Study On Argon-Argon Dating Of Ultrafine Minerals And Its Application

Argon-argon(40Ar/39Ar)dating is one of the important radiometric dating methods,the method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes.40Ar/39Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium(39K)into the radioactive 39Ar,invented to supersede potassium-argon(K/Ar)dating in accuracy,a major advantage of the argon-argon method is that themany mineral samples can be dated,the 40Ar/39Ar method is also one of the most versatile techniques with countless applications in geology,is one of the most trusted dating techniques to do that.However,because of the nuclear recoil effect during samples neutron irradiation from a nuclear reactor to convert a stable form of potassium(39K)into the radioactive 39Ar,especially ultrafine minerals,40Ar/39Ar recoil model ages are defined that provide a higher reliable age results;In general,the finer the mineral grain is,the higher the proportion of the 39Ar will loss.So the age bias is more pronounced.But when the minerals are small enough that they can't be used for the regular 40Ar/39Ar radiometric dating,So far,there has been no international agreement.When studying the age of mineralization and the ductile shear zone,the age of the relevant age was obtained by using the potassium minerals unaltered 40Ar/39Ar method.However,the effects of nuclear recoil effect should be considered when evaluating the reliability of the age results due to the small size of the potassium newborn minerals.In this paper,different granule diameters of the reference material(ZBH-25 biotite)and(SK01 sanidine)samples are prepared,by using measurement technique and experimental measurement system determining the basic particle size range and mineral processing;Through different granule diameters sample analysis and Ar gas release characteristics research,the ultra-fine mineral 40Ar/39Ar experimental process is determined.According to the 40Ar/39Ar dating analyses of reference material(ZBH-25 biotite),along with a sample of known age for a standard(SK01 sanidine),experimental results show that when the grain size to 300 mesh,the effect of the nuclear recoil can be negligible.A major advantage of the argon-argon method is that the sample can be heated incrementally.40Ar/39Ar incremental heating experiments were applied to ultra-fine sericite from metallic ores and tectonic rocks.According to the dating analyses of the samples of TS1304,when grain size is 140-160 mesh,the plateau age is 252.6±3.0 Ma,when grain size is 200-240 mesh,the plateau age is 254.2±3.0 Ma,when grain size is 600-800 mesh,the plateau age is 253.3±3.0 Ma,In the error range of the three ages,there is no significant difference,indicating that the effect of the nuclear recoil on the 40Ar/39Ar dating results is negligible for the ultrafine sericite aggregates with a grain size of 800 mesh.When grain size is 800-1000 mesh,the plateau age is 257.8±3.0 Ma.The age result is increased by about 2 percent than the coarse samples(140-160 mesh).Indicating that the effect of the nuclear recoil on the 40 Ar/39Ar dating results was significantly increased after the sample size was less than 800 mesh.The sericite from altered rocks and tectonic rock do not comply with the above rule,this could be because of the mylonite samples contain more impurities,when we use the "inverted method" to acquire sericite,it do not remove the impurities completely.According to the argon release characteristics of different grain size samples,the release peak of 40Ar*from 980? to 880? which was obtained in the sericite aggregation from quartz vein type ore.The sericite aggregation from alteration rock and tectonite rocks do not have this typical characteristic.It was also found that the advance of the 40 Ar*release peak did not affect the accuracy of the dating results.