Four international standards, Ga1550, MMhb-1, Lp-6, Bern 4M, and one domestic standard BT-1 have been intercalibrated. The repeated measurements on MMhb-1 with different mass demonstrate that MMhb-1 is inhomogeneous in age and its average age is 519.8 Ma. The results of Bern 4M and Lp-6 reflect that they have an invariable value of 40Ar*/39Ark (F) and the ages we obtained are consensus with their K-Ar age: Lp-6=127.7Ma; Bern 4M=18.2 Ma. Analyses of BT-1 age spectra, Ca/K and Cl/K spectra as well as inverse isochrons indicate that the sample is homogeneous and invariable and keeps close chemically, with its trapped argon isotope composition close to the at- mosphere. The dating results show that age values are reproducible and steady, total fusion age, step-heating age, plateau age and isochron age are in accord with each other within the error range (2σ). Therefore, we recommend 28.7 Ma as the calibrated age of BT-1. We also discuss the variation in neutron flux gradients of Beijing 49-2 reactor. It was found that the neutron flux gradient varies considerably, and more monitors (standard samples) are needed to fix the trend of variation. The coefficient of the 49-2 reactor that transfers the ratio of production rate of 37ArCa/39ArK into Ca/K ratio is 1.78. This is different from that reported earlier, 2.0, which may be caused by the reconstruction of the reactor.
WANG Fei HE Huaiyu ZHU Rixiang SANG Haiqing WANG Yinglan YANG Liekun
Quaternary K-Ar standard BB-24 is evaluated by Ar/ Ar analyses.Total-fusion results show that BB-24 is poorly ...
Fei Wang~(a,*),Ri-Xiang Zhu~a,Lie-Kun Yang~a,Huai-Yu He~a,Ching-Hua Lo~b a PGL,State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China b Department of Geosciences,National Taiwan University,Taipei,Taiwan
An attempt was made to use CO_2 laser step-heating method to date the late Pleistocene basaltic groundmass (DF-2) from Tengchong volcanic field. Among the fourteen heating steps, ten define a good inverse isochron (MSWD = 1.4) with an age of 32.2 ± 7.1 ka (2 σ).The inverse isochron also shows that the initial argon isotopic ratio is 297.1±2.0 (2σ) which is the same as the atmospheric argon at the 2σ error level. Study indicates that it is a useful means to date young volcanic groundmass with low K content by using CO_2 laser step-heating ^(40)Ar/^(39)Ar method.
WANG Fei HE Huaiyu ZHU Rixiang YANG Liekun SANG Haiqing WANG Yinglan
