无氧铜样品盘异常热本底对激光40Ar/39Ar定年的影响

激光40Ar/39Ar定年方法中扣除的本底是样品测试过程中的系统冷本底。在满足激光加热样品时样品盘升温幅度有限和样品盘已完全脱气两个条件的情况下，这一处理方式的有效性才能得到保证。本文利用具有不同大气暴露史的无氧铜样品盘结合透长石标准样品YBCs，使用相同的脱气及测试流程，对比分析了不同样品盘的冷、热本底以及放置于不同样品盘时YBCs的大气氩含量。分析结果表明，放置于暴露大气14个月的样品盘内时，YBCs透长石大气氩含量高达约34.4%，使用预先激光去气的样品盘此值可降低至约2%；暴露大气约10个月的样品盘，激光加热其两个样品孔时，40Ar脱气量可达约1.6×10−14～3.1×10−14mol；暴露时长约为26个月的样品盘，40Ar含量升高至约0.8×10−13～2.0×10−13mol；它们均远高于系统冷本底3.8×10−16～6.2×10−16mol。两个样品盘热本底40Ar/36Ar值约为310，高于大气氩比值。因此，对于暴露大气时间较长的样品盘，约150℃去气四天的流程不足以使其完全脱气。激光加热样品时会导致样品盘局部升温，脱气不完全的样品盘会释放出大量热本底。模拟以及标准样品测试均显示了这种情况会影响辐照参数J值以及年龄的计算。激光微量年轻样品40Ar/39Ar定年过程中，建议装样后对无氧铜样品盘进行300～400℃至少5h的预脱气，以保证测试数据质量。实验室不具备预脱气条件时，持续使用同一样品盘也可以有效地降低异常热本底对测试结果的影响。

Abnormal Hot Blank of Oxygen-free Copper Sample Holder and Implications for Laser 40Ar/39 Ar Dating

BACKGROUNDOxygen-free copper (OFC) tray is usually used as a container for samples of laser 40Ar/39Ar dating. The blank of the mass spectrometry system needs to be subtracted from the sample signal before age calculation. The tray’s hot blank of argon, including its amount and isotope ratio will affect the 40Ar/39Ar age calculation. However, the material and structure of the laser window make the laser chamber susceptible to temperatures higher than 150°C during the degassing procedure of laser 40Ar/39Ar dating[4,10]. This temperature may be too low to degas the OFC tray completely during the experiment. Besides, the wells loading the mineral samples limit the direct acquisition of the hot blank of the system. These two points make it very difficult to accurately deduct the blank signal of laser 40Ar/39Ar dating.