ESR dating and trapped electrons lifetime of quartz grains in loess of China

Quaternary Science Reviews, Vol. 7, pp. 533-536, 1988. Printed in Great Britain. All rights reserved.

0277-3791/88 $0.00 + .50 Copyright© 1988 Pergamon Press plc

E S R D A T I N G A N D T R A P P E D E L E C T R O N S L I F E T I M E O F Q U A R T Z G R A I N S IN L O E S S OF CHINA

H u a n g Pei-Hua, Jin Si-Zhao, Peng Zhi-Cheng, Liang R e n - Y u , Q u a n Yu-cai and W a n g Z h a o - R o n g

Department of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui, People's Republic of China

This paper reports the preliminary application of ESR dating to loess strata. The samples were collected from the 7th palaeosol layer ($7) of the Luochuan section, Shaanxi province in China. The ESR age of $7 is 736 ka (total dose 2945 Gy, annual dose 4 mGy/year). This age represents the original eolian accumulation age. The result is consistent with the palaeomagnetic data (730 ka). We have also carried out thermal annealing experiments on quartz grains from the $7 sample. ESR intensities (g = 2.0005) increase from 25°C to 320°C. It may be that trapped electrons transfer into the E' centre site. ESR intensities decrease from 340°C to 460°C due to thermal annealing. We obtained a mean-life of E' centre electrons at 20°C of 6.66 x 108 years. The activation energy is 1.35 eV and frequency factor is 3.7 x 108 min 1.

INTRODUCTION In the last few years, TL determination of loess age has developed rapidly in Europe and China. However, the use of ESR methods to determine loess age, as yet, has not been reported in published papers. The source materials of Quaternary eolian loess in the loess plateau have been transported by wind from desert and dry eolational regions of Northwest China (Xinjiang and Gansu regions), and have then been accumulated to form the loess plateau and its thickbedded loess strata during the Quaternary period. The light exposure experiments of Kalamay desert (Junggar basin, Xinjiang) and loess (Shaanxi province) samples show that an exposure equivalent to 5 days of direct sunlight or 24 hours of 50°C surface temperature of the sample is capable of reducing the TL signal to the lowest level (Nishimura et al., 1984) or 90% (Li, 1984). Hence, it is suggested that previously acquired geological ESR signals as well as the TL signal could be erased by exposure to sunlight before deposition. This problem of whether the E' signal approaches zero at zero age in loess samples will be studied continually by us. Dating is based on this assumption. The samples in this paper were collected from the 7th palaeosol layer ($7) of loess strata for the study of ESR dating. The layer $7 is located at the Brunhes/Matuyama polarity boundary of palaeomagnetic measurements (Heller and Liu, 1982; Nishida et al., 1984). Fortunately, preliminary ESR dating of $7 has given a successful result and it is in agreement with the data of palaeomagnetic measurements. In order to verify that the mean-life of trapped electrons at the ambient temperature is much longer than the age to be measured, we have carried out thermal annealing experiments at different temperatures from 25°C to 500°C for different annealing times using quartz grains from the $7 sample. The life time of E' centre electrons at an ambient temperature (20°C) is

10s years. This compares with measurements by Moiseyev and Rakov (1977) and Smolyanskiy and Masaytis (1979) who obtained the mean-life of the quartz E' centre to be of the same order.

ESR DATING OF 7TH PALAEOSOL LAYER ($7) IN LOESS STRATA The sample from the 7th palaeosol layer ($7) was taken from the Luochuan section, Shaanxi province, China. This section is located in the central part of the loess plateau and the thickness of loess strata at this section reaches 130 meters. The stratigraphic sequence of this section from top to bottom (Wang et al., 1984) is as follows: (1) ttolocene: LO, grayish yellow loess, 0.5-1.5 m; SO, black loam, 0.7-1.3 m, t4C age 5-6 ka; (2) Late Pleistocene: L1, grayish yellow loess, 8.6 m; SI, reddish brown fossil soil, 2.0 m, TL age 110 ka; (3) Middle Pleistocene: L2, grayish yellow loess, 8.1) m; $2, reddish brown fossil soil, 3.2 m, TL age 210 ka; ... L7, grayish brown loess, 3.5 m; $7, reddish brown fossil soil, 1.8 m; ... Brunhes/Matuyama polarity boundary, 730 ka; (4) Early Pleistocene: L8, grayish brown loess, 2.0 m; $8, reddish brown fossil soil, 1.5 m; ... S14, reddish brown fossil soil, 1.5 m; loess strata below L15 is 46.4 m thick. (5) Pliocene: red clay strata (Fig, 1). The results of palaeomagnetic measurements have shown that the Brunhes/Matayama boundary is close to the upper part of the 8th loess layer (Heller and Liu, 1982) or the lower part of the 7th palaeosol layer (Nishida et al., 1984). The age of the B/M boundary, in general, corresponds to 730 ka. The ESR spectra of quartz grains in the $7 sample are shown in Fig. 2. The defect signals at g = 2.0005 are increasing with the growth of artificial dose. ESR intensities appear to have good linearity (Fig. 3). The

533

Huang Pei-Hua et al.

534

I ~

6 ~

z1333 3

7~ 8

Oj

M;

r1&

_

_ _

y--=

FIG. 1. Loess section of Luochuan, Shaanxi province. (1) 04, Loess; (2) Q3, Loess; (3) Q2, Loess; (4) Ol, Loess; (5) Dark loam; (6) Palaeosol; (7) Sandy silt bed; (8) Calcareous concretions; (9) Pliocene red clay; (10) Sandstone.

ESR Intensity (arb nit) 4o

" ~ e a ~

\ ~

~roy

E

= 20005

e

t

_

~/x/X/

f

595 Gy

11

i~

>,

597 Gy

20-x ly - - 2 9 4 5 Gy

rv

= 99.9%

0 Gy

UJ

I

I

I

t

200 403 6O0 8O0 Y ray [Gy] 3467 3477 3487 Magnetic fietd (G)

FIG. 2. ESR spectra obtained from quartz grains in $7 sample.

total dose is calculated by the least squares method and is 2945 Gy. The curve is well fitted to the measured points and the linear correlation coefficient is r = 99.9%. Uranium, thorium and potassium contents in the $7 sample are 2.6 ppm, 13.5 ppm and 2.1%, respectively. The etching technique is used to remove the radiation damage of alpha rays. Based on Bell's table (1979), the annual dose is 4 mGy/year. Dividing the total dose of 2945 Gy by it gives the ESR age of the original eolian loess deposits of the 7th palaeosol layer ($7, 56 m depth) to be 736 ka if radon loss and cosmic rays were neglected. Therefore, the result of ESR dating for the $7 layer agrees with the palaeomagnetic measurements very well.

FIG. 3. Relation of signal intensity with radiation dose.

THE TRAPPED ELECTRON LIFETIME OF QUARTZ GRAINS IN LOESS We used quartz grains from loess (above mentioned sample) for a thermal annealing experiment. The quartz grains were heated at different temperatures (from 25-500°C) for different annealing times. The isochronal annealing time was 10 min, The ESR spectra were measured at room temperature, microwave frequency 9.751 GHz, microwave power 2 m W , modulation amplitude 0.8 Gpp and scan speed 200 sec. The curve of ESR intensity, g = 2.0005, as a function of annealing temperatures is shown in Fig. 4. (1) From 25°C to 200°C, ESR intensity increases very slowly; (2) from 200°C to 320°C, the intensity increases quickly, showing that a number of the trapped electrons transfer into E' centre site; (3) from 320°C to 340°C, the

535

ESR Dating and Trapped Electrons Lifetime (320,82.5)

(~0078.4)fx'x~(340,822) y "~360,73.4)

f

~

250C 200°C 240°C

(280,64.2)~

/

~((380,54.5)

2800C

t /~,-------,, ,~

300°C

fie to LIJ

320o C 340o C x

(8.~5 .

360o C

',25,14.5)

(140,15.6)

~x(440'8"8) (460,3.3)

I

I

ioo

I

200

\~

300

4(3(3

Tem. i (*C)

(480,1.7)

"-x.~ ( 5 0 0 , 0 9 )

I

380°C

5(3(3

FIG. 4. Curve of ESR intensities as a function of annealing temperatures.

intensity varies little; (4) from 340°C to 480°C, due to thermal annealing, the intensity decreases quickly. There is probably still a small transfer of charge to the E' centre but this is not important; (5) when the temperature is round about 500°C, the ESR signal decreases to its lowest level (Fig. 5).

440% .~ . . . . .

460°C 480oc 500oc

FIG. 5. ESR spectra of quartz grains at each annealing temperature.

TABLE 1. Mean-life of trapped electrons (g = 2.0005) at different annealing temperatures ($7 sample) Annealing temperature (/~C) 340 385 400 420 460

Correlation of ESR signal intensity (N) and annealing time (t, min) t N t N t N t N t N

0 78.1 0 79.1 0 68.9 0 71.0 2 23.1

10 76.1 5 65.9 3 68.4 3 62.1 4 14.8

40 68.2 10 55.3 6 64.0 8 30.8 6 12.0

i02 -

(3,68.4)

~ x

50

70 65.7 20 43.4 11 50.5 11 26.5 8 8.4

100 59.8 30 36.1 16 38.0 14 23.0 10 7.3

130 57.0 40 30.0 21 32.3 17 19.3 ---

Linear regression equation (In N)

Linear correlation coefficient

4.35-0.00244t

0.992

409.2

4.30-0.00235t

0,989

42.6

4.32-0.00392t

0.990

25.5

4.25-0.0812t

0,979

12.3

3.35-0.1440t

0.986

----26 27.2 -----

y(6,64.0 )

',_0~689)~ x ( 1 4 5 0 . 5 )

~ x - ~

Io

"'I

T " 2 5 . 4 8 rain 7"= 99.0%

5

0

I 2

I 4

I 6

I 8

I i I I I I0 12 14 16 18 Anneol, ing time (rain)

I 20

I 22

I 24

I 26

I 28

FIG. 6. Determination of mean-life of g = 2.0005 electrons at annealing temperature 400°C.

Mean-life (-r) (min)

6.97

Huang Pei-Hua et al.

536

TABLE 2. Mean-life, activation energy and frequency factor of g = 2.0005 electrons for quartz grains in sample $7 Linear regression equation (In T)

Linear correlation coefficient (r)

T 20°C (×10 s year)

E (eV)

vo (xl0 s min 1)

15591(1/T) - 19.7

0.979

66.6

1.35

3.67

Correlation of mean-life (T) and temperature T(K) -r

613 409.2

658 42.6

673 25.5

693 12.3

733 6.97

storage temperature in Kelvin, v0 is the frequency factor. According to the above data, an Arrhenius plot of the mean-life as a function of 1/T is given in Fig. 7. We obtained the mean-life o f g = 2.0005 electrons at 20°C to be 6.66 x 10s years. The activation energy is 1.35 eV and frequency factor Vo = 3.67 x 10s (rain -1) (Table 2). It can be seen that the above experiment has verified that the mean-life (108 years) of trapped electrons for quartz grains at ambient temperature (20°C) is much longer than the age of sample ($7,736 ka). Therefore, the preliminary study in this paper shows that the ESR dating method can be used to determine the age of eolian loess deposits.

T("K) 800

667

57 I

I 5

X

I

[0 2

7 = 9Z9%

102 - -

/

Vo= 3.67 x I08

/

/

p

5O

(658-~,48.631

(673- ~,25.48)

I0

1733_~,?|695-~'1232"1

5

REFERENCES

i 0001

000125

I

I

0,0015

000175

T-f(K-I)

FIG. 7. Arrhenius plot of the mean-life as a function of 1/T for quartz grains in 7th paleosol layer.

The mean-life of g = 2.0005 electrons at each annealing temperature can be determined: N t = N0 e - x t

(1)

where Nt is the number of g = 2.0005 electrons at the annealing time t; No is the initial number of g = 2.0005 electrons; k is the decay constant. The experimental results are shown in Table 1. Figure 6 shows the determination of the mean-life of g = 2.0005 electrons from the decrease of the g = 2.0005 line with annealing time when the annealing temperature was 400°C (673 K). The mean-life -r of trapped electrons is given by the expression; l/'r :

VOe - E / k T

(2)

where E is the activation energy for the release of trapped electrons, k is the Boltzmann constant, T is the

Bell, W.T. (1979). Thermoluminescence dating: radiation dose-rate data. Archaeometry, 21,243-245. Heller, F. and Liu, T.S. (1982). Magnetostratigraphical dating of loess deposits in China, Nature, 300,431-432. Hennig, G.J. and Grfin, R. (1983). ESR dating in Quaternary geology, Quaternary Science Reviews, 2, 157-238. Huang, P.H., Peng, Z.C., Jin, S.Z., Liang, R.Y. and Wang, Z.R. (1985). An attempt to determine the archaeological doses of the travertine and the deer horn with ESR. In: Ikeya, M. and Miki, T. (eds), ESR Dating and Dosimetrv, pp. 321-324. IONICS, Tokyo. Ikeya, M. (1985). Dating methods of Pleistocene deposits and their problems. Geoscience Canada, Article reprint series No. 2. Rutter, N.W. (ed.), 72-87. Li, H.H. (1984). Thermoluminescence dating of loess - - Experimental determination of shine fading and correction of experimental results. Geochimica, No. 2, 180-185. Moiseyev, B.M. and Rakov, L.T. (1977). Paleosimetry properties of E' centers in quartz. Doklady Akadernii Nauk USSR, 233, 679-683. Nishida, J.I., Yue, L.P., Kawamura, Y. and Miao, J.Y. (1984). Determination of Brunhes/Matuyama polarity boundary of Chinese loess section in Qinjiazhai, Louchuan. ln: Sasajima, S. and Wang, Y.Y. (eds), The Recent Research of Loess in China, pp. 69-78. Kyoto University and Northwest University. Smolyanskiy, P.L. and Masaytis, V.L. (1979). Reconstruction of paleotemperature anomalies of old rocks from radiation-induced defects in quartz. Doklady Akademii Nauk USSR, 248, 1428-1431. Wang, Y.Y., Sasajima, S., Teng, Z.H., Lei, X.G. and Sun, W. (1984). Loess in China and its stratigraphic sequence. In: Wang, Y.Y. and Sasajima, S. (eds), The Recent Research of Loess in China (Chinese), pp. 1-19. Northwest University and Kyoto University. Yokoyama, Y., Quaegebeur, J.P., Bibron. R. and Leger, C. (1983). ESR dating of Paleolithic calcite: Thermal annealing experiment and trapped electron life time. PACT, 9, 371-379.