Study On Geochemical Behavior And Mechanical Characteristic Of Black Shale During Weathering

The weathering of rocks is a major process that alters the Earth’s surface. It exerts important controls on the geochemical cycling of elements and affects on the human engineering activities. Compared with granite (diorite), basalt, and carbonatite, black shale is a fine-grained sedimentary rock typically rich in sulfide minerals (e.g., pyrite) and organic matter (OM). Due to high levels of sulfide and OM, black shale is susceptible to chemical weathering when exposed in oxidative supergene environment. The acidic water and expansive sulfate minerals produced in weathering process could dissolve minerals, expand fissure, degenerate shale strength, and enhance chemical weathering, etc. The chemical weathering of black shale will cause a series of engineering and environmental geological problems. Therefore, study on the chemical weathering of black shale could promote theoretical knowledge in field of rock weathering. And it also has great significance for the evaluation and prediction for engineering geological and environmental hazard.Taking the Shuijingtuo Formation black shale, Lower Cambrian, in Chengkou County in Chongqing province as research project, we had selected three profiles, including profiles A, B, and C, which are located at mid-ridge, near mountaintop, and valley floor, respectively.Firstly, the physical, porous, morphological, elemental, and mineralogical features of parent and weathered shales are analyzed. The pore evolution, microstructure changes, weathering degree, mineralogical weathering sequence, weathering evolutionary trend and elemental mobilization, redistribution, and fractionation of black shale during chemical weathering are systematically studied. Secondly, the weathering and driving mechanisms of black shale weathering are expounded synthetically. At lastly, in order to simulate the processes of acidic water-rock chemical interaction in chemical weathering of black shale, the corrosion tests were taken in which the black shale was soaked in H2SO4 acidic water with different pH value under the oxidation and dynamic nonequilibrium flow conditions. The chem-mechanical coupled characteristics of back shale are discussed during acidic water-rock chemical interaction. The achievements and conclusions are summarized as follows:(1) The sedimental environment of black shale in Chengba Block was analyzed. The results revealed that the black shale were formed in a dry, anoxic and shallow marine shelf and slope epicontinental environment belonging to passive continental margin, which is affected by the hydrothermal fluids.(2) Based on the physical and chemical properties of parent and weathered shales, the peak value of pore diameter distribution curves was shifted to right as weathering proceeds,indicating that pore diameter increased with degree of weathering. And also, the pore diameter distribution curve evolved into multiple-peaked curved characteristic in weathering proceeds. Whereas, fractal dimension of pores increased with increasing pore diameter. The microstructure showed mineral cementation became loose with weathering. Most of large mineral crystals evolved into silt particles, which appeared irregular polygon shape.(3) Oxidative weathering is a key process of converting the bedrock into saprock, and then into regolith, in black shale weathering system. The sequence of mineral decomposition during weathering contains four steps: first, oxidation of sulfides and OM; then, dissolution of carbonatite; followed by dissolution and argillization of plagioclase; lastly, transformation of clay minerals. Based on mass transfer coefficient (τTi,j) and mass flux (Mj,flux),the Ca, Mg and Na exhibited depletion patterns, but Al displayed enrichment in regolith zone. The elemental mobilization increased with degree of weathering and the sequence of elemental mobilization as follow: Ca > Mg > Na > Fe > Si > K > Al. According to the Na/K-CIA,K/Ca*-Al/Na, A-CN-K and A-CNK-FM diagram, profile A is in the weak chemical weathering stage of Ca and Mg removal; profile B reached weak to moderate chemical weathering stage of Ca, Mg, and Na removal; profile C have almost finished the stage of Ca,Mg, and Na removal with desilication and reached the moderate to intense chemical weathering stage. From perspective of different chemical weathering indexes, the chemical weathering sequence is C > B > A.(4) During chemical weathering of black shale, Mn、Sr、Ba、Pb、U、V、Cr、Co、Ni、Cu, and Zn were highly mobilized, redistributed, and fractionated from the decomposition of primary minerals, in contrast, Sc, Rb, and Th were less mobile compared to Ti. High field strength elements demonstrated Nb-Ta and Zr-Hf covariant mobilization and redistribution patterns. The mobilization of REEs increased with decreasing pH value and increasing degree of weathering. The sequence of REEs mobilization follws the order of MREE > HREE > LREE, resulting in the sequence of REEs fractionation in three profiles is C > B > A. And also, LREE displayed enrichment relative to HREE. The positive Ce anomalies (δCep>1) suggest that all three profiles are under oxidizing conditions.(5) Chemical weathering of black shale is the essence of water-rock interaction. The weathering mechanisms are mainly: oxidation of ’sulfide and OM; chemical, physical, and micro-mechanical effects of water-rock interaction. The non-equilibrium thermodynamics and dissipative theory are applied to analyze the chemical weathering processes of black shale, considering that it is the heat transfer, material flow, momentum flow, and chemical reaction to promote the irreversible process. As weathering proceeds, it is formed the frame-work of dissipative structure model by chemical oscilation to drive the weathering advance.(6) Using different pH value H2SO4 water under oxidation and dynamic nonequilibrium flow condition to simulate the processes of acidic water-shale chemical interaction. Under acidic water chemical action, uniaxial compressive strength (UCS) and elasticity modulus decreased with decreasing pH value and increasing soaking time. This interaction results in the transformation of brittle failure to ductile failure of black shale. Water-shale interaction controlled by pH, it is more obvious when decrease pH value. The secondary porosity is utilized to construction of chemical damage variable to describe the chem-mechanical coupled degeneration evolution during acidic water-shale chemical interaction.