Rare earth elements in surface soils of the Davarzan area, NE of Iran

Abstract

Rare earth element (REE) abundances, Ni–Cr content and physico-chemical characteristics in different topsoils and bedrocks of Davarzan area, NE of Iran, were determined to understand the relationships of theses parameters in an ophiolitic region. Resulted data show that REE geochemistry could be a suitable proxy for parental materials of surface soils. The soils derived by serpentinite and peridotite rocks display REE properties similar to these ultramafic rocks. They have low light REE (LREE), negative Ce anomaly (δCe=0.51–0.82) and positive Eu anomaly (Eu/Eu⁎=1.17–1.35). The REE concentrations of ultramafic-derived surface soils are significantly higher than parental rocks because of clay minerals. There is positive correlation between REE content and clay% and organic matter of these topsoils indicative of direct effect of such factors in REE abundances.The surface soils of low elevation samples in the studied area have been formed by weathering and transportation of different lithologies. Therefore, REE geochemistry is more sporadic and differs in various land uses. Agricultural surface soils have the highest organic matter and REE content (∑REE>100ppm). Chondrite-normalized REE patterns of agricultural samples depict fractionated appearance with steep LREE and flat high REE (HREE) [(La/Yb)N=15.31–16.41]; while other samples have lower REE content (∑REE=31.89–58.3ppm) and lower LREE/HREE. P-fertilizers are the major factor that produces REE enrichment in agricultural soils. Since these rocks are composed of mixed sources, there is no regular relation between REE content and other parameters such as organic matter and clay percentage.The concentrations of heavy metal elements such as Ni and Cr in surface soils of Davarzan area (693–1513ppm) are higher than all threshold standards defined for environmental assessments. The Ni–Cr concentration increases with REE content in ultramafic-related topsoils; but the relation of REE content and heavy metal composition is more complicated and influenced by other factors in the low elevation soils with mixed parental materials.