初始钍的校正是不纯沉积碳酸盐230Th234U238U测年的一个重要方面，洞穴石笋初始Th校正通常采用230Th/232Th的原子比值为（4.4±2.2）×10-6。多年来4种不同的等时线方法应用到不纯碳酸盐测年初始Th的校正中，其中全样品的等时线方法是目前公认的较为完善的一种方法。通过测定云南同一石笋2个不同层位的9个样品的U、Th同位素组成,并进行等时线分析，结果表明2层样品初始钍230Th/232Th原子比值分别为（3.5±2.8）×10-6和（10.6±2.2）×10-6，这说明即使在同一地区，由于其混入Th来源的复杂性，230Th/232Th初始比值在同一样品不同层位仍可能有较大的差异，因此不能够完全采用单一层位等时线获得的初始值对整个样品进行校正。考虑到在实验测量过程中，232Th含量的过高对于230Th的准确测定也有很大的影响，应尽量选择纯净石笋样品进行测年研究，在样品的选择和前处理过程中就减少初始钍的影响，这对获得高精度的测年结果有着重要的意义。

The correction of initial 230Th is very important for the 230Th-234U-238U dating of impure speleothem. The corrected 230Th ages usually assume an initial 230Th/232Th atomic ratio of (4.4±2.2)×10-6. During the last 30 years, various leachleach, leachresidue methodologies have been promoted to distinguish between the isotopic ratios in the authigenic and detrital phases, and the total dissolution techniques are the most reliable method to setup meaningful isochrones. In this paper, Uranium and Thorium isotopic composition of 9 subsamples in two different layers of stalagmite MN04 have been analyzed and the respective isochrones have also been setup. The results show that the initial 230Th/232Th atomic ratio of the subsamples from up layer is (3.5±2.8)×10-6，and that of from down layer is (10.6±2.2)×10-6. It suggests that, even in the same area, the initial 230Th/232Th atomic ratio in various layers of the same stalagmite could have sizable difference because of the complicated origin of the detrital thorium. Therefore, it is insufficient to use the initial 230Th/232Th ratio obtained from the single isochron to correct the whole sample as well as other samples from the same area. Considering the high content of 232Th would also have significant influence on the measurement of 230Th, choosing pure carbonates to the best in sample selecting and preprocessing will greatly reduce the age deviation caused by the incorporated detrital thorium.