フィッション・トラック

ニュースレター第 20 号

13-19

2007 年

研究ノート

Zircon fission-track technique: a laboratory procedure adopted at theInstitute of Geology, Academy of Sciences of the Czech Republic, v.v.i.
Masaki Murakami*,** and Martin Svojtka***

Department of Earth and Planetary Science, Graduate school of Science, Univ. Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo113-0033, Japan, e-mail: masaki@eps.s.u-tokyo.ac.jp**

Institute of Geology, Academy of Sciences of the Czech Republic, v.v.i., Rozvojová 269,
Praha, Czech Republic, Phone: +420 233087242, fax: +420 220922670,
e-mail: svojtka@gli.cas.cz

Intr odu c tio n

Sciences of the Czech Republic, v.v.i.

Along with many other geochronological

For thedetails of zircon fission-track method, see

methods, fission-track analysis is a technique

Tagami (2005) and references herein.

used for the reconstruction oftime-temperature evolution of rocks.

Samp les pre pa ra ti on

The fis-sion-track dating method offers easy dating of

A procedure of fission-track sample prepara-rocks without using high-priced mass spec-tion described in the following chapters (A-D)

trometers and has been applied to manyis adopted from Tagami (1988) and Tagami

low-temperature geological problems.

(2005):

This

paper describes laboratory routine proceduresfor fission-track analysis (FTA) of zircons in-

A. Mineral separation

cluding separation, mounting, grind-Rock samples collected for mineral separa-

ing/polishing, chemical etching, thermal neu-tion are in weight from 250 g to 25 kg.

tron irradiation in nuclear reactor, counting

following steps are undertaken to separate theand determination of zeta constant.

minerals required for analysis: (1) Sample

All pro-The

cedures of zircon fission-track analysis usingcrushing; (2) Wilfley table separation method;an external detector method described here is(3) Magnetic and heavy liquid separation.

applied at the Institute of Geology, Academy ofstitute of Geology (Academy of Sciences) ho-In-

Fig. 1 Equipments at the Institute of Geology for mineral separation: A) mills and jaw crushers; B) Wilfey table; C) Frantz separator for separation of magnetic and non-magnetic minerals; D) separatory funnel forheavy liquid (bromoform and diiodomethane) separation.

13

mogenization laboratory is equipped with mills

B. Handpicking and mounting

and jaw crushers, fume hoods that are used to

In order to mount the grains into teflon

ground rock samples to fine grained particles

sheets, small fraction of zircon grains is spread(Fig. 1A).

on a silica glass slide of size about 5 5 cm .

2

Next step is a separation by ashape/density separator known as the Wilfley

Silica glass is preferably used, because thevibrating table (Fig. 1B), which is used as aslide is heated during mounting on a hot plate.

quick, reliable and consistent method for

During handpicking a small quantity of alcoholde-sliming bulk powder samples.

The rock's

is put on the glass slide and with a thin paint-

mineral mixture is placed on the high side ofbrush the zircons are arranged one by one in

the surface of the Wilfey table and water issuch a manner that c-axis of all the grains areflowed over it to move the grains while thein one direction (Fig. 2).

surface is vibrated.

should be selected under the criterion of large

Heavier grains collect in

The zircon grains

the grooves while lighter grains are washedclear crystals (e.g., equant grains, prismatic

down the surface.

stubby or prismatic euhedral-needles grains)

The third step is a separa-tion by two Franz magnetic separators (Fig.

having well-defined c-axes.

1C), with variable magnetic field strength,

all the remaining grains are removed from thesample feed-through rate, and angle and tilt ofglass slide by use of a dry paintbrush andsample feed-through.

stored in a paper or a plastic pack with a

After separation of fer-romagnetic minerals (e.g., magnetite),

After handpicking,proper label.

non-ferromagnetic minerals are separated in

The silica glass slide is then shifted onto a

the isodynamic magnetic separator and com-

hot plate at a temperature of 315 C.

A second
2

posites of magnetic minerals (e.g., biotite,silica glass plate of size about 3 3 cm is also

hornblende, pyroxene, ilmenite, epidote) are

pre-heated on the same hot plate.

removed.

0.50 mm thick PFA Teflon® is cut to the size

The non-magnetic fraction that2

A sheet ofcontains datable minerals such as zircons andof 1.5 1.5 cm .

apatites (plus quartz, fluorite, etc) is then sub-tically using tweezers on the silica glass slide

jected to heavy liquid separations.

and heated for about 30 sec. until its bottom

In next

The Teflon sheet is held ver-step, a mixture of minerals is placed in theend melts slightly.

Then it is tilted to allow

separatory funnel (Fig. 1D) with heavy liquid,

falling down on the glass slide.

the grains with densities lower than that of the

pre-heated glass plate is put on the Teflon®

Theliquid will float and grains with densities
greater than the liquid will sink.

Suitable liq-

uids for density separation include bromoform
3(density = 2.84 g/cm ) and diiodomethane3(density = 3.32 g/cm ).

Final pure mineral

concentrates are achieved throughhand-picking individual grains using an opticalFig. 2 A) Photo of zircon grains mounted in PFA
2
Teflon (size of sheet is 1.5 1.5 cm ). B) Detailof previous mount with zircon grains arranged
along their c-axis in a square of 10 10.

binocular microscope (Nikon SMZ 800) withfiber-optic lights.

14

Fig. 3 Photo of individual steps during mounting procedure for zircon grains into PFA Teflon: A) Silica glass
slide with studied zircons shifted on hot plate; B) PFA Teflon sheet falling by gravitation on zircons; C)
Tweezers put pressure on small silica glass to remove bubbles from PFA Teflon. Finally, zircon grains arepushed into PFA Teflon and are ready for next procedures.

sheet and pressed gently so that the grains are

to stabilize the mount during grinding and pol-fixed into the sheet (Fig. 3).

ishing, an acrylic block is pasted on grinding

The glass slidewith the glass plate and Teflon® sheet having

and polishing machine and fixed together withzircon grains is removed from the hot plate,zircon mounted grains by using double-coatedand then pressed with a metallic plate to betape.

kept flat during cooling.

is essential to remove a certain thickness to

The sample code is

For using internal surface of zircons, it

written with a needle pen on the backside of

expose 4π geometry for FT dating.

the Teflon® mount after it is taken out fromthickness corresponds to a half of etchable

the glass slide and plate.

track length that is ~6 µm for zircon grains

In case of FTA on

zircons, it is not preferred to grind zircon

The(Krishnaswami et al., 1974).

crystals along their crystallographic c-axes,

Before 6 µm grinding, the crystal surface of

because it often produces fractures or damages

mineral grains must lie flat at the surface ofof the surface, which are enlarged during

the mount.

chemical etching and disturb the track length

gently several times in one direction perpen-

measurement and counting.

dicular to the c-axis on the wet emery paper ofThus zircon

The mount with zircons is groundgrains should be arranged with their c-axes

1500 mesh size, and observed under an optical

toward the same direction in a mount, andmicroscope for the grinding scratches.

then all grains are ground in one direction

continued until the grinding scratches are ob-(perpendicular to c-axes).

servable on the most of grains.

In our routine, 64

It is

The observ-

to 144 mineral grains (within array ranges

able area of them will be sufficiently exposed 4

from 8 8 to 12 12) are arranged and mounted

π surface after 6 µm grinding, and thus mustas mentioned above (Fig. 2).

be described and recorded by convenientphotographs or by sketches.

C. Grinding and polishing

For FT length

measurement, it is not necessary to describe

As mentioned above, FT length measure-the surface of mineral grains.

ment and counting of minerals are carried out

The next step is to grind the mount and re-on a polished internal surface of mineral grains

move a certain thickness of grains for exposing

under an optical microscope.

4π geometry.

Flat internal

Tagami et al. (1988) measured

surfaces of the mineral grains are exposed by

the depth of the grinding scratch using wetted

grinding and polishing procedures.

emery paper of 1500 mesh, which was at least

In order15

1 µm depth.

Therefore the grain minerals aretemperature of the heater in the hot plate isremoved for at least 1 µm in thickness after acontrolled by a precise thermocouple and aseries of grinding and polishing processes.

temperature controller.

Thus the series of the processes is repeated six

a stable heating up to 500 C withtimes, and then the mineral grains will beracy.

ground 6 µm at least, that is, exposed their 4π

monitored with a Teflon® coated thermocou-

surfaces.

ple, which is dipped inside the beaker.

This system provides
1 C accu-

The temperature of the etchant is also

The

After the grinding, the mount is polishedrequired temperature of the etchant is 250

with 15 µm and 3 µm diamond paste (Struers

1 C for FT length measurement and 225

Company).

for FT dating.

In case of the Institute of Geology,

1C

Approximately a half-day is

Academy of Sciences, Struers DP-Dac (for 3-6

necessary time to achieve a proposed tem-

µm abrasive grain size of diamond paste) andperature of the etchant.

DP-Dur (for 1 µm abrasive grain size of dia-

When the etchant is prepared, three or fourmond paste) polishing clothes are used forzircon mounts are put into each beaker forpolishing.

etching.

The grinding and polishing system

The etching procedure can control(MTH Company Kompakt 1031) is equipped

the width of measurable tracks between un-with a specimen mover and an extension head

der-etched and over-etched condition.

system (APX 010).

cording to etching criteria described by

The specimen mover canAc-rotate and weight itself on a polishing cloth.

Yamada et al. (1995), the width of the surface

The duration, velocity and weight of polishing

tracks is 2 µm for FT length measurement anddepend on the hardness of mineral grains.

1 µm for FT dating.

Fish Canyon Tuff zircon standards are polisheddepends on a density of tracks in zircon grainsfor 10 min. to remove the grind scratch with

(Hasebe et al., 1994).

15 µm paste using 10 N weighted specimen

Fish Canyon Tuff zircon standard (ρs ~5 10

movers (75 rounds/min.), and it also takes 10cm ) is about 25 hours and that of Bulk Mem-min. to remove the 15 µm polishing scratch

ber zircon standard (ρs ~5 10 cm ) is about

with 3 µm paste (90 rounds/min.).

50 hours in our system (1 µm criterion at 225

The etching duration

The etching time of
6

-2

6

After fin-

ishing grinding and polishing, the mount is

1 C).

-2

After taking out the mounts from thecleaned with alcohol in an ultrasonic bath andetchant, it is cleaned in 5% HCl with ultrasonicthe mount is ready for etching.

cleaner for 10-15 minutes in order to remove
the etchant from the grains and from Teflon®

D. Etching of spontaneous fission-tracks in minerals

sheet.

Fission-tracks in mineral grains can be en-

one week, and then it must be replaced by alarged and made visible in an optical microscope by chemical etching.

new one.

Latent fission

tracks are only ~1 nm in width, invisible underthe microscope.

For etching of zircon mounts,

we use an electric hot plate with aluminium
holes for keeping Teflon® beaker containing
the eutectic NaOH:KOH (1:1) etchant.

The suitable lifetime of an etchant is

The

16

After thermal neutron irradiation, the mica
detectors and plastic sheets are removed fromthe mounts.

During irradiation, the induced

fission tracks are recorded in the mica detector.
To make the induced tracks visible, the micas
are fixed in a rubber ring with several cuts forstabilization of micas (Fig. 4), and then thering with micas is put into 38% HF at 32.0 C
for 8 minutes.

After etching, all samples andmicas are washed in distilled water with an ultrasonic cleaner for a few minutes.

Afterwashing, the samples and corresponding micas
are ready for counting of the fission-tracks (Fig.

Fig. 4 Photo of muscovite external detectors attached in a rubber ring before etching mica
sheets to reveal the induced tracks.

5).

Densities and lengths of spontaneous

tracks and densities of induced tracks are
E. Packing and irradiation of samples for irra-

measured at the Institute of Geology, Academy

diation, etching of induced fission-tracksof Sciences on Axioplan 2 (Zeiss) microscope

In the external detector method (Fleischer

equipped with Autoscan™ stage.

and Hart, 1972), a mica external detector (ca.
10 10 mm; Fig. 4) is placed over the grain
mount during the thermal neutron irradiation.
After mica detector is put on the mount (thesample code is written with a needle pen on
the back of the mica), the detector is covered
with a clean plastic sheet of size slightly larger
than the mica sheet, and then they arewrapped up with a tape and the sample code iswritten on it.

The dosimeter glass (IRMM

540R, ~15 ppm U) for monitoring thermal neu-

Fig. 5 Spontaneous tracks (A) and (B) induced tracks
recorded on mica external detector in zircon Fish
Canyon standard. Scale bar is 15 µm.

tron fluence is also attached to the mica sheet
as well as the zircon mount.

Co ncl udin g re mar k s

After micas are attached to mineral mounts
and to dosimeter glasses, samples are stacked

To overcome the uncertainties that are dif-and packed in a PE-tube for irradiation in a

ficult to measure (e.g., the uncertainty of neu-

nuclear reactor.

The Oregon State University

tron flux, individual counting error), the FTTRIGA Mk. II Research Reactor (OSTR) is usedcommunity adopted a system of calibrationfor our purposes.

based on the comparison of sample measure-

The OSTR operates at

maximum steady state power of 1.1 MW and isment with the analysis of mineral standards

equipped for thermal neutron irradiations

and glass monitors̶the zeta calibration ζ

suitable for fission track geochronology.

(Hurford and Green, 1982).

17

The fission-trackzeta value is an individual constant different

181.9

between the fission-track geochronology labo-

We calculated two sample weighted mean zeta

ratories.

values (SMWZ) using three zeta values in each

The zeta value is calculated from thefollowing equation:"=

standard (165.7
for FC-3).

#iexp($Dt std %1)$D g# s # d(1.55125 10!

9.6 for FC-3; Table 1).

5.0 for BM, and 196.1

3.6

We noted that a weighted mean ofa discrete set of numbers with weights waswhere λD is total decay constant of-10

5.2, 212.1

238U

given by a sum of multiplications between in--1yr ; Steiger and Jäger, 1977);

dividual zeta and weight values.

Weight zeta

ρs is the spontaneous fission track density of avalue was a reciprocal of squared standard de-standard sample; ρi is the induced fission

viation and weighted average FT age for real

track density of a standard sample measured

samples will be calculated as T/(standard de-in a muscovite detector; g is the geometry fac-viation) .

tor (= 0.5); ρd is the induced fission of the do-(OWMZ) of our laboratory was a weighted

simeter glass measured in a muscovite detec-

mean of two zeta values from zircon standards

tor; tstd is the reference age of the standard

calculated above, and is published as 185.7

sample.

2.9 (Table 1).

2

The statistical error of zeta was cal-Finally, overall weighted zeta value

One of the recommendations

culated by the conventional analysis (Hurford

by Hurford (1990) for the standardization of FT

and Green, 1983) adopted in Trackkey pro-dating states that the standard should be irra-gram (Dunkl, 2002) that is routinely used fordiated at least three times.

apatite calculation.

sider above published zeta value as a prelimi-

The results of zeta cali-

bration are in Table 1.

Each zeta value was

Finally, we con-nary constant and for next dating will be zeta

calculated using individual ρs, ρi and ρd val-value re-counted as a personal individual

ues and reference ages of standard zircons,value.

and then six individual zeta values were obtained from two standards (188.3
9.2, 145.2
Table 1

Ac kno wledge men ts

8.6, 165.7

8.2 for BM, and 209.3

6.1,

Support provided by the grant of Ministry of

Fission-track zircon analytical data for zeta calibration and sample weighted mean zeta values(SWMZ) for fission-track laboratory at the Institute of Geology, Academy of Science, Praha.
Sample codeρs (Ns)ρi (Ni)ρd (Nd )n
P (χ2)
Zeta (
6
-2
6
-2
6
-2
[ 10 cm ][ 10 cm ][ 10 cm ](%)
BM ZR1
1.171 (649)4.477 (2482)
0.6669 (8011)
32
89
188.3
ZR2
0.883 (428)3.202 (1552)
0.7185 (8631)
31
98
165.8
ZR3
1.110 (430)3.526 (1366)
0.7185 (8631)
24
93
145.2
SWM Z
165.7
FC-3

ZR2
ZR3
ZR4

5.763 (1904)
5.630 (2154)
4.206 (725)

14.39 (4753)
12.21 (4671)
10.64 (1834)

0.6669 (8011)
0.6669 (8011)
0.6669 (8011)

8.6
9.2
8.2
5.0

12
209.3 6.1
<1
181.8 5.2
19212.1 9.6
SWM Z
196.1 3.6
OWM Z
185. 72. 9
ρs = spontaneous fission track density; Ns = total number of spontaneous fission tracks counted;
ρi = induced fission track density; Ni = total number of induced fission tracks counted; ρd = induced fission
track density in a dosimeter glass; Nd = total track number to determine ρd ; n = number of grains counted;
P (χ2) = probability of obtaining the observed value of Galbraith's (1981) χ2 parameter, for n degrees of
freedom, where n = number of crystals -1; SWMZ̶sample weight mean zeta; OWMZ̶overall mean zeta.

18

34
39
19

1σ)

Education, Youth and Sports (1K05030) is
gratefully acknowledged.

Hurford, A.J. and Green, P.F., 1982, A user's guide

We appreciate theto fission track dating calibration. Earth Planet.

detailed reviews by Noriko Hasebe and an
anonymous reviewer.

Sci. Lett., 59, 343-354.

We would like to ex-

Krishnaswami, S., Lal, D., Prabhu, N. and MacDou-press our gratitude for providing of fis-gall, D., 1974, Characteristic of fission-tracks insion-track age standards to Dr. Charles W.

zircon: applications to geochronology and cos-

Naeser for Fish Canyon Tuff (FC3) and to Dr.

mology. Earth Planet. Sci. Lett., 22, 51-59.

Andrew Carter for Buluk Member Tuff (BM).

Hurford, A. J., 1990, Standardization of fission track
dating calibration: Recommendation by the Fis-

Referen ces

sion Track Working Group of the I.U.G.S. Sub-

Dunkl, I., 2002, Trackkey: a Windows program forcommission on Geochronology. Chem. Geol.,

calculation and graphical presentation of fission

80, 171-178.

track data. Computers and Geosciences, 28, 1,

Steiger, R.H. and Jäger, E., 1977, Subcommission on

3-12.

geochronology: convention on the use of decay

Fleischer, R.L. and Hart, H.R., 1972, Fission track

constants in geo- and cosmochronology. Earthdating: techniques and problems. In: W. Bishop,

Planet. Sci. Lett., 36, 359-362.

J. Miller and S. Cole (Editors), Calibration of

Tagami, T., Lal, N., Sorkhabi, R.S., Ito, H and Ni-

Hominoid Evolution. Scottish Academic Press,

shimura, S., 1988, Fission track dating using ex-

pp. 135-170.

ternal method: a laboratory procedure. Memoirs

Galbraith, R.F., 1981, On statistical models for fis-of the Faculty of Science, Kyoto University, Ser-sion track counts. Mathematical Geology, 13,

ies of Geology and Mineralogy, vol. LIII, Nos.

471-488.

1&2.

Green, P.F., Duddy, I.R., Gleadow, A.J.W., Laslett,Tagami, T., Galbraith, R.F., Yamada, R. and Laslett,

G.M. and Tingate, P.R., 1986, Thermal anneal-

G.M., 1998, Revised annealing kinetics of fission

ing of fission tracks in apatite, 1. A qualitative

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description. Chem. Geol., 59, 237-253.

P. Van den haute and F. De Corte (Editors), Ad-

Hasebe, N., Tagami, T. & Nishimura, S. 1994, To-vances in Fission-Track Geochronology. Kluwer

wards zircon fission-track thermochronology:

Academic Publishers, pp. 99-112.

Reference framework from confined track length

Tagami, T. 2005, Zircon fission-track thermochro-measurements. Chem. Geol. (Isot. Geosci. Sec.),

nology and applications to fault studies. Reviews

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