碳酸盐矿物中稀土元素分馏特征及其获取方法研究进展

A Review of Research Progress on Fractionation Characteristics and Acquisition Methods of Rare Earth Elements in Carbonate Minerals

古地表水中自生沉积的碳酸盐矿物能够较完整地保留沉积时水体稀土元素的分馏特征,这些特征主要受水体环境条件的影响,因此可以作为沉积水体古环境信息的良好指标。本文根据前人研究,综述了古地表水pH、盐度、溶氧量和热液输入等环境因素对稀土元素分馏特征的影响,总结了碳酸盐矿物稀土元素分馏特征示踪沉积水体古环境信息和主要获取碳酸盐矿物中稀土元素分馏特征信息的方法。目前,激光剥蚀电感耦合等离子体质谱法(LA-ICP-MS)和酸溶-电感耦合等离子体质谱法是主要的获取碳酸盐矿物中稀土元素分馏特征的分析方法。单纯只含碳酸盐矿物的样品较少见,出露地表的通常是碳酸盐岩,其在形成过程中会不可避免地混入陆源矿物(如黏土、石英、长石等)和自生沉积的非碳酸盐矿物,这些矿物带入的稀土元素会扰乱碳酸盐矿物中原始稀土元素分馏特征,而这两种方法均未能找到有效地避免这些非碳酸盐矿物干扰的途径。因此,未来稀土元素分馏特征示踪古环境信息的研究首先应该探究能准确获取碳酸盐岩中碳酸盐矿物稀土元素分馏特征的方法。

BACKGROUND: Authigenic carbonate minerals deposited from paleo-surface water can retain the fractionation characteristics of rare earth elements in water when deposited. The environmental conditions of the water at the time affect these characteristics. Therefore, these characteristics can be used as a reliable indicator for the paleo-environmental information of deposited water.OBJECTIVES: Based on previous studies, the influence of pH, salinity, dissolved oxygen and hydrothermal input of paleo-surface water on the fractionation characteristics of rare earth elements are reviewed, and the methods of tracing the paleo-environment information and obtaining the characteristics information of rare earth elements in carbonate minerals are summarized.METHODS: At present, laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS) and acid dissolution-inductively coupled plasma-mass spectrometry are the mainstream analytical methods used to obtain the fractionation characteristics of rare earth elements in carbonate minerals.RESULTS: It is rare to find samples that only contain carbonate minerals, so samples are usually carbonate rocks exposed on the earth’s surface. However, carbonate rocks have been inevitably mixed with terrigenous minerals(such as clay, quartz, feldspar) and authigenic non-carbonate minerals during their formation, which will disturb the original rare earth elements fractionation characteristics of the carbonate minerals. Unfortunately, neither method mentioned earlier was successful in avoiding this interference.CONCLUSIONS: The future study on the fractionation characteristics of rare earth elements should first explore the methods to accurately obtain the fractionation characteristics of carbonate minerals in carbonate rocks.