Naturalistae 28: 7-13 (Feb. 2024)© 2024 by Okayama University of Science, PDF downloadable at http://www1.ous.ac.jp/garden/

Short Report

Yttrium mapping on fossil teeth by micro-XRF
for apatite U–Pb dating
Kazumasa AOKI1*, Kentaro CHIBA2, Tetsu KOGISO3, Kaisei MURAKAMI2
4& Khishigjav TSOGTBAATAR
Abstract: Selecting appropriate measurement points is crucial for apatite U–Pb dating of fossil teeth
due to the impact of post-fossilization alterations. Previous studies have suggested that variations
in yttrium (Y) concentrations within each sample can be qualitative indicators of such alterations(Y-screening method). To verify the validity of this method, ideal measurement areas for U–Pb datingwere selected from Y mapping using a micro-focus X-ray fluorescence spectrometer (micro-XRF)
on Late Cretaceous Tarbosaurus teeth with published apatite U–Pb ages, and trace element analysis
and U–Pb dating of apatite was performed by laser ablation and inductively coupled plasma mass
spectrometry (LA-ICP-MS). Two ages of 64.3 ± 3.4 Ma and 61.0 ± 11.0 Ma were obtained fromtwo regions of relatively different Y concentrations in a sample that had been reported as 66.7 ± 2.5
Ma. Similarly, the sample with the reported age of approximately 30 Ma yielded ages of 51.5 ± 2.7
Ma and 57.3 ± 6.1 Ma in the areas with relatively low Y concentrations. These results show that Y
concentration and an apatite U–Pb age are correlated, at least within the same sample, and support
the usefulness of the Y-screening method. Although more measurements are needed to obtain conclusive proof, the Y micro-XRF mapping would be a helpful tool for non-destructive visualization of
post-fossilization alteration in tooth fossils and could lead to the selection of suitable measuring points
for LA-ICP-MS apatite U–Pb dating.

I. Introduction

fossilization age (Greene et al. 2018, Tanabe etal. 2023). In this study, therefore, we combined Y
mapping using a microfocus X-ray fluorescence
spectrometer (micro-XRF) with U–Pb dating andtrace element analysis of two Tarbosaurus bataar
teeth performed by laser ablation inductively
coupled plasma mass spectrometry (LA-ICPMS) to validate the efficacy of this screening
method. We would like to report it here as a shortcommunication and emphasize the usefulness of
Y micro-XRF mapping for non-destructive visualization of post-fossilization alterations in fossil
teeth and for the selection of optimal measuring
points for LA-ICP-MS apatite U–Pb dating.

The lack of a rigorous temporal framework forcertain strata is often a barrier to elucidating theevolutionary history of organisms in the fossil
record. Recently, the uranium–lead (U–Pb) dating of calcites in caliches and apatites from teeth
has attracted attention as a valuable techniqueto constrain the temporal range of fossil-bearing
strata (e.g., Fassett et al. 2011, Greene et al. 2018,
Kurumada et al. 2020, Barreto et al. 2022, Tanabe
et al. 2023). However, one major concern of
this technique, especially when applied to tooth
fossils, is its susceptibility to post-fossilization.
Occasionally, the obtained apatite U–Pb ages ofteeth are affected by secondary redistributionof U and common Pb contamination and may
not always directly represent the depositional
ages of fossil-bearing strata (e.g., Romer 2001,
Kocsis et al. 2010, Greene et al. 2018). Previous studies have reported that the differencesin yttrium (Y) concentration within a sample
can serve as a proxy for post-fossilization alterations (Y-screening method) and the effect ofalteration manifesting as the rejuvenation of the

II. Samples
The joint expedition of Okayama Universityof Science and the Institute of Paleontology,
Mongolian Academy of Science was conducted
at Bugin Tsav in the Gobi Desert, Mongolia in2018 (Ishigaki et al. 2018; Fig. 1). In this locality, the middle Nemegt Formation is exposed(Jerzykiewicz & Russell 1991, Eberth 2018).
The depositional age of this formation is poorly

1.

Center for Fundamental Education, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama-shi, Okayama 700-0005, Japan. 岡山理科大学教育推進機構基盤教育センター，〒700-0005 岡山県岡山市北区理大町1-1.
2.
Department of Biosphere-Geosphere Science, Faculty of Biosphere-Geosphere Science, Okayama University of Science, 1-1 Ridai-cho, Kita-ku,
Okayama-shi, Okayama 700-0005, Japan. 岡山理科大学生物地球学部生物地球学科，〒700-0005 岡山県岡山市北区理大町1-1.
3.
Graduate School of Human and Environmental Studies, Kyoto University, Yoshidanihommatsu-cho, Sakyo-ku, Kyoto-shi, Kyoto 606-8501, Japan. 京
都大学大学院人間・環境学研究科，〒606-8501 京都府京都市左京区吉田二本松町.
4.
Institute of Paleontology, Mongolian Academy of Sciences, Ulaanbaatar 14200, Mongolia. モンゴル科学アカデミー古生物学研究センター，〒14200
モンゴル国ウランバートル.
*Correspondence: Kazumasa AOKI, E-mail: kazumasa@ous.ac.jp-7-$2.,&+,%$.2*,62085$.$0, 762*7%$$7$5

Fig. 1. Geological map of Mongolia. D 6LPSOL¿HGPDSRI0RQJROLDZLWKWKHVXUURXQGLQJFRXQWULHV E /RFDWLRQRI&UHWDFHRXVIRUPDWLRQDORXWFURSVin the Gobi Desert. Locations of the Bugin Tsav in the Nemegt Basin are also shown (Tanabe et al. 2023).

constrained due to the lack of igneous rocks
available for absolute dating (Shuvalov 2000) and
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to be late Campanian to Maastrichtian based on
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(2023) reported apatite U–Pb ages using the teethof a large theropod dinosaur Tarbosaurus, collected at Bugin Tsav during the expedition, which
supports the previously suggested relative age ofthe formation. We used two of the specimens in
Tanabe et al. (2023) for further analyses in thisstudy: MPC-D 100s/002 (Fig. 2a) and MPC-D
100s/003 (Fig. 2b). Further details of the samples
are given in Tanabe et al. (2023).
III. Analytical Method
The Y-mapping analyses were conducted using
an energy-dispersive X-ray spectrometer (EDS)
DWWDFKHGWRDPLFUR;5) ;*79+RULED
.\RWR-DSDQ DW.\RWR8QLYHUVLW\$WXEHYROWDJH
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ysis. The samples were scanned in approximately
DQGȝPLQWHUYDOV SL[HOVL]H GHSHQGLQJRQ
the range of the sample size. The intensity of Y
was determined from the cumulative intensity of
the repeated analyses (up to four times). The anal-8-

ysis of the elemental maps was conducted using
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method are given in Machida et al. (2021).
In-situ trace element analyses of the fossil
WHHWK ZHUH SHUIRUPHG ZLWK DQ L&$354 VLQgle-collector quadrupole ICP-MS (Thermo SciHQWL¿F86$ FRXSOHGWRDQ$U)([FLPHU/DVHU
Analyte G2 (Teledyne-Photon Machines, USA)at Okayama University of Science (OUS). The
laser was operated with a repetition rate of 5 Hz
DQGDODVHUVSRWGLDPHWHURIȝPZKLFKSUR
vided an estimated power density of the sample
RI  -FP2. The pulse count was 150 shots.
After shooting with the shutter closed for 30 s
while the laser warmed up, the analytical areas
were ablated for 30 s. Prior to conducting theanalysis, the analyzed spots were ablated using
DSXOVHRIWKHODVHUZLWKDGLDPHWHURIȝPWR
remove potential contaminants on their surfaces.
The ICP-MS was optimized using continuous
DEODWLRQRID1,67650JODVVVWDQGDUGWR
provide maximum sensitivity while maintaining
low oxide formation (232Th16O/232Th < 1%). 43Ca
was used as an internal standard element. In thisstudy, the Y of interest is only mentioned. Moredetailed analytical procedures are given in Tanabe
et al. (2023).
In-situ apatite U–Pb dating was also carriedout with LA-ICP-MS at OUS. The measurement

Yttrium mapping on fossil teeth

Fig. 2. Fossil teeth samples from Tarbosaurus bataar and the Y-images obtained by micro-XRF mapping. (a) Photographic image of MPC-D
100s/002. (b) Photographic image of MPC-D 100s/003. (c) Y-image of MPC-D 100s/002. (d) Y-image of MPC-D 100s/003. Scale bars equal 5 mm.

conditions were the same as those for the traceelement analysis. The analyzed masses included
six nuclides of 202Hg, 204Pb, 206Pb, 207Pb, 232Th
and 238U. The background intensities were subtracted from the subsequent intensities at theablation. OD306 apatite (Huang et al. 2015) andthe 401 apatite (Thompson et al. 2016) wereused as an external standard for the correctionof the U/Pb ratios and as a secondary standard,
UHVSHFWLYHO\$OVR1,67650JODVV -RFKXP
et al. 2005) was analyzed as the external standard
for the correction of the 207Pb/206Pb ratios. All
uncertainties are quoted at a 2-sigma level. 235U
was calculated from 238U using a 238U/235U ratioof 137.818 (Hiess et al. 2012). The apatite U–Pb
age was calculated using Isoplot/Ex 4.15 (Ludwig
2012, and its update) based on the determinationof a lower intercept age from the intersection of
the Tera-Wasserburg (TW) concordia curve anda regression line from the data and anchoredinitial common Pb (Chew et al. 2011, Aoki etal. 2021). The initial common Pb isotopic ratio
was assumed to be the terrestrial isotopic ratio
SURYLGHG E\ 6WDFH\  .UDPHUV   7KH
U–Pb analyses for the 401 (ca. 530 Ma) were
FRQGXFWHG¿YHWLPHVDQGHDFKDQDO\VLV\LHOGHG
the anchored-regression arrays (initial common
Pb = 0.872) in a TW diagram with a lower intercept age of 523 ± 24 Ma. Further details of ourapatite U–Pb analysis method are given in Tanabe

et al. (2023).
IV. Results
1. MPC-D 100s/002
Figure 2c shows the areas with Y intensities,
higher in red and lower in black, as obtained by
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suggests a heterogeneous distribution of regionswith high and low Y concentrations, and a radial
pattern of regions with high Y concentrations
can be also observed. The trace element analysis demonstrates that the Y concentrations are±ȝJJ 1  LQWKHEODFNUHJLRQVDQG
±ȝJJ 1  LQWKHUHGUHJLRQV )LJF
and Table 1). The U–Pb dating derived near themeasurement points of the trace element analysis
showed that the obtained ages from the black andred regions were 64.3 ± 3.4 Ma (N = 14, MSWD
= 1.3) and 61.0 ± 11.0 Ma (N = 7, MSWD = 5.5),
UHVSHFWLYHO\ )LJ D E  7KH ¿W RI GLVFRUGLDmodel line was anchored at the common Pb ratio
RIWKH/DWH&UHWDFHRXV ±6WDFH\ .UDPHUV 
2. MPC-D 100s/003
The red and black colors in Figure 2d obWDLQHGE\WKHPLFUR;5)PDSSLQJPHDVXUHPHQW
represent regions of relatively high and low Y
intensities, indicating that the heterogeneous

AOKI, CHIBA, KOGISO, MURAKAMI & TSOGTBAATAR
Table 1. LA-ICP-MS apatite U–Pb and Y concentration data.
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$&#%"! %!$%!%$&# %"! %and radial Y distributions are similar to MPC-D
100s/002 (Fig. 2c). The Y concentration of
±ȝJJ 1  DQG±ȝJJ 1 5) were obtained from the black and red regionsin Figure 2d, respectively (Table 1). The U–Pb
ages obtained from the model line anchored at theterrestrial isotopic ratio of the Late Cretaceous
6WDFH\ .UDPHUV ZHUH0D 1
= 12, MSWD = 3.0) from the black region (Fig.
3c) and 57.3 ± 6.1 Ma (N = 7, MSWD = 4.7) fromthe red regions (Fig. 3d).
V. Discussion and Conclusion
In MPC-D 100s/002, the U–Pb age (64.3 ± 3.4Ma) from the region with low Y concentration iscongruent with the age of this sample reported

in the previous study (66.7 ± 2.5 Ma; Tanabe
et al. 2023). The Y concentrations between theregions where the two ages were derived do not
GLIIHUVLJQL¿FDQWO\ ±ȝJJDQG±ȝJJ 7KHDJHRI0DREWDLQHGIURPthe region with relatively high Y concentration< ±ȝJJ LV\RXQJHUWKDQWKHDERYH
ages, but older than 34.7 ± 3.3 Ma, which is
obtained from a higher Y concentration region± ȝJJ 7DQDEH HW DO   7KHVH
comparisons indicate that the Y concentration
correlates with the obtained U–Pb ages and thatthe ages in areas with lower Y concentrations
tend to show older ages, which is consistent withthe results of Greene et al. (2018) and Tanabe etal. (2023). In MPC-D 100s/003, the U–Pb ageof 51.0 ± 2.7 Ma was obtained from the low Y- 10 -

Yttrium mapping on fossil teeth

Fig. 3. Tera–Wasserberg diagram of apatite analyses with an error bar (2 sigma). D %ODFNUHJLRQRI03&'V E 5HGUHJLRQRI03&'V F %ODFNUHJLRQRI03&'V G 5HGUHJLRQRI03&'V

FRQFHQWUDWLRQ UHJLRQ ± ȝJJ  DQG 
± 6.1 Ma from the high Y concentration region
±ȝJJ 7DQDEHHWDO  REWDLQHG
even younger ages (28.0 ± 4.6 Ma and 33.7 ± 2.2Ma) from the areas with higher Y concentrations
LQWKLVVDPSOH ±ȝJJDQG±
ȝJJUHVSHFWLYHO\ 7KLVWUHQGLVFRQVLVWHQWZLWKthe correlation between the U–Pb age and the
Y concentration in the MPC-D 100s/002. The
results in this study and data from Tanabe et al.
(2023) indicate that Y concentrations can be aqualitative indicator of post-fossilization alteration for apatite U–Pb dating. The regions withrelatively high concentrations of Y have beenstrongly affected by post-fossilization alterationand show younger U–Pb ages compared to therelatively low Y concentration regions within
the sample.
Although consideration of Y concentrations
FDQ EH EHQH¿FLDO IRU DSDWLWH 8±3E GDWLQJ ZH
should not overlook the fact that it is impossible
to set the universal quantitative threshold of Y
concentration to distinguish the ideal regions

for obtaining the fossilization age since our data
indicates the values and variation in the Y concentration varies depending on samples. Whenthe data are lumped together using the MPC-D
100s/002 and 100s/003 as the same sample, thecorrelation between the Y concentration and the
8±3EDJHPHQWLRQHGDERYHFDQQRWEHFRQ¿UPHG
Therefore, it should be noted that the Y-screening
method requires comparing Y concentrations only
within the same sample.
7KLVVWXG\UHFRQ¿UPVWKDWLWZRXOGEHGHVLUDEOH
to selectively measure regions of samples withthe lowest Y concentration to obtain geologically meaningful fossilization ages by apatite
U–Pb dating. The range of Y was measured bysequential spot analyses along linear transects ineach sample in Greene et al. (2018) to visualize
the trend of Y distribution from the inside to theoutside of the samples. Depending on the sample,
LWZRXOGEHGLI¿FXOWWRFDSWXUHWKHJHQHUDOVSDWLDO
trend of the Y distribution in a single linear transaction. Contrarily, the maps shown in this study(Fig. 2c, d) can non-destructively visualize the- 11 -AOKI, CHIBA, KOGISO, MURAKAMI & TSOGTBAATAR

Y distribution of the whole sampling area in 2D.
This study clearly demonstrates that Y mapping
XVLQJ PLFUR;5) LV D SRZHUIXO WHFKQLTXH IRUquick visualization of relative differences in theeffects of post-fossilization alteration within afossil tooth sample.
We would like to note that the apatite U–Pb
dating of tooth samples is easily affected bypost-fossilization alteration, and therefore, theage estimate can only be used as a lower limitof the fossilization age (Tanabe et al. 2023).
The apatite U–Pb dating combined with theY-screening method alone cannot determine a
geologically meaningful age, but the apatite U–
Pb age combined with existing and reliable ages
from other lines of evidence (e.g., biostratigraphic
data and U–Pb ages from igneous rocks and caliches) could provide important supporting datato constrain the depositional ages of fossil-bearing strata and the temporal range of the yielded
fossil remains. In conclusion, we have found
WKDWWKH<PLFUR;5)PDSSLQJLVDXVHIXOWRRO
for non-destructive visualization of the effect of
post-fossilization alteration in fossil teeth and forselecting preferable measuring points for LAICP-MS apatite U–Pb dating.
Acknowledgements
We are grateful to M. Tanabe and S. Miyake for
their support in data collection, N. Shimobayashi
IRUKLVVXSSRUWLQPLFUR;5)PDSSLQJDQG.
Brink for editorial assistance. We also thank two
anonymous reviewers for their helpful comments.
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References$RNL 6 'DWH< 1LVKLGR + $RNL .   /$
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GR 3HUHLUD 3$ $OED\UDN$ ,   83E
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%UXVDWWH 6 / &DUU7 '  1RUHOO 0$  7KH
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0F.HQQD&$   /$4 ,&306WUDFHHOHPHQW
analyses of Durango and McClure Mountain apatite

and implications for making natural LA-ICPMS mineral standards. Chemical Geology 435: 35-48.
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3DODHRHFRORJ\)DVVHWW-(+HDPDQ/0 6LPRQHWWL$  'LUHFWU-Pb dating of Cretaceous and Paleocene dinosaur
ERQHV6DQ-XDQ%DVLQ1HZ0H[LFR*HRORJ\  
Greene, S., Heaman, L. M., DuFrane, S. A., Williamson,7 &XUULH3-  ,QWURGXFLQJDJHRFKHPLFDO
screen to identify geologically meaningful U-Pb dates
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+LHVV-&RQGRQ'-0F/HDQ1 1REOH65  
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Dam, South Australia: Links with the Gairdner Large,JQHRXV 3URYLQFH 3UHFDPEULDQ 5HVHDUFK  172.
,VKLJDNL67VRJWEDDWDU.7R\RGD60DLQED\DU%
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P., Zorig, E., Bayardorj, C., Ochirjantsan, E., SaneyRVKL0+D\DVKL6 &KLED.  5HSRUWRIWKH
Okayama University of Science – Mongolian Institute
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6WROO%+HUZLJ.$PLQL0$ERXFKDPL: 
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7VRJWEDDWDU.:LQGOH\%) ,VKLJDNL6  
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scale chemostratigraphy of cross-sectioned hydrogenous ferromanganese nodules from the western North
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- 12 -

Yttrium mapping on fossil teeth

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青 木 一 勝・千 葉 謙 太 郎・小 木 曽 哲・村 上 凱 星・
TSOGTBAATAR Khishigjav：アパタイトU–Pb年代測定のためのマイクロXRFによる歯化石のイットリウ
ムマッピング要約
歯化石のアパタイトU–Pb年代測定では，化石化後の変質の影響の少ない測定試料／測定箇所を選定することが重要である．先行研究から同一試料内におけるイットリウムの濃度の違いが，定性的な変質の指標として有効であることが示唆されている．本研究ではその有用性を検証するため，既に年代値が報告されている
タルボサウルスの歯化石２試料に対し，マイクロ蛍光X線分析装置を使ったイットリウムのマッピング測定から測定領域を選定し，LA-ICP-MSアパタイトU–Pb年代測定を行った．その結果，先行研究で 66.7 ± 2.5 Ma の年代値が報告された試料からは，イットリウム濃度が相対的に異なる２つの領域からそれぞれ 64.3 ± 3.4 Ma と61.0 ± 11.0 Ma の年代値が得られた．また，約30 Ma の年代値を示した試料からは 51.5 ± 2.7 Ma と 57.3 ± 6.1
Ma の年代値が得られた．これらの結果は少なくとも同一試料内では，イットリウム濃度とアパタイトU–Pb年代
値に定性的な相関があることを示す．さらなる検証は必要ではあるが，マイクロ蛍光X線分析装置マッピング
測定が歯化石の変質の程度を非破壊で視覚化する手法として有効であり，アパタイトU–Pb年代測定における測定点の選定に利用できるだろう．

- 13 -(Accepted 5 November 2023)