Study On Acid-soluble Fractionation Of Rare Earth Elements In Carbonate Rocks

Carbonate rock,as an unconventional oil and gas reservoir and for its indicative role in the formation of the environment,has received extensive attention and research.Usually people observe the diagenetic environment through fossils,sporopollen,stratigraphic sequences,and the characteristics of minerals.Apart from this,the determination of rare earth elements of carbonate mineral in carbonate rocks has emerged as a relatively novel method,which is more accurate to speculate on the mineral deposition environment and is also more sensitive to the environment.Although rare earth elements have similar physical and chemical properties for its similar electronic configurations,In the process of geological evolution,due to the influence of various factors such as the solubility product constant,pH value,and complex stability constant,the relative abundances will change,that is Fractionation.The commonly used methods for the determination of REE in carbonate minerals of carbonate rocks include two categories: laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)and acid-soluble inductively coupled plasma mass spectrometry(acid dissolution ICP-MS)method.The LA-ICP-MS method has too much influence on the accuracy of REE determination in carbonate minerals for the large Laser ablation spot,and the Conventional acid-dissolution methods are hard to dissolve carbonate minerals completely and have no clay mineral dissolved.This paper used the elutriation-leaching method to research REE fractionation on 20 carbonate samples from Guangyuan and Chongqing,which analyzed the influence of clay minerals on the fractionation of rare earth elements,established a change rate method to evaluate the fractionation of leaching processes,and assessed the effectiveness of the rare earth element content of each acid concentration of the method by using geochemical parameters and diagrams,finally predicted the optimal acid concentration of rare earth elements.The results of the exploration of 20 carbonate samples are as follows:(1)Acetic acid is more suitable as leaching acid in elutriation-leaching process than hydrochloric acid and nitric acid.(2)The carbonate sample that leached by 5% concentration of acetic acid have some clay mineral dissolved.(3)The new method(change rate method)can better evaluate the fractionation of rare earth elements.(4)With the aid of change rate method,elutriation before leaching is conducive to more accurate to determine the rare earth elements.(5)both Ce/Ce*>0.95 in dissolved minerals of some samples leached by 5% acetic acid and Eu/Eu*<1 in dissolved minerals of some samples leached by 0.5% acetic acid are inconsistent with the actual metallogenic oxidizing environment and positive anomalies of carbonate cesium.The REE distribution pattern for dissolved mineral of some samples is left-leaning and enrich in HREE,which is inconsistent with LREE enrichment in carbonate minerals.The rare earth element content measured by the elutriation-leaching method can basically reflects the differences in the distribution pattern diagrams of different lithologies,different rock layers,and different regions samples.(6)Theoretical analysis predicted that the best acetic acid concentration to determine REE content in carbonate minerals is about 2% when elutriated samples are leached.