SHRIMP锆石铀-铅同位素定年中普通铅 204Pb对实验结果的影响

在SHRIMP锆石U-Pb同位素定年中,常用实测的204Pb校正普通铅,然后求得206Pb/238U、207Pb/206Pb等同位素比值及年龄。本文主要论述了普通铅204Pb校正的原理、来源、测定方法及其对实验结果的影响。通过实验得出普通铅204Pb测定结果不准确,会使数据在谐和图中的投影点偏离谐和线,使年龄结果偏离真实值。影响普通铅204Pb的因素有很多,主要有测试位置是否有包体或裂纹、锆石表面的清洁程度、普通铅204Pb的峰位置是否正确等;此外,一次离子束强度、密度均匀性、束斑形状、清洗时间也可能有影响。由于大多锆石样品中普通铅204Pb含量很低,在扫描普通铅204Pb峰中心时,最好在高铀锆石包体、裂隙处,或在长石等含普通铅204Pb较高的矿物上进行,同时为了避免受质量数204左侧峰拖尾叠加的影响,通常把204Pb峰位置设定在峰中心稍偏右处。

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.