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菱铁矿结核呈致密块状,结核表面呈铁褐色,抛光面显示均一的结构(图2c)。XRD分析显示菱铁矿是主要的碳酸盐岩矿物,含量为46%~92%,平均为78.63%,部分结核样品中含有极少量方解石(表1)。除此之外,菱铁矿结核还含有石英等陆源碎屑(平均含量为13.25%)和少量黏土矿物(表1)。
表 1 国姓菱铁矿主要矿物(wt.%)和同位素组成(‰,VPDB)
样品 菱铁矿 方解石 白云石 石英 钠长石 钾长石 黏土矿物 δ13C δ18O G01 65 19 8 8 -1.46 -0.93 G02 82 18 0 0 -0.84 -1.09 G03 84 16 0 0 -0.93 -0.80 G04 92 8 0 0 -0.53 -0.15 G05 87 13 0 0 -0.74 0.25 G06 81 5 14 0 0 -0.99 -0.35 G09 92 4 4 0 0 -3.69 -0.10 G10 46 11 14 14 0 16 0.08 -0.27 菱铁矿主要是以泥微晶碳酸盐岩的形式存在(图3a)。SEM观察发现,菱铁矿主要是似菱形的小颗粒,直径为2~5 μm(图3d,e)。生物碎屑和黄铁矿颗粒也随机散落在碳酸盐岩基质上(图3b,c,e)。
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主量元素分析结果见表2,样品中含量最多的元素是铁,Fe2O3平均含量为22.71%,其次是镁和钙,MgO和CaO平均含量分别为4.16%和3.25%。
表 2 国姓菱铁矿主量元素含量(%)
编号 Na2O MgO Al2O3 P2O5 K2O CaO TiO2 MnO Fe2O3 G01 0.11 3.33 0.32 0.11 0.08 1.80 0.00 0.40 19.87 G02 0.08 4.16 0.29 0.10 0.08 2.44 0.00 0.49 24.11 G03 0.11 3.94 0.32 0.12 0.08 2.15 0.00 0.49 23.68 G04 0.07 4.59 0.15 0.15 0.03 3.51 0.00 0.62 25.98 G05 0.07 4.83 0.22 0.14 0.05 3.36 0.00 0.53 25.69 G06 0.06 4.10 0.13 0.17 0.03 3.31 0.00 0.49 21.07 G09 0.05 4.18 0.13 0.11 0.04 6.16 0.00 0.46 18.54 -
稀土元素结果见表3。菱铁矿结核的总稀土元素(∑REE)含量为32×10-6~43×10-6,菱铁矿的稀土元素展示出相似的配分模式,1)中稀土(MREE)相对富集,轻稀土(LREE)相对亏损(平均(Gd/Yb)N=1.86;(Pr/Yb)N=0.83);2)轻微的La的负异常(平均(La/La*)N=0.96);3)轻微Ce异常(平均(Ce/Ce*)N=1.07);4)接近平均页岩的Y/Ho比(平均(Y/Ho)=26.93)。
表 3 国姓菱铁矿结核稀土元素含量(×10-6)
元素 G01 G02 G03 G04 G05 G06 G09 La 4.27 4.84 4.70 5.81 5.49 6.34 5.50 Ce 12.06 12.37 12.46 15.01 14.58 16.81 13.87 Pr 1.52 1.46 1.58 1.78 1.74 2.02 1.59 Nd 6.75 6.26 7.03 7.44 7.35 8.61 6.79 Sm 1.70 1.50 1.81 1.84 1.82 2.11 1.68 Eu 0.41 0.38 0.46 0.46 0.47 0.55 0.43 Gd 1.76 1.70 2.08 2.08 2.02 2.38 1.87 Tb 0.27 0.27 0.33 0.34 0.32 0.37 0.32 Dy 1.47 1.50 1.98 1.92 1.80 2.09 1.86 Ho 0.26 0.28 0.38 0.36 0.34 0.38 0.36 Er 0.67 0.74 1.04 0.94 0.86 0.95 0.96 Tm 0.09 0.10 0.14 0.12 0.11 0.11 0.13 Yb 0.49 0.55 0.88 0.68 0.63 0.63 0.76 Lu 0.07 0.08 0.13 0.10 0.09 0.09 0.11 LaN/SmN 0.37 0.47 0.38 0.46 0.44 0.44 0.48 DyN/SmN 1.03 1.19 1.30 1.24 1.17 1.17 1.31 GdN/YbN 2.17 1.87 1.42 1.85 1.96 2.28 1.48 PrN/YbN 0.99 0.85 0.57 0.83 0.89 1.02 0.67 Y/Ho 25.97 27.77 25.30 28.18 26.86 27.37 27.09 (La/La*)N 0.94 1.00 1.00 0.91 0.90 0.93 1.02 (Ce/Ce*)N 1.07 1.06 1.04 1.08 1.09 1.08 1.07 注: (Ce/Ce*)N=3CeN/(2LaN+NdN);(La/La*)N=LaN/(3PrN-2NdN);(Pr/Pr*)N=2PrN/(CeN+NdN),公式中N表示澳大利亚后太古代页岩标准化(PAAS)[24]。
Genesis of Siderite in Miocene Marine Shale in Kuohsing Area, Taiwan
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摘要: 菱铁矿很好地记录了过去地质流体的信息,能够用于示踪生物地球化学反应相关的成岩作用带。台湾国姓地区中新世海相泥页岩中发育自生的菱铁矿结核,其成因尚未厘清。野外观察发现菱铁矿以不连续透镜体平行散布于泥页岩中,主要由自生碳酸盐菱铁矿(78.63%)等矿物组成。菱铁矿的稀土元素配分模式为轻稀土亏损、中稀土富集,无Ce异常,指示菱铁矿形成于弱氧化的沉积环境,弱氧化的环境促进了菱铁矿在次氧化带的沉淀。菱铁矿的δ13CVPDB和δ18OVPDB值分别为-3.69‰~+0.08‰和-1.09‰~+0.25‰,指示菱铁矿形成于次氧化带,碳源很可能是海水和有机质降解混合产生。研究表明自生菱铁矿能够被用于识别沉积物中的生物地球化学过程和指示成岩作用带。Abstract: Siderite records historical geological information about fluids, and thus can be used to trace diagenetic zones related to biogeochemical reactions. Siderite nodules have been found in Miocene marine shale in the Kuohsing area of Taiwan, but their genesis is not clear. Field observations show that the siderite is scattered in parallel discontinuous lenses in the shale, which consists mainly of authigenic siderite (78.63%) and other minerals. The REE distributions of the siderite are characterized by MREE enrichment with no Ce anomaly. This indicates that the siderite was formed in weakly oxidizing conditions, facilitating the precipitation of siderite in a suboxic zone. The δ13C and δ18O values of this siderite are from -3.69‰ to +0.08‰ and from -1.09‰ to +0.25‰, respectively, suggesting that it was precipitated in the suboxic zone with an admixture of seawater and organic matter. Studies show that authigenic siderite can be used to identify biogeochemical processes in sediments and indicate diagenetic zones.
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Key words:
- siderite /
- carbon and oxygen isotopes /
- suboxic zone /
- REE
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表 1 国姓菱铁矿主要矿物(wt.%)和同位素组成(‰,VPDB)
样品 菱铁矿 方解石 白云石 石英 钠长石 钾长石 黏土矿物 δ13C δ18O G01 65 19 8 8 -1.46 -0.93 G02 82 18 0 0 -0.84 -1.09 G03 84 16 0 0 -0.93 -0.80 G04 92 8 0 0 -0.53 -0.15 G05 87 13 0 0 -0.74 0.25 G06 81 5 14 0 0 -0.99 -0.35 G09 92 4 4 0 0 -3.69 -0.10 G10 46 11 14 14 0 16 0.08 -0.27 表 2 国姓菱铁矿主量元素含量(%)
编号 Na2O MgO Al2O3 P2O5 K2O CaO TiO2 MnO Fe2O3 G01 0.11 3.33 0.32 0.11 0.08 1.80 0.00 0.40 19.87 G02 0.08 4.16 0.29 0.10 0.08 2.44 0.00 0.49 24.11 G03 0.11 3.94 0.32 0.12 0.08 2.15 0.00 0.49 23.68 G04 0.07 4.59 0.15 0.15 0.03 3.51 0.00 0.62 25.98 G05 0.07 4.83 0.22 0.14 0.05 3.36 0.00 0.53 25.69 G06 0.06 4.10 0.13 0.17 0.03 3.31 0.00 0.49 21.07 G09 0.05 4.18 0.13 0.11 0.04 6.16 0.00 0.46 18.54 表 3 国姓菱铁矿结核稀土元素含量(×10-6)
元素 G01 G02 G03 G04 G05 G06 G09 La 4.27 4.84 4.70 5.81 5.49 6.34 5.50 Ce 12.06 12.37 12.46 15.01 14.58 16.81 13.87 Pr 1.52 1.46 1.58 1.78 1.74 2.02 1.59 Nd 6.75 6.26 7.03 7.44 7.35 8.61 6.79 Sm 1.70 1.50 1.81 1.84 1.82 2.11 1.68 Eu 0.41 0.38 0.46 0.46 0.47 0.55 0.43 Gd 1.76 1.70 2.08 2.08 2.02 2.38 1.87 Tb 0.27 0.27 0.33 0.34 0.32 0.37 0.32 Dy 1.47 1.50 1.98 1.92 1.80 2.09 1.86 Ho 0.26 0.28 0.38 0.36 0.34 0.38 0.36 Er 0.67 0.74 1.04 0.94 0.86 0.95 0.96 Tm 0.09 0.10 0.14 0.12 0.11 0.11 0.13 Yb 0.49 0.55 0.88 0.68 0.63 0.63 0.76 Lu 0.07 0.08 0.13 0.10 0.09 0.09 0.11 LaN/SmN 0.37 0.47 0.38 0.46 0.44 0.44 0.48 DyN/SmN 1.03 1.19 1.30 1.24 1.17 1.17 1.31 GdN/YbN 2.17 1.87 1.42 1.85 1.96 2.28 1.48 PrN/YbN 0.99 0.85 0.57 0.83 0.89 1.02 0.67 Y/Ho 25.97 27.77 25.30 28.18 26.86 27.37 27.09 (La/La*)N 0.94 1.00 1.00 0.91 0.90 0.93 1.02 (Ce/Ce*)N 1.07 1.06 1.04 1.08 1.09 1.08 1.07 注: (Ce/Ce*)N=3CeN/(2LaN+NdN);(La/La*)N=LaN/(3PrN-2NdN);(Pr/Pr*)N=2PrN/(CeN+NdN),公式中N表示澳大利亚后太古代页岩标准化(PAAS)[24]。 -
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