用于新生代定年的Ar-Ar法标准样品候选样品初测结果

BT-1 Sanidine,a New Standard Sample Candidate for ^(40)Ar-^(39)Ar Dating and a Primary Analysis Result

Ar Ar法定年的特点是必须要有一套年龄从小到大的标准样。迄今用于新生代定年的国内标准样极少。为满足新生代矿物Ar Ar法定年的需要,初选了一个标准样候选者BT 1透长石。样品总重366g,粒级6080目,纯度100%,缩分为100瓶,每瓶3 66g。Ar Ar阶段加热法初测结果为:全部12个阶段给出的总气体年龄为30 8±0 9Ma,412阶段视年龄十分接近,年龄谱平坦,对应的39Ar析出量达96%,坪年龄为29 6±0 4Ma,等时年龄为29 6±0 6Ma,MSWD=1 01。40Ar/36Ar初始值为293 6±3 9,与尼尔值295 5相当。重复测定结果为:全部气体年龄为31 0±0 9Ma(全部9个阶段),坪年龄为29 5±0 4Ma,等时年龄为29 4±0 6Ma,MSWD=1 94,40Ar/36Ar初始值为282 2±6 3。这些结果表明,BT 1透长石不含过剩氩,作为新生代定年的Ar Ar法标准样品候选者是较为理想的。

The^(40)Ar-^(39)Ar dating method relies upon an independent known age of standard monitor,commonly referred to as the flux monitor,which is used to measure the fast neutron dose received by the samples during irradiation in reaction.For this purpose,a candidate BT-1 sanidine was separated from an ultrapotassic volcanic rock from Yulinshan,northern Tibet,China.About 366g of pure BT-1 sanidine obtained at the 60-80 mesh faction.It was splitted into 100 aliquots of about 3.66g each.For the^(40)Ar/^(39)Ar measurements,the sanidine was irradiated for 43.4h at Beijing 49-2 reactor using a next-to-core H8 facility which has a fast neutron flux of 6.6×10^(12)n/cm^2·sec.Samples were shieled with 0.7mm of Cd during the irradiation.Also irradiated were GA1550 biotite and FCT sanidine with a K-Ar age of 98.8±0.9Ma and 27.9±0.6Ma,respectively,to be used as the flux monitor.Analysis of argon isotopic composition under static mode was measured on an RGA10 mass spectrometer.BT-1 sanidine yields a good^(40)Ar/^(39)Ar age spectrum.Apparent ages for steps 4-12 comprising over 96% of the ^(39)Ar released defined a plateau age(4-12 steps ) of 29.6±0.4Ma,the isochron age and a total gas age(1-12 steps) of 29.6±0.6Ma and 31.0±0.9Ma,respectively(see Fig 1).Steps 4-12 yield an initial ^(40)Ar/^(39)Ar of 293.6±3.9,generally agreeing with the atmospheric value of 295.5.Above dating results suggests that BT-1 sanidine has no experience heat event and has no excess argon since it formation.It is a better Cenozoic standard sample candidate for Ar-Ar dating.