文石-方解石石笋U/Th体系的封闭性判断及意义

湖南省龙山县莲花洞两根大型石笋LLl(文石-方解石型)和LL5(文石型)ICP-MS230Th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LLl石笋全新世以来234U/238U对230Th/238U的比值具有谐和性特征并且230Th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.10～40 ka期间234U/238U与230Th/238U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60～80 ka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.

Discrimination of close U/Th system in aragonite-calcite stalagmites and their significance

This paper focuses on the relationship between the openness of systems and erroneous U-series ages based on 82 precise ICP-MS 230Th 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 234U/238U vs. 230Th /238U,lithology characteristics and 230Th 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 230Th ages during the Holocene. From 10 ka to 40 ka,U and Th isotopic system of the stalagmite LL1 was in alterations of open/close system,leading to occurrence of unexpected ages. Stalagmite LL5 essentially kept close system from 60 ka to 80 ka,resulting in 230Th 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.