- Email: [email protected]
CeIV ratios in geological materials; application for weathering, sedimentary and diagenetic processes
Earth and Planetary Science Letters 182 (2000) 201^207 www.elsevier.com/locate/epsl
A new method for the determination of CeIII/CeIV ratios in geological materials; application for weathering, sedimentary and diagenetic processes Yoshio Takahashi a; *, Hiroshi Shimizu a , Hiroyuki Kagi b , Hidekazu Yoshida c;1 , Akira Usui d , Masaharu Nomura e a
b
Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan Laboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan c Japan Nuclear Cycle Development Institute (JNC), Tono Geoscience Center, Toki, Gifu 509-5102, Japan d Geological Survey of Japan, Higashi, Tsukuba, Ibaraki 305-8567, Japan e Photon Factory, Institute of Materials Structure Science, KEK, Tsukuba, Ibaraki 305-0801, Japan Received 8 May 2000; received in revised form 4 August 2000; accepted 27 August 2000
Abstract Relative abundances of rare earth elements (REE) in geological materials are used widely to investigate diverse geochemical issues such as the origins of igneous rocks or the degree of stratification of water columns in oceans. One of the REE, cerium (Ce), can exist in either trivalent or tetravalent forms depending on the redox condition. Thus, knowledge of the oxidation state of Ce in rocks and minerals could potentially be used to constrain the redox states of past and present geological environments. However, the use of Ce for this purpose has been hampered by an inability to measure its oxidation state directly. Here, we present a new method, employing the X-ray absorption near-edge structure, for making such determinations in samples with Ce concentrations as low as 15 ppm. By analyzing a range of diverse geological materials (granites, manganese nodules and cherts), we showed that the method could identify significant differences in Ce speciation between materials formed under widely differing redox conditions. Used together with the degrees of Ce anomalies measured in REE patterns, this information has promise for refining our knowledge of various geochemical processes such as weathering, sedimentary and diagenetic processes that control the REE behavior. Such a refinement should in turn improve geological interpretations based upon REE data. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: cerium; isotope ratios; X-ray analysis; geochemical anomalies; granites; nodules; chert
1. Introduction
* Corresponding author. Tel.: +81-824-24-7460; Fax: +81-824-24-0735; E-mail: [email protected] 1
Present address: Nagoya University Museum, Nagoya University, Chikusa, Nagoya 464-8601, Japan.
The cerium (Ce) anomaly is frequently observed as an anomalous value of Ce away from the trend de¢ned by the other rare earth elements (REE) on the REE pattern [1,2]. This is presumably due to
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 2 5 0 - 8
EPSL 5604 13-10-00
202
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207
the formation of CeIV under oxidizing conditions at and near the earth's surface. Therefore, Ce behavior di¡ers from other REE, which are exclusively trivalent (except for EuIII /EuII ). In spite of the great usefulness of the Ce anomaly as an index to elucidate geochemical processes such as redox reactions in marine and freshwaters, the actual ratio of CeIII and CeIV species has not been determined directly in geological samples, because of typically low Ce abundances in a poly-component matrix. Although the existence of CeIV in geological samples was observed for Ce-enriched phases [3,4], the actual ratio of CeIII and CeIV has not been obtained for ordinary geological samples. In this letter, we ¢rst report the actual ratio of CeIII and CeIV determined by Xray absorption near-edge structure (XANES) in the natural samples of (weathered) granitic rocks, manganese nodules and cherts. Although we recently identi¢ed CeIV by XANES in manganese nodules [4], we found that the method is also applicable to obtain the actual ratio of CeIII and CeIV in the sample where Ce abundance is around 15 ppm. These results provide various examples of
relationships between Ce anomalies and the oxidation states of Ce in these samples. Based on such an overview, it is shown that the new information on the oxidation state of Ce given by XANES enables us to interpret the behavior and geochemical processes of Ce and other REE more precisely than hitherto known. 2. Materials and methods Granitic rocks (Toki granite) that have undergone various degrees of weathering were obtained from outcrops in the Tono district, Gifu, Central Japan (ML1, ML2). A fresh sample of the same rock body was taken from a core collected from borehole DH3 at 504 m depth. The manganese nodules employed in this study were recovered from the Central Paci¢c Ocean [5]. Chert samples were collected from the Unuma area in the Mino terrane (Mino-09 and -17, Triassic cherts) and from the Saiki area in the Southern Chichibu terrane (N19, Permian chert) in Japan [6^8]. Abundances of REE are given in separate pa-
Table 1 REE abundances (ppm) and the degree of Ce anomalies (CeCN /CeCN *)a for granitic rocks, manganese nodules and cherts examined in this study Granitic rocks La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ce/Ce* a
Manganese nodules
ML2
DH3
CD25b
D535b
D886
Mino-9c
Mino-17c
N19d
7.54 58.5 2.17 8.19 2.70 0.129 3.99 0.903 6.54 1.48 4.83 0.772 5.45 0.744
45.1 66.0 12.2 48.9 12.4 0.521 14.0 2.42 15.6 3.17 9.17 1.35 8.18 0.880
97.6 199 23.4 85.1 20.2 0.339 21.4 3.95 24.2 4.99 14.7 2.14 13.6 2.05
303 844 70.8 283 62.2 15.1 64.9 9.43 58.0 11.5 32.1 4.39 27.3 4.16
177 1230 41.3 164 38.3 8.97 40.3 5.90 33.8 6.89 20.2 2.88 19.3 2.95
257 951 53.5 228 53.5 12.8 58.2 8.28 48.3 9.82 28.8 3.86 25.1 3.80
7.03 15.2 1.65 6.09 1.22 0.270 1.09 0.174 1.06 0.213 0.633 0.0941 0.644 0.0984
6.07 15.9 1.62 6.39 1.37 0.293 1.26 0.211 1.22 0.256 0.749 0.109 0.700 0.111
7.73 13.8 1.88 7.09 1.36 0.291 1.09 0.171 0.980 0.194 0.538 0.0781 0.519 0.0773
3.47
0.674
1.00
1.38
3.45
1.07
1.22
0.868
CN denotes the abundance normalized by a C1 chondrite [15]; From Takahashi et al. [4]. c From Shimizu et al. [8]. d From Kunimaru et al. [6]. b
Cherts
ML1
1=2 CeCN * = LaCN
EPSL 5604 13-10-00
1.94 1=2 PrCN .
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207
pers [4,6,8] or determined by the method reported elsewhere [6]. The precision and accuracy were better than 5% for the analyses of REE abundances. The XANES spectra at the LIII edge of Ce were obtained by using beam line 12C connected to a synchrotron (2.5 GeV) at the KEK-Photon Factory in Tsukuba, Japan [9]. The £uorescence yield from Ce was measured using a 19 element pureGe solid-state detector (SSD), which provided a high energy-resolution su¤cient to separate signals of Ce from the scattering signal and the £uorescent X-rays of other elements [10]. The XANES spectra were recorded with a 0.5 or 1.0 eV step from 5700 to 5800 eV. The measurement time varied from 1 to 3 h depending on the Ce abundance. Small chips of manganese nodules and cherts were analyzed directly, whereas powdered samples of granite were analyzed. The source X-ray beam was small ( 6 1 mm2 ) [10] so that we could avoid analyzing REE in other materials in the samples, such as veins. We can apply the beam for the direct analysis of large mm-sized crystals in rocks, in the future. In order to estimate the percentages of CeIII and CeIV in these specimens, a calibration was undertaken in which the peak areas in the XANES spectra were determined for di¡erent mechanical mixtures of CeIII (CeCl3 )+CeIV (Ce(SO4 )2 ) and silica sand (Fig. 1) [11^14]. These spectra were deconvoluted to obtain contributions of CeIII and CeIV species, as shown in the sample containing 70% of CeIV (see captions of Fig. 1 for the details). The calibration obtained was used to estimate the percentage of CeIV and CeIII in the natural specimens with an error of about þ 10% estimated from the repeated analyses. 3. Results and discussion The REE patterns for the granitic rocks, normalized by C1 chondrite [15], are shown in Fig. 2a. The REE abundances are given in Table 1 accompanied with those for manganese nodules and cherts. The degree of Ce anomalies (CeCN / CeCN *) for each sample are also included in Table 1, where CN denotes chondrite-normalized value.
203
Fig. 1. XANES spectra of mixtures of CeIII (CeCl3 ) or CeIV (Ce(SO4 )2 ) with silica sand. The results of various mixtures containing 0, 10, 30, 50, 70, 90 and 100% of CeIV are shown. The interpretation of the Ce spectra of CeCl3 (CeIV 0%) and Ce(SO4 )2 (CeIV 100%) has been well studied [4,11^14,21]. The CeIII species show a single peak around 5726.5 eV (2p3=2 C(4f1 )5d). The CeIV species have one minor peak around 5720 eV (2p3=2 C4f1 ) and two intense peaks at 5737.5 eV (2p3=2 C(4f0 )5d) and 5729.5 eV (2p3=2 C(4f1 L)5d), where L denotes a hole in the ligand shell. The spectrum of the sample containing only CeIII or CeIV can be ¢tted well by the combination of a Lorentzian function and an arctangent function [4,11^14]. One Lorentzian function and one arctangent function are assigned to each peak (i.e. three Lorentzian functions and three arctangent functions are used to ¢t CeIV species). The spectra for the mixtures could be deconvoluted to give the spectra of CeIII and CeIV species. An example of the deconvolution is shown by the sample containing 70% of CeIV . The peak area ratio is the ratio of the areas corresponding to the Lorentzian functions. The bold solid curve is the calculated spectrum given by the ¢tting. The solid curve shows the contribution of Lorentzian functions for CeIII and CeIV species, while the dotted curve indicates the contribution of the arctan function.
In the present study, CeCN * is de¢ned as CeCN * = (LaCN )1=2 (CeCN )1=2 . Large positive and negative Ce anomalies were found in ML1 and ML2, respectively. This shows that some weathering processes, including redox reactions, have altered the original REE patterns of ML1 and ML2, as estimated from similar results on Ce anomalies brought about during granite weather-
EPSL 5604 13-10-00
204
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207
Fig. 2. Chondrite-normalized REE patterns of (a) granitic rocks, (b) manganese nodules and (c) cherts. The REE abundances in a C1 chondrite [15] were used for normalization. The abundances of Ce in the samples are also noted. The numbers with arrows indicate the magnitude of the Ce 1=2 1=2 anomaly de¢ned as CeCN /CeCN * (CeCN * = LaCN PrCN ), where CN denotes the abundance normalized by the C1 chondrite. For the pattern of ML1 (a), the abundance of CeIII is also indicated by a dotted curve based on the XANES results.
ing [3,16]. From the deconvoluted XANES spectra for the granitic rocks (Fig. 3a), the percentages of CeIII were estimated to be 30, 70 and 100% for ML1, ML2 and DH3, respectively. For DH3, Ce is exclusively trivalent, which agrees with the fact that the REE pattern of DH3 has no Ce anomaly. Since ML1, ML2 and DH3 are taken from an identical granitic body (Toki granite), it is estimated that Ce was exclusively involved as CeIII
in the rocks at the time of their crystallization [17]. For the ML1 having intense positive Ce anomaly, CeIII is still involved in the sample whose percentage is estimated to be ca. 30%. When the CeIII abundance is plotted in the REE pattern of ML1, as indicated by the dotted curve (Fig. 2a), a smooth pattern through the other REE and CeIII is obtained. This means that we can obtain the CeIII abundance by interpolating normalized values of La and Pr for the ML1 granite. This fact suggests that the REE pattern including the CeIII abundance corresponds to speci¢c phases in ML1 that have not been a¡ected by water^rock reactions. That is, CeIV in ML1 is involved in a di¡erent fraction from the phase containing CeIII and other REE. This suggests that CeIV was formed by oxidation in the aqueous phase after being dissolved initially as CeIII , accompanied by other REE, during weathering. The dissolved fraction of REE other than Ce was removed from ML1, while most of the Ce was ¢xed to ML1 as a result of oxidation, due to the low solubility and high adsorptive activity of CeIV . For ML2, showing a negative Ce anomaly, about 30% of Ce was present in the rock as CeIV . The existence of CeIV shows that the weathering environment around ML2 was also oxidizing enough to produce CeIV . Since the CeIV species was generated during weathering, at least 30% of Ce was redistributed from the aqueous phase to ML2 during water^rock interactions. If REE were removed from ML2 during weathering, the low mobility of CeIV should bring about a positive Ce anomaly in ML2. This suggests that REE were not removed but added to ML2 granite in the weathering processes. The negative Ce anomaly indicates that the dissolved REE exhibiting negative Ce anomaly was provided to ML2 during weathering processes. These ¢ndings show that the actual ratio of CeIII and CeIV determined by XANES can be used as a quantitative index of REE migration in natural systems, and can shed light on the processes controlling this migration. The manganese nodules employed in this study are of hydrogenetic origin, estimated from their occurrences, mineralogy and chemistry [5]. Each REE pattern for the manganese nodules (Fig. 2b) shows a positive Ce anomaly, which is a common
EPSL 5604 13-10-00
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207
Fig. 3. XANES spectra at the LIII edge of Ce in (a) granitic rocks, (b) manganese nodules and (c) cherts. The contributions of CeIII and CeIV species were shown in the spectra. For manganese nodules and cherts, the result of the least square ¢tting to one sample was shown, since other samples showed similar results. Each curve shows the results of the deconvolution described in the caption of Fig. 1.
feature of a hydrogenetic manganese nodule [18,19]. Although the degree of the Ce anomaly di¡ers from one sample to another, the XANES spectra show that in all the samples only CeIV is found (Fig. 3b). This is possibly due to the oxidative sorption of dissolved CeIII in the seawater
205
by the surface of manganese oxide [4,18,20]. Most of the Ce sorbed on the manganese oxides is immediately oxidized to CeIV . With the aid of XANES, it is possible to gain an improved understanding of the mechanism that enriched Ce in the manganese nodules and the behavior of REE in the marine environment [4]. Among the cherts employed in this study, we can ¢nd both positive and negative chondrite-normalized Ce anomalies (Fig. 2c). These anomalies are related to the depositional environments of the cherts; the negative anomaly is related to deep sea environment while the positive to shallow sea [6,7]. Since there are only small amounts of detrital material in the chert, the various magnitudes of the Ce anomalies should be caused by the redox reactions that a¡ect CeIV concentrations in the marine environment. By adopting the same approach as that used for the ML1 granite, the REE pattern indicates that in Mino-17, ca. 18% of the Ce should be CeIV . However, least square ¢ttings of the cherts' XANES spectra (Fig. 3c) reveal that CeIV contributes less than 5% of the total Ce in spite of the various Ce anomalies. If we assume that Ce was incorporated into the cherts as CeIV during their formation, the present results show that diagenetic processes with reductive reaction of CeIV to CeIII have occurred in the cherts. This suggests that the bulk REE pattern of the chert is rather conservative and resistant to diagenesis, while the Ce valence is more sensitive to the diagenetic processes. If a dissolution process of Ce in the cherts is needed to reduce Ce, as in the case of the weathered granitic rocks, it is estimated that a part of REE in the cherts had been leached in the diagenetic processes. However, the present results suggest that the dissolution process did not alter the REE pattern of the whole rock of the cherts completely, irrespective of the diagenesis. Further study is needed to elucidate a general conclusion on REE behavior, including Ce redox changes in cherts during diagenesis. However, it is clear that the combination of REE patterns and the oxidation state of Ce revealed by XANES provides a new perspective on Ce and REE behavior during processes such as weathering and diagenesis. In particular, if we can ¢nd such a di¡erence between the results of
EPSL 5604 13-10-00
206
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207
REE pattern and XANES, the discrepancy will give us valuable knowledge about the initial redox condition and the post-depositional processes such as diagenesis, metamorphism and alteration.
[3] [4]
4. Summary Here we show various relationships between Ce anomalies and the oxidation states of Ce given by XANES. It is noteworthy that the XANES spectra at the LIII edge of Ce could be obtained even for cherts containing Ce abundances of ca. 15 ppm. The XANES technique, based on synchrotron radiation and the detection of £uorescent Xray by SSD, can clarify the Ce valence in many geological samples. Cerium in cherts and fresh granite is trivalent, while most of the Ce in manganese nodules is tetravalent. Both CeIII and CeIV species are found in weathered granite. This study shows that the oxidation state of Ce given by XANES, coupled with the Ce anomaly in REE patterns, can be used to provide a new view on REE geochemistry and hence clarify various geochemical processes. Acknowledgements
[5]
[6]
[7] [8]
[9] [10] [11]
We are grateful to N. Sato, T. Kunimaru and E. Nagashio for their help with the ICP-MS measurement and XANES experiment. We thank H. Ishisako and Y. Shibata for their technical support. We also thank Dr. R. Metcalfe (JNC) for his great contribution in improving this manuscript. Constructive comments by S. McLennan, M. Bau and another anonymous reviewer are greatly acknowledged.[AH]
[12]
[13] [14]
[15]
References [1] P. Henderson, Rare Earth Element Geochemistry, Elsevier, Amsterdam, 1984. [2] S.R. Taylor, S.M. McLennan, The signi¢cance of the rare earths in geochemistry and cosmochemistry, in: K.A. Gschneider, L. Eyring (Eds.), Handbook on the Physics
[16] [17]
and Chemistry of Rare Earths, vol. 11, Elsevier Sci. Publ., 1988, pp. 485^580. J.J. Braun, M. Pagel, J.P. Muller, P. Bilong, A. Michard, B. Guillet, Cerium anomalies in lateritic pro¢les, Geochim. Cosmochim. Acta 51 (1990) 597^605. Y. Takahashi, H. Shimizu, A. Usui, H. Kagi, M. Nomura, Direct observation of tetravalent cerium in ferromanganese nodules and crusts by X-ray-absorption nearedge structure (XANES), Geochim. Cosmochim. Acta 64 (2000) 2929^2935. A. Usui, A. Nishimura, N. Mita, Composition and growth history of sur¢cial and buried manganese nodules in the Penrhyn Basin, Southwestern Paci¢c, Mar. Geol. 114 (1993) 133^153. T. Kunimaru, H. Shimizu, K. Takahashi, S. Yabuki, Difference in geochemical features between Permian and Triassic cherts from the Southern Chichibu terrane, Southwest Japan: REE abundances, major element compositions and Sr isotopic ratios, Sediment. Geol. 119 (1998) 195^217. H. Shimizu, A. Masuda, Cerium in cherts as an indication of marine environment of its formation, Nature 266 (1977) 346^348. H. Shimizu, T. Kunimaru, S. Yoneda, M. Adachi, Sources and depositional environments of some Permian and Triassic cherts: signi¢cance of Rb^Sr and Sm^Nd isotopic and REE abundances data, J. Geol., in press. M. Nomura, A. Koyama, Design and performance of a new XAFS beamline at the photon factory; BL-12C, KEK Report 95-15, 1996. M. Nomura, Dead-time correction of a multi-element SSD for £uorescent XAFS, J. Synchrotron Rad. 5 (1998) 851^853. A. Bianconi, A. Marcelli, H. Dexpert, R. Karnatak, A. Kotani, T. Jo, J. Petiau, Speci¢c intermediate-valence state of insulating 4f compounds detected by L3 X-ray absorption, Phys. Rev. B 35 (1987) 806^812. G. Kaindl, G.K. Wertheim, G. Schmiester, E.V. Sampathkumaran, Mixed valency versus covalency in rare-earth core-electron spectroscopy, Phys. Rev. Lett. 58 (1987) 606^609. Z. Hu, S. Bertram, G. Kaindl, X-ray-absorption study of PrO2 at high pressure, Phys. Rev. B 49 (1994) 39^43. H. Shigekawa, M. Ishida, K. Miyake, R. Shioda, Y. Iijima, T. Imai, H. Takahashi, J. Sumaoka, M. Komiyama, Extended X-ray absorption ¢ne structure study on the cerium(IV)-induced DNA hydrolysis: implication to the roles of 4f orbitals in the catalysis, Appl. Phys. Lett. 74 (1999) 460^462. E. Anders, N. Grevesse, Abundances of the elements: meteoritic and solar, Geochim. Cosmochim. Acta 57 (1989) 197^214. J.F. Ban¢eld, R.A. Eggleton, Apatite replacement and rare earth mobilization, fractionation, and ¢xation during weathering, Clay Clay Miner. 37 (1989) 113^127. H.D. Schreiber, H.V. Luer, T. Thanyasiri, The redox state of cerium in basaltic magmas: an experimental study of
EPSL 5604 13-10-00
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207 iron^cerium interactions in silicate melts, Geochim. Cosmochim. Acta 44 (1980) 1599^1612. [18] N. Takematsu, Mangan Dankai (Manganese nodule), Koseisha Kouseikaku, Tokyo, 1998, pp. 67^74 (in Japanese). [19] E.H. De Carlo, G.M. McMurtry, Rare-earth element geochemistry of ferromanganese crusts from the Hawaiian Archipelago, central Paci¢c, Chem. Geol. 95 (1992) 235^ 250.
207
[20] E.D. Goldberg, Chemistry of the Oceans, in: M. Sears (Ed.), Oceanography, vol. 67, Am. Assoc. Adv. Sci. Publ., 1961, pp. 538^597. [21] A.J. Davenport, H.S. Isaacs, M.W. Kendig, XANES investigation of the role of cerium compounds as corrosion inhibitors for aluminum, Corros. Sci. 32 (1991) 653.
EPSL 5604 13-10-00
A new method for the determination of CeIII/CeIV ratios in geological materials; application for weathering, sedimentary and diagenetic processes Yoshio Takahashi a; *, Hiroshi Shimizu a , Hiroyuki Kagi b , Hidekazu Yoshida c;1 , Akira Usui d , Masaharu Nomura e a
b
Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan Laboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan c Japan Nuclear Cycle Development Institute (JNC), Tono Geoscience Center, Toki, Gifu 509-5102, Japan d Geological Survey of Japan, Higashi, Tsukuba, Ibaraki 305-8567, Japan e Photon Factory, Institute of Materials Structure Science, KEK, Tsukuba, Ibaraki 305-0801, Japan Received 8 May 2000; received in revised form 4 August 2000; accepted 27 August 2000
Abstract Relative abundances of rare earth elements (REE) in geological materials are used widely to investigate diverse geochemical issues such as the origins of igneous rocks or the degree of stratification of water columns in oceans. One of the REE, cerium (Ce), can exist in either trivalent or tetravalent forms depending on the redox condition. Thus, knowledge of the oxidation state of Ce in rocks and minerals could potentially be used to constrain the redox states of past and present geological environments. However, the use of Ce for this purpose has been hampered by an inability to measure its oxidation state directly. Here, we present a new method, employing the X-ray absorption near-edge structure, for making such determinations in samples with Ce concentrations as low as 15 ppm. By analyzing a range of diverse geological materials (granites, manganese nodules and cherts), we showed that the method could identify significant differences in Ce speciation between materials formed under widely differing redox conditions. Used together with the degrees of Ce anomalies measured in REE patterns, this information has promise for refining our knowledge of various geochemical processes such as weathering, sedimentary and diagenetic processes that control the REE behavior. Such a refinement should in turn improve geological interpretations based upon REE data. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: cerium; isotope ratios; X-ray analysis; geochemical anomalies; granites; nodules; chert
1. Introduction
* Corresponding author. Tel.: +81-824-24-7460; Fax: +81-824-24-0735; E-mail: [email protected] 1
Present address: Nagoya University Museum, Nagoya University, Chikusa, Nagoya 464-8601, Japan.
The cerium (Ce) anomaly is frequently observed as an anomalous value of Ce away from the trend de¢ned by the other rare earth elements (REE) on the REE pattern [1,2]. This is presumably due to
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 2 5 0 - 8
EPSL 5604 13-10-00
202
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207
the formation of CeIV under oxidizing conditions at and near the earth's surface. Therefore, Ce behavior di¡ers from other REE, which are exclusively trivalent (except for EuIII /EuII ). In spite of the great usefulness of the Ce anomaly as an index to elucidate geochemical processes such as redox reactions in marine and freshwaters, the actual ratio of CeIII and CeIV species has not been determined directly in geological samples, because of typically low Ce abundances in a poly-component matrix. Although the existence of CeIV in geological samples was observed for Ce-enriched phases [3,4], the actual ratio of CeIII and CeIV has not been obtained for ordinary geological samples. In this letter, we ¢rst report the actual ratio of CeIII and CeIV determined by Xray absorption near-edge structure (XANES) in the natural samples of (weathered) granitic rocks, manganese nodules and cherts. Although we recently identi¢ed CeIV by XANES in manganese nodules [4], we found that the method is also applicable to obtain the actual ratio of CeIII and CeIV in the sample where Ce abundance is around 15 ppm. These results provide various examples of
relationships between Ce anomalies and the oxidation states of Ce in these samples. Based on such an overview, it is shown that the new information on the oxidation state of Ce given by XANES enables us to interpret the behavior and geochemical processes of Ce and other REE more precisely than hitherto known. 2. Materials and methods Granitic rocks (Toki granite) that have undergone various degrees of weathering were obtained from outcrops in the Tono district, Gifu, Central Japan (ML1, ML2). A fresh sample of the same rock body was taken from a core collected from borehole DH3 at 504 m depth. The manganese nodules employed in this study were recovered from the Central Paci¢c Ocean [5]. Chert samples were collected from the Unuma area in the Mino terrane (Mino-09 and -17, Triassic cherts) and from the Saiki area in the Southern Chichibu terrane (N19, Permian chert) in Japan [6^8]. Abundances of REE are given in separate pa-
Table 1 REE abundances (ppm) and the degree of Ce anomalies (CeCN /CeCN *)a for granitic rocks, manganese nodules and cherts examined in this study Granitic rocks La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ce/Ce* a
Manganese nodules
ML2
DH3
CD25b
D535b
D886
Mino-9c
Mino-17c
N19d
7.54 58.5 2.17 8.19 2.70 0.129 3.99 0.903 6.54 1.48 4.83 0.772 5.45 0.744
45.1 66.0 12.2 48.9 12.4 0.521 14.0 2.42 15.6 3.17 9.17 1.35 8.18 0.880
97.6 199 23.4 85.1 20.2 0.339 21.4 3.95 24.2 4.99 14.7 2.14 13.6 2.05
303 844 70.8 283 62.2 15.1 64.9 9.43 58.0 11.5 32.1 4.39 27.3 4.16
177 1230 41.3 164 38.3 8.97 40.3 5.90 33.8 6.89 20.2 2.88 19.3 2.95
257 951 53.5 228 53.5 12.8 58.2 8.28 48.3 9.82 28.8 3.86 25.1 3.80
7.03 15.2 1.65 6.09 1.22 0.270 1.09 0.174 1.06 0.213 0.633 0.0941 0.644 0.0984
6.07 15.9 1.62 6.39 1.37 0.293 1.26 0.211 1.22 0.256 0.749 0.109 0.700 0.111
7.73 13.8 1.88 7.09 1.36 0.291 1.09 0.171 0.980 0.194 0.538 0.0781 0.519 0.0773
3.47
0.674
1.00
1.38
3.45
1.07
1.22
0.868
CN denotes the abundance normalized by a C1 chondrite [15]; From Takahashi et al. [4]. c From Shimizu et al. [8]. d From Kunimaru et al. [6]. b
Cherts
ML1
1=2 CeCN * = LaCN
EPSL 5604 13-10-00
1.94 1=2 PrCN .
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207
pers [4,6,8] or determined by the method reported elsewhere [6]. The precision and accuracy were better than 5% for the analyses of REE abundances. The XANES spectra at the LIII edge of Ce were obtained by using beam line 12C connected to a synchrotron (2.5 GeV) at the KEK-Photon Factory in Tsukuba, Japan [9]. The £uorescence yield from Ce was measured using a 19 element pureGe solid-state detector (SSD), which provided a high energy-resolution su¤cient to separate signals of Ce from the scattering signal and the £uorescent X-rays of other elements [10]. The XANES spectra were recorded with a 0.5 or 1.0 eV step from 5700 to 5800 eV. The measurement time varied from 1 to 3 h depending on the Ce abundance. Small chips of manganese nodules and cherts were analyzed directly, whereas powdered samples of granite were analyzed. The source X-ray beam was small ( 6 1 mm2 ) [10] so that we could avoid analyzing REE in other materials in the samples, such as veins. We can apply the beam for the direct analysis of large mm-sized crystals in rocks, in the future. In order to estimate the percentages of CeIII and CeIV in these specimens, a calibration was undertaken in which the peak areas in the XANES spectra were determined for di¡erent mechanical mixtures of CeIII (CeCl3 )+CeIV (Ce(SO4 )2 ) and silica sand (Fig. 1) [11^14]. These spectra were deconvoluted to obtain contributions of CeIII and CeIV species, as shown in the sample containing 70% of CeIV (see captions of Fig. 1 for the details). The calibration obtained was used to estimate the percentage of CeIV and CeIII in the natural specimens with an error of about þ 10% estimated from the repeated analyses. 3. Results and discussion The REE patterns for the granitic rocks, normalized by C1 chondrite [15], are shown in Fig. 2a. The REE abundances are given in Table 1 accompanied with those for manganese nodules and cherts. The degree of Ce anomalies (CeCN / CeCN *) for each sample are also included in Table 1, where CN denotes chondrite-normalized value.
203
Fig. 1. XANES spectra of mixtures of CeIII (CeCl3 ) or CeIV (Ce(SO4 )2 ) with silica sand. The results of various mixtures containing 0, 10, 30, 50, 70, 90 and 100% of CeIV are shown. The interpretation of the Ce spectra of CeCl3 (CeIV 0%) and Ce(SO4 )2 (CeIV 100%) has been well studied [4,11^14,21]. The CeIII species show a single peak around 5726.5 eV (2p3=2 C(4f1 )5d). The CeIV species have one minor peak around 5720 eV (2p3=2 C4f1 ) and two intense peaks at 5737.5 eV (2p3=2 C(4f0 )5d) and 5729.5 eV (2p3=2 C(4f1 L)5d), where L denotes a hole in the ligand shell. The spectrum of the sample containing only CeIII or CeIV can be ¢tted well by the combination of a Lorentzian function and an arctangent function [4,11^14]. One Lorentzian function and one arctangent function are assigned to each peak (i.e. three Lorentzian functions and three arctangent functions are used to ¢t CeIV species). The spectra for the mixtures could be deconvoluted to give the spectra of CeIII and CeIV species. An example of the deconvolution is shown by the sample containing 70% of CeIV . The peak area ratio is the ratio of the areas corresponding to the Lorentzian functions. The bold solid curve is the calculated spectrum given by the ¢tting. The solid curve shows the contribution of Lorentzian functions for CeIII and CeIV species, while the dotted curve indicates the contribution of the arctan function.
In the present study, CeCN * is de¢ned as CeCN * = (LaCN )1=2 (CeCN )1=2 . Large positive and negative Ce anomalies were found in ML1 and ML2, respectively. This shows that some weathering processes, including redox reactions, have altered the original REE patterns of ML1 and ML2, as estimated from similar results on Ce anomalies brought about during granite weather-
EPSL 5604 13-10-00
204
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207
Fig. 2. Chondrite-normalized REE patterns of (a) granitic rocks, (b) manganese nodules and (c) cherts. The REE abundances in a C1 chondrite [15] were used for normalization. The abundances of Ce in the samples are also noted. The numbers with arrows indicate the magnitude of the Ce 1=2 1=2 anomaly de¢ned as CeCN /CeCN * (CeCN * = LaCN PrCN ), where CN denotes the abundance normalized by the C1 chondrite. For the pattern of ML1 (a), the abundance of CeIII is also indicated by a dotted curve based on the XANES results.
ing [3,16]. From the deconvoluted XANES spectra for the granitic rocks (Fig. 3a), the percentages of CeIII were estimated to be 30, 70 and 100% for ML1, ML2 and DH3, respectively. For DH3, Ce is exclusively trivalent, which agrees with the fact that the REE pattern of DH3 has no Ce anomaly. Since ML1, ML2 and DH3 are taken from an identical granitic body (Toki granite), it is estimated that Ce was exclusively involved as CeIII
in the rocks at the time of their crystallization [17]. For the ML1 having intense positive Ce anomaly, CeIII is still involved in the sample whose percentage is estimated to be ca. 30%. When the CeIII abundance is plotted in the REE pattern of ML1, as indicated by the dotted curve (Fig. 2a), a smooth pattern through the other REE and CeIII is obtained. This means that we can obtain the CeIII abundance by interpolating normalized values of La and Pr for the ML1 granite. This fact suggests that the REE pattern including the CeIII abundance corresponds to speci¢c phases in ML1 that have not been a¡ected by water^rock reactions. That is, CeIV in ML1 is involved in a di¡erent fraction from the phase containing CeIII and other REE. This suggests that CeIV was formed by oxidation in the aqueous phase after being dissolved initially as CeIII , accompanied by other REE, during weathering. The dissolved fraction of REE other than Ce was removed from ML1, while most of the Ce was ¢xed to ML1 as a result of oxidation, due to the low solubility and high adsorptive activity of CeIV . For ML2, showing a negative Ce anomaly, about 30% of Ce was present in the rock as CeIV . The existence of CeIV shows that the weathering environment around ML2 was also oxidizing enough to produce CeIV . Since the CeIV species was generated during weathering, at least 30% of Ce was redistributed from the aqueous phase to ML2 during water^rock interactions. If REE were removed from ML2 during weathering, the low mobility of CeIV should bring about a positive Ce anomaly in ML2. This suggests that REE were not removed but added to ML2 granite in the weathering processes. The negative Ce anomaly indicates that the dissolved REE exhibiting negative Ce anomaly was provided to ML2 during weathering processes. These ¢ndings show that the actual ratio of CeIII and CeIV determined by XANES can be used as a quantitative index of REE migration in natural systems, and can shed light on the processes controlling this migration. The manganese nodules employed in this study are of hydrogenetic origin, estimated from their occurrences, mineralogy and chemistry [5]. Each REE pattern for the manganese nodules (Fig. 2b) shows a positive Ce anomaly, which is a common
EPSL 5604 13-10-00
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207
Fig. 3. XANES spectra at the LIII edge of Ce in (a) granitic rocks, (b) manganese nodules and (c) cherts. The contributions of CeIII and CeIV species were shown in the spectra. For manganese nodules and cherts, the result of the least square ¢tting to one sample was shown, since other samples showed similar results. Each curve shows the results of the deconvolution described in the caption of Fig. 1.
feature of a hydrogenetic manganese nodule [18,19]. Although the degree of the Ce anomaly di¡ers from one sample to another, the XANES spectra show that in all the samples only CeIV is found (Fig. 3b). This is possibly due to the oxidative sorption of dissolved CeIII in the seawater
205
by the surface of manganese oxide [4,18,20]. Most of the Ce sorbed on the manganese oxides is immediately oxidized to CeIV . With the aid of XANES, it is possible to gain an improved understanding of the mechanism that enriched Ce in the manganese nodules and the behavior of REE in the marine environment [4]. Among the cherts employed in this study, we can ¢nd both positive and negative chondrite-normalized Ce anomalies (Fig. 2c). These anomalies are related to the depositional environments of the cherts; the negative anomaly is related to deep sea environment while the positive to shallow sea [6,7]. Since there are only small amounts of detrital material in the chert, the various magnitudes of the Ce anomalies should be caused by the redox reactions that a¡ect CeIV concentrations in the marine environment. By adopting the same approach as that used for the ML1 granite, the REE pattern indicates that in Mino-17, ca. 18% of the Ce should be CeIV . However, least square ¢ttings of the cherts' XANES spectra (Fig. 3c) reveal that CeIV contributes less than 5% of the total Ce in spite of the various Ce anomalies. If we assume that Ce was incorporated into the cherts as CeIV during their formation, the present results show that diagenetic processes with reductive reaction of CeIV to CeIII have occurred in the cherts. This suggests that the bulk REE pattern of the chert is rather conservative and resistant to diagenesis, while the Ce valence is more sensitive to the diagenetic processes. If a dissolution process of Ce in the cherts is needed to reduce Ce, as in the case of the weathered granitic rocks, it is estimated that a part of REE in the cherts had been leached in the diagenetic processes. However, the present results suggest that the dissolution process did not alter the REE pattern of the whole rock of the cherts completely, irrespective of the diagenesis. Further study is needed to elucidate a general conclusion on REE behavior, including Ce redox changes in cherts during diagenesis. However, it is clear that the combination of REE patterns and the oxidation state of Ce revealed by XANES provides a new perspective on Ce and REE behavior during processes such as weathering and diagenesis. In particular, if we can ¢nd such a di¡erence between the results of
EPSL 5604 13-10-00
206
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207
REE pattern and XANES, the discrepancy will give us valuable knowledge about the initial redox condition and the post-depositional processes such as diagenesis, metamorphism and alteration.
[3] [4]
4. Summary Here we show various relationships between Ce anomalies and the oxidation states of Ce given by XANES. It is noteworthy that the XANES spectra at the LIII edge of Ce could be obtained even for cherts containing Ce abundances of ca. 15 ppm. The XANES technique, based on synchrotron radiation and the detection of £uorescent Xray by SSD, can clarify the Ce valence in many geological samples. Cerium in cherts and fresh granite is trivalent, while most of the Ce in manganese nodules is tetravalent. Both CeIII and CeIV species are found in weathered granite. This study shows that the oxidation state of Ce given by XANES, coupled with the Ce anomaly in REE patterns, can be used to provide a new view on REE geochemistry and hence clarify various geochemical processes. Acknowledgements
[5]
[6]
[7] [8]
[9] [10] [11]
We are grateful to N. Sato, T. Kunimaru and E. Nagashio for their help with the ICP-MS measurement and XANES experiment. We thank H. Ishisako and Y. Shibata for their technical support. We also thank Dr. R. Metcalfe (JNC) for his great contribution in improving this manuscript. Constructive comments by S. McLennan, M. Bau and another anonymous reviewer are greatly acknowledged.[AH]
[12]
[13] [14]
[15]
References [1] P. Henderson, Rare Earth Element Geochemistry, Elsevier, Amsterdam, 1984. [2] S.R. Taylor, S.M. McLennan, The signi¢cance of the rare earths in geochemistry and cosmochemistry, in: K.A. Gschneider, L. Eyring (Eds.), Handbook on the Physics
[16] [17]
and Chemistry of Rare Earths, vol. 11, Elsevier Sci. Publ., 1988, pp. 485^580. J.J. Braun, M. Pagel, J.P. Muller, P. Bilong, A. Michard, B. Guillet, Cerium anomalies in lateritic pro¢les, Geochim. Cosmochim. Acta 51 (1990) 597^605. Y. Takahashi, H. Shimizu, A. Usui, H. Kagi, M. Nomura, Direct observation of tetravalent cerium in ferromanganese nodules and crusts by X-ray-absorption nearedge structure (XANES), Geochim. Cosmochim. Acta 64 (2000) 2929^2935. A. Usui, A. Nishimura, N. Mita, Composition and growth history of sur¢cial and buried manganese nodules in the Penrhyn Basin, Southwestern Paci¢c, Mar. Geol. 114 (1993) 133^153. T. Kunimaru, H. Shimizu, K. Takahashi, S. Yabuki, Difference in geochemical features between Permian and Triassic cherts from the Southern Chichibu terrane, Southwest Japan: REE abundances, major element compositions and Sr isotopic ratios, Sediment. Geol. 119 (1998) 195^217. H. Shimizu, A. Masuda, Cerium in cherts as an indication of marine environment of its formation, Nature 266 (1977) 346^348. H. Shimizu, T. Kunimaru, S. Yoneda, M. Adachi, Sources and depositional environments of some Permian and Triassic cherts: signi¢cance of Rb^Sr and Sm^Nd isotopic and REE abundances data, J. Geol., in press. M. Nomura, A. Koyama, Design and performance of a new XAFS beamline at the photon factory; BL-12C, KEK Report 95-15, 1996. M. Nomura, Dead-time correction of a multi-element SSD for £uorescent XAFS, J. Synchrotron Rad. 5 (1998) 851^853. A. Bianconi, A. Marcelli, H. Dexpert, R. Karnatak, A. Kotani, T. Jo, J. Petiau, Speci¢c intermediate-valence state of insulating 4f compounds detected by L3 X-ray absorption, Phys. Rev. B 35 (1987) 806^812. G. Kaindl, G.K. Wertheim, G. Schmiester, E.V. Sampathkumaran, Mixed valency versus covalency in rare-earth core-electron spectroscopy, Phys. Rev. Lett. 58 (1987) 606^609. Z. Hu, S. Bertram, G. Kaindl, X-ray-absorption study of PrO2 at high pressure, Phys. Rev. B 49 (1994) 39^43. H. Shigekawa, M. Ishida, K. Miyake, R. Shioda, Y. Iijima, T. Imai, H. Takahashi, J. Sumaoka, M. Komiyama, Extended X-ray absorption ¢ne structure study on the cerium(IV)-induced DNA hydrolysis: implication to the roles of 4f orbitals in the catalysis, Appl. Phys. Lett. 74 (1999) 460^462. E. Anders, N. Grevesse, Abundances of the elements: meteoritic and solar, Geochim. Cosmochim. Acta 57 (1989) 197^214. J.F. Ban¢eld, R.A. Eggleton, Apatite replacement and rare earth mobilization, fractionation, and ¢xation during weathering, Clay Clay Miner. 37 (1989) 113^127. H.D. Schreiber, H.V. Luer, T. Thanyasiri, The redox state of cerium in basaltic magmas: an experimental study of
EPSL 5604 13-10-00
Y. Takahashi et al. / Earth and Planetary Science Letters 182 (2000) 201^207 iron^cerium interactions in silicate melts, Geochim. Cosmochim. Acta 44 (1980) 1599^1612. [18] N. Takematsu, Mangan Dankai (Manganese nodule), Koseisha Kouseikaku, Tokyo, 1998, pp. 67^74 (in Japanese). [19] E.H. De Carlo, G.M. McMurtry, Rare-earth element geochemistry of ferromanganese crusts from the Hawaiian Archipelago, central Paci¢c, Chem. Geol. 95 (1992) 235^ 250.
207
[20] E.D. Goldberg, Chemistry of the Oceans, in: M. Sears (Ed.), Oceanography, vol. 67, Am. Assoc. Adv. Sci. Publ., 1961, pp. 538^597. [21] A.J. Davenport, H.S. Isaacs, M.W. Kendig, XANES investigation of the role of cerium compounds as corrosion inhibitors for aluminum, Corros. Sci. 32 (1991) 653.
EPSL 5604 13-10-00