^(40)Ar/^(39)Ar测年法表观年龄计算中的误差分析

APPARENT AGE ERROR ANALYSIS IN ^(40)Ar/^(39)Ar DATING TECHNIQUE

除了样品因素外,第四纪地质样品^(40)Ar/^(39)Ar测年的关键在于测试和年龄计算中的误差控制。信号强度(随测量时刻)的拟合值可以采用最小二乘法进行直线或二次多项式拟合计算,但拟合值的误差不能通过拟合方程系数的误差进行计算,而应选择合适的计算方法以便获得与实测值误差相协调的拟合值误差。减小质量歧视系数D的相对误差并控制^(36)Ar的原始误差,才能有效的降低质量歧视校正带给^(39)Ar的相对误差。^(40)Ar_(rad)的相对误差来源于^(40)Ar测量值的相对误差、^(36)Ar测量值的相对误差、^(36)Ar_(Ca)的相对误差、吸附大气氩^(36)Ar_(air)的相对误差以及样品经反应堆辐照后产生的^(39)Ar_K的相对误差等。当样品极年轻且含有较多吸附大气氩时,^(40)Ar_(rad)的相对误差就会变大;当样品极年轻且受到过度辐照时,^(39)Ar_K的相对误差对^(40)Ar_(rad)相对误差的贡献也会增大;当反应堆中热中子比例较高且样品辐照过程中没有Cd屏蔽时,校正系数α的相对误差对其贡献也不可忽略。当标样年龄小于100Ma时,J值相对误差等于标样测量值R_S的相对误差平方、衰变常数相对误差平方及标样年龄相对误差平方加和的平方根。当标样年龄大于100Ma时,衰变常数相对误差平方及标样年龄相对误差平方将被不同程度的放大(1倍多到几倍)后传递到J值相对误差中,从而增大了这两个来源的误差对J值相对误差的影响。

Apart from the sample related factors, error control in both the measurement and afterward data processing is the key element in dating Quaternary geological samples by ^40Ar/^39Ar technique. Signals are usually linear or quadratic polynomial fitted. The adjusted value error will not be calculated via the error of coefficients in whatever the fitting equation, but will be obtained through a suitable procedure so as to be in harmony with the errors of the measured data. The error reduction in discrimination Correction coefficient（D） ,together with the error control in the measured ^36Ar, will effectively cut down the ^36Ar error caused by this correction. Errors in the ^40Ar measurement, in the ^36Ar measurement, in the calculated ^36Arca, in the calculated ^36Arair, and finally in the calculated ^39Ark, are all non-neglected sources of and will all be propagated into the calculated ^40Arrad error. Samples with more atmospheric argon contamination and over-irradiation in younger rocks will cause more contribution of relative errors from ^39Ark to ^40Arrad. The fractional error in correction coefficient of is also an important source of errors when samples are irradiated in reactors with higher thermal neutron ratio and without Cd shielding. When using standard with ages older than 100Ma, the relative error of measured Rs keeps the same while that of decay constant and of standard age will be one to several times magnified （compared to the error of Rs and the amplification factors respectively when standard age is less than 100Ma） will be one to several times magnified（compared to the amplification factor when standard age is less than 100Ma） and then be propagated into the relative error of J value and therefore increases the influence of these two sources to the error in J.