不纯碳酸盐U-Th等时线定年及同位素分馏对年龄的影响

应用U-系L/R等时线模式测定了用天然碎屑沉积物与已知年龄的纯CaCO3按不同比例混合而成的不纯碳酸盐样品的年龄。实验分两个序列进行:其一研究碎屑沉积物化学前处理过程中U、Th同位素的萃取行为及其分馏效应。另一序列用不同浓度的溶剂对均匀混合的不纯碳酸盐样品进行溶解,确定稀酸淋滤碳酸盐的效率。各组分的238U、234U、230Th和232Th含量分析数据表明,稀酸溶解碎屑沉积物样品将U和Th同位素等比例的提取出来,不会发生U和Th同位素分馏现象。对于不纯碳酸盐样品,所有数据点落在一条直线上,良好的线性关系反映真实年龄含意。3个不纯碳酸盐样品的等时线年龄与纯碳酸盐年龄一致表明实验技术是可信的。同时,也证明对于碎屑含量较高的不纯碳酸盐U-Th定年,L/R等时线技术是适应的。

U-Th isochron dating of impure carbonates and the possible effect of isotopic fractionation during leaching

U-series isochron technique has been applied to determine the age of impure carbonates that were blended by one natural detritus and a pure CaCO3 with known age in different proportions. The experiment was designed two sets: The first one was to evaluate the extracting behaviour and fractionating effect of isotopes U and Th of detritus during leaching samples. Another set was to examine the effect of acid attacks on both the carbonate and the detritus fractions in a mixture of different proportions when treated with hydrochlority acid of different concentrations. Both the leachates and residuals was analyzed for 238U, 234U, 230Th and 232Th concentrations, which indicated that the detritus dissolved by diluted acid have extracted U and Th isotopes based on constant proportion, thus the isotopic fractionation would not occur. For impure carbonates samples, all data points fell on the straight line, such good linear relationship between the isotopic ratios of the leachate and residue fractions could reflect the meaning of true age. The L/R method was applied to the artificial impure carbonate and the resulting age is found in agreement with the age of the pure carbonate sample, which showed that the technique was reliable. Therefore, for the U/Th age determination of impure carbonates with a higher content of detritus, the L/R isochron dating would be adaptable.