碳酸盐岩风化过程中高场强元素的地球化学行为研究——来自碳酸盐岩淋溶实验的证据

Study on geochemical behavior of high field strength elements during weathering of carbonate rocks:Evidence from leaching experiment on carbonate rock

选择黔中地区的一条白云岩原位风化剖面(平坝剖面)作为研究对象,通过对岩-土界面之下的岩粉层(砂状碳酸盐岩)动态淋溶过程中高场强元素(HFSE)地球化学行为的研究,并结合其在风化壳剖面的分布特征,获得了以下主要认识:(1)碳酸盐岩风化过程中,HFSE间存在明显的分馏,而且元素分馏主要出现在岩-土界面作用过程中,即碳酸盐岩溶蚀形成残积土阶段;元素的地球化学惰性由强到弱的顺序依次为Zr>Hf>Nb>Sc>Th>Ta>Ti>Y,其中,Zr是最稳定的元素,Hf仅次于Zr,Nb和Sc也相对较为惰性,而Th、Ta、Ti、Y呈现出明显的活性;(2)对于碳酸盐岩风化剖面的质量平衡计算,Zr是理想的参比元素(即惰性元素);(3)由基岩酸不溶物至风化壳剖面,元素对Nb-Ta、Zr-Hf显示出较好的协变性,没有明显分馏,因此,在利用这类元素对岩溶区风化壳的物源进行示踪时,碳酸盐岩作为潜在母岩,宜采用其酸不溶物作为参比对象;(4)碳酸盐岩风化过程中,虽然Sc也是一个较为稳定的HFSE,但在风化母岩中分布不均匀,不宜用于岩溶区风化壳的物源示踪。

It is generally believed that the high field strength elements(HFSE,including Sc,Ti,Y,Zr,Nb,Hf,Ta,Th,etc.)are extremely inert in the epigenetic environment.However,it had been reported that there was a significant variation in the content ratio between above elements during the transition from the rock to soil in weathering profiles developed on carbonate rocks.At present,little is known about whether the fractionation between HFSE occurs or not during weathering of carbonate rocks,which needs further research.The karst area in southwestern China,centered on Guizhou Province,is the largest carbonate rock continuous distribution area in the world,with an area of 5×10 5 km 2.Under the subtropical humid monsoon climate,a set of 1-10 m thick red weathering crusts widely cover over the gently sloping hilly area,which is an ideal place for the above research.In this paper,we selected an in-situ weathering profile derived from carbonate rock(i.e.,dolomite)in central Guizhou Province as the study area,by probing geochemical behaviors of the HFSE due to dynamic leaching of the arenilitic carbonate rock happens at the rock-soil interface and by combining the distribution characteristics of HFSE in the profile to preliminarily reveal geochemical behavior of the HFSE during weathering of the carbonate rocks.This study draws the conclusions as follows,(1)During weathering of carbonate rocks,there is distinct fractionation between HFSE;and their fractionation mainly occurs in the geochemical reaction at the rock-soil interface,i.e.,at the stage of residual soil formation by carbonate dissolution.Geochemical inertia of these elements from strong to weak is in the order of Zr>Hf>Nb>Sc>Th>Ta>Ti>Y.Among them,Zr is the most immobile,while Hf is second only to Zr.Nb and Sc are relatively more immobile and Th,Ta,Ti and Y show obvious mobility;(2)For mass balance calculation of the weathering profile of carbonate rocks,Zr is an ideal reference element(i.e.,inert element);(3)The element pairs such as Nb-Ta and Zr-Hf display good covariances from the acid-insoluble residues of the bedrock to the weathering profile,without evident fractionation.Therefore,when using these elements to trace the provenance of weathering covers in karst areas,and when carbonate bedrocks are used as the potential parent rocks,their acid-insoluble residues should be used as the reference object;(4)Although Sc is also relatively inert during weathering of carbonate rocks,it is not suitable for tracing the source of weathering covers in karst areas owing to its non-even distribution in the parent rocks of the weathering profile.