Fish Canyon Tuff榍石He扩散及(U-Th)/He年代学研究

Helium diffusion and(U-Th)/He thermochronology on Fish Canyon Tuff titanite

榍石富含U和Th,是(U-Th)/He定年的理想矿物之一。本文以Fish Canyon Tuff榍石为例,开展了榍石He扩散行为和榍石(U-Th)/He定年实验方法研究。榍石分步加热扩散实验结果表明He扩散系数ln(D/a2)与温度倒数呈负相关,与期望的热活化扩散过程一致。测试Fish Canyon Tuff榍石(U-Th)/He年龄分布在28.3~24.6 Ma之间,平均值为26.7±1.2 Ma(1σ),Th/U分布在4.6~5.5之间,平均值为5.2±0.2,在误差范围内与国际上已出版数据一致,表明建立的榍石(U-Th)/He定年实验方法可靠。本次测试15粒榍石碎片外表层(~20μm)存在不同程度的磨蚀(即不完整晶体),且榍石表层磨蚀厚度随着等效半径的增加而增大。榍石碎片(U-Th)/He年龄介于完整晶体(U-Th)/He年龄和真实(U-Th)/He年龄之间,且随着榍石等效半径及表层磨蚀厚度(<20μm)的增大,(U-Th)/He年龄更接近真实年龄,这表明榍石(U-Th)/He年龄不确定度与等效半径大小和表层磨蚀厚度有关。

Titanite is an ideal mineral for(U-Th)/He dating because of its relatively high U and Th.In this paper,diffusion of helium has been characterized in the Fish Canyon Tuff(FCT)titanite based on high-precision laboratory step heating experiments.We present analyses of FCT titanite(U-Th)/He ages,evaluate data uncertainty,and discuss the influences on(U-Th)/He ages.The ln(D/a~2)in FCT titanites determined from laboratory step heating experiments was negatively correlated with reciprocal temperature,as expected for thermally activated volume diffusion.The titanite(U-Th)/He ages and Th/U ratio are clustered at 28.3~24.6 Ma and 4.6~5.5,respectively,with a mean value of 26.7±1.2 Ma and 5.2±0.2,respectively,which agree within analytical uncertainty with previously reported data,respectively,suggesting that the experimental procedure we established is reliable.The outer 20μm of the 15 original titanite crystals had been partially or completely abraded and the abrasion thickness increased with increasing equivalent radius.The measured(U-Th)/He ages from FCT titanite fragment are older than the original bulk crystal ages(uncorrected for ejection),and some are approximately equivalent to the true(U-Th)/He ages of the whole crystals.The(U-Th)/He ages are closer to the true age of the original bulk crystal due to the larger size and thicker outer abrasion thickness of original crystals,suggesting that the uncertainty of FCT titanites(U-Th)/He age is correlated with grain size and abrasion thickness of the original crystal face.