摘要
同位素地质年代学是探索地质体时空演化及地球动力学等问题的基础学科,应用最为广泛的当属含铀副矿物的U-Pb定年技术。榍石具有相对较低的U-Pb体系封闭温度,并广泛发育于岩浆岩、各类变质岩、热液成因岩石以及少量沉积岩中,是一种理想的中高温地质事件定年矿物。利用电感耦合等离子体质谱法(ICP-MS)测定榍石U-Pb年龄时,不可避免地要解决高普通铅以及元素分馏效应对测试的影响。本文对榍石LA-ICP-MS实验过程中的元素分馏行为进行研究,采用相同基体的标准样品与未知样品对比,发现了榍石不同颗粒之间元素分馏行为不一致的现象;同时采用不同的元素分馏校正方法,分别应用于锆石、独居石和榍石进行对比研究,认为分馏行为一致的副矿物定年可以采用“指数法”和“均值法”对数据进行校正,但是对于榍石这种分馏行为不一致的副矿物,定年时只有采用“截距法”对数据进行校正才可以获得正确的年龄。进而将此结论应用于秦岭造山带老牛山地区岩浆成因榍石样品,得到的结果与锆石年龄一致,表明“截距法”可以避免分馏行为不一致导致的校正不准确的问题。本研究成果为榍石LA-ICP-MS U-Pb定年方法的完善提供了一种思路。
BACKGROUND:Isotope geochronology is a basic subject to explore the temporal and special evolution of geological bodies and geodynamics.The U-Pb dating technology of accessory minerals is the most widely used in isotope geochronology.Titanite has a relatively low closed temperature for U-Pb isotopic system and is common in magmatic rocks,metamorphic rocks,hydrothermal-related rocks and a few sedimentary rocks.This indicates that it is an ideal mineral for U-Pb dating to constrain the medium to high temperature geological event.OBJECTIVES:To understand the element fractionation behavior during laser ablation-inductively coupled plasms-mass spectrometry(LA-ICP-MS)U-Pb dating and propose corresponding correction methods.METHODS:In situ U-Pb dating were performed using a Geolas Pro laser-ablation system and a 7700x quadrupole ICP-MS.A stationary laser ablation spot with a beam diameter of 24μm was used for the analyses.The ablated aerosol was carried by helium and then combined with argon via a T-connector before being introduced to the ICP-MS plasma.After smoothed,the sample gas will go into quadrupole ICP-MS for U-Pb dating.Each analysis incorporated a background acquisition of approximately 10s(gas blank)followed by 40s data acquisition from the sample.After the experiments,the fractionation behavior of elements of titanite during the laser ablation were compared for using different fractionation correction methods to correct zircon,monazite and titanite separately.These correction methods were based on different mathematical model such as quadratic curve,power function and so on.The data was processed with different softwares such as GLITTER and BUSTER based on different mathematical equations,in order to look for the appropriate correction methods for different uranium-rich minerals based on different fractionation characteristics.RESULTS:The inconsistent fractionation behavior of elements between different titanite particles was revealed.After comparison,it was proposed that the‘Exponential Function Method’and the‘Average Data method’can only be used for uranium-rich minerals minerals that have consistent fractionation behavior,but it was not pragmatic for inconsistent ones.For these uranium-rich minerals minerals such as tianite,the‘Intercept Method’was an improvement on the current method,in order to ascertain the correct age.CONCLUSIONS:It is indicated that the‘Intercept Method’can avoid inaccurate correction caused by inconsistent fractionation behavior.
作者
靳梦琪
李艳广
王鹏
汪双双
黎卫亮
JIN Meng-qi;LI Yan-guang;WANG Peng;WANG Shuang-shuang;LI Wei-liang(Xi’an Center of China Geological Survey,China Geological Survey,Xi’an 710054,China;Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits,Ministry of Natural Resources,Xi’an 710054,China;Northwest China Center for Geoscience Innovation,Xi’an 710054,China;Center for Orogenic Belt Geology,China Geological Survey,Xi’an 710054,China)
出处
《岩矿测试》
CAS
CSCD
北大核心
2020年第2期274-284,共11页
Rock and Mineral Analysis