2O3 contents.
c: Theoretical basis of chemical datingUraninite and thorite are abundant in U-Th
system radioisotopes, which finally disintegrateinto the stable Pb isotope through a and b de-cays. As the contents of nonradiogenic Pb inuraninite and thorite are negligible comparedwith the amount of radiogenic Pb, the age (t) ofindividual mineral was calculated using equation(2)
�����������������������(2)
where W symbolizes the gram-molecular weightof each oxide (WPb�224, WTh�264, WU�270).l is the decay constant of each isotope(l232�4.9475�10�11 yr�1, l235�9.8485�10�10
yr�1 and l238�1.55125�10�10 yr�1 according toSteiger and Jager, 1977). Although the solutionof the complex equation for age (t) is given indifferent forms by each researcher (e.g. Bowles1990: Suzuki and Adachi, 1990), the results arethe same.
In younger granites the age is roughly calculat-ed using the equation (1) of Holmes (1931) or, as
far as uraninite is concerned, the age may be cal-culated as PbO�80 (Ma) as discussed later. Thisshows that the standard deviation of each analyti-cal point depends on the count statistics of Pb.
d: Zircon age (SHRIMP method)Zircon grains from the samples and the zircon
standard QGNG, with a TIMS U/Pb age of1850�2 Ma (2s), were cemented in an epoxyresin and polished until their centers were largelyexposed on a flat surface. U–Pb dating was per-formed using a SHRIMP II installed at Hiroshi-ma University, Japan. Instrumental conditionsand measurement procedures are described inSano et al. (2000). The spot size of the primaryion beam was about 20 mm. Both back-scatteredimages and cathodoluminescense images wereused to select sites for SHRIMP analysis. The206Pb/238U ratios of the samples were calibratedusing the empirical relationship described byClaoue-Long et al. (1995). In this procedure, it isessential to subtract initial Pb from measured Pbto estimate the age accurately. The measured206Pb/204Pb ratio was used for the correction ofinitial Pb, whose isotopic composition was as-sumed using a single-stage model (Compston etal., 1984).
Results
Data for U, Th and Pb in each analytical pointfor the age analyses are listed in Table 3. Afterage was obtained from each point, all the data inthe granitic rock were summarized as an averageand standard deviation. The standard deviation isusually 1 sigma calculated from a Gaussian dis-tribution in probability theory. All the age data ofeleven granitic rocks in the Japanese Islands aresummarized in Table 1 as well as the results bySHRIMP analyses.
Thorite was found in most of the granitic rocksin the Japanese Islands. All the samples in Table1 contained thorite but most of them were alteredstrongly into hydrous thorite, especially in theold rocks. Hence, thorite age was obtained onlyfrom the young granitic rocks. The standard de-
PbO
W
ThO
WTht) 1}
UO
W
exp(Pb
2
Th
2
U
235
� � �
�
{exp( 232
UUt) 138exp( Ut)
1391
235��
⎡
⎣⎢⎢
⎤
⎦⎥⎥
12 Kazumi Yokoyama et al.
Fig. 3. Correlation of Y2O3 content and totalother REE content in uraninite and thorite.
U-Th-total Pb ages of Uraninite and Thorite from Granititc Rocks in the Japanese Islands 13
Table 3. UO2, ThO2, PbO contents and age for each analytical point in uraninites and thorites in the granitic rocks.
Uraninite in Hida 98112501No. UO2 ThO2 PbO age (Ma)
viation for uraninite was much lower than that ofthorite, simply due to the very high U contentwhich produced radiogenic Pb. Hurtado et al.(2007) obtained 0.04 Ma as 2s for 18 Ma urani-nite. Numerically, the error depends on the probecurrent, measurement time and number of analy-ses. In addition, error may depend on machinedrift, standard material conditions and surfaceconditions. In the present study, the standard de-viations of uraninites were found to be around 4Ma at around 200 Ma, 1 Ma at 66 Ma and lessthan 1 Ma at 2.4 Ma and 5.2 Ma, comparable tothose obtained by the isotopic methods.
Correlation of the EPMA and isotopic ages ispresented in Fig. 4, showing a concordant rela-tionship between them. The coincidence of theages is evidence that EPMA age is comparable to
those yielded by the isotopic methods. Using thesame machine in the National Museum of Natureand Science and the same analytical conditions,
U-Th-total Pb ages of Uraninite and Thorite from Granititc Rocks in the Japanese Islands 15
Fig. 4. Correlation of ages obtained by EPMAand isotopic methods.
Fig. 5. A & B: Partly recrystallized uraninite in the grey granite from the Hida Terrane (KM1331-1). C: the old-est uraninite, 2.46 Ga, included in a monazite grain from the Yellow River.
such a concordant relationship had previouslybeen reported for uraninite and huttonite in India(Santosh et al. 2003) and thorianite (ThO2) inBohemian Massif (Naemura et al. 2008). Hut-tonite is a monoclinic polymorph of thorite. InIndia, uraninite and huttonite grains in gneiss,granulite and granite are zoned in the same rangefrom 460 to 580 Ma as the coexisting monazite.In Bohemian Massif, the EPMA age of thorianitein peridotite was 333.8�4.5 Ma, quite compara-ble with the values of 338�1 Ma and 339.8�2.6Ma obtained by isotopic methods for the associ-ated granulites. As a rare case, we found urani-nite included in monazite grain from the riversands (Fig. 5). The uraninite age was determinedto be 2.46 Ga, slightly lower than the monaziteage from 2.5 to 2.56 Ga.
During study of the other granitic rocks in theJapanese Islands, bimodal age distributions ofuraninite have been occasionally found. Age dataof grey granite from the Hida Terrane are shownin Fig. 5 and 6. One uraninite grain exhibitedages of 191�1.8 Ma and 233�3.4 Ma. The zon-ing texture of this sample has not been recog-nized, but high and low Pb areas are distributedin a patchy fashion. Any micro-scale diffusionbetween these areas has not been observed. Theolder age is similar to that of coexisting monaziteage at 237�6.8 Ma (Fig. 6).
Discussion
Age data obtained by EPMA analyses are con-sistent with those of the isotopic methods. Stan-dard deviation of the derived ages is around 1Main the range from 1.5 to 100 Ma, comparablewith that of SHRIMP dating. The EPMA systemis the most commonly used machine for mineralanalyses and has high spatial resolution, i.e. 1–2m , compared with 10–20 m of SHRIMP. Althoughuraninite and thorite are trace minerals and occurin selected rocks, it is more preferable to useyoung granite with uraninite or thorite. The coin-cidence also supports the assumption that the ini-tial Pb content is negligible in the minerals at thetime of the mineral crystallization, as stated firstby Holmes (1911). As reported in previous stud-ies using the same machine, this method is alsoapplicable to thorianite and huttonite (Naemuraet al., 2007: Santosh et al., 2003).
Granitic rocks have a thermal history, i.e. cool-ing or overprinting by later stage granite. Hence,the age result cannot be accepted simply as therepresentative age of the igneous stage. Since ingeneral the closure temperature of zircon isaround 900°C, it has been considered that theSHRIMP age of zircon in granitic rock shows theage of an early plutonic event. Monazite has alsohigh closure temperature, i.e. around 800°C. Onthe other hand, there is no experimental datumabout the closure temperature of uraninite andthorite. In the grey granite from the Hida Terrane(Fig. 6), uraninite has bimodal age distributionand monazite is consistent with the older of thetwo ages. These facts apparently show that urani-nite is lower in closure temperature than mon-azite. However, uraninite grains in gneiss andgranulite from India are similar in range to theassociated monazite (Santosh et al., 2003). Al-though a few problems including closure temper-ature and meaning of the obtained age have notsolved, the age data by EPMA are reliable.
Many researchers have reported the ages ofminerals such as monazite and uraninite analyzedusing EPMA, assuming that their initial Pb con-tents were negligible. It is amazing that the pre-
16 Kazumi Yokoyama et al.
Fig. 6. Bimodal age distribution of uraninite inthe grey granite from the Hida Terrane(KM1331-1). Associated monazite is similarin age to the older uraninite.
sent method is essentially the same as that devel-oped first by Holmes (1911). The ages obtainedby the simplified calculation of Holmes (1931)are similar for thorite and higher for uraninitethan those given by the present complex calcula-tion (Fig. 7). The difference is mainly due to theeffect of 235U. In 1931, 235U content in total Uwas estimated to be less than 0.28%, far less thanthe present accepted value, 0.72%. In uraninite, amore simplified equation from equation (1) is
Age (Ma)�80�PbO (wt%)This is only applicable to young uraninite (Fig.
8). This simplified equation is also similar to thatpresented by Holmes (1911), i.e. age�8200�
Pb/U (Ma) where Pb and U are wt%. This simpleequation means that the standard deviation is to-tally dependent on the error of Pb analyses. In
our normal analytical conditions for uraniniteand thorite, the standard deviation is around 1Ma for the samples dating from less than 100Ma. High probe current and measurement timewill reduce the error.
Acknowledgements
We wish to express our thanks to Prof. M.Akaogi and Prof. T. Kawasaki for syntheses ofUO2 and ThO2 and also to Prof. K. Kunugiza forgiving us granitic rocks from the Hida Terrane.
References
Bowles, J. F. W. (1990) Age dating of individual grains ofuraninite in rocks from electron microprobe analyses.Chemical Geology, 83: 47–53.
Claoue-Long, J. C., Compston, W., Roberts, J. and Fan-ning, C. M. (1995) Two Carboniferous ages: a compari-son of SHRIMP zircon dating with conventional zirconages and 40Ar/39Ar analysis. In Geochronology, TimeScales and Global Stratigraphic Correlation (Berggren,A., Kent D. V., Aubly, M.-P., Hardenbol, J. Eds). Soci-ety for sedimentary Geology Special Publication, 54,Society for Sedimentary Geology, Tulsa, USA, 3–21.
Compston, W., Williams, I. S. and Meyer, C. (1984) U–Pbgeochronology of zircons from lunar breccia 73217using a sensitive high mass-resolution ion microprobe.Journal of Geophysical Research, 89: Supplement,B525–534.
Foster, G., Kinny, P., Vance, D., Prince, C. and Harris, N.(2000a) The significance of monazite U–Th–Pb agedata in metamorphic assemblages: A combined studyof monazite and garnet chronology. EPSL, 181:327–340.
Foster, H. J., Harlov, D. E. and Milke, R. (2000b) compo-sition and Th-U-total Pb ages of huttonite and thoritefrom Gillespie’s beach, Soouth island, New Zealand.Canadian Mineralogist. 38: 675–684.
Holmes (1911) The association of lead with uranium inrock minerals and its application to the measurement ofgeological time. Proc. Roy. Soc., London, A 85:248–256.
Holmes, A. (1931) Radioactivity and geological time.Bull. National Research Council., 80: 124–459.
Holmes, A. (1960) A revised geological time-scale.Transactions of the Edinburgh Geological Society, 17:183–215.
Hurtada, H., Chatterjee, N., Ramezani, J., Hodges, K. andBowring, S. (2007) Electron microprobe chemical dat-
U-Th-total Pb ages of Uraninite and Thorite from Granititc Rocks in the Japanese Islands 17
Fig. 7. Relationships between present age calcu-lation and simplified age of Holmes (1931) foruraninite and thorite. Pb, U, and Th as wt%.
Fig. 8. Relationships between present age calcu-lation and simplified age, 80�PbO, for urani-nite. PbO as wt%.
ing of uraninite as a reconnaissance tool for leucogran-ite geochronology. Nature Precedings �http://hdl.han-dle.net/10101/npre.2007.655.1�
Iimori, T. (1941) The microgranular uraninite from Iisakaand its geologic age. Scientifc paper of Inst. Phy.Chem. Research, 89: 208–210.
Kawano, Y. and Ueda, Y. (1966) K-Ar dating on the ig-neous rocks in Japan (IV)-Granitic rocks in northeast-ern Japan. J. Min. Petr. Econ. Geol., 56: 41–55. (inJapanese with English abstract).
Montel, J. M., Doret, S., Veschambre, M., Nicollet, C. andProvpst, A. (1996) Electronic dating of monazite.Chemical Geology, 131: 37–53.
Nagashima, O. and Nagashima, K. (1960) Rare elementminerals in Japan. Koubutsu shumino kai press. (inJapanese)
Naemura K., Yokoyama, K., Hirajima, T. and Svojtka, M.(2008) Age determination of thorianite in phlogopite-bearing spinel-garnet peridotite in the Gföhl Unit,Moldanubian Zone of the Bohemian Massif. Journal ofMineralogical and Petrological Sciences, 103: 285–290.
Pyle, J. M., Spear, F. S., Wark, D. A., (2002). Electron mi-croprobe analysis of REE in apatite, monazite andxenotime: protocols and pitfalls. Phosphates: Geo-chemical, Geobiological and Materials Importance, Re-views in Mineralogy and Geochemistry, 337–362.
Sakashima, T., Terada, K., Takeshita, T. and Sano, Y.(2003) Large-scale displacement along Median Tecton-ic Line, Japan: evidence from SHRIMP zircon U-Pbdating of granites and gneiss from the South Kitakamiand paleo-Ryoke belt. Journal of Asian Earth Science,21: 10199–1039.
Sano, Y., Hidaka, H., Terada, K., Shimizu, H. and Suzuki,M. (2000) Ion microprobe U–Pb zircon geochronologyof the Hida gneiss: Finding of the oldest minerals inJapan. Geochemical Journal 34: 135–153.
Sano, Y., Tsutsumi, Y., Terada, K. and Kaneoka, I. (2002)Ion microprobe U–Pb dating of Quaternary zircon: im-plication for magma cooling and residence time. J. Vol-canol. Geotherm. Res. 117: 285–296.
Santosh, M., Yokoyama, K., Biju-Sekhar, S. and Rogers,J. J. W., (2003) Multiple tectonothermal events in thegranulite blocks of Southern India revealed fromEPMA dating: Implications on the history of supercon-tinents. Gondwana Research 6: 29–63.
Santosh, M., Morimoto, T. and Tsutsumi, Y. (2006)Geochronology of the khondalite belt of TrivandrumBlock, Southern India: Electron probe age and implica-tions for Gondwana tectonics. Gondwana Research, 9:261–278.
Shibata, K., Kato, Y. and Mimura, K. (1984) K–Ar agesof granites and related rocks from the northern Kofuarea. Bull. Geol. Surv. Japan, 35: 19–24. (in Japanesewith English abstract)
Steiger, R. H. and Jäger, E. (1977). Subcommission ongeochronology: convention on the use of decay con-stants on geo- and cosmochronology. Earth and Plane-tary Science Letters, 36: 359–362.
Uchiumi, S. Uto, K. and Shibata, K. (1990) K–Ar age re-sults-3-New data from the Geological Survey of Japan.Bull. Geol. Surv. Japan, 41: 567–575. (in Japanese withEnglish abstract)
Watanabe, T., Ireland, T, Tainosho, Y. and Nakai, Y.(2000) Zircon U–Pb sensitive high mass-resolution ionmicroprobe dating of granitoids in the Ryoke metamor-phic belt, Kinki District, Southwest Japan. Island Arc,9: 55–63.
Watanabe, T., Fanning, C. M., Uruno, K. and Kano, H.(1995) Pre-Middle Silurian granitic magmatism and as-sociated metamorphism in northern Japan: SHRIMP U-Pb zircon chronology. Geological Journal, 30: 273–280.
Williams, M. L., Jercinovic, M. J. and Terry, M. (1999)Age mapping and dating of monazite on the Electronmicroprobe deconvoluting multistage tectonic histories.Geology, 27: 1023–1026.
Yokoyama, K., Tsutsumi, Y., Nhung, N. T. and Quynh, P.V. (2010) Age distribution of monazites from the ninerivers of Vietnam. Mem. Natl. Mus. Nat. Sci., Tokyo,46: 97–108.
![AReviewonInfraredSpectroscopyofBorateGlasseswith ...ISRN Ceramics 3 Table 1: The molar compositions of PbO-B 2O 3 of various glass samples [34]. No. PB-1 PB-2 PB-3 PB-4 PB-5 PB-6 PB-7](https://static.documents.pub/doc/80x56/611d3182f1d5a60ff83c4a72/areviewoninfraredspectroscopyofborateglasseswith-isrn-ceramics-3-table-1-the.jpg)
