海相碳酸盐－稀土元素共沉淀过程中的分异作用研究

IOCAS-IR > 海洋环流与波动重点实验室

	海相碳酸盐－稀土元素共沉淀过程中的分异作用研究
其他题名	Experimental evaluation of yttrium and rare earth element (YREE) coprecipitation with calcium carbonates in seawater at 5℃, 15℃, 25 °C and 1 atm., and low dissolved concentrations
	刘刚
学位类型	博士
导师	李铁刚 ; 于心科
	2007-06-07
学位授予单位	中国科学院海洋研究所
学位授予地点	海洋研究所
学位专业	海洋地质学
关键词	稀土元素钇海水碳酸岩文石方解石共沉淀分异分馏
摘要	本文主要运用稳定加液－反应系统对海水中方解石和文石形成时稀土元素的共沉淀现象进行了分析，研究了稀土元素在固-液体系中的迁移、转化和分配。进而在对其定量描述的前提下，研究了稀土元素共沉淀对各种反应条件的响应，并对共沉淀行为的机制进行了探讨。本实验首先运用pH测试、高精度滴定分析等手段测定了实验中的一些基本参数，如[H+]、碱度和[Ca2+]，根据计算结果获得了各碳酸体系要素，并以此为基础建立了5℃、15℃和25℃及pCO2=0.003atm下海水中方解石或文石的沉淀动力学方程。实验结果表明： 1）在各条件下，方解石或文石的沉淀速率（R）和其在海水中过饱和度（Ω）存在很好的线性相关性，即海相碳酸盐的沉淀动力学方程可以通过下面的基本表达式来表示：LogR=k*Log(Ω-1)+b ； 2）过高的稀土元素浓度会对文石或方解石的沉淀产生抑制作用，进而对共沉淀过程中YREEs的分异和分馏产生一定的影响。相比方解石而言，文石的沉淀动力学过程承受稀土元素的干扰能力更强； 3）不同温度下得到的方解石或文石各自的沉淀动力学方程存在明显的差异，表明这一过程受热力学因素控制。相对于方解石而言，温度对文石的沉淀动力学的影响更为显著。与前人研究不同的是，本实验中YREEs的浓度设定在非常低的范围内，从而避免了过高浓度YREEs对方解石或文石沉淀动力学过程的干扰。在最终的反应液中，各种实验条件非常接近自然环境。有关稀土元素的共沉淀行为主要得出以下定性或定量化结论： 1）YREEs在随方解石或文石的共沉淀过程中，均发生了强烈的分异作用。在方解石实验中，稀土元素的分异系数分布曲线呈凸状分布；而在文石实验中，稀土元素的分异系数随原子序数的增加逐渐减小，遵循镧系收缩的规律。总的来说，稀土元素，尤其轻稀土元素在文石中的分异作用要强于方解石。 2）无论是方解石还是文石，沉淀速率对YREEs的分异作用都有着明显的影响。在方解石中，YREEs的分异系数随沉淀速率的增加呈一致性递减趋势；而在文石中，其分异系数对文石沉淀速率有着截然不同的响应：轻稀土元素（La, Ce, Nd, Sm, Eu, Gd）的分异系数随文石沉淀速率的增加而下降，而重稀土元素（Ho, Y, Tm, Yb , Lu）的分异系数则随文石沉淀速率的增加呈上升趋势。 3）在方解石中YREEs的分异系数之间存在非常好的相互关系，表明这些元素是以成比例的方式参与共沉淀。整个谱系呈现中等强度的分馏，MREE相对于LREE和HREE要更为富集；在文石中由于沉淀速率的作用不同，只有Y、Ho、Yb、Lu等元素的分异系数之间有较好的相互关系。YREEs出现了差异性的强烈分馏，在新生成沉淀中轻稀土元素相对于重稀土元素强烈富集。 4）YREEs在溶液中和碳酸盐晶体表面的碳酸根配位形式对YREEs在共沉淀过程中的分异作用极为重要，YREEs在碳酸盐晶体表面的吸附是整个谱系发生分馏效应的关键环节。对于文石来讲，晶体中有效YREE离子和Ca离子半价大小之间的相近程度是其分馏效应的关键因素；而对于方解石来说，YREEs在方解石晶格中的安置就是其分馏效应的关键控制因子，但在晶格安置中起到关键作用的是YREEs和方解石中O原子之间离子键M-O的键长，而非离子半径。 5）综合YREEs在方解石中的分异作用和分馏效应，我们认为M2(CO3)3-CaCO3和MNa(CO3)2-CaCO3是最为可能的两种固体溶液形成模式。最为重要的是，对比我们的实验结果与前人在灰岩、叠层石、微生物成因碳酸盐等方解石质载体中的研究成果，两者之间出现了非常好的一致性。我们认为方解石质载体将是重建古海水中稀土元素相关信息的重要工具。相比之下，文石质载体不适合作为类似的载体。
其他摘要	Using the Constant Addition System, the coprecipitation of YREEs with calcite or aragonite in seawater was simulated, with concerning on the migration, transformation and reallocation of these elements in such water-solid interactions. Based on quantitative descriptions of their partitioning between CaCO3 and seawater, we studied their responses to experimental conditions, with attempts to study the key factors that controlling the partitioning and extract the mechanism. Basic parameters of experiments, namely [H+], [Ca2+] and total alkalinity were first measured by pH measurements and high precision titrations. Subsequently key factors of the carbonic system were calculated and the knetic expressions of calcite or aragonite precipitation in seawater at 5℃, 15℃, 25℃ and pCO2=0.003atm were built. Our results presented that: 1) Under any given condition, the precipitation rates of calcite or aragonite have excellent correlativity with CaCO3 saturation states of seawater, which can be expressed as LogR=k*Log(Ω-1)+b 2) High concentration of YREEs will inhibit the precipitation of aragonite or calcite, therefore effect the partitioning and fractionation of YREEs during the coprecipitation. The precipitation of aragonite, as compared with that of calcite, appears to be more endurable to YREEs. 3) The kinetic expressions of calcite or aragonite at different temperature are quite different, proving that relative processes are thermodynamic. Different from former experiments, the concentrations of individual YREEs was initialized to a considerable low level that no significant disturbance to the kinetic of carbonate by YREEs was observed. And the final condition of the steady solution was much closed to the nature situation. Some conclusion can be drive from this study as flows: 1) All YREEs are strongly partitioned into calcite or aragonite. The partitioning coefficient of YREEs were about 3000 for calcite, while 3000~10000 for aragonite. 2) The precipitation rates of calcite or aragonite have a kinetic control over the partitioning of YREEs. In calcite, partition coefficients of all YREEs decrease with the increase of calcite precipitation rates. In aragonite, the precipitation rates are have observed to have distinct impacts on YREE partitioning, which is negative for elements from La to Gd and positive for Y, Ho, Tm, Yb and Lu. 3) In calcite, partition coefficients of YREEs have excellent correlation with each other. The YREE spectrum is slightly fractionated in calcite with a convex kink at the position from Ce to Eu. In aragonite, YREEs are severely but differentially fractionated due to their different response to aragonite precipitation rates. Over investigated temperature scales, well restricted ratios are only found among Y, Ho, Tm, Yb and Lu. 4) The carbonate complexes of YREEs in solutions and on calcite surfaces are vital to YREE partitioning between calcite and seawater solutions. The adsorption of YREEs on calcite surface dominates the fractionation of YREE spectrum. The bong length between YREE ions and O, instead of ionic radii, is the regulator of YREE coordination in calcite. 5) M2(CO3)3-CaCO3 and MNa(CO3)2-CaCO3 are two possible models of YREE-calcite solid solutions in this experiments. It is more inspiring to observe that our DYREE patterns display striking consistency with those inferred from natural calcites, including limestones, microbialites and foraminifers. Therefore, these natural calcites formed in seawater are promising proxy recorders for the reconstruction of ancient seawater YREE patterns.
页数	133
语种	中文
文献类型	学位论文
条目标识符	http://ir.qdio.ac.cn/handle/337002/977
专题	海洋环流与波动重点实验室
推荐引用方式 GB/T 7714	刘刚. 海相碳酸盐－稀土元素共沉淀过程中的分异作用研究[D]. 海洋研究所. 中国科学院海洋研究所,2007.