碎屑沉积岩沉积作用的高精度定年——自生磷钇矿离子探针UPb年龄测定

沉积岩沉积作用准确时间的厘定是目前同位素年代学研究中的一大难题 ,尽管目前可采用多种方法对成岩过程中的自生矿物进行定年 ,但由于技术上的原因 ,这项研究一直发展很慢。文中详细介绍了近年来发展起来的运用高精度离子探针 (SHRIMP)技术确定自生磷钇矿形成年龄 ,进而确定沉积作用年代的新方法。如 ,澳大利亚西北部Kimberley盆地中未变质的古元古代砂岩中自生磷钇矿的SHRIMP定年将成岩作用限定在 7Ma的误差范围之内。相比之下 ,对非洲南部太古宙Witwa tersrand超群和澳大利亚西南部MountBarren群绿片岩相变质砂岩中磷钇矿的研究 ,不仅确定了所研究岩石的成岩作用时代 ,而且恢复了该区后期的复杂热演化历史。研究还表明 ,这种方法同样适用于显生宙岩石。这些实例表明 ,磷钇矿的SHRIMP定年不仅可以测定从太古宙到第四系所有年代碎屑沉积岩的沉积时代 ,而且可以实现极小尺度上的微区定年 ,从而可以研究岩石自沉积成岩以后的演化历史 ,显示这一方法在沉积作用及相关热事件问题研究上的巨大潜力。

HIGH PRECISION DATING OF DEPOSITION OF CLASTIC SEDIMENTARY ROCKS--U-Pb SHRIMP DATING ON AUTHIGENIC XENOTIME

The determination of accurate time of sedimentary deposition is a big problem in the present study of isotopic geochronology. Although there are many isotopic clocks could be applied to date the authigenic minerals formed during the diagenesis, this kind of study develops slowly owing to the technological reason. This paper introduces in detail a newly developed SHRIMP method to date the authigenic xenotime, with which we can determine the age of the sedimentation. The SHRIMP dating on the authigenic xenotimes in Paleo Proterozoic un metamorphosed sandstone in Kimberley Basin of Northwestern Australia limits the depositional age within an error of 7 Ma. In comparison, for the Archean Witwatersrand Supergroup in the Southern Africa and the greenschist facies metamorphosed Mount Barren Group in the Southwestern Australia, the SHRIMP dating on xenotime provides not only the age of the diagenesis, but also the complex post depositional thermal evolution. The experience also indicates that this method could be used to date the Phanerozoic rock, which suggested that the SHRIMP dating on xenotime could be applied to constrain the depositional age of Archean to Quaternary clastic sedimentary rocks, and the dating on micro scale makes it possible to study the history of rock after sedimentary diagenesis. Therefore, the SHRIMP dating on xenotime has great potential to study the histories of sedimentation and correlated thermal events.