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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马里亚纳弧前发育目前全球已知的、唯一仍在活跃的蛇纹岩泥火山群，马里亚纳俯冲带属于非增生型板块汇聚边缘，蛇纹岩泥火山作用喷发的物质未经历弧前增生楔的强烈改造作用，是示踪俯冲带深部过程的绝佳载体[1-4]，因此，蛇纹岩泥火山自被发现以来受到科学界的持续关注[1-2,5-10]。大型蛇纹岩泥火山喷发活动停止或者间歇期，泥火山顶部可发育低温、强碱性流体渗漏；渗漏流体性质受泥火山与海沟距离控制，与泥火山之下不同俯冲深度所发生的地球化学过程有关[11-14]。渗漏流体上升到海底附近，在泥火山顶部发育烟囱状、手指状以及结壳状等自生沉积物，其矿物成分也受泥火山与海沟距离控制[1,14-15]，说明蛇纹岩泥火山顶的自生沉积物记录了渗漏流体活动，甚至记录了泥火山之下与俯冲作用有关的地球化学过程的信息。
马里亚纳弧前蛇纹岩泥火山顶发育的流体渗漏成因的自生沉积物几乎不含碎屑物质[4,15-16]。在矿物组成上，与海沟较近的泥火山顶部发育的自生沉积物主要由水镁石组成；与海沟较远的泥火山顶部发育的自生沉积物主要由碳酸盐矿物组成，偶尔也发育硅酸盐矿物[4,15,17]。目前研究主要集中于南Chamorro、Conical及Quaker蛇纹岩泥火山发育的自生碳酸盐岩[4,12,15-16]。蛇纹岩泥火山顶的自生碳酸盐岩一般以较低的δ13C值及较高的δ18O值为特征，指示渗漏流体具有较重的氧及较轻碳同位素组成，有学者认为这种较轻的碳同位素组成可能与无机成因甲烷的缺氧氧化作用有关[4,13,15,18-19]，较重的氧同位素组成与俯冲带的板片源流体的演化有关[4,15]。自生碳酸盐岩的87Sr/86Sr值一般低于海水值，指示渗漏流体具有贫放射成因锶的特征[4,15,20]。未见关于这些自生碳酸盐岩稀土元素等微量元素特征的研究。Conical蛇纹岩泥火山是目前唯一报道发育硅酸盐质烟囱状自生沉积物的蛇纹岩泥火山，其成分主要为一种罕见的含水无定形富镁硅酸盐[17]，但只有岩石学和微观形貌的报道[4,15]，未见关于该硅酸盐矿物地球化学特征的研究。这些自生沉积物元素地球化学研究的缺失可能与早期研究未对自生沉积物的研究意义给予足够重视，以及样品匮乏有关，但地球化学研究的缺失直接影响对渗漏流体的示踪。
因此，本文对马里亚纳弧前Conical蛇纹岩泥火山顶发育的自生沉积物开展研究，在岩石学、矿物学及主量元素分析基础上，结合稀土元素分析，对比不同类型自生沉积物及通过渗漏流体与海水两端元混合模型模拟的稀土元素特征间的差异，揭示Conical蛇纹岩泥火山自生沉积物所记录的渗漏流体信息。
1. 地质背景
马里亚纳弧前是指马里亚纳海沟和岛弧之间的狭长地带（图1a），由向西北俯冲的太平洋板块和上覆的菲律宾板块形成。马里亚纳弧前蛇纹岩泥火山主要由未固结的粉砂质、泥质蛇纹石胶结块状及砾状蛇纹岩或蛇纹石化超基性岩和变质岩等组成[1,5-7,21]。在马里亚纳俯冲带，太平洋洋壳自海沟处向下俯冲，随着俯冲深度的增加，温度和压力逐渐增加，俯冲板片逐渐发生压实、脱水等过程，产生的流体与上盘地幔楔橄榄岩发生蛇纹石化[22-26]，在拉伸构造背景下，蛇纹岩化的地幔楔沿弧前深断裂上升，喷发至海底形成蛇纹岩泥火山[1,8-9,27-29]。此类蛇纹岩泥火山主要呈串珠状分布于马里亚纳弧前12°~20°N区间，距离海沟轴线30～100 km宽的狭长弧前范围内，其直径约10～30 km、高度约0.5～2 km[1,8,13-14,30]。
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Conical蛇纹岩泥火山位于马里亚纳弧前北部，19°32′N、146°40′E范围内（图1b），大致呈现圆锥形，底部直径约为20 km，高出正常海底1500 m，山顶水深约3 100 m[4,7,15,31]。距离海沟轴线约86 km，位于俯冲界面上方19 km，是距离海沟最远、对应俯冲深度最深的一座蛇纹岩泥火山，山顶或裂缝处发育活跃的流体渗漏活动[1,13-15,17]。
2. 样品与方法
研究样品由2003年夏威夷大学组织的马里亚纳弧前航次（RI/OCE0002584）利用无人深潜器（ROV）Jason II，在第32次下潜期间从Conical蛇纹岩泥火山顶部采集，水深约3 100 m。样品在海底呈厚结壳状，覆盖并突出于蛇纹岩泥质沉积物之上。采集的Conical蛇纹岩泥火山自生沉积物用去离子水清洗后自然风干。样品呈疏松多孔状，中间发育不规则型的残余流体通道，通道周围发育黄色球粒状沉积物，远离通道白色沉积物含量增加（图2a）。样品极易碎，碎后样品呈黄或白色的不规则球粒状及小块状（图2b—c），以及含有针状矿物的薄片状（图2d）。使用牙钻或镊子分别对不同产状样品进行分类取样，并用玛瑙研钵研磨至200目用于元素地球化学分析。挑选典型样品制作光学薄片并进行扫描电镜观察。
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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 volcanoa. Loose porous authigenic deposition with irregular residual fluid path marker by the arrow; b. lamellar fragment; c. yellow spherical fragment; d. white spherical fragment.岩石学和矿物学观察在上海海洋大学海洋沉积地球化学分析实验室完成，所用仪器为Olympusa偏光显微镜和库塞姆EM-30台式扫描电镜。主量及微量元素分析在中国科学院地球化学研究所完成，主量元素的分析使用700 Series ICP-OES，微量元素的分析使用Plasma Quant MS。由于Conical蛇纹岩泥火山的自生沉积物几乎不含碎屑物质，主量及微量元素分析采用全溶消解法。称量50 mg样品放入15 mL特氟龙杯，加入二次蒸馏的优纯级HF和HNO3各1 mL，185℃烘箱加热36小时；冷却后蒸干，加入4 mL去离子水、2 mL HNO3及1 mL内标（微量元素加入Rh内标，主量元素加入Cd内标）后上机测试。主量元素及稀土元素平均相对标准偏差优于5%。
3. 结果
3.1 岩石学及矿物学
Conical蛇纹岩泥火山自生沉积物主要由黄色或白色球粒状组分及白色薄片状组分构成，黄色球粒状组分主要集中在残余流体通道附近，几乎不含有碎屑沉积物（图3a）。样品极易碎，碎裂后的碎块宏观产状呈白色薄片状和黄色或白色球粒状（图2b—d）。薄层片状碎块的显微结构观察显示由针状文石和短柱状方解石组成（图3a—c），局部发育镁硅酸盐，碳酸盐矿物与镁硅酸盐矿物间孔隙明显（图3c），说明二者间可能存在沉积间断。黄色或白色球粒状样品由不同大小的不规则球状物质组成（图3d—f），单个球体直径约几微米到几百微米，表面光滑（图3e—f），能谱分析显示由Si、O和Mg组成（图3f）。Conical蛇纹岩泥火山以往的研究均显示自生碳酸盐岩烟囱及镁硅酸盐烟囱均含有无定形镁硅酸盐[4, 13, 15, 17]，因此，球状集合体组成的黄色或白色球粒状沉积物主要成分为无定形的镁硅酸盐。
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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 volcanoa. 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.3.2 主量元素
Conical蛇纹岩泥火山自生沉积物主量元素分析结果（表1）显示，薄片状碎块CaO含量为49.3%～53.3%，MgO含量为2.3%～4.5%。球粒状碎块CaO含量为0.5%～2.9%，MgO含量为25.5%～29.1%。薄片状和球粒状碎块的混合样品CaO含量为11.2%～41.2%，MgO含量为7.8%～22.5%。样品的MgO含量与CaO含量显示了强烈负相关性，与Na2O和K2O含量显示了较好的正相关性（图4b、c）。TiO2和MnO含量均低于0.01%。
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。 ![]()
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 volcano3.3 稀土元素
Conical蛇纹岩泥火山自生沉积物均显示了较低的稀土元素（REE）含量（表2），且类薄片状碎块的总稀土含量（ΣREE为227.2～4 136.6 ng/g，平均值1 306.3 ng/g）略高于球粒状碎块的总稀土含量（ΣREE为115.4～3 64.9 ng/g，平均值192.5 ng/g）。薄片状和球粒状碎块的混合样品的稀土元素特征与薄片状碎块相似，具有相对高的总稀土含量（ΣREE含量为156.7～514.8 ng/g，平均值285.5 ng/g）。所有样品均显示了Eu正异常特征（薄片状碎块的Eu/Eu*为1.17～8.00，粒状碎块的Eu/Eu*为2.46～37.14，混合碎块的Eu/Eu*为1.88～5.96）。
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 volcanong/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。 4. 讨论
4.1 渗漏流体与海水的贡献比例
马里亚纳弧前蛇纹岩泥火山渗漏流体的稀土含量较低，配分模式以轻稀土略微富集为特征[14]，而Conical蛇纹岩泥火山自生沉积物显示了轻微的重稀土富集特征（图5），说明其形成过程中同时受到了渗漏流体和海水稀土元素组成的影响。Conical蛇纹岩泥火山自生沉积物中方解石和文石的形成被认为与碳酸盐矿物沉淀过程中海水与渗漏流体贡献比例差异有关[15]。但是，除碳酸盐矿物外，本文研究的Conical蛇纹岩泥火山自生沉积物中还发育较多黄色或白色球粒状无定形镁硅酸盐（图2），主要由镁和硅组成（图3f、 表1），自生沉积物样品镁含量与钠和钾含量具有非常好的正相关性（图4b、c），说明无定形镁硅酸盐中还含有少量钠和钾。
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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 ShaleData of seepage fluids and seawater after reference [14].Fleet等[32]曾利用稀土元素混合模型对铁锰结核中不同成因组分进行了定量研究，我们把类似方法应用于马里亚纳弧前蛇纹岩泥火山自生沉积物，利用渗漏流体和海水的稀土元素组成建立两端元混合模型，获得不同比例混合后流体的稀土元素含量，并将其稀土配分模式与自生沉积物的稀土配分模式进行比较（图6a）。由于尚未有Conical蛇纹岩泥火山渗漏流体稀土元素含量的报道，本文选取与Conical蛇纹岩泥火山地质背景类似的南Chamorro蛇纹岩泥火山的渗漏流体的稀土元素组成作为参考端元值[14,33]，海水端元的稀土元素含量根据蛇纹岩泥火山底层海水值[14]。与海水典型的重稀土元素富集特征不同，南Chamorro蛇纹岩泥火山的渗漏流体显示轻微轻稀土富集的平坦型稀土配分模式，以及显著Eu正异常的特征[14]。两端元混合模型显示，如果混合流体中海水比例超过了约10%（渗漏流体比例低于90%），混合流体的稀土配分模式就显示了类似海水配分模式的形态特征，即显著的重稀土富集，La正异常以及显著Ce负异常特征（图6a）。这是由于海水端元稀土含量（ΣREE为1.86×10−2 ng/g）显著高于渗漏流体端元（ΣREE为1.85×10−3 ng/g）[14]。但是，Conical蛇纹岩泥火山发育的大部分自生沉积物的稀土配分模式显示了与海水显著不同的特征（图5），说明这些自生沉积物沉淀于与海水稀土特征显著不同的流体环境中，且这种流体中的海水比例应低于10%，渗漏流体贡献应超过90%。因此，Conical蛇纹岩泥火山的大部分自生沉积物形成于较强的流体渗漏环境。
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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.Conical蛇纹岩泥火山自生沉积物中绝大部分样品（除两个稀土含量相对较高样品外）均具有非常显著的Eu正异常特征（图5）。Eu正异常是南Chamorro蛇纹岩泥火山渗漏流体的典型特征[14]。尽管超过10%海水的混合流体就显示了重稀土富集的海水型特征，但是就Eu元素特征来说，在混合模型中，即使海水比例达20%，混合流体的稀土配分模式仍显示轻微的Eu正异常特征（图6a）。因此，Eu/Eu*值似乎更加适合用于进一步半定量估算形成Conical蛇纹岩泥火山形成自生沉积物样品海水与渗漏流体所占的比例。拟合结果显示，稀土含量最高的两个碳酸盐型样品具有最低的Eu/Eu*值（1.17与1.48），其形成流体中的海水所占比例最高（0.2与0.34，图6b），说明这两个样品形成过程中受海水影响更加显著。部分样品的Eu/Eu*值高于渗漏流体端元值取值范围（图6b），说明尽管南Chamorro蛇纹岩泥火山渗漏流体与Conical蛇纹岩泥火山渗漏流体均具有显著Eu正异常特征，但后者可能具有更高的Eu/Eu*值，这可能导致流体混合比例反演结果对海水比例的低估。尽管海水比例最高的样品出现在薄片状碳酸盐岩碎块样品中，但总体而言薄片状碳酸盐岩碎块、球粒状镁硅酸盐碎块和混合碎块样品的Eu/Eu*值及其拟合的海水比例并不存在类型间的系统性差异（图6b），说明薄片状碎块和球粒状碎块均形成于较强的渗漏环境，海水参与程度及渗漏强度并非是自生沉积物中碳酸盐和无定形镁硅酸盐矿物同时发育的原因。
4.2 渗漏流体类型
马里亚纳弧前Conical蛇纹岩泥火山自生沉积物主要由碳酸盐矿物和无定形镁硅酸盐组成，自生沉积物中的碳酸盐矿物常结合成薄片状，而无定形镁硅酸盐呈小球状，在宏观上基本可区分（图2），混合碎块样品可能是样品破碎时的物理混合。扫描电镜下观察也显示镁硅酸盐与碳酸盐矿物之间有明显的沉积间隔（图3c），说明碳酸盐矿物和无定型镁硅酸盐可能并非共沉淀。前人也曾报道在Conical蛇纹岩泥火山顶的碳酸盐烟囱晶洞中发育少量凝胶状镁硅酸盐，说明镁硅酸盐沉淀晚于碳酸盐，为不同期次流体作用的结果[9,17]。热力学上，碳酸盐矿物在碱性条件下稳定，而镁硅酸盐矿物在碱性条件下稳定性差，二者形成于不同的酸碱条件。这些特征说明，Conical蛇纹岩泥火山自生沉积物中碳酸盐矿物碎块和无定形镁硅酸盐碎块可能分别是两种不同性质流体的渗漏活动的产物。
在Conical蛇纹岩泥火山顶碳酸盐烟囱状自生沉积物发育的邻近区域报道有烟囱状硅酸盐自生沉积物发育，且硅酸盐烟囱一般比碳酸盐岩烟囱更加粗大，成分与碳酸盐烟囱晶洞中的硅酸盐成分一致[17]。形成碳酸盐岩质烟囱的渗漏流体的碳酸盐碱度极高，可达海水值的20倍以上（52～69 meq/Kg）[13-14]，其硅含量低于或者略高于底层海水值[13-14]。沉淀硅酸盐质烟囱的渗漏流体的pH值（9.28）也高于海水，但碱度（约6 meq/Kg）仅略高于海水值，其硅含量却高于底层海水数倍[4, 17]，这说明Conical蛇纹岩泥火山顶发育“低硅型”和“高硅型”两种性质明显不同的渗漏流体。因此，基于本研究自生沉积物中薄片状碳酸盐碎块和球粒状硅酸盐碎块样品间显著的岩石学和矿物学差异，以及二者并非形成于同种渗漏流体、不同渗漏强度产物的特性，推测本研究中Conical蛇纹岩泥火山的薄片状碳酸盐样品应形成于“低硅型”渗漏流体活跃阶段，球粒状无定形镁硅酸盐样品应形成于“高硅型”渗漏流体活跃阶段，Conical蛇纹岩泥火山在同一渗漏点发育“低硅型”和“高硅型”两种渗漏流体的动态演化。
在Conical蛇纹岩泥火山顶海底的不同位置已报道发育“低硅型”和“高硅型”两种流体的渗漏[13-14,17]，但是关于同一个渗漏点发育“低硅型”和“高硅型”两种类型流体动态演化的机制目前尚不清楚。地震被认为是除蛇纹岩浮力与俯冲带深部超压以外，引起蛇纹岩泥火山间歇性喷发的重要原因[1]，地震等脉冲事件是否可能为引起多种性质渗漏流体转换及动态演化的原因需要进一步深入研究。
5. 结论
马里亚纳弧前Conical蛇纹岩泥火山自生沉积物由两种矿物组成，分别是白色薄片状碳酸盐矿物以及黄色或白色球粒状无定型镁硅酸盐。前者CaO含量较高，MgO含量较低，主要由针状文石和棱柱状方解石组成，后者MgO含量较高，CaO含量较低，微观下呈球状集合体。二者均形成于较强的渗漏环境，渗漏流体与海水稀土元素混合模型显示形成大部分自生沉积物的流体中的渗漏流体比例应高于90%，碳酸盐及无定形镁硅酸盐可能分别由“低硅型”和“高硅型”两种性质流体渗漏所引起。自生沉积物的这些特征说明，Conical蛇纹岩泥火山同一渗漏点可能发育“低硅型”和“高硅型”两种性质渗漏流体的动态演化，但诱发二种流体转换的机制有待进一步研究。
致谢：感谢美国夏威夷大学Fryer P教授提供样品和采样信息，感谢审稿人提出的宝贵意见。
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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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