LA-ICP-MS独居石U-Th-Pb测年方法研究

Analytical Conditions for U-Th-Pb Dating of Monazite by LA-ICP-MS

相比LA-ICP-MS锆石U-Pb测年,独居石在一些年轻地质体或流体作用下的矿物定年中更具优势,具有很好的应用前景。然而,大多数独居石Th含量较高(可达7%),包裹体较多,另外随着独居石定年标样不断消耗,存量越来越少,也限制了独居石U-Th-Pb同位素测年的发展与应用。前人利用LA-ICP-MS探究合适的独居石U-Th-Pb测年实验条件,主要是改变激光器的参数,而未对ICP-MS的参数进行系统研究。本文通过改变激光器参数(束斑直径和激光频率)和ICP-MS参数(^(232)Th驻留时间),分别在束斑直径为24μm、16μm和10μm,激光频率为3Hz、4Hz和5Hz,^(232)Th驻留时间为10ms、6ms、3ms和1ms的条件下进行U-Th-Pb测年。最后以独居石RW-1为标样,对独居石样品Bananeira进行校正,期望得到独居石U-Th-Pb测年的最佳条件。结果表明:束斑直径为16μm,^(232)Th驻留时间为3ms或1ms,能量密度为4J/cm^(2),激光频率为5Hz,载气He流速为0.35L/min,载气Ar流速为0.95L/min的实验条件下适合独居石测年,这两种条件下Bananeira的^(207)Pb/^(235)U加权平均年龄分别为510.7±8.6Ma(MSWD=0.87)、513.8±5.7 Ma(MSWD=0.38,推荐值507.7±1.3Ma),误差在0.59%和1.20%左右;208 Pb/^(232)Th加权平均年龄分别为496.9±8.6Ma(MSWD=0.596)、499.8±5.6Ma(MSWD=0.37,推荐值497.6±1.6Ma),误差在0.14%和0.44%左右。并利用此条件对黄山花岗岩(HS-1)进行独居石U-Th-Pb测年,其^(207)Pb/^(235)U加权平均值在128.3±2.4Ma(MSWD=0.73),与本次测定该岩体的锆石年龄数据(127.0±2.1Ma,MSWD=0.93)在误差范围内一致,验证了本实验建立的独居石U-Th-Pb定年方法可靠。

BACKGROUND:Compared with zircon U-Pb dating,monazite has more advantages in the dating of some metamorphic rocks or highly differentiated rocks.However,the U-Th-Pb isotopic dating of monazite has been limited,because of high Th concentration(commonly>7%),many mineral inclusions,and rare dating standard samples.Previous research on LA-ICP-MS U-Th-Pb dating of monazite mainly focused on laser parameters,and lacked attention on ICP-MS conditions.OBJECTIVES:To find suitable experimental LA-ICP-MS monazite dating conditions.METHODS:Using 193nm ArF excimer laser ablation system and Agilent 7500a inductively coupled plasma-mass spectrometer,laser parameters(laser beam spot diameter and laser frequency)and ICP-MS parameters(dwelling time of ^(232)Th)were investigated.The laser beam spot diameter was set to 24μm,16μm and 10μm,and the laser frequency was set to 3Hz,4Hz and 5Hz.The dwelling time of ^(232)Th was set to 10ms,6ms,3ms and 1ms for U-Th-Pb dating.Finally,the monazite sample RW-1 was used as the standard sample to calibrate the monazite sample Bananeira.RESULTS:It was suitable for monazite dating when the beam spot diameter was 16μm,the dwelling time of ^(232)Th was 3ms or 1ms,the energy density was 4J/cm^(2),the laser frequency was 5Hz,the carrier gas(He)flow rate was 0.35 L/min,and the carrier gas(Ar)flow rate was 0.95L/min.Under these conditions,Bananeira’s weighted average ages of ^(207)Pb/^(235)U were 510.7±8.6Ma(MSWD=0.87),and 513.8±5.7Ma(MSWD=0.38),which were consistent with the recommended age of 507.7±1.3Ma with respective errors of 0.59%and 1.20%.208 Pb/^(232)Th weighted average ages were 496.9±8.6Ma(MSWD=0.596)and 499.8±5.6Ma(MSWD=0.37),which were consistent with the recommended age of 497.6±1.6Ma,with respective errors of 0.14%and 0.44%.The ^(207)Pb/^(235)U weighted average age of Huangshan sample HS-1 was 128.3±2.4Ma(MSWD=0.73),which were close to the zircon age of 127.0±2.1Ma(MSWD=0.93)in the Huangshan area.CONCLUSIONS:The optimal laser and ICP-MS conditions are suitable for monazite U-Th-Pb isotopic age determination.