A high precision thermal ionization mass spectrometric (HP-TIMS) technique is used to determine 238U,234U,232Th,230Th concentrations and their ratios in whole rocks and minerals separated from Ouaternary Maanshan, Day...A high precision thermal ionization mass spectrometric (HP-TIMS) technique is used to determine 238U,234U,232Th,230Th concentrations and their ratios in whole rocks and minerals separated from Ouaternary Maanshan, Dayingshan and Heikongshan volcanic rocks of Tengchong volcanic field .Yunnan Province, China. The 238U-230Th isochrons are given, yielding four age values (227 ? 20) ka (D-1, Dayingshan), (79.6 ±5.5) ka (D-7, Dayingshan), (21.9 ± 3.0) ka (h-1, Heikongshan), and (7.5 ± 1.0) ka (M-1, Maanshan). The result is not only consistent with but also preciser than those measured by the K-Ar method and the alpha spectrometry U-series method, indicating that the HP-TIMS method is reliable and has high precision. Besides, a procedure of HP-TIMS analysis of young volcanic rocks in China is set up preliminarily.展开更多
samples of lava and pumice from the cone of Changbaishan Tianchi Volcano, Jiling, China, were dated by using high precision U-series TIMS method. We conclude that the bottom of the cone formed before 350 ka, the middl...samples of lava and pumice from the cone of Changbaishan Tianchi Volcano, Jiling, China, were dated by using high precision U-series TIMS method. We conclude that the bottom of the cone formed before 350 ka, the middle part in 70～80 ka, the upper during 20～1ka, and the top less than 1ka, and the age based periods of the volcano eruption since Late Pleistocene is given as follows: >350ka,70ka,18～25ka,10ka, 4C5ka, 1～0.75ka, which may offer the basis for the study of volcanic disaster in future. In addition, the principle of dating young volcanic rocks by using U-series TIMS method is introduced briefly. Differentiation characteristics of U and Th in different minerals of the volcanic rocks are discussed, and the ability producing isochrons, based on U and Th differentiation, are discussed. In the last part of the paper, the closure of samples to the elements U and Th, which is important for age results, is discussed by using ( 234 U/ 238 U) radioactivity ratio which can be used to monitor if the samples have been weathered or eroded or leached since the time they formed. In this study, all samples have ( 234 U/ 238 U) activity ratios within 1% of secular equilibrium (( 234 U/ 238 U) radioactivity ratios are unity), indicating no disturbance of the 234 U～ 238 U system. All of these discussions show that the TIMS method is good to date Tianchi volcanics and the results are reliable.展开更多
Two national stalagmite standard （GBW04412, GBW04413） and one international coral standard （RKM4） have been determined by using thermal ionization mass spectrometry （TIMS） method. The values of （ 234 U/ 238 U）...Two national stalagmite standard （GBW04412, GBW04413） and one international coral standard （RKM4） have been determined by using thermal ionization mass spectrometry （TIMS） method. The values of （ 234 U/ 238 U） act , （ 230 Th/ 234 U） act and age are all consistent in error range with the standard values obtained by using α spectrometry.展开更多
Calculations, according to some open-system models, point out that while a statistically significant discrepancy between the results of two U-series methods, 230Th/234U and 227Th/230Th (or 231Pa/235U), attests a relat...Calculations, according to some open-system models, point out that while a statistically significant discrepancy between the results of two U-series methods, 230Th/234U and 227Th/230Th (or 231Pa/235U), attests a relatively recent and important uranium migration,concordant dates cannot guarantee closed-system behavior of a sample. The results of 20 fossil bones from 10 Chinese sites, 19 of which are determined by two U-series methods, are given.Judging from independent age controls, 8 out of the 11 concordant age sets are unacceptable.The results in this paper suggest that uranium may cycle into or out of fossil bones, such geochemical events may take place at any time and no known preserving condition may securely protect them from being affected. So for the sites we have studied, the U-series dating of fossil bones is of limited reliability.展开更多
文摘A high precision thermal ionization mass spectrometric (HP-TIMS) technique is used to determine 238U,234U,232Th,230Th concentrations and their ratios in whole rocks and minerals separated from Ouaternary Maanshan, Dayingshan and Heikongshan volcanic rocks of Tengchong volcanic field .Yunnan Province, China. The 238U-230Th isochrons are given, yielding four age values (227 ? 20) ka (D-1, Dayingshan), (79.6 ±5.5) ka (D-7, Dayingshan), (21.9 ± 3.0) ka (h-1, Heikongshan), and (7.5 ± 1.0) ka (M-1, Maanshan). The result is not only consistent with but also preciser than those measured by the K-Ar method and the alpha spectrometry U-series method, indicating that the HP-TIMS method is reliable and has high precision. Besides, a procedure of HP-TIMS analysis of young volcanic rocks in China is set up preliminarily.
文摘samples of lava and pumice from the cone of Changbaishan Tianchi Volcano, Jiling, China, were dated by using high precision U-series TIMS method. We conclude that the bottom of the cone formed before 350 ka, the middle part in 70～80 ka, the upper during 20～1ka, and the top less than 1ka, and the age based periods of the volcano eruption since Late Pleistocene is given as follows: >350ka,70ka,18～25ka,10ka, 4C5ka, 1～0.75ka, which may offer the basis for the study of volcanic disaster in future. In addition, the principle of dating young volcanic rocks by using U-series TIMS method is introduced briefly. Differentiation characteristics of U and Th in different minerals of the volcanic rocks are discussed, and the ability producing isochrons, based on U and Th differentiation, are discussed. In the last part of the paper, the closure of samples to the elements U and Th, which is important for age results, is discussed by using ( 234 U/ 238 U) radioactivity ratio which can be used to monitor if the samples have been weathered or eroded or leached since the time they formed. In this study, all samples have ( 234 U/ 238 U) activity ratios within 1% of secular equilibrium (( 234 U/ 238 U) radioactivity ratios are unity), indicating no disturbance of the 234 U～ 238 U system. All of these discussions show that the TIMS method is good to date Tianchi volcanics and the results are reliable.
文摘Two national stalagmite standard （GBW04412, GBW04413） and one international coral standard （RKM4） have been determined by using thermal ionization mass spectrometry （TIMS） method. The values of （ 234 U/ 238 U） act , （ 230 Th/ 234 U） act and age are all consistent in error range with the standard values obtained by using α spectrometry.
文摘Calculations, according to some open-system models, point out that while a statistically significant discrepancy between the results of two U-series methods, 230Th/234U and 227Th/230Th (or 231Pa/235U), attests a relatively recent and important uranium migration,concordant dates cannot guarantee closed-system behavior of a sample. The results of 20 fossil bones from 10 Chinese sites, 19 of which are determined by two U-series methods, are given.Judging from independent age controls, 8 out of the 11 concordant age sets are unacceptable.The results in this paper suggest that uranium may cycle into or out of fossil bones, such geochemical events may take place at any time and no known preserving condition may securely protect them from being affected. So for the sites we have studied, the U-series dating of fossil bones is of limited reliability.
