Occurrence of Excess ^40Ar in Amphibole： Implications of ^40Ar/^39Ar Dating by Laser Stepwise Heating and in vacuo Crushing

作     者：Rong-Guo Hu Xiu-Juan Bai Jan Wijbrans Fraukje Brouwer Yi-Lai Zhao Hua-Ning Qiu 

作者机构：College of Earth Sciences Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration Guilin University of Technology Guilin 541004 China Geology & Geochemistry Cluster Department of Earth Sciences VU Amsterdam De Boelelaan 1085 1081 HV Amsterdam The Netherlands Key Laboratory of Tectonics and Petroleum Resources Ministry of Education China University of Geosciences Wuhan 430074 China State Key Laboratory of Isotope Geochemistry Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou 510640 China 

出 版 物：《Journal of Earth Science》 (地球科学学刊（英文版）)

年 卷 期：2018年第29卷第2期

页      面：416-426页

核心收录：

学科分类：0709[理学-地质学] 070901[理学-矿物学、岩石学、矿床学] 07[理学] 08[工学] 0827[工学-核科学与技术] 082701[工学-核能科学与工程] 

基　　金：funded by the National Natural Science Foundation of China (Nos. 41703054, 41503053) the Guangxi Natural Science Foundation Program (Nos. 2016GXNSFCA380022, 2014GXNSFBA118231) the Chinese Academy of Sciences-Royal Netherlands Academy of Arts and Sciences Joint PhD Training Programme (No. O8PhD-08) 

主　　题：^40Ar/^39Ar dating in vacuo crushing amphibole fluid inclusions. 

摘      要：ABSTRACT： The joint methods of ^40Ar/^39Ar laser stepwise heating and in vacuo crushing have been applied to date amphiboles from the North Qaidam ultra-high pressure metamorphic amphibolites. Two amphibole samples analyzed by laser heating yielded saddle-shaped age spectra with total gas ages of 574.5±2.5 and 562.5=±2.5 Ma. These ages are much older than the reported zircon U-Pb ages （-495 Ma） from Yuka eclogite, indicating the presence of excess ^40Ar. In order to decipher the occur- rence of excess ^40Ar and constrain the age of amphibolite-facies retrogression, two duplicate amphibole samples were further employed for ^40Ar/^39Ar in vacuo crushing analyses. Both samples exhibit similar monotonically declining release spectra, which are characterized by rapid decline of anomalously old apparent ages in the early steps. The data of the late steps yielded concordant apparent ages with pla- teau ages of 460.9±1.2 and 459.6±1.8 Ma. We interpret that gases released in the early steps derive from the significant excess ^40Ar containing secondary fluid inclusions （SFIs） due to their distribution characteristics along cracks leading to be easily extracted, whereas those released in the later steps rep- resent the contribution of the small primary fluid inclusions （PFIs）.