2018, 37(3): 315-329.
doi: 10.11932/karst20180301
Abstract:
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 5×105 km2. 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.
