Determination of ²³⁸U, ²³²Th and ¹⁴⁷Sm Isotopes in Apatites by Inductively Coupled Plasma-Mass Spectrometry with Non-spike method

The determination of ²³⁸U, ²³²Th, ¹⁴⁷Sm and ⁴He contents in apatite is the key to the (U-Th)/He dating. This paper introduces the measurement of ²³⁸U, ²³²Th and ¹⁴⁷Sm contents by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) with non-spike method. This method assumes that the abundances of ²³⁸U, ²³²Th, and ¹⁴⁷Sm in samples are identical to the natural isotopic abundances. The ²³⁸U, ²³²Th and ¹⁴⁷Sm contents were obtained by the ²³⁸U, ²³²Th, and ¹⁴⁷Sm natural abundances and total measured U, Th, and Sm contents. In order to test this method, total U, Th and Sm contents of basalt standard and Durango apatite standard were measured and subsequently used as an analytical standard in (U-Th)/He laboratory. The results show that the obtained total U, Th and Sm contents for basalt standard are consistent with certified values. From the ²³⁸U, ²³²Th, ¹⁴⁷Sm and ⁴He contents of the Durango apatite, (U-Th)/He was calculated to have an age of 30.0 Ma, which agreed with the certified age of (31.02±1.01) Ma(1σ) within uncertainties obtained by McDowell. The above results indicate that this method of measuring ²³⁸U, ²³²Th and ¹⁴⁷Sm contents is feasible. The method is simple and applicable for the labs having no spike.