SHRIMP Zircon U-Pb Dating: Effects of Common ²⁰⁴Pb on the Result

When performing SHRIMP Zircon U-Pb Dating, ²⁰⁴Pb is often used to make common Pb correction to obtain the zircon ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb ages. This paper mainly discusses theory of ²⁰⁴Pb correction, sources and measuring method of ²⁰⁴Pb, and effect of ²⁰⁴Pb on the result. The study test indicates that the inaccurate measurement of ²⁰⁴Pb can drift age spots from the concordant line, making the average age higher or lower. Many factors will influence measurements of ²⁰⁴Pb, mainly: fractures and inclusions on the testing points, cleanliness level of the zircon mount surface and peak position of ²⁰⁴Pb. In addition, the intensity of the primary beam, uniformity, shape of the beam spot, and raster time may also affect the measurement of common Pb. Due to the low content of ²⁰⁴Pb in most zircon samples, is more beneficial to scan the ²⁰⁴Pb peak at the fractures, the inclusions on high-uranium zircon, or the ²⁰⁴Pb-rich minerals, like feldbar. The peak position of ²⁰⁴Pb is usually slightly offset to the right of the peak center in order to avoid peak interference.