长兴“金钉子”灰岩Re-Os定年方法及Os同位素反演古环境研究

The study of high accurate radiometric dating and isotopic variation are essential to the improvement of the stratigraphic chart and the study of stratigraphy. Currently the main method for the strata dating is Zircon U-Pb dating using SHRIMP (ion-microprobe) or LA-ICP-MS. And the Re-Os isotope system to the geochronology of black shales is also popular and acceptable all over the world. This study is focus on the Re-Os dating of Changxing limestone which is located at the Meishan Section, Changxing County, Zhejiang Province(GSSP). The main purpose of this study is to establish the method of Re-Os isotopic dating of limestone and to verify the feasibility at the same time. The prepared techniques are dissolving samples using Carius tube, the preconcentration and separation for rhenium and osmium using solvent extraction and distillation with microdistillation, the determination for Re-Os using TRITON (N-TIMS). The second purpose of this study is to explore the correlation between 187Os/188Os variation and palaeoenvironment according to the systematic analysis of 187Os/188Os which the sample intervals spanning the Permian-Triassic boundary. The key concerns now for this study are a) the systematic samples and the closure,b) the occurrence state of Re-Os,c) the solution of the contradiction between sample volume, safe operation and high recovery, d) the effective reduction of the total experimental procedure blanks, e) the measurement of mass spectrometry for requiring high sensitivity and precision.

高精度的同位素测年以及同位素变化研究，是完善地层表及进行地层综合研究不可或缺的组成部分，凝灰岩中锆石离子探针质谱（及激光剥蚀等离子体质谱) U-Pb 测年，是目前地层定年的主要手段，黑色页岩的Re-Os同位素定年也是当前的国际热点。本研究将以已有精确年龄的长兴"金钉子"灰岩为对象，以高温密闭氧化法溶解样品，溶剂萃取分离富集Re，直接和微蒸馏结合分离富集Os，热表面电离质谱仪（TRITON）精确测定Re-Os的同位素为基本手段，研究建立灰岩Re-Os 同位素定年方法并验证其可行性，为地层高精度定年研究提供一种新的技术手段。通过P-T界线层的系统取样，研究187Os/188Os同位素初始比值的变化趋势，反演古环境。关键问题在于样品的封闭性以及系统采集，研究解决取样量、安全操作和高回收率之间的矛盾；样品处理时Re-Os是否有丢失，有效地降低实验空白；高精度质谱测量以及Re-Os赋存状态。

项目的目标是以已有精确年龄的长兴 “金钉子”灰岩为对象，建立灰岩 Re-Os 同位素定年技术方法并验证其可行性，为地层高精度定年及古环境研究提供一种新的技术手段。研究内容包括界线层灰岩 Re-Os 年龄厘定、Os同位素古环境的反演、灰岩消解方法研究及Re-Os 赋存状态研究等。. 通过对比灰岩取样量、化学试剂加入量和Re-Os同位素结果以及Carius管爆炸之间的平衡点研究，确定灰岩样品Re-Os同位素测定消解方法。取样量为1g，在Carius管封闭之前先加入2mL 12moL/L盐酸与灰岩反应，待CO2释放完毕后加入1 mL过氧化氢、1 mL 12mol/L盐酸、5 mL 16mol/L硝酸和定量185Re 190Os混合稀释剂， 220℃加热24h溶样。对长兴“金钉子”剖面灰岩进行Re-Os定年分析，获得了三条等时线年龄分别为1～6层（257.3±4.0）Ma、18～22层（253.3±6.0）Ma、23～24层（251.8±4.7）Ma，与锆石U-Pb年龄相互吻合。结果表明Re-Os同位素能够实现对灰岩地层沉积时代进行直接厘定。对灰岩Re-Os 赋存状态研究发现，Re、Os主要受氧化还原作用影响。还原环境沉积的灰岩易富集Re，易获得Re-Os同位素年龄；氧化环境下海水中Re几乎不被灰岩富集，氧化环境沉积的灰岩不适用Re-Os同位素定年。但氧化环境下，部分Os可以被含铁、铜等自生矿物吸附富集，自生矿物Os同位素代表沉积时海水Os同位素特征。Re、Os在不同氧化还原条件下的富集差异导致了还原环境沉积的灰岩中Re/Os比值大于氧化环境，灰岩中Re/Os变化对古海洋沉积氧化还原环境可能具有一定指示意义。其次，获得P-T 界线层两侧灰岩Os同位素初始比值的变化趋势曲线，有效反演了晚二叠-早三叠期间发生的重大全球地质事件，支持西伯利亚玄武岩喷发是导致二叠/三叠之交生物绝灭事件的主控因素。