投稿时间：2013-05-29
修订日期：2014-05-18
网络发布日期：2014-06-28
中文摘要:铀是自然界中天然存在的最重的放射性元素。传统观点认为238U/235U是不分馏的,但近些年的研究发现自然界中铀同位素分馏δ238/235U可达1.3‰,远大于多接收电感耦合等离子体质谱仪（MC-ICPMS）的测试精度±0.2‰（2σ）。铀同位素的分馏机制与核场效应有关,自然界中最大的δ238/235U分馏发生在氧化性沉积物（形成于氧化环境下的大洋锰结核富集轻铀同位素）和还原性沉积物（形成于还原环境下的黑色页岩富集重铀同位素）中,次氧化性沉积物和海水中δ238/235U的分馏则小于前两者。由于具有对氧化-还原环境较敏感的特性,铀同位素被认为是理想的反演古海洋及古大气氧化还原环境变化的手段之一。δ238/235U在形成于不同氧化-还原环境的高温型铀矿床（比如岩浆型）和低温型铀矿床（比如砂岩型）之间存在明显的同位素分馏,此特征可以作为确定铀矿床类型的重要依据。铀同位素分馏现象的发现,提示人们在应用Pb-Pb体系进行年轻地质体高精度测年时需要考虑到铀同位素分馏对测年精确度的影响。文章对近年来有关238U/235U的研究成果做了详细介绍,以推动铀同位素体系在地球科学领域的研究和应用。
Abstract:Uranium is the heaviest radioactive element in nature. The 238U/235U isotope ratio has long been considered invariant in nature; nevertheless, it has been proved in recent years that the δ238/235U fractionation between euxinic and oxic sediments can reach up to 1.3‰, much higher than the analytical precision of ±0.2‰ (2σ) for the multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Experimental results and theoretical calculation indicate that the mechanism of 238U/235U fractionation is a mass-independent, volume-dependent fractionation mechanism called Nuclear Field Shift. The largest isotope variations found in nature are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Heavy δ238/235U values were observed for black shales precipitated under reducing environments. In contrast, light δ238/235U isotope compositions were observed for manganese curst representing for oxidized sedimentary environment. Due to the redox-sensitive property, U isotope fractionation is a promising proxy for paleo-redox conditions of the Earth's oceanic system and atmosphere. Moreover, U isotope fractionations were observed between redox sensitive high temperature and low temperature uranium deposits (represented by magmatic and sandstone type, respectively), which can be applied to ore genesis identification and ore-forming process studies. The 238U/235U isotope ratio is a cornerstone of the high-precision U series dating that defines the absolute age of the solar system. The discovery of 238U/235U variations in nature implies that the U isotope fractionation should be considered during high-precision U-Pb dating, especially for young U-bearing minerals. The author here provides an overview of the uranium geochemistry and its isotope system with the purpose of promoting applications both as the redox proxy and the indicator of ore genesis.
keywords:geochemistry uranium isotope isotope fractionation nuclear field shift redox environment black shale high resolution U-Pb geochronology
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中图分类号：P597+.1
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基金项目:本文得到中央级公益性科研院所基本科研业务费项目（编号：K1205）、地质过程与矿产资源国家重点实验室开放课题基金项目（编号：GPMR2011）、国土资源大调查项目（编号：12120114034301, 1212011220869）和人力资源和社会保障部留学人员科技活动择优资助项目的联合资助
引用文本：
徐林刚.2014.238U/235U分馏及其地质应用[J].矿床地质,33(3):497~510XU LinGang.2014.238U/235U isotope fractionation in nature and its geological applications[J].Mineral Deposits33(3):497~510
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