高黏土含量沉积岩古环境指标适用性讨论

Discussion on the applicability of paleoenvironmental index for sedimentary rocks with high clay content

黏土矿物普遍具有吸附的特性,构成指示沉积古环境指标的微量元素很可能受黏土矿物吸附产生富集,对指标的指示精度产生不同程度的影响。本文通过研究微量元素在矿物脱硅演化过程中的富集情况,以判断黏土矿物对各元素吸附(或类质同象)富集的影响程度。研究发现,黏土矿物的吸附(或类质同象)作用会对一些元素(V、Cr、Th、Li、Sr、Ga、B)产生较强的富集(脱硅演化过程中出现明显的富集峰),导致其构成的指标出现指示意义不一致的情况;而黏土矿物的吸附(或类质同象)作用对另一些元素(Ni、Co、U、Ce、Pr、Nd)产生的富集作用较弱(脱硅演化过程中产生的富集峰不明显),由其构成的指标指示意义较一致,因此判断由富集程度低的元素构成的古环境指标具有较准确的指示意义。这一结论较好地运用到了黔中地区黏土岩剖面的古环境重建中。因此,对含有较多黏土矿物的沉积岩进行古环境重建时,使用微量元素指标时应慎重,须注意黏土矿物吸附(或类质同象)富集作用对微量元素指标的影响。本文通过讨论,推荐使用Ni/Co指标指示氧化还原及沉积环境封闭开放情况,指标δU和Ceanom指示意义可对Ni/Co指示意义进行对比参考;Ni指示古盐度变化情况。

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.TheδU 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.