Geological partial annealing zone of zircon fission-track system: additional constrains from the deep drilling MITI-Nishikubiki and MITI-Mishima

Chemical Geology 199 (2003) 45 – 52 www.elsevier.com/locate/chemgeo

Geological partial annealing zone of zircon fission-track system: additional constrains from the deep drilling MITI-Nishikubiki and MITI-Mishima Noriko Hasebe a,*, Satoshi Mori b, Takahiro Tagami c, Ryoichi Matsui d a

Institute of Nature and Environmental Technology, Kanazawa University, Kanazawa 920-1192, Japan Department of Earth Sciences, Faculty of Science, Kanazawa University, Kanazawa 920-1192, Japan c Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto, Japan d Technical Department, Japan National Oil Corporation, Japan b

Received 23 July 2002; accepted 4 February 2003

Abstract Fission track (FT) analysis of zircon from rhyolite and sandstone samples of Ministry of International Trade and Industry (MITI)-Nishikubiki and MITI-Mishima boreholes has given additional constraints on the temperature range of the zircon partial annealing zone (ZPAZ) over a geological time scale. The advantages of these samples are the following: (1) they have been exposed to a stable temperature for f 1 million years, and (2) it is possible to obtain data at successive temperatures. Ages and confined track lengths were measured in samples from depths of f 3.5 – 5.6 km with a current environmental temperature of f 223 jC at the deepest. Track lengths in samples below 200 jC are still long and FT ages are compatible with their sedimentary ages, showing no evidence of annealing for a heating duration of 1 million years. Samples at 205 jC seem to yield shortened tracks as a result of annealing. Because of a low number of zircon crystals, only four track lengths were measured for the deepest sample whose mean length was shortest among analysed samples. These data are consistent with previous results from ultra-deep boreholes. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Fission track; Zircon; Long-term annealing; Boring core

1. Introduction Fission Track (FT) analysis is an effective technique to estimate the thermal histories of rock bodies.

* Corresponding author. Present address: Department of Geological Sciences, University College London, Gower Street, WC1E 6BT, London, UK. Tel.: +44-020-7679-2418; fax: +44-020-78132802. E-mail address: [email protected] (N. Hasebe).

Although short-term annealing characteristics of FTs in zircon have been studied, at various temperatures, using laboratory experiments (350 – 750 jC for 10 1 – 104 h: Tagami et al., 1998; Yamada et al., 1995b), the influence of long-term heating under natural geological conditions has not been adequately investigated. One of the previous geological annealing studies is on samples from basement sandstone around a granitic intrusion (Tagami and Shimada, 1996). They divided a track length distribution into a short component, which underwent heating by the granitic

0009-2541/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2541(03)00053-6

46

N. Hasebe et al. / Chemical Geology 199 (2003) 45–52

intrusion, and a long component, which formed after the intrusive event. They found that the shortened component had a similar track length distribution with distributions obtained by laboratory annealing experiments, and concluded that annealing behaviour for short- and long-term heating should follow the same kinetics, ensuring the reliability of extrapolation of laboratory results to geological time scales. In addition, a limited data set from ultra-deep borehole samples indicated that FTs in zircon are still stable around f 200 jC (Zaun and Wagner, 1984; Coyle and Wagner, 1996; Green et al., 1996; Tagami et al., 1996). Meanwhile, an extrapolation of laboratory data to geological timescales provides an estimate for the zircon partial annealing temperature of f 170 –390 jC with heating duration of 1 million years (Tagami et al., 1998). The purpose of this study is to give further constraints on the temperature range of the zircon partial annealing zone (ZPAZ) over a geological time scale, using zircons from rhyolite and sandstone samples of the Ministry of International Trade and Industry (MITI)-Nishikubiki and MITI-Mishima deep boreholes, Niigata Prefecture, Japan (Fig. 1). Samples were collected from depths of f 3.5 – 6.2 km, with a present environment temperature of f 238 jC at the deepest, to measure confined track lengths and ages.

2. Sample description Niigata Prefecture is one of the most important oil and gas provinces in Japan. Accordingly, the structure and the developmental histories of reservoir rocks are well studied by extensive drilling through the sedimentary basin. The typical stratigraphy of the basin is summarized in Table 1 (JNOC (Japan National Oil Corporation), 1993, 1996). Sediments are mainly volcanogenic with a rhyolite basement. In middle Miocene, the basin subsided to the middle to lower bathyal zone, resulting in the deposition of thick mudstones (the Nanatani formation). Andesite volcanism occurred in the area and huge amounts of coarse-grained andesitic sediments were deposited in late Miocene to Pliocene (the Teradomari and Shiiya formations), followed by shallow marine conditions (the Haizume formation). There was no remarkable unconformity or terrestrial erosion, although gentle folding, which did not cause a long vertical displacement, developed mainly in the Pliocene (Kishi and Miyawaki, 1996). Since the Pleistocene, the basin has remained stable. The Basin paleo-geothermal gradients were similar to the present geothermal gradients from vitrinite reflectance and illite crystallinity (Akiyama and Hirai, 1997). These lines of evidence suggest that the heating duration of

Fig. 1. Locations of deep MITI-Nishikubiki and MITI-Mishima drillings in Japan.

N. Hasebe et al. / Chemical Geology 199 (2003) 45–52

47

Table 1 Typical stratigraphy of the sedimentary basin where boring cores are obtained (JNOC, 1993, 1996) Formation

Rock facies

Diagnostic fossil (fossil zone of benthic foraminifera)

Uonuma

sandstone and siltstone alternation of sandstone and siltstone mudstone

not found

Haizume

Nishiyama

Shiiya

dacitic tuff breccia and tuff

Upper Teradomari

alternation of mudstone and sandstone alternation of mudstone and fine-grained sandstone and tuff mudstone and muddy tuff

Lower Teradomari

Nanatani

rhyolite

Sedimentary environment

Stratigraphic ages Pleistocene

Ammonia japonica – Cibicides cf. Refulgens A. japonica – Cribrononion clavatum C. clavatum – Islandiella japonica Epistominella pulchella – Islandiella japonica U. akitaensis – E. pulchella U. akitaensis – Trifarina kokozuraensis U. akitaensis – Cribrostomoides cf. Evoluta Martinottiella communis – U. spp. Cribrostomoides cf. Subglobosum – U. akitaensis Cribrostomoides sp. – Uvigerina sp. M. communis – Cribrostomoides sp. Dorothia spp. – Cribrostomoides spp.

upper shallow sea middle shallow sea lower shallow sea upper bathyal zone upper – middle bathyal upper – middle bathyal upper – middle bathyal upper – middle bathyal upper – middle bathyal middle – lower bathyal middle – lower bathyal middle – lower bathyal

Pleistocene

Pleistocene zone zone zone zone zone zone zone zone

Rarely Found ?

Pliocene Pliocene Late Miocene Late Miocene

Late Miocene Mid Miocene

H. sinboi – Sigmoilopsis schlumbergeri H. morimachiensis – S. schlumbergeri H. nanataniensis – S. schlumbergeri Not Found

upper – middle bathyal zone upper – middle bathyal zone upper – middle bathyal zone

Mid Miocene

Early Miocene

Shallow sea: f 140 m. Bathyal zone: 100 – 200 to 3000 m.

the samples at the current temperature is to the order of f 106 years. Sampling depths, rock facies and current temperatures are shown in Fig. 2. Of these, NSK01 and 11 reached 6.0 km in depth, at which the present geothermal temperatures are up to 238 jC. Assuming the surface temperature to be 15 jC, the highest geothermal gradients of these wells are f 5.17 jC/ 100 m, which is estimated from the deepest part of MITIMishima. Both the stratigraphic ages of all samples and the zircon FT ages of shallower samples than those analysed in this study are Miocene (JNOC, 1993, 1996), suggesting the zircon grains included were mostly derived from syndepositional volcanism.Accordingly, the advantages of these samples are the following: (1) these samples have been exposed to a stable temperature for at least 1 million years because there is little crustal movement in this region; and

(2) a series of data can be obtained at successive temperatures of f 120– 238 jC.

3. Experimental method Zircon separates were obtained using conventional crushing, sieving, heavy-liquid and magnetic separation techniques. Subsequently, zircon grains were mounted in polytetrafluoroethylene-perfluoralkoxyethlene (PFA) TeflonR sheets and their external prismatic surfaces were ground and polished. Samples were prepared for length measurement following the procedures described by Yamada et al. (1995a). FTs were etched in NaOH/KOH ( = 1:1) eutectic etchant (Gleadow et al., 1976) at 248 F 1 jC until the surface tracks perpendicular to c-axis attained the width of 2 Am. Although this etching criteria over-etched the surface, making the grains unsuitable for age determination, the number of measurable track lengths can

48

N. Hasebe et al. / Chemical Geology 199 (2003) 45–52

Fig. 2. Sampling depth from MITI-Nishikubiki (upper) and MITI-Mishima (lower). Present borehole temperature (left) and rock facies (middle) are also shown. The present-day temperature of each sample was estimated from 3 or 4 measured points, which are indicated by open circles with temperature. Sst: sandstone, Mst>Sst: alternation of mudstone and sandstone (predominately mudstone).

be increased. Further, this procedure follows the same experimental conditions as the short-term annealing study by Yamada et al. (1995b), allowing direct comparison of data between laboratory and geological timescales and avoiding problems associated with track etching efficiency (Yamada et al., 1995a). Track lengths were measured for horizontal confined tracks (HCTs; Laslett et al., 1982) consisting of both track-intracks and track-in-cleavages (TINTs and TINCLEs, respectively; Laslett et al., 1982). Tracks with widths

of 1 F 0.5 Am were selectively measured (Yamada et al., 1993). Lengths and angles were measured using a digital screen interfaced with the HamamatsuR C2500 Image Processing System combined with a NikonR Biophot microscope at  925 magnification with a dry object lens or using Zeiss Axioplan microscope at  1250 magnification combining with Houston Instruments digitising tablet projected onto the field of view using a drawing tube. The error for both measurement system is F 0.1 Am.

N. Hasebe et al. / Chemical Geology 199 (2003) 45–52

For age determination, samples were prepared following the procedures described by Tagami et al. (1988) and a zeta calibration (e.g., Hurford, 1990) is adopted. Etching duration was 30 h with NaOH/KOH ( = 1:1) eutectic etchant at 225 F 1 jC. Samples were irradiated at the Thermal Column Pneumatic Tube [TC-Pn] facility of the Kyoto University Research Reactor [KUR-1]. Using an SRM 612 dosimeter glass, the zeta value is 358.9 F 14.0 (2r).

4. Results and discussion Analytical data of confined FT length measurement are listed in Table 2. No zircon crystals were found in samples MSM01-04 and NSK11-14 by conventional mineral separation. We could not measure HCTs in zircon from NSK01 and 02, because there were too few zircon crystals and most had low track densities. Because of the anisotropic etching nature of zircon, mean lengths were calculated only for tracks whose crystallographic orientations are >60j to the c-axis (Yamada et al., 1993). Data from depths with the similar temperatures (i.e. MSM05&06) were combined to improve the number of measured HCTs. Fig. 3 shows the mean track length profile against present-day geothermal temperature with the track length distribution patterns for each sample. Ages

49

were determined on samples MSM08 and MSM11 (Table 3) from the temperature range of < 200 jC, and radial plots (Galbraith, 1988) of these samples are also shown in Fig. 3. The ages measured here are equivalent to their sedimentary ages with a small number of grains older than Eocene (Fig. 3), suggesting no annealing of tracks at current temperatures. Other samples do not have enough zircon grains for both length measurement and age determination. Except for NSK03&04 and NSK07&15 which have a lower number of measurable tracks, the mean track lengths fall in a range of f 9.8– 10.8 Am with standard deviation of f 0.5 –1.9 Am (Table 2). Most of the track length distributions are characterized by a unimodal shape with peak length range of 9– 12 Am accompanied by a few short tracks of about 6 Am (Fig. 3). The tracks in zircon which did not undergo annealing show a unimodal length distribution ranging from 8 to 13 Am with a mean value of about 10.5 Am (Hasebe et al., 1994). When annealing occurs, the peak of the unimodal distribution shifts left because all tracks shorten (Yamada et al., 1995b). Considering the data provided by this study, the peak positions of the track length distributions for low-temperature ( < 200 jC) samples correspond to the original positions for unannealed samples. This observation suggests that the shortening of tracks from the lowtemperature ( < 200 jC) samples did not occur under

Table 2 Analytical results of confined FT length measurement Sample code

All tracks N

L (Am)

sd (Am)

se (Am)

Tracks >60j N

L (Am)

sd (Am)

se (Am)

Temperature (jC)

Depth (m)

Formation

Rock facies

MSM11 MSM10 MSM09 MSM08 MSM07 MSM05&06

62 64 41 59 53 38

10.6 10.3 10.7 10.8 10.2 10.2

1.59 1.92 1.62 1.27 1.83 1.60

0.2 0.2 0.3 0.2 0.3 0.3

38 40 27 33 50 22

10.4 10.1 10.4 10.8 10.3 10.5

1.64 1.96 1.55 0.98 1.82 1.59

0.3 0.3 0.3 0.2 0.3 0.3

124 136 147 164 174 200

Lower Lower Lower Lower Lower Lower

Sst Sst Sst Sst Sst Mst>Sst

0.56

0.2

187

9.8

0.81

0.1

205

9.2

1.18

0.7

223

3500.70 – 3501.00 3986.70 – 3987.00 4515.00 – 4515.10 4881.00 – 4881.15 5196.50 – 5196.75 5561.45 – 5561.55, 5564.45 – 5564.70 4830.56 – 4830.78, 4835.71 – 4836.00 5250.03 – 5250.25, 5253.97 – 5254.25 5643.03 – 5643.32, 5644.45 – 5644.62

NSK07&15

17

9.9

0.71

0.2

12

10.1

NSK05&06

53

9.9

0.89

0.1

34

NSK03&04

4

9.2

0.97

0.5

3

Teradomari Teradomari Teradomari Teradomari Teradomari Teradomari

Lower Teradomari

Sst

Lower Teradomari

Sst

Lower Teradomari

Sst

N: number of measured tracks; L: mean length of measured tracks; sd: standard deviation of length distribution; se: standard error of length distribution; Sst: sandstone; Mst>Sst: mudstone dominant alternation of sandstone of mudstone.

50

N. Hasebe et al. / Chemical Geology 199 (2003) 45–52

Fig. 3. Mean track length (for tracks >60j) profiles against current temperatures where samples were collected. Error bars represent F 2 standard error. Track length distributions of each sample are also shown with. Black columns are for tracks whose crystallographic angle is >60j, and white columns for tracks < 60j against c-axis. Two age data are shown as radial plots in which an age (Ma) of each grain is represented as the slope of the line from the origin to an individual point (Galbraith, 1988). Grains subtracted to calculate second group in Table 3 are shown as open circles.

the current geothermal condition. The short tracks found would reflect some thermal influence on the crystals before deposition, because the samples contain a few older grains, which may have undergone heating before deposition. One shortened horizontal

confined track was observed in the old grain from MSM08. Hence, we can conclude that tracks are not being shortened at the current geothermal temperatures below 200 jC. This is consistent with the previous results from ultra-deep boreholes (e.g. KTB

Table 3 Results of age determination qs (106 cm MSM11 MSM08 MSM11 MSM08

3.20 2.55 2.05 2.12

Ns 2

) 297 302 174 226

qi (106 cm 4.75 4.71 4.74 4.91

Ni 2

) 441 558 403 524

qd (106 cm 0.1023 0.1023 0.1023 0.1023

Nd

T (Ma)

r (Ma)

N

P(v2) (%)

2224 2224 2224 2224

12.4 9.9 7.9 7.9

1.0 0.8 0.8 0.7

8 14 7 12

0.00 0.00 0.00 5

2

)

qs: density of spontaneous tracks; Ns: number of spontaneous tracks counted to determine rs; qi: density of induced tracks in a sample; Ni: number of induced tracks counted in a muscovite external detector to determine qi; qd: density of induced tracks in NBS-SRM612 dosimeter glass; Nd: number of induced tracks counted in a muscovite external detector to determine qd; T: FT pooled age calculated from pooled Ns and Ni for all grains counted; N: number of counted grains; r: standard error of FT pooled age; P(m2): Probability of v2 values for n degrees of freedom (n = N 1). Upper two lines are calculated using all obtained grain data. Lower two are calculated after withdrawing grains whose ages are Eocene or older.

N. Hasebe et al. / Chemical Geology 199 (2003) 45–52

HB, Kola, and Vienna, respectively; Coyle and Wagner, 1996; Green et al., 1996; Tagami et al., 1996). The track length distribution of NSK05&06, which are from 205 jC depth, have a peak shorter than that of the original unannealed distribution, showing similarities with the laboratory-annealed sample at 600 jC for 1 h (Yamada et al., 1995b). Hence, it could be undergoing annealing at the present ambient temperature, suggesting the beginning of track annealing at f 200 jC. Although shortened tracks in samples from 223 jC depth show a possibility of annealing at the current temperature, the low number of measured tracks prevent detailed analysis. Although it is clear that age determination on those bottom samples could give further evidence of annealing, we could not find enough zircon grains to determine ages. Track annealing temperatures in nature can be estimated by extrapolating the fitted kinetic model to geological time scales (Fig. 4). The characteristics of geological track annealing were described for zircon using a contact metamorphic aureole around a granitic pluton with estimated heating durations of f 105 – 106 years (Tagami and Shimada, 1996). They found

51

that the observed track length reduction characteristics are indistinguishable from those in laboratory heating experiments (e.g. Tagami et al., 1998; Yamada et al., 1995b). It is noted here that the characteristics of track annealing in zircon are very similar to those reported by Green et al. (1986) for apatite, and that kinetic models based on laboratory data work well for apatite FT analysis of geological samples (Green et al., 1989). These facts suggest that the extrapolation is reasonable also for zircon. The data from NSK05&06 with a mean track length of 9.8 Am (cf. NSK03&04) are plotted in Fig. 4, along with the results from three previous borehole samples. Considering the measurement error and contamination of original short tracks which is deduced from the length distribution from unannealed samples at low temperatures, the data fit the range extrapolated from the results of laboratory experiments using a fanning model on the Arrhenius plot. Further work on obtaining and analyzing samples from deep borehole core is necessary to provide more adequate information on the temperature range of the ZPAZ.

Fig. 4. The Arrhenius plot showing the design points of the annealing experiments and contour lines fitted using the fanning model extrapolating to geological time. The approximate locations of the previous three borehole samples and results of this study are also shown. A star symbol indicates the data from NSK03&04 with a mean track length of 9.2 Am. Modified from Tagami et al. (1998). See also Yamada et al. (1995b).

52

N. Hasebe et al. / Chemical Geology 199 (2003) 45–52

Acknowledgements This study was supported by the grant from the Japan National Oil Cooperation. We are grateful that Drs. Paul Green, Gunter Wagner and Andrew Carter who gave us critical comments on this manuscript. Prof. Andrew J. W. Gleadow also read through the manuscript and gave us comments. [PD]

References Akiyama, M., Hirai, A., 1997. Maximum paleotemperature gradient using vitrinite reflectance mainly of the government MITI exploratory test wells. J. Jpn. Assoc. Pet. Technol. 62, 69 – 79 (in Japanese). Coyle, D.A., Wagner, G.A., 1996. Fission-track dating of zircon andtitanite from the 9101 m deep KTB: observed fundamentals of track stability and thermal history reconstruction. Abstract of presentation at International Workshop on Fission Track Dating, Gent. Galbraith, R.F., 1988. Graphical display of estimates having differing standard errors. Technometrics 30, 271 – 281. Gleadow, A.J.W., Hurford, A.J., Quaife, R.D., 1976. Fission track dating of zircon: improved etching techniques. Earth Planet. Sci. 33, 273 – 276. Green, P.F., Gleadow, A.J.W., Tigate, 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. Green, P.F., Duddy, I.R., Laslett, G.M., Hegarty, K.A., Gleadow, A.J.W., Lovering, J.F., 1989. Thermal annealing of fission tracks in apatite: 4. Quantitative modeling techniques and extension to geological time scales. Chem. Geol., Isot. Geosci. Sect. 79, 155 – 182. Green, P.F., Hegarty, K.A., Duddy, I.R., Foland, S.S., Gorbachev, V., 1996. Geological constraints on fission track annealing in zircon. Abstract of presentation at International Workshop on Fission Track Dating, Gent. Hasebe, N., Tagami, T., Nishimura, S., 1994. Towards zircon fission-track thermochronology: reference framework for confined track length measurement. Chem. Geol., Isot. Geosci. Sect. 112, 169 – 178.

Hurford, A.J., 1990. Standardization of fission track dating calibration: recommendation by the Fission Track Working Group of the I. U. G. S. Subcommission on Geochronology. Chem. Geol., Isot. Geosci. Sect. 80, 171 – 178. JNOC (Japan National Oil Corporation), 1993. ‘‘MITI-Mishima’’ survey report. Basis, survey of domestic petroleum, natural gas in 1991 (in Japanese). JNOC (Japan National Oil Corporation), 1996. ‘‘MITI-Nishikubiki’’ survey report. Basis, survey of domestic petroleum, natural gas in 1995 (in Japanese). Kishi, K., Miyawaki, R., 1996. Plio-Pleistocene fold development in the Kashiwazaki plain and vicinity, Niigata Prefecture. J. Geogr. 105, 88 – 112 (in Japanese). 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. Tagami, T., Shimada, C., 1996. Natural long-term annealing of the zircon fission track system around a granitic pluton. J. Geophys. Res. 101, 8245 – 8255. Tagami, T., Lal, N., Sorkhabi, R.B., Ito, H., Nishimura, S., 1988. Fission track dating using external detector method: a laboratory procedure. Mem. Fac. Sci., Kyoto Univ. 53, 14 – 30. Tagami, T., Carter, A., Hurford, A.J., 1996. Natural long-term annealing of the zircon fission-track system in Vienna Basin deep borehole samples: constraints upon the partial annealing zone and closure temperature. Chem. Geol., Isot. Geosci. Sect. 130, 147 – 157. Tagami, T., Galbraith, R.F., Yamada, R., Laslett, G.M., 1998. Revised annealing kinetics of fission tracks in zircon and geological implications. Advances in Fission-Track Geochronology. Kluwer Academic Publishing, The Netherlands, pp. 99 – 112. Yamada, R., Tagami, T., Nishimura, S., 1993. Assessment of overetching factor for confined fission-track length measurement in zircon. Chem. Geol., Isot. Geosci. Sect. 104, 251 – 259. Yamada, R., Tagami, T., Nishimura, S., 1995a. Confined fissiontrack length measurement of zircon: assessment of factors affecting the paleo-temperature estimate. Chem. Geol., Isot. Geosci. Sect. 119, 293 – 306. Yamada, R., Tagami, T., Nishimura, S., Ito, H., 1995b. Annealing kinetics of fission tracks in zircon: an experimental study. Chem. Geol., Isot. Geosci. Sect. 122, 249 – 258. Zaun, P.E., Wagner, G.A., 1984. Fission-track stability in zircons under geological conditions. Nucl. Tracks 10, 303 – 307.