锡石40Ar/39Ar 法直接定年探讨

赣南漂塘钨矿共生白云母和锡石⁴⁰Ar/³⁹Ar 定年对比研究, 前者采用激光阶段加热分析,后者采用真空击碎法提取流体包裹体. 白云母形成平坦的年龄谱, 坪年龄为159.3±1.5 Ma(2σ). 锡石形成下降型年龄谱, 最初6 个阶段表观年龄明显偏老且迅速下降, 从3954 Ma 降至196 Ma, 表明锡石中次生包裹体含有过剩氩; 第7~18 阶段形成了年龄坪, 坪年龄为159.1±1.8Ma, 对应的数据点构成高度线性相关的等时线, 等时年龄为158.6±2.9 Ma, 代表了原生包裹体的年龄. 锡石原生包裹体年龄与白云母年龄一致, 代表了漂塘钨矿的形成年龄. 研究结果表明锡石是⁴⁰Ar/³⁹Ar 真空击碎分析理想的定年对象, 其原生包裹体年龄代表了成矿年龄. 继闪锌矿之后, 再次拓展了流体包裹体⁴⁰Ar/³⁹Ar 法应用于矿石矿物直接定年测试对象.

Direct dating of cassiterite by ~(40)Ar/~(39)Ar progressive crushing

The aim of this study is to investigate the possibility of dating fluid inclusions in cassiterite using the ⁴⁰Ar/³⁹Ar technique by crushing in vacuo. Paragenous muscovite and cassiterite in a piece of tungsten ore from the Piaotang tungsten deposit in southern Jiangxi Province were selected. Laser stepwise heating of the muscovite yields a flat age spectrum with a plateau age of 159.3±1.5 Ma. The ⁴⁰Ar/³⁹Ar dating result of cassiterite by crushing forms a declining age spectrum marked by the highest apparent age of 3954 Ma at the 1st stage, then shows an apparent decline step-by-step, and finally a plateau appears with a plateau age of 159.1±1.8 Ma in the last 12 stages (from 7 to 18, or about 83% of the total ³⁹Ar). The data points of cassiterite clearly distribute into two groups corresponding to secondary and primary fluid inclusions, and their mixtures. The secondary ones contain excess ⁴⁰Ar and their data points scatter on the inverse isochron diagram of ³⁶Ar/⁴⁰Ar vs. ³⁹Ar/⁴⁰Ar, while the primary ones do not contain excess ⁴⁰Ar and their data points yield an excellent isochron line corresponding to an age of 158.6±2.9 Ma. The ⁴⁰Ar/³⁹Ar isochron age of the cassiterite by crushing is well concordant with the age of the paragenous muscovite by ⁴⁰Ar/³⁹Ar stepwise heating, indicating that mineralization occurred at 159 Ma. This study reveals that cassiterite is a suitable dating target for ⁴⁰Ar/³⁹Ar stepwise crushing. Moreover, the isochron age of the primary inclusions represent the mineralization age of the Piaotang tungsten deposit.