Mobility of high field strength elements during weathering of black shales in central Hunan Province, China

This study took a geochemical investigation on distribution of major elements and high field strength elements(HFSE)in rocks and soils from different weathering zones along the paraller bedding cutting MZ(with thickness of 10.5 m)and the perpendicularly bedding cutting M(with thickness of 5.4 m)profiles(28°04′N, 111°15′E)developed at the Muzi Village, Anhua County, central Hunan Province, China. The mobility of HFSE was estimated using geochemical analysis results which were comprehensively explained by the sequential extraction analyses and the principal component analyses. The results showed that concentrations of major elements Fe2O3, MnO, MgO, CaO、P2O5, and LOI in rocks from the two profiles varied significantly with Cv(coefficient of variation)>0.2, but CaO=0.13 in M profile, and while those of SiO2 and Al2O3 were relatively less variable(Cv≤0.2) in the weathered black shales (WBS) along the two profiles. Concentrations of all the major elements in rocks from the MZ profile were more variable than those from the M profile. The mobile major element components in rocks from the two profiles were significantly depleted relative to those of the UCC(upper continental crust). Major elements Fe2O3, P2O5, and LOI were significantly enriched in the fresh black shales, and less enriched in the weathered black shales along the two profiles. The major element compositions of different rocks along the profiles suggested the rocks underwent a high degree(the chemical index of alteration from 64.08 to 84.49)of chemical weathering. The HFSEs including Ti, Zr, Hf, Nb, Ta, Ga, and Sc had concentrations in the weathered black shales increasing significantly with the elevated CIA values in rocks along the profiles, however they were less significantly enriched in rocks of the profiles(EF<2.0)compared to the NASC(North American shale composition). While elements Ge and U were distinctly enriched in rocks(EF>2.0)of the profiles. The sequential extraction analyses showed that HFSEs including Ti, Zr, Hf, Nb, Ta, Ga, and Ge in fresh and weathered black shales and soils were mostly in residual fraction with proportions higher than 90 %, while elements U and Sc were in variable speciation in different rocks along the profiles. The mass balance migration index calculation suggested that the HFSEs were leached and mobile where/when the fresh black shales were weathered into the weathered black shale; and then they were less mobile and were secondarily enriched in the secondary Fe-Mn oxides after they were released from their original hosting minerals. Element pairs of Zr vs. Hf, and Nb vs. Ta displayed very similar mobility patterns during black shale weathering, and the pairs of Ga vs. Ge had different mobility features and they were fractionated during weathering. Element Th were significantly mobile during black shale weathering. The mobility of these HFSEs were caused by dissolution of their host minerals due to both the acids produced by oxidation of organic matters and sulfide minerals(e.g., pyrite)during weathering. The secondary enrichment of HFSEs in weathered black shales and soils resulted from reaccumulation of these elements in secondary minerals(e.g., Fe-Mn oxide)formed due to pyrite oxidation during weathering.