榍石LA-SF-ICP-MS U-Pb定年及对结晶和封闭温度的指示

In situ U-Pb dating of titanite by LA-SF-ICP-MS and insights into titanite crystallization and closure temperature

榍石富含U、Th,贫Pb,是U-Pb定年的理想矿物之一。本文采用激光剥蚀-高分辨等离子体质谱建立榍石U-Pb定年方法,采用25~30μm激光斑束,准确测定榍石U-Pb标准样品BLR-1 (~1048Ma)、OLT-1 (~1014Ma)和Pakistan(~21.4Ma),以及年轻榍石样品(<100Ma) U-Pb年龄,提高了检测准确性和空间分辨率。综合对比共生锆石和榍石U-Pb年龄、榍石颗粒微量元素和U-Pb年龄环带及不同成分岩浆岩(SiO_2含量48.1%~77.0%)中岩浆榍石的结晶温度,结果表明:岩浆演化过程中,榍石具有宽泛的结晶温度(600~900℃,峰值~750℃),主要集中于岩浆演化中晚期结晶,榍石U-Pb同位素封闭温度接近或略低于锆石,因此同一岩浆体系中锆石与榍石的年龄差异可能反映了该熔体较晚达到榍石饱和,而非熔体热演化历史或封闭温度信息。

Titanite is an ideal mineral for U-Pb isotopic dating because of its relatively high U,Th,and low common Pb contents.In this paper,we developed a technique for in situ U-Pb dating of titanite using laser ablation sector field inductively coupled plasma mass spectrometry(LA-SF-ICP-MS),which could improve the accuracy and spatial resolution efficiently for its high sensitivity.The ages of titanite U-Pb standards BLR-1(~1048 Ma),OLT-1(~1014 Ma)and Pakistan(~21.4 Ma)and young titanite samples(<100 Ma)were determined accurately with a laser spot size of 25~30μm.We performed U-Pb dating on titanite from Gangdese and Namche Barwa area and associated zircon as well as on the titanite grains with age zoning.Then calculated the crystallization temperature of titanites in magmatic rocks with different components(SiO2 contents range from 48.1%to 77.0%)by Zr-in-titanite thermobarometer.Comprehensive of these results indicate that titanite has a wide range of crystallization temperature(600~900℃,with a peak at^750℃),mainly during the middle to the late stage of magmatic evolution.The U-Pb closure temperature of titanite is close to or slightly lower than that of zircon.Therefore,the age difference between zircon and associated titanite may reflect that the melt reached titanite saturation later,rather than the thermal evolution history or the closure temperature.