Short-term annealing characteristics of spontaneous fission tracks in zircon: A qualitative description

Chemical Geology 227 (2006) 214 – 222 www.elsevier.com/locate/chemgeo

Short-term annealing characteristics of spontaneous fission tracks in zircon: A qualitative description Masaki Murakami a,*, Ryuji Yamada b, Takahiro Tagami a a

b

Department of Geology and Mineralogy, Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan Solid Earth Research Group National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Ibaragi 305-0006, Japan Received 25 October 2004; received in revised form 18 August 2005; accepted 5 October 2005

Abstract Thermal annealing behavior of fission tracks (FT) has not yet been documented under short-term (b 4 min) heating condition. We performed a new series of 17 laboratory heating experiments on spontaneous FTs in zircon at 550–910 8C for ~4, 10 and 100 s under strict controls of time and temperature using a graphite furnace. FTs were almost completely annealed at 912 8C for 3.9 s, at 858 8C for 10.4 s and at 805 8C for 100.6 s. These results suggest that FTs in zircon are totally annealed at time–temperature conditions of ordinary pseudotachylyte formation (i.e. at 1000 8C for 5 s). The measured FT lengths in ~4 and 10 s heating runs are shorter than that predicted by previous annealing kinetic models, suggesting a more rapid annealing therein. The component of short tracks in these samples increases suddenly as the annealing proceeds, which has not been observed for longer heating runs including the results of previous studies. Such rapid shortening for ~4 and 10 s heating is caused by earlier emergence of the annealing process of segmentation. D 2005 Elsevier B.V. All rights reserved. Keywords: Fission track annealing; Zircon; Graphite furnace; Pseudotachylyte; Fault dating; Short-term heating

1. Introduction Horizontal confined fission track (FT) length has been used as a diagnostic parameter for thermal history analysis in the apatite and zircon FT method. A series of laboratory studies of temperature-dependent track retention in zircon was conducted in the 1990s, using confined spontaneous track lengths as a measure of annealing, as well as etched track widths for standard* Corresponding author. Present address: Institute of Geology, Academy of Sciences, 16502 Prague 6, Czech Republic. E-mail addresses: [email protected], murakami@kueps. kyoto-u.ac.jp (M. Murakami), [email protected] (R. Yamada), [email protected] (T. Tagami). 0009-2541/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2005.10.002

ization of track revelation (Tagami et al., 1990, 1998; Yamada et al., 1993, 1995a,b, 1998; Hasebe et al., 1994). Kinetic parameters that describe annealing at laboratory time scales (4.5 min — 400 days) were established for zircon (Yamada et al., 1995b; Galbraith and Laslett, 1997; Tagami et al., 1998). Pseudotachylytes are generated by frictional melting during seismic slip along a fault with heating at about 1000 8C for 5 s (e.g., Otsuki et al., 2003; Kikuchi and Kanamori, 1996). Zircon annealing models predict that tracks should be totally annealed in such short-term heating conditions and therefore in principle the zircon FT method should be useful for dating fault movements. Such conditions are, however, outside the realm of laboratory data using conventional furnaces,

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and hence at present application must rely on extrapolation of annealing models. Therefore, we have carried out laboratory heating experiments to directly constrain annealing behavior within the plausible time–temperature conditions of pseudotachylyte formation. In this study, we performed laboratory heating experiments of spontaneous FTs in zircon at 550 to 910 8C for ~4, 10 and 100 s. For such short-term heating experiments, a new experimental scheme was introduced to achieve strict control on heating times and temperature conditions using a graphite furnace and infrared thermometers. The resultant uncertainties were 0.4 s and b 6 8C for time and temperature, respectively. The applicability of the zircon FT system to fault dating was also assessed by comparing heating conditions between the pseudotachylyte formation and FT fading. 2. Experimental methods Zircons from the Nisatai Dacite (named NST) were employed for laboratory annealing experiments using spontaneous track length. The grain size was ~50–100 Am in width. NST has a K–Ar biotite age of ca. 21 Ma and a spontaneous track density of ca. 4 d 106 cm 2 (Tagami et al., 1995; Yamada et al., 1995b). These spontaneous tracks have experienced little or no geological annealing since their formation. Detailed description of the sample and experimental procedures was given elsewhere (e.g., Yamada et al., 1995b) except the sample heating and length measurement procedures. In this study, 17 annealing experiments were performed from 550 to 910 8C for ~4, 10, 100 s, as listed in Table 1. These heating conditions were designed to cover those assumed for pseudotachylyte formation (e.g., Otsuki et al., 2003). In order to heat samples, we used a graphite furnace system equipped with an atomic absorption spectrometer (SAS7500, Seiko Instruments Inc., Japan). The main part is a cylindrical graphite cuvette of 8 mm in diameter, 1 mm thick and 30 mm long with a hole of 5 mm in diameter at the center on the top (Fig. 1a). This provides the stable heating at up to 3000 8C, which is achieved within 1 s and kept within F3 8C at the plateau by conducting up to 10 A of electricity. The argon gas was flowed around the cuvette at the rate of 1 l/min to prevent the graphite cuvette from oxidization during heating. For measuring temperatures of the graphite cuvette, we used the ChinoR IR-FL infrared sensors with specified response time of 0.01 s. The IR-I sensor (focal length = 100 mm, spot diameter = 1 mm) was located

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Table 1 Analytical data of FT short-term annealing experiments of NST zircon t

T

s

8C

3.6 3.5 4.1 3.6 3.9 10.6 10.8 10.8 10.5 10.1 10.4 101.1 100.6 100.8 100.3 100.6 100.1

599 700 751 800 912 599 649 700 750 800 858 549 649 698 750 805 858

Nall

L all

N

Am 56 54 63 62 10 50 56 47 58 56 5 56 20 53 44 11 3

10.90 10.08 9.32 8.58 5.68 10.65 10.28 9.55 8.53 7.23 5.21 10.85 9.60 9.01 7.65 5.57 4.17

37 38 38 38 4 31 35 30 35 29 3 38 14 25 26 6 1

L

SD

SE

Am

Am

Am

10.72 9.89 8.92 8.50 5.86 10.57 10.13 9.40 8.35 7.31 5.71 10.72 9.34 8.74 7.78 5.23 4.05

0.56 0.72 1.36 1.29 0.97 0.86 1.18 0.99 1.18 1.31 0.27 0.76 0.64 0.88 1.01 1.19 –

0.09 0.12 0.22 0.21 0.49 0.15 0.20 0.18 0.20 0.24 0.16 0.12 0.17 0.18 0.20 0.49 –

L / L0 0.97 0.90 0.81 0.77 0.53 0.96 0.92 0.85 0.76 0.66 0.52 0.97 0.85 0.79 0.70 0.47 0.37

t = annealing duration; T = annealing temperature; N all = number of all measured tracks; L all = mean length of all measured tracks; N = number of tracks N608 to crystallographic c-axis; L = mean length of tracks N608; SD = standard deviation for the length distribution of tracks N608; SE = standard error of L; L / L 0 = length reduction ratio (L 0 = 11.05 Am, mean HCT length for an unannealed sample; Yamada et al., 1995a). Note that HCTs with angles to the crystallographic c-axis of N608 are selectively used for analysis hereafter, considering the various orientation factors which affect the track length measurement.

over the cuvette at the distance of its focal length from the cuvette surface to measure heating temperature near the grains through the top hole (Fig. 1a). The signals of the infrared emission from the cuvette surface were sampled at the frequency of 8 Hz and recorded by the YokogawaR MV100 data recorder. The calibration of infrared thermometers was performed by using a reference thermocouple certificated by the Japanese Industrial Standards, and a specially equipped muffle furnace having small holes on the sidewall. The cylindrical graphite cuvette was placed at the center of the preheated muffle furnace. The standard thermocouple was positioned very close to the cuvette to measure its temperature. An infrared sensor was located outside the furnace at the distance of its focal length from the cuvette surface to detect the infrared emission from the cuvette through the small hole of the furnace. A good correlation between infrared signals and cuvette temperatures was recognized within the temperature range from 350 to 1200 8C, and the accuracy was estimated at F2.5 8C at 1000 8C. Overall, the uncertainty of the heating temperature measurement was assessed at 6 8C, by summing up the graphite furnace instability of 3 8C and the infrared sensor accuracy of 2.5 8C.

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Fig. 1. (a) Figure of the measuring system of heating temperature and duration. IR-I measures the heating temperature, IR-II records the heating duration. The position of dropped grains in the cuvette is monitored with the IR-II sensor continuously. (b) IR-II signal intensity against time of the annealing experiment. The signal keeps stable until the time when zircon grains are dropped into the cuvette preheated at scheduled temperature, but changes discontinuously as soon as grains drop onto the monitored area by IR-II. This discontinuous time was clearly assessed as the initiation of sample heating. The subtle change in IR-II signal is caused by the change in the surface material from graphite to mineral of which the infrared emission is monitored, not due to the temperature itself. The end of the heating period was defined by the rapid cool down of the graphite cuvette after accomplishment of the scheduled duration. The annealing effect of the cooling process (gray area) is equivalent to a heating of b0.25 s at the scheduled heating temperature, as estimated by the model of Tagami et al. (1998).

In addition, we assessed the response time to reach equilibrium between an environmental temperature and the K-type thermocouple covered with the stainless steel sheath of 3.2 mm in diameter that had been used in the previous experiments (Yamada et al., 1995b; Tagami et al., 1998). The use of thermocouples has the trade-off between the sheath diameters and the

response times; the thinner is the quicker. When measuring the temperature at several hundred degrees Celsius, the sheath should be thicker than 3 mm to endure the environmental temperature. The same setting as the calibration of infrared sensor was used for the assessment of the response time. The test thermocouple was inserted into the muffle furnace through a small hole in

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order that its tip was placed close to that of the standard thermocouple located at the center of the furnace preheated at 500 or 600 8C. The output of the test thermocouple showed equivalent value of the furnace temperature ~1.5 min later than the insertion. It was indicated that thermocouples are not suitable to monitor the temperature change when the heating temperature is several hundred degrees Celsius and the heating time is on the order of minutes or less. Infrared sensors do not have such fatal problems because the response time is relatively short in general. However, calibration with a standard thermometer is necessary because their indication varies due to the difference in emissivity from the surface of various materials. When loading samples, a batch of hundreds of grains without wrapping was dropped on to the inside wall of the cuvette through the top hole using a pair of bead handling tweezers that have semiglobular tips of 2 mm in diameter. The cuvette was heated at scheduled temperature. The grains were dropped all at once from the height of a few centimeters so that they distributed evenly without piling up on the inside wall of the cuvette. Preliminary investigation on the temperature distribution in the cuvette confirmed that cuvette was in

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thermally equilibrium along its length while heating at ~1000 8C, and that the temperature of the cuvette did not change before and after the sample loading. Although the capacity of the cuvette was very small compared with a muffle furnace, it was large enough to ensure good contact of hundreds of grains to the inside wall, and the heating condition among grains was expected to be well homogenous. The time to reach thermal equilibrium between the individual grains and the cuvette surface was identical with the time estimated for a single grain heated conductively. It was estimated to be b 0.009 s, based on the one-dimensional heat conduction under the conditions such that single grain size of 100 Am, thermal diffusivity of 1 mm2 s 1, environmental temperature of 1000 8C, and achievement temperature at the center of a grain of 999.5 8C. We assessed the effective heating time for each experimental run by reading the chart of the change in infrared emission monitored with another infrared sensor (Fig. 1b). The IR-II sensor (focal length = 200 mm, spot diameter = 2 mm) was set almost horizontal to detect the subtle change in infrared emissivity from the cuvette when the sample grains were dropped (Fig. 1a). This change marked the onset of the grains heating. The end

Fig. 2. Photographs illustrating the short-term FT annealing in zircon. FTs are shortened with increasing annealing temperature, and surface FTs fade nearly completely with very few tracks remaining visible in samples heated at 912 8C for 3.9 s.

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of heating was achieved by rapid temperature drop by switching off the furnace power after a scheduled duration passed. The sources of uncertainty for effective heating time of samples are the sampling intervals at 8 Hz, the time delay to reach thermal equilibrium between the individual grains and the cuvette surface, and the remnant heat after switching off the furnace. The time delay is b 0.009 s as estimated above. The equivalent time (Duddy et al., 1988) for the remnant heat was estimated to be b0.25 s for all the heating conditions in this study, based on the calculation using the kinetics of Tagami et al. (1998). Overall, the effect of uncertainty of these sources was 0.4 s at most, negligibly small even in the case of the shortest heating durations (~4 s). FT lengths were measured as horizontal confined tracks (HCTs; Laslett et al., 1982). Because of the rapid heating and cooling in the experiments, the zircons tended to have many cracks so that a lot of trackin-cleavages (TINCLE; Lal et al., 1969) were revealed by etching, compared to their fraction observed in ordinary samples. Therefore, track-in-tracks (TINT; Lal et al., 1969) were selectively measured in order to avoid possible sampling biases caused by measuring unusually large numbers of TINCLEs. Etched FTs were observed using a Nikon EclipseR E600 microscope with 100 dry objectives and 10 eyepieces. Digital images of HCTs were taken with the NikonR DXM1200 CCD camera mounted on the NikonR microscope. The length and crystallographic orientation were measured on the digital image of each HCT displayed on a PC monitor with commercial image processing software. This measurement system was intercalibrated with the one used previously (e.g., Yamada et al., 1995b) by measuring the same track sets having various length and angles (Yamada et al., in preparation). Significant bias was not found between the two systems.

in zircon are presented in Fig. 2. General shapes of FTs on an etched grain-internal surface appear indistinguishable from those of previous long-term laboratory experiments (Yamada et al., 1995a, Fig. 8). In order to compare the annealing behavior for shortterm heating with that of previous experiments, the ratio between the mean length of measured HCTs (L) and that predicted by the Tagami et al. (1998) model (L p) was plotted against the length reduction ratio (L / L 0) in Fig. 3. The mean length was calculated using HCTs with crystallographic orientation N608 to the grains caxis to minimize the overetching bias (Yamada et al., 1993, 1995a). The plotted data in Fig. 3 consist of previous (open symbols; Yamada et al., 1995b; Tagami et al., 1998) and new (filled symbols) results. The mean HCT lengths from the short-term heating experiments are not perfectly concordant with those predicted by the previous kinetic model. The departure from that model increases as the annealing proceeds toward L / L 0 = ~0.7. This suggests that the annealing in the short-term heating proceeds more rapidly than predicted by the previous model.

3. Result and discussion Analytical results of the laboratory annealing experiments performed on untreated NST zircon grains under various time–temperature conditions are listed in Table 1. Except for three heating experiments at 599 8C for 3.6 s, 599 8C for 10.6 s and at 549 8C for 101.1 s, FTs of all other samples were partially to totally annealed where the partial annealing zone (PAZ) was defined here as the length reduction ratio (L / L 0) between 0.4 and 0.95. Surface FTs almost faded and only a few tracks remained visible in four samples heated at 912 8C for 3.9 s, at 858 8C for 10.4 s, at 805 8C for 100.6 s and at 858 8C for 100.1 s. The photographs of the FTs

Fig. 3. Variations of the ratio between mean HCT lengths of N608 angle to the c-axis (L) and the length L p predicted by the model of Tagami et al. (1998) plotted against the length reduction ratio L / L 0 (L 0 = mean HCT length for an unannealed sample; Yamada et al., 1995a). The gray box along L / L 0 = 1 represents L / L 0 = 0.97 to 1.03, which indicates the maximum effect of the uncertainty in the temperature of up to 6 8C throughout the experiments. The measured FT lengths in this study (filled marks) are not concordant with those predicted by the Tagami et al. (1998) kinetic model. Error bars represent F 1r. The data at 858 8C for 100.1 s was excluded because its predicted length is a negative value.

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Such rapid annealing is also suggested in Fig. 4, which shows track length distributions of 15 annealing runs. Distributions of the 100 s annealing runs show unimodal and narrow patterns below 649 8C (Fig. 4k and l), but broader patterns appear at higher temperatures (Fig. 4m–o) like the case in previous long-term annealing experiments (Yamada et al., 1995b). Distributions of ~4 s annealing runs, however, have a component of short tracks that appears suddenly at 751 8C, resulting in a unimodal distribution with negative skew-

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ness (Fig. 4c). When shapes of individual distributions are compared for L / L 0 = 0.76 to 0.79 among three series of isochronal annealing experiments (i.e., at 800 8C for 3.6 s, at 750 8C for 10.5 s and at 698 8C for 100.8 s; Fig. 4d, i and m), the two distributions of the ~4 and 10 s annealing runs are characterized by a unimodal one with negative skewness (Fig. 4d and i), in contrast with the distribution with positive skewness of the 100 s annealing run (Fig. 4m). These lines of evidence suggest that the rapid annealing at ~4 and

Fig. 4. Track length distributions of 15 annealing experiments with various heating conditions. The distributions of ~4, 10 and 100 s isochronal annealing runs are disposed from (a) to (e), (f) to (j) and (k) to (o), respectively.

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10 s heating runs is due to a rapid increase of short tracks 5–7 Am in length. The L / L 0 values of such rapid shortening are between 0.8 and 0.9 for ~4 s heating, and between 0.75 and 0.85 for 10 s heating. Standard deviations of the measured HCT lengths were plotted against the length reduction ratio in Fig. 5. Previous data and new results of the 100 s annealing runs show a gradual increase in standard deviation from L / L 0 = 1 to 0.7, followed by a rapid increase (Yamada et al., 1995a; Tagami et al., 1998). The new data of the ~4 and 10 s annealing runs, however, show a more rapid increase at earlier stages of track annealing, i.e., L / L 0 = ~0.9 to 0.8, yielding a significant departure of data points from a trend described by data of longer heating. This is consistent with what suggested by track length distributions we described above. What causes such rapid shortening in ~4 and 10 s annealing experiments? The nature of annealing of a FT has been recognized as the primary shrinking of its length followed by the secondary segmentation (Green et al., 1986; Yamada et al., 1995a). Fig. 6 shows the length of individual tracks plotted against their orientation to the crystallographic c-axis during ~4 s isochronal annealing. The length yields a rather negative correlation to the angle to c-axis, and this trend is more emphasized with increasing temperature from 599 to 751 8C like the case of previous longterm annealing experiments (Green et al., 1986; Yamada et al., 1995a). Whereas some of the tracks at high angles to c-axis (N60 8) were shortened rapidly from 700 to 751 8C, some at low angles to caxis (b 60 8) were kept long (N 11 Am) below 751 8C and then shortened at 800 8C. Overall, the anisotropic

Fig. 5. Standard deviation of HCT length distributions plotted against degree of length reduction (L / L 0 = 0.6 to 1.0). Plots of long-term (open marks) and 100 s (filled star) annealing data tend to generally increase in standard deviation for L / L 0 b0.8 (gray zone). However, plots of ~4 and 10 s annealing data show a more rapid increase in standard deviation at L / L 0 N0.8 (arrow).

trend at 751 8C is the most obvious of the four annealing runs. For tracks N 608 at 751 8C, however, there is no clear anisotropy in length between 608 and 908, merely showing a greater scatter in individual lengths. This suggests that the rapid shortening is caused by earlier emergence of the secondary process of segmentation. Fig. 7 shows an Arrhenius diagram in order to compare previous long-term annealing data and new short-term annealing data with their respective upper zircon PAZ boundary as estimated by the L / L 0 = 0.4 line. The alignment of data points on the Arrhenius plot seems to prefer polygonal lines or curved lines than simple linear lines in order to describe contours of relative reduction ratio of annealed FTs. This may reflect a change in the transition of the dominant process of annealing between shrinking and segmentation. We need to contrive how to build in such a change when we model annealing kinetics of zircon FT using experimental data in this article together with data from previous laboratory annealing experiments, namely Yamada et al. (1995a,b) and Tagami et al. (1998) that used a common zircon sample and experimental criteria. Through the analysis to develop the kinetic model, more details would be possibly obtained, such as the factor that is the most responsible for the emergence of segmentation. 4. Implication for pseudotachylyte dating Is the heating associated with pseudotachylyte formation sufficiently strong to reset of the zircon FT system? Fig. 7 also shows that the temperature condition of frictional heating to form pseudotachylyte was estimated as ~750–1280 8C of the Nojima fault (Otsuki et al., 2003) and as N 1000 8C in the Eastern Peninsular Ranges of California (Wenk et al., 2000). The duration of the formation was estimated as ~5–6 s by teleseismic waves in the 1995 Kobe earthquake (Kikuchi and Kanamori, 1996) and as 3–4 s in the 1999 Chi–Chi earthquake (Chi et al., 2001). The present results of laboratory annealing suggest that zircon FTs are annealed partially or totally at those time–temperature conditions of pseudotachylyte formation. Murakami and Tagami (2004) succeeded in separating zircons from the pseudotachylyte layer occurring along the Nojima fault activated during the 1995 Kobe earthquake in Japan, as well as in dating them by the zircon FT method. It was found that the layer has a significantly younger age than the initial cooling age of the host rock. The layer has mean lengths of ~11 Am and unimodal length distributions in contrast with its neigh-

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Fig. 6. Lengths and crystallographic angles to the c-axis of individual confined tracks of the NST zircons heated at four different temperatures for ~4 s. The anisotropic trend at 751 8C is the most obvious of the four annealing runs.

boring samples having components of short tracks of 4– 9 Am length. The combination of the two parameters, i.e., FT length and age, is thus useful to verify the total

reset of the thermochronometer during short-term heating and offers a robust tool to determine the age of pseudotachylyte formation.

Fig. 7. Arrhenius diagram of the previous long-term (open circles) and the new short-term (filled circles) annealing experiments. Their respective upper partial annealing zone (ZPAZ) are also drawn where they are defined by the L / L 0 = 0.4. The two lines show different slopes and the downward extrapolation of long-term PAZ boundary indeed indicates total annealing of the fission tracks at 1000 8C and 1 s (open square), the maximum temperature of ordinary pseudotachylyte formation. When the plausible variation in maximum temperature is taken into account within as well as between individual pseudotachylyte layers, however, the conditions estimated for the pseudotachylyte formation show a gross overlap with those of the short-term PAZ for zircon fission tracks.

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Acknowledgement We acknowledge Dr. Guanhong Tao for suggesting the graphite furnace as the short-term heating device. Without his advice we could not carry out these experiments. We appreciate detailed reviews by P. Green and an anonymous person. This study has been supported by a Grant-in Aid (no. 12440137) as well as by a Grantin-Aid for the 21st Century COE Program (Kyoto University, G3) from the Japanese Ministry of Education, Culture, Sports, Science and Technology. [PD] References Chi, W.–C., Dreger, D., Kaverina, A., 2001. Finite-source modeling of the 1999 Taiwan (Chi–Chi) earthquake derived from a dence strong-motion network. Bull. Seismol. Soc. Am. 91, 995 – 1012. Duddy, I.R., Green, P.F., Laslett, G.M., 1988. Thermal annealing of fission tracks in apatite, 3. Variable temperature annealing. Chem. Geol., Isot. Geosci. Sect. 73, 25 – 38. Galbraith, R.F., Laslett, G.M., 1997. Statistical modelling of thermal annealing of fission tracks in zircon. Chem. Geol., Isot. Geosci. Sect. 140, 123 – 135. Green, P.F., Duddy, I.R., Gleadow, A.J.W., Tingate, P.R., Laslett, G.M., 1986. Thermal annealing of fission tracks in apatite: 1. A qualitative description. Chem. Geol., Isot. Geosci. Sect. 59, 237 – 253. Hasebe, N., Tagami, T., Nishimura, S., 1994. Towards zircon fission-track thermochronology: reference framework for confined track length measurements. Chem. Geol., Isot. Geosci. Sect. 112, 169 – 178. Kikuchi, M., Kanamori, M., 1996. Rupture process of Kobe, Japan, earthquake of Jan. 17, 1995, determined from teleseismic body waves. J. Phys. Earth 44, 429 – 436. Lal, D., Rajan, R.S., Tamhane, A.S., 1969. Chemical composition of nuclei of z N 22 in cosmic rays using meteoritic minerals as detectors. Nature 221, 33 – 37. Laslett, G.M., Kendall, W.S., Gleadow, A.J.W., Duddy, I.R., 1982. Bias in measurement of fission-track length distributions. Nucl. Tracks 6, 79 – 85.

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