Constrains of seepage fluids based on the characteristics of authigenic deposition from Conical serpentinite mud volcano in the Mariana forearc
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摘要: 马里亚纳弧前蛇纹岩泥火山顶部海底发育由低温碱性流体渗漏形成的自生沉积物，记录了渗漏流体特征，对俯冲带的物质循环研究有重要意义。但目前对复杂矿物组成的自生沉积物及其所指示的渗漏流体信息仍不清楚。对采自马里亚纳弧前Conical蛇纹岩泥火山的自生沉积物进行了岩石学、矿物学及主量和微量元素分析。结果表明，Conical蛇纹岩泥火山自生沉积物呈疏松多孔状，极易碎，碎块主要呈薄片状和球粒状。薄片状碎块呈白色，主要由针状文石和棱柱状方解石组成，CaO含量较高（49.3%～53.3%），MgO含量较低（2.3%～4.5%）。球粒状碎块呈黄色或白色，为无定形镁硅酸盐，MgO含量较高 （25.5%～29.1%），CaO 含量较低（0.5%～2.9%）。碳酸盐岩碎块的总稀土含量（ΣREE）为227.2～4 136.6 ng/g；无定形镁硅酸盐碎块的ΣREE为115.4～364.9 ng/g，均显示轻微重稀土富集的平坦型配分模式。自生沉积物的稀土配分模式显示，除两个稀土含量相对较高的碳酸盐岩样品外，渗漏流体的贡献高于90%，说明两类样品均形成于较强的渗漏环境，并且碳酸盐及镁硅酸盐可能分别形成于“低硅型”和“高硅型”不同的流体活跃期。Abstract: Authigenic depositions induced by low-temperature alkaline seepage fluids occur on the top of the Mariana forearc serpentinite mud volcanoes, which are archives of the seepage fluids and are significant for studying the material circulation of the subduction zone. However, little is known about the features of the authigenic depositions composed of multiple minerals and their recording of seepage fluids. In this paper, we investigated the petrology, mineralogy and major and trace element compositions of authigenic depositions collected from Conical serpentinite mud volcano in Mariana forearc. The authigenic depositions from Conical serpentinite mud volcano are loose and extremely friable into lamellar and spherical fragments. The lamellar fragments are white, mainly composed of needle-like aragonite and prismatic calcite, with high CaO contents (49.3%~53.3%) and low MgO contents (2.3%~4.5%). The spherical fragments are yellow or white, made of amorphous magnesium silicate, with high MgO contents (25.5%~29.1%) and low CaO contents (0.5%~2.9%). ΣREE of the carbonate fragments range from 227.2 ng/g to 4136.6 ng/g, while the ΣREE of the amorphous magnesium silicate fragments are from 115.4 ng/g to 364.9 ng/g. All samples show flat distribution patterns with slight enrichment of heavy rare earth elements. The rare earth element distribution patterns of authigenic depositions indicate that the contribution of seepage fluids is higher than 90% except for two carbonate samples with relatively high rare earth element contents. This suggests that all samples should form in the intense seepage environments, but the carbonates and magnesium silicates may be induced by varied types of seepage fluid, namely, "low-silica type" and "high-silica type".
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图 2 Conical蛇纹岩泥火山自生沉积物手标本
a. 具有不规则型残余流体通道的疏松多孔自生沉积物，箭头指示残余流体通道；b. 薄片状碎块；c. 黄色球粒状碎块；d白色球粒状碎块。
Figure 2. Authigenic deposition from Conical serpentine mud volcano
a. Loose porous authigenic deposition with irregular residual fluid path marker by the arrow; b. lamellar fragment; c. yellow spherical fragment; d. white spherical fragment.
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图 3 Conical蛇纹岩泥火山自生沉积物显微结构特征
a. 薄片状碎块发育针状文石的显微薄片照片（正交偏光）及b扫描电镜照片；c. 薄片状碎块中棱柱状方解石与针状文石伴生发育，碳酸盐矿物与镁硅酸盐矿物间孔隙明显，能谱分析显示1、2和3分别为文石，镁硅酸盐和方解石；d. 球粒状碎块由块状及边缘的球状（箭头）无定形镁硅酸盐组成（单偏光）；e—f. 为球状无定形镁硅酸盐扫描电镜照片，能谱分析结果显示主要元素组成为Si、O、Mg。红色小圈为能谱测试点。
Figure 3. The microstructure features of authigenic deposition from Conical serpentinite mud volcano
a. Thin section photo of lamellar fragments showing acicular aragonite (polarized light); b. Photo showing acicular aragonite under scanning electron microscope (SEM); c. SEM photo showing prismatic calcite associated with acicular aragonite, but separated with irregular magnesium silicate by obvious porosity; the energy dispersive spectrometer (EDS) results indicate that 1, 2 and 3 are aragonite, magnesium silicate and calcite, respectively; d. Thin section photo showing spherical fragments are composed of massive parts and spherular parts at edges (single polarized light); e-f. SEM photos of amorphous magnesium silicates showing spherular structure; EDS results show that spherular fractures are composed of Si, O, Mg elements.
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图 4 Conical蛇纹岩泥火山自生沉积物部分主量元素含量图
Figure 4. Part of the major element contents in authigenic deposition from Conical serpentine mud volcano
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图 5 Conical蛇纹岩泥火山自生沉积物及马里亚纳弧前蛇纹岩泥火山渗漏流体澳大利亚后太古代页岩标准化稀土配分模式图
海水与渗漏流体数据来自文献[14]。
Figure 5. Rare earth element patterns of authigenic deposition from Conical serpentinite mud volcano and of seepage fluids from Marianas forearc serpentinite mud volcanoes standardized by Post-Archean Australian Shale
Data of seepage fluids and seawater after reference [14].
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图 6 渗漏流体与海水混合流体的澳大利亚后太古代页岩标准化稀土配分模式（a）及Eu/Eu*值与拟合海水贡献比例（b）
a中0%代表南Chamorro渗漏流体，b中曲线根据海水与南Chamorro渗漏流体定量混合后与其对应的Eu/Eu*值拟合；样品中海水贡献比例根据混合流体曲线对应拟合函数计算，不在曲线上的点默认海水贡献为0。
Figure 6. The rare earth partitioning patterns of mixed fluids of seepage fluids and seawater standardized by Post-Archean Australian Shale (a) and Eu/Eu* values V.S. modeled seawater contribution ratios (b)
a. 0% represents the REE pattern of the seepage fluid from South Chamorro serpentinite mud volcano; b.The curve is deduced according to the Eu/Eu* values of mixed fluids of seawater and the seepage fluid from South Chamorro serpentinite mud volcano; The proportion of seawater contribution in the sample is calculated according to the corresponding fitting function of the mixed fluid curve, The seawater ratios of the points with Eu/Eu* values beyond the curve were taken as 0.
表 1 Conical蛇纹岩泥火山自生沉积物主量元素特征
Table 1 Characteristics of major elements in authigenic deposition from Conical serpentine mud volcano
% 样品编号 碎块类型 MgO CaO Na2O Al2O3 P2O5 K2O Fe2O3-T h1 薄片状 2.3 53.3 1.1 0.02 0.02 0.01 0.010 9 h2 薄片状 3.8 50.1 1.2 0.11 0.03 0.02 0.073 6 h3 薄片状 4.5 49.3 1.3 0.00 0.03 0.02 0.0030 h4 薄片状 3.4 49.7 1.1 0.01 0.02 0.01 0.006 3 h5 薄片状 2.5 52.8 1.2 0.22 0.03 0.02 0.053 0 h6 薄片状 2.3 50.9 1.1 0.27 0.06 0.02 0.130 2 h7 混合碎块 8.6 39.3 1.3 0.02 0.03 0.05 0.011 8 h8 混合碎块 18.9 18.2 1.8 0.06 0.03 0.10 0.025 5 h9 混合碎块 11.6 33.9 1.4 0.01 0.02 0.06 0.003 0 h10 混合碎块 7.8 41.1 1.4 0.01 0.03 0.04 0.006 8 h11 混合碎块 22.5 11.2 2.1 0.02 0.03 0.11 0.008 0 h12 混合碎块 8.0 40.6 1.4 0.01 0.03 0.04 0.003 3 h13 球粒状 28.1 0.7 2.0 0.01 0.02 0.14 0.013 0 h14 球粒状 27.2 2.1 2.1 0.00 0.03 0.13 0.006 7 h15 球粒状 28.6 0.7 2.2 0.00 0.02 0.13 0.001 1 h16 球粒状 28.1 0.7 2.2 0.00 0.02 0.13 0.001 5 h17 球粒状 27.9 0.5 2.3 0.01 0.02 0.13 0.002 2 h18 球粒状 27.7 0.9 2.6 0.00 0.03 0.14 0.000 2 h19 球粒状 27.0 0.8 2.5 0.00 0.02 0.13 0.002 7 h20 球粒状 25.5 2.9 2.2 0.00 0.03 0.13 0.002 3 h21 球粒状 29.1 0.5 2.5 0.01 0.02 0.14 0.002 3 h22 球粒状 27.0 2.2 2.4 0.00 0.03 0.13 0.003 4 注：主量元素分析结果未包含碳和硅元素含量，以及部分氧元素含量。 
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表 2 Conical蛇纹岩泥火山自生沉积物稀土元素含量及特征
Table 2 Contents and characteristics of REE in authigenic deposition of Conical serpentinite mud volcano
ng/g 样品编号 La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu ΣREE Ce/Ce* Eu/Eu* h1 40.5 155.8 7.3 89.7 8.8 10.0 5.8 3.1 9.3 125.0 3.0 9.3 0.4 6.2 1.4 350.6 2.07 5.40 h2 89.1 168.9 29.4 121.6 14.0 16.7 37.4 6.3 28.6 371.7 10.3 26.4 2.3 29.8 3.0 583.7 0.75 2.36 h3 51.9 97.7 6.5 36.2 9.2 5.5 2.0 1.4 7.2 99.0 2.9 3.7 0.3 2.8 0.3 227.2 1.17 8.00 h4 51.0 147.4 15.6 66.6 4.6 13.9 12.3 1.9 11.8 118.4 3.2 7.0 − 8.5 1.1 344.9 1.19 5.33 h5 848.4 161.5 121.1 516.9 84.9 22.6 89.4 12.9 107.9 1 201.3 24.0 67.3 14.0 105.3 18.4 2 194.8 0.11 1.17 h6 1 620.9 547.2 245.7 1 020.1 174.9 45.2 127.2 16.7 125.7 1 079.9 26.7 70.5 16.2 85.2 14.6 4 136.6 0.20 1.48 h7 47.7 91.5 9.2 37.4 9.0 7.3 12.0 2.0 22.3 160.1 3.2 7.8 1.4 9.8 2.1 262.7 1.00 3.22 h8 55.8 127.5 20.0 82.3 30.3 12.9 38.6 6.5 56.0 461.0 13.5 27.1 5.9 33.3 5.2 514.8 0.86 1.88 h9 16.9 69.1 3.6 20.6 8.0 10.1 11.9 1.0 5.6 89.0 2.2 4.5 0.4 0.9 1.9 156.7 2.06 4.88 h10 35.0 67.8 8.6 31.0 2.0 7.1 5.6 2.8 11.0 114.3 3.1 7.5 0.4 10.8 1.5 194.3 0.90 5.96 h11 54.8 124.6 8.0 48.3 19.4 11.9 13.6 4.2 14.1 120.7 5.3 7.8 − 12.4 1.4 325.7 1.34 4.56 h12 49.2 86.0 5.5 42.1 15.1 8.4 24.7 1.8 11.1 104.6 4.1 6.0 − 4.1 0.8 258.8 1.13 1.96 h13 55.7 142.5 6.1 54.4 11.6 12.4 17.8 4.4 16.0 319.6 6.3 16.0 1.8 18.4 1.3 364.9 1.67 3.71 h14 32.6 77.2 5.3 23.1 6.4 4.1 6.0 1.1 9.9 80.0 4.3 7.4 0.5 2.7 2.5 183.2 1.33 3.47 h15 21.9 50.9 2.5 25.1 4.8 3.6 2.5 1.6 11.4 139.5 3.6 8.0 − 6.1 1.3 143.3 1.49 5.50 h16 9.8 70.8 1.0 17.0 − 6.6 − 0.3 0.7 58.8 2.1 3.9 0.5 1.6 1.2 115.4 4.78 37.14 h17 57.0 81.8 9.0 42.0 2.9 9.1 8.2 2.1 8.6 158.3 3.4 6.3 0.4 12.1 1.8 244.7 0.82 5.29 h18 8.5 105.6 3.2 8.1 3.1 9.5 8.2 1.7 8.0 109.1 1.6 3.7 0.1 5.0 0.7 167.0 4.57 7.01 h19 8.3 97.9 − − − 4.3 8.3 0.3 14.0 61.8 2.9 5.2 − 2.6 0.6 144.5 11.32 3.36 h20 16.8 58.1 4.1 4.0 1.0 4.6 8.0 1.0 5.1 121.1 2.9 7.3 2.2 8.9 3.8 127.9 1.61 3.61 h21 48.8 135.2 6.4 67.3 9.5 5.5 9.9 0.6 5.3 126.1 2.6 7.6 0.1 2.5 1.2 302.6 1.69 2.46 h22 10.0 77.6 1.7 6.0 0.9 7.3 4.0 0.6 9.8 74.2 2.5 5.5 0.8 4.4 0.3 131.5 4.30 10.67 Ce/Ce*=2CeN/(LaN+PrN)，Pr/Pr*=2PrN/(CeN+NdN)，Eu/Eu*=EuN/(0.33NdN+0.67GdN)，ΣREE不包括Y。“−”表示未检出。
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