渝东北黑色页岩元素迁移特征及化学风化程度

黑色页岩是富含有机质和硫化矿物的特殊沉积岩，但人们对其风化过程的元素活动性及矿物风化机制关注较少.为探讨不同地形位置的黑色页岩化学风化过程，采集了渝东北城口某山脊 (A)、近山顶 (B) 和沟谷 (C) 的下寒武统水井沱组黑色页岩风化剖面岩样，利用XRD、XRF和化学分析手段对采集样品的矿物成分、主量元素进行测定分析.元素和矿物的质量迁移系数 (τ) 和质量迁移通量 (M_{j, flux}) 的计算结果表明，黑色页岩风化过程中Ca、Mg和Na元素具有明显的贫化现象，近地表处存在Al元素的富集现象；矿物成分方面，黄铁矿和有机质氧化后形成的酸性水环境，造成方解石、白云石、斜长石等不稳定矿物溶解，并生成含水石膏、铁质氧化物、黏土矿物等次生矿物.不同赋存位置的黑色页岩风化程度有所差异，Na/K-CIA、K/Ca*-Al/Na、A-CN-K和A-CNK-FM图解显示:A剖面处于脱Ca过程的初级风化阶段，B剖面处于脱Ca、Na初期的初等-中等风化阶段，C剖面已发生脱Ca、Na过程，并伴随脱Si作用的中等-强烈风化阶段，结合不同风化指数 (如:CIA、CIW、PIA、MWPI等)，得出各剖面的化学风化强弱程度依次为C＞B＞A. 

Elemental Migration Characteristics and Chemical Weathering Degree of Black Shale in Northeast Chongqing,China

Black shales are special sedimentary rocks that contain significant amounts of organic matter and sulfide minerals; whereas, less is known about the elemental mobility and weathering mechanism in black shales during chemical weathering. For understanding the chemical weathering processes of black shale in different geographic locations, a number of Shuijingtuo Formation black shale samples were collected from three weathering profiles, namely, profile A (mid-ridge), profile B (near mountaintop), and profile C (valley) at Chengkou County, northeastern Chongqing Province in this study. The major element concentration, mineral composition, and bulk density of each sample were analyzed by XRF, XRD and chemical analysis methods. The results of geochemical analysis indicate that these weathering profiles were significantly depleted of Ca, Mg and Na in all samples but enriched in Al on the surface weathering zone by calculated mass transfer coefficient (τ) and mass transfer flux (M_{j, flux}) in all samples. From the perspective of mineralogy, the calcite, dolomite and plagioclase were dissolved in acid solution produced from pyrite and organic matter, and the illite, gypsum and Fe-oxides were formed during chemical weathering process. From the graphical representations of Na/K-CIA, K/Ca*-Al/Na, A-CN-K, and A-CNK-FM show that profile A is located in the weak chemical weathering stage of Ca removal. Profile B has reached the weak to moderate chemical weathering stage and being initial stage of Ca and Na removal. Profile C has undergone the early stage of Ca and Na removal, and reached the Si removal (desilication) stage of moderate to strong chemical weathering. Different chemical weathering indexes of CIA, CIW, PIA, and MWPI were calculated in three profiles, showing that the chemical weathering sequence in all profiles is C > B > A.