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 

机构地区：[1]College of Earth Sciences, Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration, Guilin University of Technology, Guilin 541004, China [2]Geology & Geochemistry Cluster, Department of Earth Sciences, VU Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands [3]Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China [4]State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

出　　处：《Journal of Earth Science》2018年第2期416-426,共11页地球科学学刊（英文版）

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)

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）.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）.

关 键 词：^40Ar/^39Ar dating in vacuo crushing amphibole fluid inclusions. 

分 类 号：P578.955[天文地球—矿物学] O571.6[天文地球—地质学]