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文石-方解石石笋U／Th体系的封闭性判断及意义

ISSN号：0379-1726
期刊名称：《地球化学》
时间：0
分类：P597[天文地球—地球化学;天文地球—地质学] P532[天文地球—古生物学与地层学;天文地球—地质学]
作者机构：[1]西南大学地理科学学院,重庆400715, [2]中国地质科学院岩溶地质研究所,国土资源部岩溶动力学重点实验室,广西桂林541004, [3]西南大学地球化学与同位素实验室,重庆400715, [4]Geology and Geophysics Department, Minnesota UniVersity, MN55455, USA
相关基金：国家自然科学基金（40231008,90511004）;中国西南大学博士后科学基金（122060-20701640）;西南大学自然地理学重点学科基金（124490-20300103）美国明尼苏达大学地质与地球物理系同位素实验室Larry Edwards教授为本文石笋年龄样品的测试顺利完成提供了大力的技术支持,该实验室王先锋博士、Carolyn博士对本人在美实验期间提供了大量帮助,在此向他们致以深深的谢意.最后还要感谢两位匿名评审专家,他们提出的宝贵意见对于本文的修改有非常大的帮助.

作者：杨琰[1,2,3], 袁道先[1,2,3], 程海[4], 覃嘉铭[2], 张美良[2], 林玉石[2], 朱晓燕[2]

关键词：石笋, 铀系法定年, 体系开放度, ICP-MS, 古气候, 莲花洞, stalagmite, U-series dating, openness of system, ICP-MS, paleoclimate, Lianhua Cave

中文摘要：

湖南省龙山县莲花洞两根大型石笋LL1（文石-方解石型）和LL5（文石型）ICP-MS 230↑Th结果表明，82个年龄数据并不完全符合石笋生长层序律。根据U／Th同位素比值、沉积和矿物学特征，分析了同位素体系开放度对建立石笋正确年代学模式的影响。莲花洞LL1石笋全新世以来。234↑U／238↑U对230↑Th／238↑U的比值具有谐和性特征并且230↑Th年龄层序正常，说明文石矿物基本接近U／Th同位素封闭系统，实测年龄基本可靠。10-40ka期间234↑U／238↑U与230Th／238↑U离散度较大和矿物具有溶蚀、风化现象，表明体系发生U加入／流失作用。LL5石笋60-80ka期间封闭性较好，实测年龄可信。上述结果表明，同一洞穴中文石石笋U／Th同位素体系开放度与时间的关系并不是线性关系，沉积时水文和物理化学性质以及随后的保存状况是决定洞穴文石石笋同位素封闭性的关键因素。

英文摘要：

This paper focuses on the relationship between the openness of systems and erroneous U-series ages based on 82 precise ICP-MS 230↑Th dating measurements of two big stalagmites LL1 （aragonite-calcite stalagmite） and LL5 （aragonite stalagmite） from Lianhua Cave, Longshan County, Hunan Province, China. The concordant changes of 234↑U/238↑U vs. 230↑Th/238↑U, lithology characteristics and 230↑Th ages in the stalagmites, suggest that aragonite minerals of stalagmite LL1 in Lianhua Cave kept close system during U/Th isotopes decay and provide credible 230↑Th ages during the Holocene. From 10 ka to 40 ka, U and Th isotopic system of the stalagmite LL1 was in alterations of open/dose system, leading to occurrence of unexpected ages. Stalagmite LL5 essentially kept close system from 60 ka to 80 ka, resulting in 230↑Th ages in order of stratigraphic sequence. A conclusion can be drawn that the openness of U/Th isotopic systems in aragonite stalagmites in one cave do not always increase as age increases. Many complicated factors, such as cave hydrologic conditions, geochemical property of solutes and diagenesis after its formation, may contribute to the erroneous U-series ages. Our preliminary study on U and Th isotopic system provides great potential for calibrating erroneous U-series ages in aragonite-calcite stalagmites and thus establishing a correct age-growth length model.

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