碳酸盐(岩)的稀土元素特征及其古环境指示意义

稀土元素主要通过交代碳酸盐矿物的Ca^{2+0进入碳酸盐格架,所以沉积碳酸盐(岩}的稀土元素特征能够很好的指示沉积流体来源和古环境。常用的稀土元素指标包括稀土元素总量(ΣREE)、稀土元素配分型式、以及La、Ce、Eu、Gd和Y等元素的异常指数。碳酸盐(岩)的稀土元素含量可能受到硅酸盐矿物、Fe-Mn氧化物/氢氧化物和磷酸盐等非碳酸盐组分以及成岩蚀变作用的影响。因此,在分析过程中,我们只有排除这些影响因素,才能用碳酸盐(岩)的稀土元素指标来探讨流体来源和古环境。这要求我们采集新鲜剖面上的样品,并用适当浓度的弱酸进行分步溶样,提取适当的组分,避免样品中的非碳酸盐组分干扰原始沉积组分的稀土元素特征。不同的沉积水体和沉积相下形成的碳酸盐(岩)具有不同的稀土元素特征:从太古宙到全新世的海相碳酸盐(岩)记录了LREE亏损、La正异常和高Y/Ho值的稀土元素特征;海底孔隙水的稀土元素特征则受氧化-还原条件、离子络合形式、孔隙流体来源的制约;热液流体具有LREE富集、Eu正异常的稀土元素特征;河水和湖泊有相对平坦的稀土元素特征。因此,碳酸盐(岩)的稀土元素特征具有重要的古环境指示意义。

Rare Earth Element Geochemistry of Carbonate and its Paleoenvironmental Implications

The rare earth elements can enter into the lattices of carbonate minearls by the replacement of Ca^2+,which means that the REE+Y features of carbonate are promising proxies to trace fluid chemistry and establish paleoenvironmental conditions.The useful geochemical proxies of REE+Y mainly consist of the total REE concentrations,REE+Y patterns,and the anomalies of some elements,such as La,Ce,Eu and Y,etc.However,the REE+Y characteristics can be influenced by non-carbonate components,such as siliciclastic materials,Fe-Mn oxides/oxydydroxides,phosphatic materials,and be altered by diagenetic alteration.In order to preclude such contamination and alteration,the samples should be collected from the newly exposed sections,and dissolved in relatively weak acid by sequential steps.It has been shown that the different precipitation fluids for carbonate are characterized by distinct REE+Y features.The marine carbonates from Archean to present are characterized by uniform LREE depletion,positive La anomalies and high Y/Ho ratios.The REE+Y patterns of pore waters in the sediments are controlled by redox conditions,the forms of carbonate complexations and the origin of the pore water.The hydrothermal fluids commonly have LREE enrichment and positive Eu anomalies.The rivers and lacustrine are usually characterized by relatively flat REE+Y patterns.