Age dependence of luminescence signals from granitic and mylonitic quartz

Quaternary Science Reviews (Quaternary Geochronology), Vol. 16, pp. 427-430, 1997. © 1997 Elsevier Science Ltd. All rights reserved. Printed in Great Britain.

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AGE DEPENDENCE OF LUMINESCENCE SIGNALS FROM GRANITIC AND MYLONITIC QUARTZ ZHI-YONG HAN, SHENG-HUA LI* and MAN-YIN W. T S O

Radioisotope Unit, The University of Hong Kong, Pokfulam Road, Hong Kong Abstract - - Granite and mylonitic samples with an age range of 11-3300 Ma were collected from different regions of China. Several luminescence signals were measured from the extracted quartz. Age-dependence has been observed in their luminescence signals, including the sensitivity of the 110°C TL-peak, the temperature of the high-temperature TL peak and the regenerated TL with/3 dose. Other luminescence signals show little age-dependence. The TL reaches maximum at about 160°C in the quartz samples, which is significantly different from signals from sediments. It is concluded that the luminescence signals may lead to the establishment of a new geochronological method for the whole earth history. © 1997 Elsevier Science Ltd

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temperature rises. Granitic quartz crystals are big enough to be separated easily. Apart from the luminescence signals studied by Rink (1994), 110°C TL and photon transferred thermoluminescence (PTTL) have been studied in order to find signals with a strong correlation with age. Both signals have been studied and widely used for conventional dating (Aitken, 1985).

INTRODUCTION At present, luminescence dating methods are mainly applied to archaeological and Quaternary samples using the signals accumulated with time. It is required that the lifetime of the signal is substantially longer than the age to be dated. This has limited the luminescence dating to be applied to samples younger than 1 Ma (Aitken, 1985). However, age-dependence may still exist for the luminescence signals because long-term effects may change the lattice structure of the mineral and hence the response of the luminescence signals. It has been reported that some luminescence signals from granitic quartz are age-dependent (Rink, 1994). He observed that the thermoluminescence (TL) peak temperature was positively correlated with age for the bleached samples, whereas the regenerated optically simulated luminescence (OSL) signals decrease with age. This conclusion was drawn based on four granite samples with an age range from 25 to 1040 Ma. Granitic quartz has several advantages over other quartz samples. The age of samples can be measured with isotope chronological methods, such as the K-Ar method. The quartz grains are from a single origin. Granitic quartz has a high purity because of slow growth and experiences relatively stable conditions in terms of irradiation. Granitic quartz was cooled from about 700°C through to near-surface temperatures, whereas mylonitic rock experienced single or multiple post-crystallization

SAMPLE PREPARATION AND EXPERIMENTAL CONDITIONS Rock samples from different regions of China were collected from several geologists (Table. 1). Parallel samples have been studied previously with isotope chronological methods. The listed ages were interpreted as the crystallization ages. They have been accepted by geologists, as they are consistent with geological settings (Zhang and Liu pers. commun.). Block samples were cleaned before crushing with a jaw crusher. The fraction between 177 and 121 gm was acquired by sieving. Heavy liquid separation has been applied to the grains. The fraction of quartz and plagioclase (density between 2.62 and 2.75 g/cm 3) was etched with 40% HF at ambient temperature for 40 min. The grains were rinsed with 1 M HC1 and distilled water. Further HF etching was applied if there was an IRSL signal from the grains. The quartz grains were loaded on aluminium discs with silkspray. All of the sample preparation processes were carried out in daylight. Before luminescence measurement, samples were bleached for

*Corresponding author.

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1 hr with Oriel 1000 W solar simulator, which has the similar spectrum as sunlight and the power at sample position is about 980 mW/cm 2. The TL was measured using Daybreak 1000 system with filters of Schott BG-39 and Coring 7-59. Green light stimulated luminescence (GLSL) was measured on a Ris0 reader with Schott BG-39 and U-340 filters. The stimulating source was a wavelength band of 5 1 5 560 nm. Unless specified, bleached and irradiated samples were stored in dark room for at least 24 hr prior to measurement. All results are the averages of more than three discs and all aliquots are weight-normalized. For most luminescence signals, the scatter among the discs is less than ± 1 0 % of the average value.

FIG. 3. Age-dependence of regenerated TL integrated intensity from 230 to 500°C for the age range of less than 400 Ma (inserted diagram shows all of the samples).

applied to all samples. In order to minimize contribution from other temperature TL peaks, the TL intensity was the integration of 60°C to 110°C. For samples younger than 200 Ma, this signal decreases approximately linearly with age down to a m i n i m u m level (Fig. I). For the two samples at the top right of the graph, their 110°C TL peak overlapped with other TL peaks. One sample at the bottom (128 Ma) has suffered detbrmation.

Regenerated TL Peak Temperature The regenerated TL glow curves were recorded with a ramp rate of 5°C/s to a maximum temperature of 500°C after 222 Gy /3 dose irradiation. The peak with the highest temperature would be chosen for multiple peaks cases. This peak temperature increases with age (Fig. 2).

RESULTS

Sensitivity of llO°C TL Peak The with a heated 40 sec

sensitivity of the I I0°C TL peak was measured 6 G y / 3 irradiation as a test dose. Samples were to 200°C with a ramp rate of 10°C/s. A delay of between irradiation and TL measurement was

Regenerated TL Intensity The regenerated TL intensity is the integral of 230°C to 500°C TL induced with 222 Gy/3 dose. For samples younger than 400 Ma, this signal increases with age (Fig. 3).

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Z.-Y. Han et ah Age Dependence of Luminescence Signals from Granitic and Mylonitic Quartz

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FIG. 7. Comparison of 6 Gy ~ dose induced TL and green light PTTL for temperatures less than 200°C from the same samples. Residual TL Peak Temperature

The residual TL peak temperature is the highest temperature of separable TL peaks via a 5°C/s ramp rate after the sample was bleached for l hr with a solar simulator. This TL temperature increases with age for samples younger than 400 Ma (Fig. 4).

Green Light Stimulated Luminescence (GLSL) After 432 Gy ,~3 irradiation and 5 min preheating at 220°C, the GLSL signals were measured with 100 sec of green light stimulation from a filtered He lamp. The GLSL intensity was the integrated count of 100 sec after subtracting the equivalent count of those from 80 to 100 sec. The results are shown in Fig. 5. Little agedependence was observed.

that the PTTL peak temperatures from granitic quartz are different with those from quartz of Quaternary sediment• Although there is a l l0°C TL peak for 3-irradiated samples, no 110°C peak was recorded in the PTTL of the granitic samples, but PTTL at l l0°C has been reported for quartz from Quaternary sediments (Franklin et al., 1995). The PTTL maximum was around 160°C (Fig. 7). Similar PTTL peak temperature was also observed from vein quartz. We have also measured the luminescence signals of a few samples of gneiss, diorite and tonalite samples with known age. Similar age-dependence was observed. This may lead to the application of the luminescence methods to other intrusive rock.

DISCUSSION Photon Transferred Thermoluminescence ( P T T L )

PTTL was measured immediately after GLSL measurement via a ramp rate of 10°C/s to a maximum temperature of 200°C. The PTTL intensity was the integrated TL signal from 50 to 200°C (Fig. 6). No obvious correlation was observed between PTTL intensity and the age of the samples. One significant distinction is

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The age-dependence observed in these luminescence signals demonstrates the potential of establishing a chronological method for the range of earth history. The sensitivity of the 110°C TL peak shows a clear decrease for samples with ages less than 200 Ma. Both regenerated TL temperature and intensity increase with sample age. A combined study of the luminescence signals could give an age estimation of granitic rocks. The method is relatively efficient and economical compared with isotopic dating methods. Observations of age-dependence and age-independence of the signals cannot be explained by the current theory or models established for luminescence dating (McKeever, 1985). It is not clear what mechanism is involved after the granite formation• It is thought that the defects created when granitic quartz crystallized decay with geological time and result in the reduction in concentrations of traps and luminescence centres. This can explain the sensitivity of the l l0°C TL peak decreasing with age. However, it cannot explain the age-dependence found in the TL peak temperature and the results of luminescence signals showing little correlation with age. The scattering of data in the diagrams may be due to inaccurate ages quoted from other workers (Table 1)

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Quaternar3, Science Reviews (Quaternao, Geochronology): Volume 16 TABLE 1. Granite samples collected from China

Sample

Rock type

Host unit

Location

Age (Ma)

A9013 A9138 Q94110 94-20 JS93-1 - 1 Q9417 JR5 SW 1 Q9402 Q9483 Q9464 Q9461 MA-3 91-4-1 HK 95-16 91 - 13-1 BZ-1 $93-3-3 H Y-02 Y-01 Do Y-04 Y-03

Gneissoid granite Poxphritic granite Plagiogranite Granitic mylonite Plagiogranite

Chentaigou Tiejiashan Shancha Baituman Wuduoshan Fuliushan Guangtoushan Laocheng Yanzhiba Duoguding -

Anshan Anshan Shanaxi lueyang Shandong Jiaonan Xixia Henan Wuduoshan Xinjiang Xinjiang Henan Lushan South Qinling South Qinling South Qinling Neimeng Rushan Hong Kong Taihangshan Rushan Xixia Fujiang Tibet Qushui Tibet Nimu Tibet Tibet Lasa Tibet Lasa

3300 (zircon single grain) 3000 (zircon single grain) 926+ 10 (zircon Pb-Pb) 800 391 340-385 297 294 280 20O 170.9+0.7 (Ar-Ar) 162.7+ 1.4 (At-At) 153 147 (K-Ar) 138-155 140 131 128 105 91.3+0.72 (Rb-Sr) 5O 50 15 (zircon U-Pb) 11 11

Alkali granite Alkali granite Gneissoid granodiorite Granite Gneissoid biotite adamellite Granitic mylonite Biotite granite Alkali granite Biotite granite Biotite granite Porphyritic Porphyritic

Washanshuidao Beijing Miyun Heiyanzhen Kuiqi Qushui Lasa Lasa

and/or deformation found in several samples. The luminescence signature may be affected if the quartz was subjected to significant heating, recrystallization and/or metamorphism after initial cooling. This may have led to some of the scatter in the age correlation. The concentration of radioactive elements may vary considerable among granites. Measurements of U, Th, K and other radioactive elements have been planned. The PTTL peak temperature for all granitic quartz samples is different with, and higher than, that observed for quartz from sediments and archaeological materials (Franklin et al., 1995). We have also noticed that the sensitivity of the l l0°C TL peak to # dose tbr these samples is significantly lower than that of quartz from sediments. In the process of becoming grains in the sediments, granitic quartz might undergo sensitization.

A CKNOWLEDGEMENTS The author wish to thank Prof. Zhang Zongqing of the Chinese Academy of Geological Scienceand Dr Liu Yulin of Peking University tbr providingthe samples. Z.Y.Han was supported by a studentship.

REFERENCES Aitken, M.J. (1985) Thermoluminescence Dating. Academic Press, London. Franklin, A.D., Prescott, J.R. and Scholefield, R.B. (1995) The mechanism of thermoluminescence in an Australian sedimentary quartz. Journal of Luminescence 63, 317-326. McKeever, S.W.S. (1985) Thermoluminescence of Solids. Cambridge University Press, Cambridge. Rink, W.J. (1994) Billion-year age dependence of luminescence in grantic quartz. Radiation Measurements 23, 419-422.