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On epidote fission track dating
Radiation Measurements 39 (2005) 641 – 645 www.elsevier.com/locate/radmeas
On epidote fission track dating E.A.C. Curvoa,∗ , J.C. Hadler Netoa , P.J. Iunesa , S. Guedesa , C.A. Tello S.a , S.R. Paulob , P.C. Hackspacherc , R. Palissaria , P.A.F.P. Moreiraa a Instituto de Física “Gleb Wataghin”, Universidade Estadual de Campinas, UNICAMP, 13083-970 Campinas, SP, Brazil b Departamento de Física, Instituto de Ciências Exatas e da Terra, Universidade Federal de Mato Grosso, UFMT, 78060-900,
Cuiabá, MT, Brazil c Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, UNESP, 13506-900, Rio Claro, SP, Brazil
Received 13 February 2004; accepted 30 June 2004
Abstract The use of epidote in fission track dating was abandoned since the beginning of the 1980s due to difficulties like absence of a standard etching procedure, obtainment of different closure temperatures and the percentage of the datable samples. The results become much more reproducible when restricting fission track analysis to a peculiar kind of track. We are also studying confined track length, what makes possible to obtain information about fossil track annealing. Fission tracks in epidote were successfully etched with 48% HF at 35 ◦ C for 12.5 min. Dating samples by the external detector method was not possible due to problems in measuring the efficiency factor held between the number of fossil fission tracks and tracks induced in mica. Dating a sample from Brejuí, RN, Brazil with the population method gave a corrected age of 510 ± 69 Ma, in agreement with published U/Th–Pb ages. From the fact that the fossil track length histogram was bimodal, we were able to infer that this sample registered a thermal episode during its history. These preliminary results indicate that epidote deserves further studies to establish whether it can be employed as a thermochronological tool. © 2005 Elsevier Ltd. All rights reserved. Keywords: Epidote; Fission track dating; Brejuí epidote; Borborema Province—Brazil
1. Introduction Studies about dating epidote using the fission track method started in the 1970s and were abandoned in the early 1980s. The reasons were the difficulties in dealing with this mineral, that can be summarized as: (i) a standard etching procedure for this mineral was not developed, (ii) very different values were found for the closure temperature (around 260 ◦ C according to Haack (1976), Saini ∗ Corresponding author. Tel.: +55 19 37885362; fax: +55 19 37885512. E-mail address: curvo@ifi.unicamp.br (E.A.C. Curvo).
1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2004.06.016
et al. (1978) and 630 ◦ C according to Naeser et al. (1970)) and (iii) the percentage of datable samples was found to be very variable, for instance Reimer (1972) dated 8 out of 12 collected samples while Haack (1976) succeeded in dating only 9 out of a total of 125. In item (iii) a variety of problems can be summarized: the crystals have all kinds of imperfections and inclusions, the U distribution is extremely heterogeneous, zoning is common and the U content could be too low or too high. In spite of these difficulties, this mineral presents interesting characteristics that justify further investigation. Epidote is a hydrothermal mineral, which, in principle, should permit the dating of geological reactivation of faulting zones. Its
E.A.C. Curvo et al. / Radiation Measurements 39 (2005) 641 – 645
closure temperature is higher than that of apatite (∼ 100 ◦ C; Wagner and Van Den Haute, 1992) and zircon (∼ 210 ◦ C; Wagner and Van Den Haute, 1992). A high closure temperature is useful for dating geological events that erased all the fossil tracks in apatite and zircon. Thus, annealing effects detected in epidote reflect a high-temperature thermal history which is inaccessible by fission-track analysis of apatite and zircon. In this work we restarted the application of the fission track dating method to the epidote. An old problem often described in former works, etching anisotropy, was investigated and a solution is proposed. 2. Experimental procedure
8
<ρ> = (1.103 ± 0.073)x106 tracks/cm2
6 4 2 0 0
Table 1 Etching experiment carried out under HF 48% at 35 ◦ C 5 min
10 min
12.5 min
15 min
Total density — 0.906 ± 0.041 1.103 ± 0.073 0.787 ± 0.055 Dark tracks — 0.524 ± 0.021 0.519 ± 0.027 0.475 ± 0.025 Length (m) 8.30 ± 0.25 9.77 ± 0.24 10.53 ± 0.21 10.52 ± 0.21 12.5 min was the time chosen as standard in this work.
5
10
15 20 25 tracks/field
30
35
40
Fig. 1. Fossil track density histogram for the Brejuí sample. x means the average track density per field, S is the standard deviation of the distribution and is the average track density per square centimeter.
Fig. 2. Epidote fossil-tracks photographs. Some “dark tracks” are highlighted.
18 = 6.00 S = 3.64 tracknumber = 804 <ρ> = (0.519 ± 0.027)x106 tracks/cm2
15 12 N
The sample studied in this work is a monocrystal from Brejuí-RN, located in the Northern part of Borborema Province, Northeast Brazil. It was broken and crushed into 200 m grains. After this, the grains were mounted in epoxy resin, polished and etched for 12.5 min in HF 48% at 35 ◦ C. This etching time was obtained through an etching experiment with progressive etching time (Table 1), where fossil track density and fossil confined track length were measured. NaOH is also used for etching tracks in epidote (Naeser et al., 1970; Bar et al., 1974; Haack, 1976; Saini et al., 1978), but etching 25 N NaOH, for 300 min at 75 ◦ C, did not succeeded in revealing any track in our sample. As shown in Fig. 1, the track density distribution is broad, and deviates from the Poisson distribution, expected for a radioactive decay where the uranium distribution of the source is homogeneous. The most probable explanation of this is the etching anisotropy. Bal et al. (1982) have shown that different crystalline faces presented very different track densities, relating this to an etching anisotropy. To solve this problem, we choose a specific kind of tracks oriented in a short angular distribution, usually darker than others random oriented in the same grain, “dark tracks” (Fig. 2). The criterium to set up what is or is not a dark track is observational, based upon the track angular distribution and their opacity. We had named the existence of these special tracks a “fission-track-window”. By counting only them the distribution became less broad (Fig. 3). In Table 2 the reduced 2 values of these distributions are shown together
= 12.75 S = 8.63 track number = 1326
N
642
9 6 3 0 0
5
10
15 20 25 tracks/field
30
35
40
Fig. 3. Fossil-dark track density histogram. x means the average track density per field, S is the standard deviation of the distribution and is the average track density per square centimeter.
E.A.C. Curvo et al. / Radiation Measurements 39 (2005) 641 – 645 Table 2 Reduced 2 values for total and dark track density histograms, for 3 etching times 10 min
12.5 min
15 min
Total density Dark tracks
6.02 2.12
5.84 2.21
4.53 1.94
= 10.53 S = 2.18 σ = 0.21 track number = 107
25 20 15 N
2
643
10 16
5
14
= 14.17 S = 1.27 σ = 0.20 track number = 39
12
N
10
0 0
8
2
4
6
8
10 12 L (µm)
14
16
18
20
Fig. 5. Confined track length histogram. x means the average track length, S means the standard deviation of the distribution and means the standard deviation of the mean.
6 4 2 0 0
2
4
6
8
10 12 L (µm)
14
16
18
20
Fig. 4. Induced track length histogram. x means the average track length, S means the standard deviation of the distribution and means the standard deviation of the mean.
with the distributions of total track density. It is clear that the density of dark tracks presents distribution close to Poisson. By this way we had adopted the dark tracks like our counting criterium, regarding that they are easily counted and that they give easily comparative results. 2.1. Brejuí-RN dating by the population method In this method the samples’s grains are divided in two fractions one for counting the fossil tracks and the other for the induced ones. The latter is irradiated with thermal neutrons, after its fossil tracks were previously erased in an oven. After this, both fractions are submitted to the same polishing, etching and analysed under the optical microscope. The grains of Brejuí sample were irradiated with a thermal neutron fluence of 1.95 × 1015 cm−2 (±3.6%), in the IPEN/CNEN—São Paulo nuclear reactor. The dark induced track density (I ) obtained was (0.1361 ± 0.0083) × 106 tracks/cm2 (1). The mean confined induced track length (L0 ) was (14.17 ± 0.20) m (1) and the histogram of induced tracks can be seen in Fig. 4.
3. Results An interesting fact is that a bimodal fossil length histogram was obtained, as shown in Fig. 5. This means that
the sample was submitted to an annealing during its thermal history that shortened part of its fossil tracks. Using the dating equation adapted to our neutron dosimetry (Iunes et al., 2002): 1 RM t = ln 1 + 238 s , (1) i f C238 where is the decay constant of 238 U, f is the decay constant for spontaneous fission of 238 U, RM is the fraction of fission events per uranium target nucleus of the mineral, 238 is the mineral detection efficiency, i.e., the ratio between the number of observed tracks per unit area and the number of 238 U spontaneous fission, per unit volume, occurring in the mineral, is the corresponding detection efficiency for the 235 U induced fissions, C238 is the 238 U isotopic concentration in natural uranium, s is the number of spontaneous (or fossil) tracks per unit area, i is the number of induced tracks per unit area. The following values were used: = 1.55 × 10−10 yr −1 (Jaffey et al., 1971), f = 8.35 × 10−17 yr −1 (Guedes et al., 2000) in agreement with IUPAC recommendation (Holden and Hoffman, 2000) and with another determination in which neutron dosimetry was not needed (Guedes et al., 2003); C238 = 0.99275 (Lederer and Shirley, 1978); 238 /=1 (Bigazzi et al., 2000); RM = (8.15 ± 0.29) × 10−9 (±3.6%). In this way an apparent age (tap ) of 367 ± 32 Ma was obtained. The correspondent corrected age, 496 ± 66 Ma, was taken via size correction method (Storzer and Wagner, 1969), by using the data from the correction curve (/0 versus L/L0 ) proposed by Saini et al. (1978), where 238 / was taken as /0 . A Poisson error was assumed for the track densities and a 10% error was assumed to the 238 / values. Considering that the track length histogram is bimodal (Fig. 5) two independent histograms can be obtained if we
644
E.A.C. Curvo et al. / Radiation Measurements 39 (2005) 641 – 645 Table 3 Characteristics of short and long track histograms
1.1 1.0 0.9
Short track histogram
Long track histogram
7.21 ± 0.21 21 0.510 (±3.2%)
11.34 ± 0.16 86 0.800 (±2.0%)
19.6 (±21.8%)
80.4 (±10.8%)
tap (Ma)
21.7 (±21.8%) 1.12 × 105 (±22.4%) 79 (±22.7%)
78.3 (±10.8%) 4.06 × 105 (±12.0%) 284 (±12.5%)
238 / tcorr (Ma)
0.510 (±10%) 155 (±24.8%)
0.80 (±10%) 355 (±16.0%)
0.8
ρ/ρ
0
0.7 0.6 0.5 0.4 0.3
Simulated confined tracks
0.2
Projected tracks
0.1
Polynomial fit
0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 l/l0
L(m) Track number L/L0 Confined tracks (%) Projected tracks (%) s (cm−2 )
Fig. 6. Comparison between efficiencies for revelation of confined and projected tracks.
equation split it at 9 m. By aiming to obtain the density of projected tracks for both histograms, from the total projected p track density (total = (0.519 ± 0.027) × 106 tracks/cm2 (1), measured density of tracks that cross the mineral surface), it is necessary to know how the density of projected and confined tracks are related. It can be analytically showed that the density of projected tracks is directly proportional to the length of the fission tracks (Wagner and Van Den Haute, 1992). We did not succeed to analytically show how the density of confined tracks changes with their length. To access this information, a computational simulation was carried out. The program works as showed below: (i) A 4×4 matrix of defects, representing the surface tracks, is established. The separation between defects was calculated taking into account the measured surface track density, supposing that the tracks were uniformly distributed in a square arrangement. The value used was 9.5 m. (ii) After this different sized tracks are systematically simulated in the region of the four central defects. At first, only tracks with 1 m are simulated. The whole area is thus covered through steps of 0.5 m. To each point 1000 directions are drawn and if it meets a defect the track is virtually revealed. The process is repeated, with increases of 0.5 m in the track length, until the confined track length reaches 15 m. The results are presented in Fig. 6, where a third-order polynomial fit brought the mean curve. It can be noted that the confined tracks are not revealed with the same geometrical efficiency that the projected tracks. Thus, it is necessary to correct the distribution of tracks in the length histogram before calculating the splitted histogram ages. The correction can be understood through the
Nl = Nl
total , l
(2)
where Nl is the new number of confined tracks with length l, Nl is the old one, l is the length efficiency, i.e., how easily a track of length l is revealed and total is the average total efficiency, i.e., an average over all l . Now the densities of projected tracks can be calculated p
s =
Ns p , Ntotal total p
(3)
where s is the density of projected tracks in the short track p histogram, total is the total density of projected tracks, Ns is the number of confined tracks in the short track histogram and Ntotal is the total number of confined tracks. The density of projected tracks in the long track histogram is taken via Eq. (3) by replacing the “s” quantities by “l” ones. The results of the correction are presented in Table 3. The sum of the corrected time values (tcorr ) brings to the fission track retention age of this epidote sample: (510 ± 69) Ma. No significant thermal event took place during the time related to the large track histogram (355 ± 57) Ma. To obtain the corrected times (tcorr ) in Table 3 we used the size correction method, with Saini et al. (1978) data. The computational simulation did not calculated the error propagation, so the error from the percentage of confined tracks was adopted. A Sm–Nd age of approximately 590 Ma is found in the literature (Cordani and Sato, 1999) being attributed to the Brazilian Orogeny. In 2001, Souza et al. by using the U/Th–Pb method in monazites, dated samples from the Northeast of Borborema Province. They obtained an event dated as (403 ± 28) Ma besides the Brazilian Orogeny (578 ± 25) Ma.
E.A.C. Curvo et al. / Radiation Measurements 39 (2005) 641 – 645
4. Discussion and conclusion A counting method, based on a special kind of tracks which have a higher opacity and a short angular distribution, was developed and applied to epidote fission track dating. These dark tracks appear because of the strength of the etching anisotropy, that is to say that some crystalline directions have a faster etching rate. This method showed to be very useful when there is a high fission track etching anisotropy, providing a better control above it. It is worth noting that although the dark track approach improves the epidote dating, problems with uranium inhomogeneity should not be discarded. By applying the population dating method to the BrejuíRN sample a corrected age of (496 ± 66) Ma was obtained. This value is in agreement with the Sm–Nd dating, that is a method presenting higher closure temperature (650–680 ◦ C; Thöni, 2002). This age has an accepted geological interpretation (Brazilian Orogeny). Besides this, the confined track length histogram shows a bimodal distribution which could be interpreted as a thermal event that took place up to (355 ± 57) Ma ago and the age compatible with the Brazilian Orogeny (510 ± 69) Ma. The (355 ± 57) Ma age could be related with the younger age obtained by Souza et al. (2001), of (403 ± 28) Ma. However, a problem related with the closure temperatures arise. If the thermal event reset the U/Th–Pb clock, which have a higher closure temperature (> 750 ◦ C; Braun et al., 1998), it should have erased all the tracks in epidote. A plausible explanation is that the monazite was heated more strongly during the event, while the epidote was far away sufficiently to suffer only a partial annealing. This younger event has no geological interpretation yet. This work with confined track length indicates that epidote fission track dating has the potential to be used as a thermochronological tool, provided the anisotropy effects are taken into consideration. Acknowledgements The authors are grateful to Dr. Adalberto José Soares, from IPEN/CNEN, São Paulo, Brazil, for performing the neutron irradiations, to Luiz Felipe Brandini Ribeiro, from Instituto de Geociências e Ciência Exatas, UNESP, Rio Claro, for providing the epidote sample, and to FAPESP (State of São Paulo Research Foundation) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for Eduardo Augusto Campos Curvo scholarship. This work is part of the Fapesp Thematic Project 00/03960-5. References Bal, K.D., Lal, N., Nagpaul, K.K., 1982. Fission track etching studies of different planes of epidote. Phys. Chem. Miner. 8, 158–160.
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Bar, M., Kolodny, Y., Bentor, Y.K., 1974. Dating faults by fission track dating of epidotes—an attempt. Earth Plan. Sci. Lett. 22, 157–162. Bigazzi, G., Guedes, S., Hadler Neto, J.C., Iunes, P.J., Paulo, S.R., Tello S., C.A., 2000. Application of neutron dosimetry by natural uranium and thorium thin films in fission track dating. In: Proceedings of the Ninth International Conference on Fission Track Dating and Thermochronology, pp. 33–35. Braun, I., Montel, J., Nicollet, C., 1998. Electron microprobe dating of monazites from high-grade gneisses and pegmatites of the Kerala Khondalite Belt, southern India. Chem. Geol. 146, 65–85. Cordani, U.G., Sato, K., 1999. Crustal evolution of the South American plataform, based on Nd isotopic systematics on granitoid rocks. Episodes 22 (3), 167–173. Guedes, S., Hadler Neto, J.C., Iunes, P.J., Paulo, S.R., Zuñiga, A., 2000. The spontaneous fission decay constant of 238U using SSNTD. J. Radioanal. Nucl. Chem. 245 (1), 441–442. Guedes, S., Hadler Neto, J.C., Sarkis, J.E., Oliveira, K.M., Kakazu, M.H., Iunes, P.J., Saiki, M., Tello S., C.A., Paulo, S.R., 2003. Spontaneous-fission decay constant of 238U measured by nuclear track techniques without neutron irradiation. J. Radioanal. Nucl. Chem. 258 (1), 117–122. Haack, U., 1976. Experiences with dating garnet, epidote, vesuvianite (idiocrase) and apatite by fission tracks. N. Jb. Min. Abh. 127, 143–155. Holden, N.E., Hoffman, D.C., 2000. Spontaneous fission half-lives for ground-state nuclides. Pure Appl. Chem. 72, 1525. 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 thorium thin films and natural U-doped glasses calibrated through natural uranium thin films. Chem. Geol. 187, 201–211. Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C., Essling, A.M., 1971. Precision measurements of half-lives and specific activities of 235U and 238U. Phys. Rev. C4, 1889–1906. Lederer, C.M., Shirley, V.M., 1978. Table of Isotopes. seventh ed. Wiley-Interscience, New York. Naeser, C.W., Engels, J.C., Dodge, F.C., 1970. Fission track annealing and age determination of epidote minerals. J. Geoph. Res. 75 (8), 1579–1584. Reimer, M., 1972. Fission track geochronology: method for tectonic interpretation of apatite studies with examples from the central and southern Alps. Dissertation University of Pennsylvania, Philadelphia. Saini, H.S., Sharma, O.P., Parshad, R., Nagpaul, K.K., 1978. Fission track annealing characteristics of epidote: applications to geochronology and geology. Nucl. Track Detect. 2, 133–140. Souza, S.Z., Montel, J.M., Hollanda, M.H., Sá, E.F., Pimentel, M.M., Lardeaux, J.M., Amaro, V.E., Veschambre, M., Nascimento, M.A., Menezes, L., 2001. Idade do metamorfismo Neoproterozóico de alta temperatura no Maciço São José de Campestre, NE da Província Borborema, com base em datações U–Th–Pb em monazitas e isócronas internas Sm–Nd. VIII Simpósio Nacional de Estudos Tectônicos, Recife, PE, Brasil, pp. 115–118. Storzer, D., Wagner, G.A., 1969. Correction of thermally lowered fission track ages of tektites. Earth Planet. Sci. Lett. 5, 463–468. Thöni, M., 2002. Sm–Nd isotope systematics in garnet from different lithologies (Eastern Alps): age results, and an evaluation of potential problems for garnet Sm–Nd chronometry. Chem. Geol. 185, 255–281. Wagner, G., Van Den Haute, P., 1992. Fission track dating. Solid Earth Sciences Library, 285pp.
On epidote fission track dating E.A.C. Curvoa,∗ , J.C. Hadler Netoa , P.J. Iunesa , S. Guedesa , C.A. Tello S.a , S.R. Paulob , P.C. Hackspacherc , R. Palissaria , P.A.F.P. Moreiraa a Instituto de Física “Gleb Wataghin”, Universidade Estadual de Campinas, UNICAMP, 13083-970 Campinas, SP, Brazil b Departamento de Física, Instituto de Ciências Exatas e da Terra, Universidade Federal de Mato Grosso, UFMT, 78060-900,
Cuiabá, MT, Brazil c Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, UNESP, 13506-900, Rio Claro, SP, Brazil
Received 13 February 2004; accepted 30 June 2004
Abstract The use of epidote in fission track dating was abandoned since the beginning of the 1980s due to difficulties like absence of a standard etching procedure, obtainment of different closure temperatures and the percentage of the datable samples. The results become much more reproducible when restricting fission track analysis to a peculiar kind of track. We are also studying confined track length, what makes possible to obtain information about fossil track annealing. Fission tracks in epidote were successfully etched with 48% HF at 35 ◦ C for 12.5 min. Dating samples by the external detector method was not possible due to problems in measuring the efficiency factor held between the number of fossil fission tracks and tracks induced in mica. Dating a sample from Brejuí, RN, Brazil with the population method gave a corrected age of 510 ± 69 Ma, in agreement with published U/Th–Pb ages. From the fact that the fossil track length histogram was bimodal, we were able to infer that this sample registered a thermal episode during its history. These preliminary results indicate that epidote deserves further studies to establish whether it can be employed as a thermochronological tool. © 2005 Elsevier Ltd. All rights reserved. Keywords: Epidote; Fission track dating; Brejuí epidote; Borborema Province—Brazil
1. Introduction Studies about dating epidote using the fission track method started in the 1970s and were abandoned in the early 1980s. The reasons were the difficulties in dealing with this mineral, that can be summarized as: (i) a standard etching procedure for this mineral was not developed, (ii) very different values were found for the closure temperature (around 260 ◦ C according to Haack (1976), Saini ∗ Corresponding author. Tel.: +55 19 37885362; fax: +55 19 37885512. E-mail address: curvo@ifi.unicamp.br (E.A.C. Curvo).
1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2004.06.016
et al. (1978) and 630 ◦ C according to Naeser et al. (1970)) and (iii) the percentage of datable samples was found to be very variable, for instance Reimer (1972) dated 8 out of 12 collected samples while Haack (1976) succeeded in dating only 9 out of a total of 125. In item (iii) a variety of problems can be summarized: the crystals have all kinds of imperfections and inclusions, the U distribution is extremely heterogeneous, zoning is common and the U content could be too low or too high. In spite of these difficulties, this mineral presents interesting characteristics that justify further investigation. Epidote is a hydrothermal mineral, which, in principle, should permit the dating of geological reactivation of faulting zones. Its
E.A.C. Curvo et al. / Radiation Measurements 39 (2005) 641 – 645
closure temperature is higher than that of apatite (∼ 100 ◦ C; Wagner and Van Den Haute, 1992) and zircon (∼ 210 ◦ C; Wagner and Van Den Haute, 1992). A high closure temperature is useful for dating geological events that erased all the fossil tracks in apatite and zircon. Thus, annealing effects detected in epidote reflect a high-temperature thermal history which is inaccessible by fission-track analysis of apatite and zircon. In this work we restarted the application of the fission track dating method to the epidote. An old problem often described in former works, etching anisotropy, was investigated and a solution is proposed. 2. Experimental procedure
8
<ρ> = (1.103 ± 0.073)x106 tracks/cm2
6 4 2 0 0
Table 1 Etching experiment carried out under HF 48% at 35 ◦ C 5 min
10 min
12.5 min
15 min
Total density — 0.906 ± 0.041 1.103 ± 0.073 0.787 ± 0.055 Dark tracks — 0.524 ± 0.021 0.519 ± 0.027 0.475 ± 0.025 Length (m) 8.30 ± 0.25 9.77 ± 0.24 10.53 ± 0.21 10.52 ± 0.21 12.5 min was the time chosen as standard in this work.
5
10
15 20 25 tracks/field
30
35
40
Fig. 1. Fossil track density histogram for the Brejuí sample. x means the average track density per field, S is the standard deviation of the distribution and is the average track density per square centimeter.
Fig. 2. Epidote fossil-tracks photographs. Some “dark tracks” are highlighted.
18
15 12 N
The sample studied in this work is a monocrystal from Brejuí-RN, located in the Northern part of Borborema Province, Northeast Brazil. It was broken and crushed into 200 m grains. After this, the grains were mounted in epoxy resin, polished and etched for 12.5 min in HF 48% at 35 ◦ C. This etching time was obtained through an etching experiment with progressive etching time (Table 1), where fossil track density and fossil confined track length were measured. NaOH is also used for etching tracks in epidote (Naeser et al., 1970; Bar et al., 1974; Haack, 1976; Saini et al., 1978), but etching 25 N NaOH, for 300 min at 75 ◦ C, did not succeeded in revealing any track in our sample. As shown in Fig. 1, the track density distribution is broad, and deviates from the Poisson distribution, expected for a radioactive decay where the uranium distribution of the source is homogeneous. The most probable explanation of this is the etching anisotropy. Bal et al. (1982) have shown that different crystalline faces presented very different track densities, relating this to an etching anisotropy. To solve this problem, we choose a specific kind of tracks oriented in a short angular distribution, usually darker than others random oriented in the same grain, “dark tracks” (Fig. 2). The criterium to set up what is or is not a dark track is observational, based upon the track angular distribution and their opacity. We had named the existence of these special tracks a “fission-track-window”. By counting only them the distribution became less broad (Fig. 3). In Table 2 the reduced 2 values of these distributions are shown together
N
642
9 6 3 0 0
5
10
15 20 25 tracks/field
30
35
40
Fig. 3. Fossil-dark track density histogram. x means the average track density per field, S is the standard deviation of the distribution and is the average track density per square centimeter.
E.A.C. Curvo et al. / Radiation Measurements 39 (2005) 641 – 645 Table 2 Reduced 2 values for total and dark track density histograms, for 3 etching times 10 min
12.5 min
15 min
Total density Dark tracks
6.02 2.12
5.84 2.21
4.53 1.94
25 20 15 N
2
643
10 16
5
14
12
N
10
0 0
8
2
4
6
8
10 12 L (µm)
14
16
18
20
Fig. 5. Confined track length histogram. x means the average track length, S means the standard deviation of the distribution and means the standard deviation of the mean.
6 4 2 0 0
2
4
6
8
10 12 L (µm)
14
16
18
20
Fig. 4. Induced track length histogram. x means the average track length, S means the standard deviation of the distribution and means the standard deviation of the mean.
with the distributions of total track density. It is clear that the density of dark tracks presents distribution close to Poisson. By this way we had adopted the dark tracks like our counting criterium, regarding that they are easily counted and that they give easily comparative results. 2.1. Brejuí-RN dating by the population method In this method the samples’s grains are divided in two fractions one for counting the fossil tracks and the other for the induced ones. The latter is irradiated with thermal neutrons, after its fossil tracks were previously erased in an oven. After this, both fractions are submitted to the same polishing, etching and analysed under the optical microscope. The grains of Brejuí sample were irradiated with a thermal neutron fluence of 1.95 × 1015 cm−2 (±3.6%), in the IPEN/CNEN—São Paulo nuclear reactor. The dark induced track density (I ) obtained was (0.1361 ± 0.0083) × 106 tracks/cm2 (1). The mean confined induced track length (L0 ) was (14.17 ± 0.20) m (1) and the histogram of induced tracks can be seen in Fig. 4.
3. Results An interesting fact is that a bimodal fossil length histogram was obtained, as shown in Fig. 5. This means that
the sample was submitted to an annealing during its thermal history that shortened part of its fossil tracks. Using the dating equation adapted to our neutron dosimetry (Iunes et al., 2002): 1 RM t = ln 1 + 238 s , (1) i f C238 where is the decay constant of 238 U, f is the decay constant for spontaneous fission of 238 U, RM is the fraction of fission events per uranium target nucleus of the mineral, 238 is the mineral detection efficiency, i.e., the ratio between the number of observed tracks per unit area and the number of 238 U spontaneous fission, per unit volume, occurring in the mineral, is the corresponding detection efficiency for the 235 U induced fissions, C238 is the 238 U isotopic concentration in natural uranium, s is the number of spontaneous (or fossil) tracks per unit area, i is the number of induced tracks per unit area. The following values were used: = 1.55 × 10−10 yr −1 (Jaffey et al., 1971), f = 8.35 × 10−17 yr −1 (Guedes et al., 2000) in agreement with IUPAC recommendation (Holden and Hoffman, 2000) and with another determination in which neutron dosimetry was not needed (Guedes et al., 2003); C238 = 0.99275 (Lederer and Shirley, 1978); 238 /=1 (Bigazzi et al., 2000); RM = (8.15 ± 0.29) × 10−9 (±3.6%). In this way an apparent age (tap ) of 367 ± 32 Ma was obtained. The correspondent corrected age, 496 ± 66 Ma, was taken via size correction method (Storzer and Wagner, 1969), by using the data from the correction curve (/0 versus L/L0 ) proposed by Saini et al. (1978), where 238 / was taken as /0 . A Poisson error was assumed for the track densities and a 10% error was assumed to the 238 / values. Considering that the track length histogram is bimodal (Fig. 5) two independent histograms can be obtained if we
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E.A.C. Curvo et al. / Radiation Measurements 39 (2005) 641 – 645 Table 3 Characteristics of short and long track histograms
1.1 1.0 0.9
Short track histogram
Long track histogram
7.21 ± 0.21 21 0.510 (±3.2%)
11.34 ± 0.16 86 0.800 (±2.0%)
19.6 (±21.8%)
80.4 (±10.8%)
tap (Ma)
21.7 (±21.8%) 1.12 × 105 (±22.4%) 79 (±22.7%)
78.3 (±10.8%) 4.06 × 105 (±12.0%) 284 (±12.5%)
238 / tcorr (Ma)
0.510 (±10%) 155 (±24.8%)
0.80 (±10%) 355 (±16.0%)
0.8
ρ/ρ
0
0.7 0.6 0.5 0.4 0.3
Simulated confined tracks
0.2
Projected tracks
0.1
Polynomial fit
0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 l/l0
L(m) Track number L/L0 Confined tracks (%) Projected tracks (%) s (cm−2 )
Fig. 6. Comparison between efficiencies for revelation of confined and projected tracks.
equation split it at 9 m. By aiming to obtain the density of projected tracks for both histograms, from the total projected p track density (total = (0.519 ± 0.027) × 106 tracks/cm2 (1), measured density of tracks that cross the mineral surface), it is necessary to know how the density of projected and confined tracks are related. It can be analytically showed that the density of projected tracks is directly proportional to the length of the fission tracks (Wagner and Van Den Haute, 1992). We did not succeed to analytically show how the density of confined tracks changes with their length. To access this information, a computational simulation was carried out. The program works as showed below: (i) A 4×4 matrix of defects, representing the surface tracks, is established. The separation between defects was calculated taking into account the measured surface track density, supposing that the tracks were uniformly distributed in a square arrangement. The value used was 9.5 m. (ii) After this different sized tracks are systematically simulated in the region of the four central defects. At first, only tracks with 1 m are simulated. The whole area is thus covered through steps of 0.5 m. To each point 1000 directions are drawn and if it meets a defect the track is virtually revealed. The process is repeated, with increases of 0.5 m in the track length, until the confined track length reaches 15 m. The results are presented in Fig. 6, where a third-order polynomial fit brought the mean curve. It can be noted that the confined tracks are not revealed with the same geometrical efficiency that the projected tracks. Thus, it is necessary to correct the distribution of tracks in the length histogram before calculating the splitted histogram ages. The correction can be understood through the
Nl = Nl
total , l
(2)
where Nl is the new number of confined tracks with length l, Nl is the old one, l is the length efficiency, i.e., how easily a track of length l is revealed and total is the average total efficiency, i.e., an average over all l . Now the densities of projected tracks can be calculated p
s =
Ns p , Ntotal total p
(3)
where s is the density of projected tracks in the short track p histogram, total is the total density of projected tracks, Ns is the number of confined tracks in the short track histogram and Ntotal is the total number of confined tracks. The density of projected tracks in the long track histogram is taken via Eq. (3) by replacing the “s” quantities by “l” ones. The results of the correction are presented in Table 3. The sum of the corrected time values (tcorr ) brings to the fission track retention age of this epidote sample: (510 ± 69) Ma. No significant thermal event took place during the time related to the large track histogram (355 ± 57) Ma. To obtain the corrected times (tcorr ) in Table 3 we used the size correction method, with Saini et al. (1978) data. The computational simulation did not calculated the error propagation, so the error from the percentage of confined tracks was adopted. A Sm–Nd age of approximately 590 Ma is found in the literature (Cordani and Sato, 1999) being attributed to the Brazilian Orogeny. In 2001, Souza et al. by using the U/Th–Pb method in monazites, dated samples from the Northeast of Borborema Province. They obtained an event dated as (403 ± 28) Ma besides the Brazilian Orogeny (578 ± 25) Ma.
E.A.C. Curvo et al. / Radiation Measurements 39 (2005) 641 – 645
4. Discussion and conclusion A counting method, based on a special kind of tracks which have a higher opacity and a short angular distribution, was developed and applied to epidote fission track dating. These dark tracks appear because of the strength of the etching anisotropy, that is to say that some crystalline directions have a faster etching rate. This method showed to be very useful when there is a high fission track etching anisotropy, providing a better control above it. It is worth noting that although the dark track approach improves the epidote dating, problems with uranium inhomogeneity should not be discarded. By applying the population dating method to the BrejuíRN sample a corrected age of (496 ± 66) Ma was obtained. This value is in agreement with the Sm–Nd dating, that is a method presenting higher closure temperature (650–680 ◦ C; Thöni, 2002). This age has an accepted geological interpretation (Brazilian Orogeny). Besides this, the confined track length histogram shows a bimodal distribution which could be interpreted as a thermal event that took place up to (355 ± 57) Ma ago and the age compatible with the Brazilian Orogeny (510 ± 69) Ma. The (355 ± 57) Ma age could be related with the younger age obtained by Souza et al. (2001), of (403 ± 28) Ma. However, a problem related with the closure temperatures arise. If the thermal event reset the U/Th–Pb clock, which have a higher closure temperature (> 750 ◦ C; Braun et al., 1998), it should have erased all the tracks in epidote. A plausible explanation is that the monazite was heated more strongly during the event, while the epidote was far away sufficiently to suffer only a partial annealing. This younger event has no geological interpretation yet. This work with confined track length indicates that epidote fission track dating has the potential to be used as a thermochronological tool, provided the anisotropy effects are taken into consideration. Acknowledgements The authors are grateful to Dr. Adalberto José Soares, from IPEN/CNEN, São Paulo, Brazil, for performing the neutron irradiations, to Luiz Felipe Brandini Ribeiro, from Instituto de Geociências e Ciência Exatas, UNESP, Rio Claro, for providing the epidote sample, and to FAPESP (State of São Paulo Research Foundation) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for Eduardo Augusto Campos Curvo scholarship. This work is part of the Fapesp Thematic Project 00/03960-5. References Bal, K.D., Lal, N., Nagpaul, K.K., 1982. Fission track etching studies of different planes of epidote. Phys. Chem. Miner. 8, 158–160.
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