High Precision Thermal Ionization Mass

Introduction In the 1970's Thompson (1973) determined the ages of cave formations using the 230U/234U dating method at the Laboratory of U-series Dating and Stable Isotopic Analysis, McMaste University, marking a success in dating of impure calcium carbonate. In the 1980 ' s Henning et al. (1980) conducted a comparative study on the dates of calcium carbonates using the 14 C, U-series, thermal luminescence and electron spin resonance techniques . Ivanovich et al . (1982) presented the complete U-series dating equation, making the U-series disequilibrium dating method approaching to its perfection. In the late 1980' s Edwards (1986) determined precisely the ages of coral samples using the thermal ionization mass spectrometric U-series method. In the Vanuatu area, for example, TAN corals were dated at 180 ± 5a, and CWS corals yielded an age of 845 ± 8a. Such a high precision of detection aroused great attention at that time. In the late 1980's scientists of the California University of the United States successfully determined the extremely low abundances of 234 U and 230 Th in seawater and coral using, for the first time, the TIMS -U-series techniques, marking a breakthrough in disequilibrium U-series dating ( Chen, J . H . et al. , 1986; Edwards et al. , 1986/87) . As compared with traditional a-spectroscopy, TIMS is characterized not only by high sensitivity, but also by small sample requirement and short analytical duration. The TIMS -U-series techniques can be applied to determining the geological ages ranging from several hundreds to five hundred thousands of years, and unaltered Quaternary corals and Quaternary carbonate deposits are the best objects of study. It has been realized that seafloor sediments such
238U-234U-230Th,,,-238 U:
If A 8 , X4 , and Xo are taken as the decay constants of 238 U, 234 U , 230 Th , respectively, then A8 = 1.55125 X 10 -10 /a, A4 = 2 .835 X 10 - 6 / a, and X = 9.195 X 10 - 6 /a. If the system meets the following conditions, (1) the initial radioactive ratio 230 Th/ 238 U approaches to zero; (2) the samples are a close system relative to the U series and the nuclides between 238 U and 230 Th; and (3) the initial ratio 234U/238U must be known. Then we can obtain the following formula ( Edwards,
Vol. 20 No. 2
CHINESE JOURNAL OF GEOCHEMISTRY
2001
High Precision Thermal Ionization Mass Spectrometric Dating of Corals and Its Application to P Study
WANG ZHAORONG ( ZHANG ZHAOFENG (3E JL ) " PENG ZICHENG (Si $ G )1' 2) ) 1'
2) 2) ,
1, 2) (Department of Earth and Space Science, University of Science and Technology of China, Hefei 230026, China) ()3)
AND MA ZHIBANG
3) ( Institute of Geology, Chinese Academy of Sciences, Beijing 100029, China)
skeleton Sr/Ca thermometer is considered as one of the most outstanding advances in the field of paleo-oceanography and also the most effective way to reconstruct the SST records in the past. The TIMS-U-series -techniques can be used to determine geological ages ranging from several hundreds to five hundred thousands of years and diagenetically unaltered Quaternary corals and Quaternary carbonate formations are the best objects of study. Fundamental Principle 34 U and 230 Th are two intermediate products of the radioactive decay series of
ISSN 1000-9426 * This research project was granted jointly by the National Natural Science Foundation of China ( Grant No. 49872061) and the Open Lab. of Guilin Karst Institute, Ministry of State Land and Resources.
No. 2
CHINESE JOURNAL OF GEOCHEMISTRY
139
as foraminifera, calcic nodules, corals, etc. have high-resolution records of environmental changes in the past owing to their high sensitivity to environmental change . High-precision and high-resolution investigations of these . seaf loor sediments are helpful to get a better understanding of global change and environmental evolution in the long past and are of great significance both in theory and in practice in predicting the trends of climate fluctuation and environmental change in the future time and rationally arranging the life and development of mankind. In the early 1990' s Beck (1992) and de Villiers (1994) analyzed and examined coral samples using the high-resolution sampling method and high-precision TIMS technique, providing further evidence suggesting a good correlation between Sr/ Ca in marine coral skeletons and seawater tem -peratu.ThniomefSradCswtviehnragdf0.45%(Xi Ming, 1989) , permitting in principle the reconstruction of paleo-SST for corals on a time scale of 10 5 a. Because of fast continuous growth of corals, they can record seasonal or annual SST varia -tionsahrgwevls.Byuingth-reolsampingthd,evroal parameters can also be obtained for even shorter growth intervals . So the applicatct: Global change has become a hot spot in Quaternary geology, and high-precision, high-sensitivity dating is also an urgent problem which needs to be solved. This paper presents some achievements in U-series dating of marine corals by thermal ionization mass spectrometry (TIMS) and its application to the study of paleo-environments. Recently, coral samples were determined for their ages on a MAT-262 mass spectrometer and satisfactory results have been obtained. Key words: thermal ionization mass spectrometric-U-series dating; coral; paleo-temperature; paleo-environment