SHRIMP锆石铀-铅同位素定年中普通铅 204Pb对实验结果的影响
SHRIMP Zircon U-Pb Dating: Effects of Common 204Pb on the Result
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摘要: 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。Abstract: 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.
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