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U and Th thin film neutron dosimetry for fission-track dating: application to the age standard Moldavite
Radiation Measurements 39 (2005) 665 – 668 www.elsevier.com/locate/radmeas
U and Th thin film neutron dosimetry for fission-track dating: application to the age standard Moldavite P.J. Iunesa,∗ , G. Bigazzib , J.C. Hadler Netoa , M.A. Laurenzib , M.L. Balestrierib , P. Norellib , A.M. Osorio Arayac , S. Guedesa , C.A. Tello S.c , S.R. Paulod , P.A.F.P. Moreiraa , R. Palissaria , E.A.C. Curvoa a Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas, UNICAMP, 13083-970, Campinas—SP, Brazil b Istituto di Geoscienze e Georisorse, CNR, Area della Ricerca di Pisa, Via G. Moruzzi 1, 56124 Pisa, Italy c Departamento de Ciências Ambientais, Universidade do Estado de São Paulo, 19060-900 Presidente Prudente—SP, Brazil d Instituto de Ciências Exatas e da Terra, Universidade Federal do Mato Grosso, UFMT, 78070-900 Cuiabá—MT, Brazil
Received 13 February 2004; accepted 3 August 2004
Abstract Neutron dosimetry based on U and Th thin films was used for fission-track dating of the age standard Moldavite, the central European tektite, from the Middle Miocene deposit of Jankov (southern Bohemia, Czech Republic). Our fission-track age (13.98 ± 0.58 Ma) agrees with a recent 40Ar/39Ar age, 14.34 ± 0.04 Ma, based on several determinations on Moldavites from different sediments, including the Jankov deposit. This result indicates that the U and Th thin film neutron dosimetry represents a reliable alternative for an absolute approach in fission-track dating. © 2005 Elsevier Ltd. All rights reserved. Keywords: Fission-track dating; Neutron dosimetry; U thin film; Th thin film; Moldavite
1. Introduction During the last 20 years most fission-track (FT) groups adopted the system calibration known as “zeta calibration” (Hurford and Green, 1983). The use of the zeta calibration, based upon age standards, is a way of overcoming difficulties related to uncertainties in the fundamentals (such as in the value of the 238 U spontaneous fission decay constant or in the approach to be adopted for adequate neutron dosimetry) as well as in parameters related to some experimental procedures (such as the external detector method, Gleadow, 1981). However, the zeta calibration makes the FT method
∗ Corresponding author. Fax: +55 193 788 5314.
E-mail address: pjiunes@ifi.unicamp.br (P.J. Iunes). 1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2004.08.007
dependent on other isotopic techniques employed for determining the independent reference age of the standards used for the determination of the zeta factor. For this reason, research on procedures devoted to enabling FT dating to be an independent system with an absolute calibration was encouraged (see for instance Hurford, 1998). One of the crucial points for setting an absolute system calibration is neutron dosimetry. Bigazzi et al. (1999) have shown that the use of natural U and Th thin films can be an efficient alternative for accurate neutron dosimetry. In addition, it has been shown that U and Th thin films make possible to apply the FT method also when only weakly thermalized irradiation facilities are available, such as in many research nuclear reactors. The International Union of Geological Sciences (IUGS) had discouraged the use of such facilities (Hurford, 1990), because of the difficulties related
666
P.J. Iunes et al. / Radiation Measurements 39 (2005) 665 – 668
to the determination of the contribution (to the induced track density) of the U and Th fissions induced by epithermal and fast neutrons. Recently, neutron dosimetry based on U and Th thin films was adopted for dating Durango apatite (Iunes et al., 2002), one of the age standards recommended by the IUGS subcommission on geochronology for FT dating of apatite (Hurford, 1990). Our result, 29.7 ± 1.1 Ma, was consistent with the recommended reference age, 31.4±0.6 Ma, average of K–Ar determinations of the Carpintero group (McDowell and Keizer, 1977). In the present work, the neutron dosimetry based on U and Th thin films was used for dating Moldavite, the central European tektite, the unique glass recommended as the age standard for FT dating of natural glasses (Hurford, 1990).
2. Experimental Neutron irradiations were performed in three nuclear reactors: IPEN/CNEN—São Paulo, Brazil (irradiation I27MD and I32MD), LENA—Pavia, Italy (irradiation P4MD) and IPEN—Lima, Peru (irradiation L19MD). The sample-set present in these irradiations was made up of thin Th films, natural U-doped glasses (Cornig glass CN1 in irradiations I32MD and P4MD, and Cornig glasses CN2 and CN5 in irradiations I27MD and L19MD, respectively) and two Moldavites (MJ1 and MJ2) from the Middle Miocene deposit of Jankov, southern Bohemia (Bouška, 1994; Balestrieri et al., 1998). Moldavites MJ1 and MJ2 were included in irradiation I32MD, but only MJ1 was included in the other irradiations. Only in case of irradiation I32MD, Moldavite samples were pre-heated for 10 h at 560 ◦ C in order to erase spontaneous tracks. Ages were computed using the following equation: 1 (S /I )RM T = ln +1 , (1) C238 F where R M = RU +
NTh NU
RTh
RU = C235 A235 + C238 A238 ,
(2) RTh = A232 ,
(3)
where is the 238 U -decay constant; S /I is the spontaneous to induced track areal density ratio; C235 (C238 ) is the 235 U (238 U) abundance in natural U; F is the 238 U spontaneous fission decay constant; NTh /NU is the sample Th/U ratio; and A235 , A238 and A232 are the numbers of fissions per 235 U, 238 U and 232 Th target nucleus, respectively. The following values for the constants of Eq. (1) were adopted: = 1.55125 × 10−10 a−1 , C238 = 0.99275 and F =8.46×10−17 a−1 (Spadavecchia and Hahn, 1967). This F “historical” value, widely adopted through the FT community by people working with an absolute approach, is in close agreement with the F value of (8.5±0.1)×10−17 a−1
recently recommended by Holden and Hoffman (2000) and is also confirmed by a new measurement which is the average of nine determinations carried out using neutron dosimetry based on natural U thin films, (8.37 ± 0.17) × 10−17 a−1 (Guedes et al., 2003). The RM values used in the present work were computed from the determinations of the RU and RTh parameters, using in Eq. (2) a Th/U ratio of 4.41 for Jankov Moldavite (Bouška, 1994). The RU values were obtained through natural U-doped glasses present in the irradiations quoted above. The calibration of these glasses was carried out by irradiating them together with natural U thin films (Bigazzi et al., 1999), which played the role of primary calibrators. The RTh values were obtained through Th thin films, also present in the irradiations (Bigazzi et al., 1999). The calibration parameters of the glasses and thorium thin films used in this work are shown in Iunes et al. (2002).
3. Results and discussion Results of age determinations of Moldavites MJ1 and MJ2 from the Jankov deposit are shown in Table 1. It must be noted that all results shown in this work are quoted at 1, where is the standard error of the mean. These samples showed a certain degree of reduction of the spontaneous track sizes in comparison with the induced ones (DS /DI < 1). Reduced track sizes indicate that glass suffered a certain amount of partial annealing of spontaneous tracks, which are etched with reduced efficiency. Therefore, FT “raw” ages of MJ1 and MJ2 Moldavites are partially rejuvenated. In this work the plateau method introduced by Storzer and Poupeau (1973) was used for correcting thermally lowered ages. To apply this age correction technique, two groups from each sample—one for spontaneous track counting and the other for induced track counting (the irradiated one)—were heated for 24 h at 350 ◦ C. This heating step produced a significant reduction in track densities. The attainment of the plateau condition—an identical etching efficiency for spontaneous and induced tracks—is proved by the DS /DI ratio value that became close to 1. Considering only the plateau age determinations of irradiation I32MD, the weighted mean of the S /I ratios is 0.422 ± 0.019. Using this value and the RM value of Table 1, a mean plateau age of 13.31 ± 0.83 Ma was computed using Eq. (1). For irradiation P4MD, two thermal treatments were imposed for the plateau age determination—4 and 24 h at 350 ◦ C, respectively. In both cases, the plateau condition was attained (DS /DI ∼ 1). The weighted mean of the S /I ratios is 0.688 ± 0.043, and the corresponding mean plateau age is 14.2 ± 1.1 Ma. Considering the mean ages from irradiations I32MD and P4MD and the ages determined through irradiations L19MD and I27MD as independent determinations, the weighted
P.J. Iunes et al. / Radiation Measurements 39 (2005) 665 – 668
667
Table 1 Fission-track dating of Moldavites from the Middle Miocene deposit of Jankov Irradiat Glass
U
Th film
Th
(105 cm−2 ) (105 cm−2 )
CN1
ThE
3.74 ±2.7%
5.90 ±2.8%
I32MD
CN1
RM 10−9
Sample (heating)
342 0.30 80 515 0.20 200
1.145 ±5.4% 0.689 ±4.4%
577 0.50 56 505 0.50 76
2.76 ±4.2% 1.788 ±4.4%
0.901 ±3.1% 0.991 ±3.0%
13.1 ± 1.0 ±8.0% 12.23 ± 0.92 ±7.5%
MJ2 (no heating) MJ2 (24 h, 350 ◦ C)
950 0.50 195 537 0.60 150
1.304 ±5.1% 0.959 ±4.3%
788 0.40 70 452 0.60 60
3.01 ±3.6% 2.017 ±4.7%
0.876 ±3.3% 0.985 ±3.5%
13.66 ± 0.87 ±6.4% 15.0 ± 1.2 ±7.7%
322 0.40 98
0.880 ±5.6%
538a 0.75a 66a
1.30 ±8.2%
0.985 14.0 ± 1.5 ±3.2% ±10.9%
MJ1 497 0.60 (24 h, 350 ◦ C) 177
0.752 ±4.5%
835a 0.75a 122a
1.081 ±6.7%
1.061 14.4 ± 1.3 ±3.1% ±9.3%
0.949 MJ1 540 0.90 ±5.2% (24 h, 350 ◦ C) 193
0.750 ±4.3%
627a 0.10a 133a
0.512 ±11.5%
1.021 16.5 ± 2.2 ±2.5% ±13.4%
1.591 MJ1 352 0.40 ±5.3% (24 h, 350 ◦ C) 127
0.742 ±5.3%
923a 0.60a 145a
0.962 ±7.1%
0.957 14.6 ± 1.5 ±3.4% ±10.4%
2.929 ±2.8%
ThR
L19MD 0.366 ±2.9%
2.453 ±2.9%
CN2
ThJ
I27MD 2.075 ±3.6%
P (2 ) I DS /DI T (104 cm−2 ) (Ma)
MJ1 (no heating) 2.65 MJ1 ±4.2% (24 h, 350 ◦ C)
1.733 MJ1 ±4.6% (4 h, 350 ◦ C)
CN5
P ( 2 ) S NI (104 cm−2 ) nI
ThA
P4MD 2.45 ±3.3%
NS nS
4.33 ±3.3%
U is the surface track density on the mica coupled with the U-doped glass; Th is the surface track density on the mica coupled with the thorium thin film; RM is the value computed through Eq. (2); Heating is the thermal treatment imposed for the plateau age determination; NS (NI ) and nS (nI ) are the number of counted spontaneous (induced) tracks and the corresponding number of observed fields of view; P (2 ) is the probability of obtaining 2 value testing induced track counts against a Poisson distribution; S (I ) is the spontaneous (induced) track density; DS /DI is the spontaneous to induced track size ratio; T is the apparent (determined only for irradiation I32MD) or plateau age. Errors are ±1, where is the standard error of the mean. a Values refer to S+I measurements.
mean of these four plateau ages is 13.98 ± 0.58 Ma, with a 2 of 2.2 for three degrees of freedom (p(2 ) ∼ 0.50). In other words, the four independent determinations agree within experimental errors. The mean value of the plateau FT ages of the Jankov Moldavites studied in this work agrees with the mean value of 17 determinations made using the K/Ar and 40Ar/39Ar methods before 1995 (14.82 ± 0.32 Ma, Storzer et al., 1995)
and with the results of a new 40Ar/39Ar study aimed to better constrain the age of Moldavite. Several determinations on 7 samples from Bohemian and Moravian sediments, including the Jankov deposit, yielded an average of 14.34 ± 0.04 Ma (Laurenzi et al., 2003). This new age is significantly younger than the 40Ar/39Ar of 15.21 ± 0.08 Ma, published by Staudacher et al. (1982), and is in close agreement with recently published 40Ar/39Ar data (14.40 ± 0.25 Ma (Lange
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P.J. Iunes et al. / Radiation Measurements 39 (2005) 665 – 668
et al., 1995) 14.50±0.21 Ma and 14.38±0.12 Ma (Schwarz and Lippolt, 2002)). However, it is significantly more precise.
4. Conclusions The age of the Moldavite from the Middle Pliocene deposit of Jankov determined in this work is in close agreement with the more recent independent 40Ar/39Ar determinations made on Moldavites. The results of this work confirm those previously obtained through the analysis of another age standard—Durango apatite—and indicate that neutron dosimetry based on natural U and Th thin films turns out to be a reliable tool to be used for an absolute approach in FT dating.
Acknowledgements Three of us (P.J.I., C.A.T.S. and S.G.) are grateful to FAPESP, the Research Foundation of São Paulo State, for financial support.
References Balestrieri, M.L., Bigazzi, G., Bouška, V., Labrin, E., Hadler Neto, J.C., Kitada, N., Osorio Araya, A.M., Poupeau, G., Wadatsumi, K., Zúñiga, A., 1998. Potential glass age standards for fissiontrack dating: an overview. In: Van den haute, P., De Corte, F. (Eds.), Advances in Fission-Track Geochronology. Kluwer Academic Publishers, Dordrecht, pp. 287–304. Bigazzi, G., Guedes, S., Hadler Neto, J.C., Iunes, P.J., Oddone, M., Osorio Araya, A.M., Paulo, S.R., Zúñiga, A., 1999. Potentialities and practical limitations of an absolute neutron dosimetry using thin films of uranium and thorium applied to the fission track method. Radiat. Meas. 31, 651–656. Bouška, V., 1994. Moldavites. The Czech tektites. Stylizace, Prague. Gleadow, A.J.W., 1981. Fission track dating methods: what are the real alternatives? Nucl. Tracks 5, 3–14. Guedes, S., Hadler Neto, J.C., Iunes, P.J., Zúñiga, A., Tello S, C.A., Paulo, S.R., 2003. The use of the U(n, f) reaction dosimetry
in the determination of the F value through fission-track techniques. Nucl. Instrum. Methods Phys. Res. A 496, 215–221. Holden, N.E., Hoffman, D.C., 2000. Spontaneous fission half-lives for ground-state nuclides. Pure Appl. Chem. 72, 1525–1562. Hurford, A.J., 1990. Standardization of fission track dating calibration: recommendation by the Fission-Track Working Group of the IUGS subcommission on Geochronology. Chem. Geology (Isot. Geosci. Section) 80, 171–178. Hurford, A.J., 1998. Zeta: the ultimate solution or just an interim measure?. In: Van den haute, P., De Corte, F. (Eds.), Advances in Fission-Track Geochronology. Kluwer Academic Publishers, Dordrecht, pp. 19–32. Hurford, A.J., Green, P.F., 1983. The Zeta age calibration of fission track dating. Chem. Geol. (Isot. Geosci. Section) 1, 285–317. Iunes, P.J., Hadler Neto, J.C., Bigazzi, G., Tello S., C.A., Guedes, S., Paulo, S.R., 2002. Durango apatite fission-track dating using length-based age corrections and neutron fluence measurements by natural thorium thin films and natural U-doped glasses calibrated through natural uranium thin films. Chem. Geol. 187, 201–211. Lange, J.-M., Bollinger, H., Horn, P., Jessberger, E.K., Schaaf, P., Storzer, D., 1995. Moldavites from Lusatia (Germany) III: Sr isotope, 40Ar/39Ar, and fission track studies. 26th Lunar and Planetary Science Conference, pp. 823–824. Laurenzi, M.A., Bigazzi, G., Balestrieri, M.L., Bouška, V., 2003. 40Ar/39Ar laser probe dating of the Central European tektiteproducing impact event. Meteor. Planet. Sci. 38, 887–894. McDowell, F.W., Keizer, R.P., 1977. Timing of mid-tertiary volcanism in the Sierra Madre occidental between Durango city and Mazatlan, Mex. Geol. Soc. Am. Bull. 88, 1479–1487. Schwarz, W.H., Lippolt, H.J., 2002. Coeval 40Ar/39Ar ages of moldavites from the Bohemian and Lusatian strewn fields. Meteor. Planet. Sci. 37, 1757–1763. Spadavecchia, A., Hahn, B., 1967. Die Rotationskammer und einigen Anwendungen. Helv. Phys. Acta 40, 1063–1069. Staudacher, T., Jessberger, E.K., Dominik, B., Kirsten, T., Schaeffer, O.A., 1982. 40Ar/39Ar ages of rocks and glasses from the Nördlinger Ries crater and the temperature history of impact breccias. J. Geophys. 51, 1–11. Storzer, D., Poupeau, G., 1973. Ages-plateaux de minéraux et verres par la méthode des traçes de fission. Comp. Rend. Acad. Sci. Paris 276 (D), 137–139. Storzer, D., Jessberger, E.K., Kunz, J., Lange, J.-M., 1995. Synopsis von spaltspuren- und kalium–argon–datierungen an ries-impaktgläsern und Moldaviten. Exkurs. f. u. Veröfftl. GGW 195, 79–80.
U and Th thin film neutron dosimetry for fission-track dating: application to the age standard Moldavite P.J. Iunesa,∗ , G. Bigazzib , J.C. Hadler Netoa , M.A. Laurenzib , M.L. Balestrierib , P. Norellib , A.M. Osorio Arayac , S. Guedesa , C.A. Tello S.c , S.R. Paulod , P.A.F.P. Moreiraa , R. Palissaria , E.A.C. Curvoa a Instituto de Fisica Gleb Wataghin, Universidade Estadual de Campinas, UNICAMP, 13083-970, Campinas—SP, Brazil b Istituto di Geoscienze e Georisorse, CNR, Area della Ricerca di Pisa, Via G. Moruzzi 1, 56124 Pisa, Italy c Departamento de Ciências Ambientais, Universidade do Estado de São Paulo, 19060-900 Presidente Prudente—SP, Brazil d Instituto de Ciências Exatas e da Terra, Universidade Federal do Mato Grosso, UFMT, 78070-900 Cuiabá—MT, Brazil
Received 13 February 2004; accepted 3 August 2004
Abstract Neutron dosimetry based on U and Th thin films was used for fission-track dating of the age standard Moldavite, the central European tektite, from the Middle Miocene deposit of Jankov (southern Bohemia, Czech Republic). Our fission-track age (13.98 ± 0.58 Ma) agrees with a recent 40Ar/39Ar age, 14.34 ± 0.04 Ma, based on several determinations on Moldavites from different sediments, including the Jankov deposit. This result indicates that the U and Th thin film neutron dosimetry represents a reliable alternative for an absolute approach in fission-track dating. © 2005 Elsevier Ltd. All rights reserved. Keywords: Fission-track dating; Neutron dosimetry; U thin film; Th thin film; Moldavite
1. Introduction During the last 20 years most fission-track (FT) groups adopted the system calibration known as “zeta calibration” (Hurford and Green, 1983). The use of the zeta calibration, based upon age standards, is a way of overcoming difficulties related to uncertainties in the fundamentals (such as in the value of the 238 U spontaneous fission decay constant or in the approach to be adopted for adequate neutron dosimetry) as well as in parameters related to some experimental procedures (such as the external detector method, Gleadow, 1981). However, the zeta calibration makes the FT method
∗ Corresponding author. Fax: +55 193 788 5314.
E-mail address: pjiunes@ifi.unicamp.br (P.J. Iunes). 1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2004.08.007
dependent on other isotopic techniques employed for determining the independent reference age of the standards used for the determination of the zeta factor. For this reason, research on procedures devoted to enabling FT dating to be an independent system with an absolute calibration was encouraged (see for instance Hurford, 1998). One of the crucial points for setting an absolute system calibration is neutron dosimetry. Bigazzi et al. (1999) have shown that the use of natural U and Th thin films can be an efficient alternative for accurate neutron dosimetry. In addition, it has been shown that U and Th thin films make possible to apply the FT method also when only weakly thermalized irradiation facilities are available, such as in many research nuclear reactors. The International Union of Geological Sciences (IUGS) had discouraged the use of such facilities (Hurford, 1990), because of the difficulties related
666
P.J. Iunes et al. / Radiation Measurements 39 (2005) 665 – 668
to the determination of the contribution (to the induced track density) of the U and Th fissions induced by epithermal and fast neutrons. Recently, neutron dosimetry based on U and Th thin films was adopted for dating Durango apatite (Iunes et al., 2002), one of the age standards recommended by the IUGS subcommission on geochronology for FT dating of apatite (Hurford, 1990). Our result, 29.7 ± 1.1 Ma, was consistent with the recommended reference age, 31.4±0.6 Ma, average of K–Ar determinations of the Carpintero group (McDowell and Keizer, 1977). In the present work, the neutron dosimetry based on U and Th thin films was used for dating Moldavite, the central European tektite, the unique glass recommended as the age standard for FT dating of natural glasses (Hurford, 1990).
2. Experimental Neutron irradiations were performed in three nuclear reactors: IPEN/CNEN—São Paulo, Brazil (irradiation I27MD and I32MD), LENA—Pavia, Italy (irradiation P4MD) and IPEN—Lima, Peru (irradiation L19MD). The sample-set present in these irradiations was made up of thin Th films, natural U-doped glasses (Cornig glass CN1 in irradiations I32MD and P4MD, and Cornig glasses CN2 and CN5 in irradiations I27MD and L19MD, respectively) and two Moldavites (MJ1 and MJ2) from the Middle Miocene deposit of Jankov, southern Bohemia (Bouška, 1994; Balestrieri et al., 1998). Moldavites MJ1 and MJ2 were included in irradiation I32MD, but only MJ1 was included in the other irradiations. Only in case of irradiation I32MD, Moldavite samples were pre-heated for 10 h at 560 ◦ C in order to erase spontaneous tracks. Ages were computed using the following equation: 1 (S /I )RM T = ln +1 , (1) C238 F where R M = RU +
NTh NU
RTh
RU = C235 A235 + C238 A238 ,
(2) RTh = A232 ,
(3)
where is the 238 U -decay constant; S /I is the spontaneous to induced track areal density ratio; C235 (C238 ) is the 235 U (238 U) abundance in natural U; F is the 238 U spontaneous fission decay constant; NTh /NU is the sample Th/U ratio; and A235 , A238 and A232 are the numbers of fissions per 235 U, 238 U and 232 Th target nucleus, respectively. The following values for the constants of Eq. (1) were adopted: = 1.55125 × 10−10 a−1 , C238 = 0.99275 and F =8.46×10−17 a−1 (Spadavecchia and Hahn, 1967). This F “historical” value, widely adopted through the FT community by people working with an absolute approach, is in close agreement with the F value of (8.5±0.1)×10−17 a−1
recently recommended by Holden and Hoffman (2000) and is also confirmed by a new measurement which is the average of nine determinations carried out using neutron dosimetry based on natural U thin films, (8.37 ± 0.17) × 10−17 a−1 (Guedes et al., 2003). The RM values used in the present work were computed from the determinations of the RU and RTh parameters, using in Eq. (2) a Th/U ratio of 4.41 for Jankov Moldavite (Bouška, 1994). The RU values were obtained through natural U-doped glasses present in the irradiations quoted above. The calibration of these glasses was carried out by irradiating them together with natural U thin films (Bigazzi et al., 1999), which played the role of primary calibrators. The RTh values were obtained through Th thin films, also present in the irradiations (Bigazzi et al., 1999). The calibration parameters of the glasses and thorium thin films used in this work are shown in Iunes et al. (2002).
3. Results and discussion Results of age determinations of Moldavites MJ1 and MJ2 from the Jankov deposit are shown in Table 1. It must be noted that all results shown in this work are quoted at 1, where is the standard error of the mean. These samples showed a certain degree of reduction of the spontaneous track sizes in comparison with the induced ones (DS /DI < 1). Reduced track sizes indicate that glass suffered a certain amount of partial annealing of spontaneous tracks, which are etched with reduced efficiency. Therefore, FT “raw” ages of MJ1 and MJ2 Moldavites are partially rejuvenated. In this work the plateau method introduced by Storzer and Poupeau (1973) was used for correcting thermally lowered ages. To apply this age correction technique, two groups from each sample—one for spontaneous track counting and the other for induced track counting (the irradiated one)—were heated for 24 h at 350 ◦ C. This heating step produced a significant reduction in track densities. The attainment of the plateau condition—an identical etching efficiency for spontaneous and induced tracks—is proved by the DS /DI ratio value that became close to 1. Considering only the plateau age determinations of irradiation I32MD, the weighted mean of the S /I ratios is 0.422 ± 0.019. Using this value and the RM value of Table 1, a mean plateau age of 13.31 ± 0.83 Ma was computed using Eq. (1). For irradiation P4MD, two thermal treatments were imposed for the plateau age determination—4 and 24 h at 350 ◦ C, respectively. In both cases, the plateau condition was attained (DS /DI ∼ 1). The weighted mean of the S /I ratios is 0.688 ± 0.043, and the corresponding mean plateau age is 14.2 ± 1.1 Ma. Considering the mean ages from irradiations I32MD and P4MD and the ages determined through irradiations L19MD and I27MD as independent determinations, the weighted
P.J. Iunes et al. / Radiation Measurements 39 (2005) 665 – 668
667
Table 1 Fission-track dating of Moldavites from the Middle Miocene deposit of Jankov Irradiat Glass
U
Th film
Th
(105 cm−2 ) (105 cm−2 )
CN1
ThE
3.74 ±2.7%
5.90 ±2.8%
I32MD
CN1
RM 10−9
Sample (heating)
342 0.30 80 515 0.20 200
1.145 ±5.4% 0.689 ±4.4%
577 0.50 56 505 0.50 76
2.76 ±4.2% 1.788 ±4.4%
0.901 ±3.1% 0.991 ±3.0%
13.1 ± 1.0 ±8.0% 12.23 ± 0.92 ±7.5%
MJ2 (no heating) MJ2 (24 h, 350 ◦ C)
950 0.50 195 537 0.60 150
1.304 ±5.1% 0.959 ±4.3%
788 0.40 70 452 0.60 60
3.01 ±3.6% 2.017 ±4.7%
0.876 ±3.3% 0.985 ±3.5%
13.66 ± 0.87 ±6.4% 15.0 ± 1.2 ±7.7%
322 0.40 98
0.880 ±5.6%
538a 0.75a 66a
1.30 ±8.2%
0.985 14.0 ± 1.5 ±3.2% ±10.9%
MJ1 497 0.60 (24 h, 350 ◦ C) 177
0.752 ±4.5%
835a 0.75a 122a
1.081 ±6.7%
1.061 14.4 ± 1.3 ±3.1% ±9.3%
0.949 MJ1 540 0.90 ±5.2% (24 h, 350 ◦ C) 193
0.750 ±4.3%
627a 0.10a 133a
0.512 ±11.5%
1.021 16.5 ± 2.2 ±2.5% ±13.4%
1.591 MJ1 352 0.40 ±5.3% (24 h, 350 ◦ C) 127
0.742 ±5.3%
923a 0.60a 145a
0.962 ±7.1%
0.957 14.6 ± 1.5 ±3.4% ±10.4%
2.929 ±2.8%
ThR
L19MD 0.366 ±2.9%
2.453 ±2.9%
CN2
ThJ
I27MD 2.075 ±3.6%
P (2 ) I DS /DI T (104 cm−2 ) (Ma)
MJ1 (no heating) 2.65 MJ1 ±4.2% (24 h, 350 ◦ C)
1.733 MJ1 ±4.6% (4 h, 350 ◦ C)
CN5
P ( 2 ) S NI (104 cm−2 ) nI
ThA
P4MD 2.45 ±3.3%
NS nS
4.33 ±3.3%
U is the surface track density on the mica coupled with the U-doped glass; Th is the surface track density on the mica coupled with the thorium thin film; RM is the value computed through Eq. (2); Heating is the thermal treatment imposed for the plateau age determination; NS (NI ) and nS (nI ) are the number of counted spontaneous (induced) tracks and the corresponding number of observed fields of view; P (2 ) is the probability of obtaining 2 value testing induced track counts against a Poisson distribution; S (I ) is the spontaneous (induced) track density; DS /DI is the spontaneous to induced track size ratio; T is the apparent (determined only for irradiation I32MD) or plateau age. Errors are ±1, where is the standard error of the mean. a Values refer to S+I measurements.
mean of these four plateau ages is 13.98 ± 0.58 Ma, with a 2 of 2.2 for three degrees of freedom (p(2 ) ∼ 0.50). In other words, the four independent determinations agree within experimental errors. The mean value of the plateau FT ages of the Jankov Moldavites studied in this work agrees with the mean value of 17 determinations made using the K/Ar and 40Ar/39Ar methods before 1995 (14.82 ± 0.32 Ma, Storzer et al., 1995)
and with the results of a new 40Ar/39Ar study aimed to better constrain the age of Moldavite. Several determinations on 7 samples from Bohemian and Moravian sediments, including the Jankov deposit, yielded an average of 14.34 ± 0.04 Ma (Laurenzi et al., 2003). This new age is significantly younger than the 40Ar/39Ar of 15.21 ± 0.08 Ma, published by Staudacher et al. (1982), and is in close agreement with recently published 40Ar/39Ar data (14.40 ± 0.25 Ma (Lange
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et al., 1995) 14.50±0.21 Ma and 14.38±0.12 Ma (Schwarz and Lippolt, 2002)). However, it is significantly more precise.
4. Conclusions The age of the Moldavite from the Middle Pliocene deposit of Jankov determined in this work is in close agreement with the more recent independent 40Ar/39Ar determinations made on Moldavites. The results of this work confirm those previously obtained through the analysis of another age standard—Durango apatite—and indicate that neutron dosimetry based on natural U and Th thin films turns out to be a reliable tool to be used for an absolute approach in FT dating.
Acknowledgements Three of us (P.J.I., C.A.T.S. and S.G.) are grateful to FAPESP, the Research Foundation of São Paulo State, for financial support.
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