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Discussion on the applicability of paleoenvironmental index for sedimentary rocks with high clay content (Vol. 40, No.6, 2020) TEXT SIZE: A A A

YANG Ji-hua1,2, LUO Chong-guang1, DU Sheng-jiang3, YU Wen-xiu4, YANG Yang1,2, WEN Han-jie1,2*

(1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China; 2. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China; 3. State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, China; 4. Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China)

Abstract: Clay minerals generally have the characteristics of adsorption.The trace elements that constitute the indicators of sedimentary paleoenvironment are likely to be enriched by the adsorption of clay minerals. The enriched trace elements in clay minerals will cause different degrees of influence on the indicators’ accuracy. In this paper, we have carried out research on the enrichment of trace elements in the evolution process of silicate mineral desilication, to determine degrees of the influence on the enrichment of each element caused by the adsorption (or isomorphism) of clay minerals. It is found that the adsorption (or isomorphism) of clay minerals could cause the relatively strong enrichment of some elements including V, Cr, Th, Li, Sr, Ga, and B as obvious enrichment peaks of these elements are appeared in the evolution process of the silicate mineral desilication. This will cause inconsistent paleoenvironmental significances indicated by indexes of those trace elements. However, other elements of Ni, Co, U, Ce, Pr, and Nd are relative weakly enriched in rocks by the adsorption (or isomorphism) of clay minerals as the enrichment peaks of these elements are not obviously appeared in the evolution process of silicate mineral desilication. The paleoenvironmental significances indicated by indexes of these elements are relatively consistent. This means that the indexes of these weakly enriched elements can be used to judge the paleoenvironment with relatively accurate instruction significance. This conclusion has been applied to reconstruct the sedimentary paleoenvironment of the clay rock in a profile in the central Guizhou. Therefore, the paleoenvironmental reconstruction of sedimentary rocks containing high contents of clay minerals by using trace element indexes should be careful as the attention should be paid to the effect on some indexes due to the adsorption (or isomorphism) enrichment of some trace elements by clay minerals. Through the discussion in this paper, it is recommended to use the Ni/Co ratio to indicate the REDOX condition and open/closed situation of the sedimentary paleoenvironment. The δU and Ceanom indexes can be used as referencing indexes for comparing the instruction significance indicated by the Ni/Co indicator. The Ni index cane be used to indicate the salinity change of sedimentary paleoenvironment.

Keywords: Clay minerals; enrichment; trace elements; paleoenvironment; index

ACTA MINERALOGICA SINICA Vol. 40, No.6, 2020, page 723-733

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