Intercalibration of internatioal and domestic ~(40)Ar/~(39)Ar dating standards

【摘要】：正Four international standards,Gal 550,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 the average age of 519.8Ma of it is obtained.The results of Bern 4M and Lp-6 reflect they have invariable value of ~(40)Ar*/~(39)Ar_k(F) and the ages we obtained are consensus with their K-Ar age:Lp-6=127.7Ma;Bern 4M=18.2Ma. 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 keep close chemically,with its trapped argon isotope composition close to the atmosphere.The dating results show age values are reproducible and steady,its total fusion age,step-heating age,plateau age and isochron age are in accordant with each other within the error range(2a).Therefore,we recommend 28.7Ma 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 transfer the ratio of production rate of ~(37)Ar_(Ca)/~(39)Ar_K into Ca/K ratio is 1.78.This is different from 2.0 reported earlier, which may be caused by the reconstruction of the reactor.