A review of studies on the magmatism at Southwest Indian Ridge from petrological and geochemical perspectives
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摘要: 作为超慢速扩张脊的代表，西南印度洋中脊(SWIR)因其独一无二的地形地貌特征、洋壳结构、洋壳增生机制、岩浆和热液活动以及深部动力学过程，近30年来成为国内外研究的热点区域。基于近年来对SWIR玄武岩、辉长岩及橄榄岩的岩石学和地球化学研究成果总结，重点探讨了沿SWIR轴向（大尺度）以及单个洋脊分段（小尺度）的岩石地球化学变化特征及其影响因素，阐述了SWIR的岩浆供应及洋壳增生模式。其中，在9°～16°E斜向扩张脊，以构造作用为主的洋脊扩张模式导致了更宽的洋壳增生带和显著的地球化学异常；而在50°～51°E脊段，发育了强烈的火山活动，其成因机制包括克洛泽热点与洋中脊相互作用、微热点、古老熔融事件的残留地幔再熔融等几种观点。此外，西南印度洋中脊龙旂热液区（～49.7°E）的最新研究表明，其热液循环路径与拆离断层的发育密不可分，热液流体循环最深可达莫霍面以下6 km。因此，在今后的一段时间，应进一步加强SWIR不同空间尺度地幔源区性质、洋中脊构造与岩浆作用过程、热点-洋中脊相互作用和岩浆-热液活动与成矿等主要科学问题的研究。Abstract: The Southwest Indian Ridge (SWIR), as an ultraslow spreading ridge, has attracted great attentions from the geo-society of the world in the past three decades due to its unique morphology, crustal architecture, crustal accretion mode, volcanism, hydrothermal activities and deep mantle processes. This paper is devoted to the recent research progress on the petrology and geochemistry of basalt,gabbro and mantle peridotite collected from the SWIR. The geochemical data well revealed the variations of the whole ridge and ridge segments. Based on the data mentioned above, we described and discussed the main factors, which control the geochemical variations, magma supply and crustal accretion. In the oblique spreading ridge segment of 9°～16°E, the tectonics-dominated ocean ridge spreading patterns resulted in the wider oceanic crust accretion zone with significant geochemical anomalies; in the 50°～51°E ridge segment, strong volcanic activities occur, and its genetic mechanism includes different points of view, such as the interaction between the Crozet hotspot and SWIR, the micro hotspot, and the remelting of the residual mantle left behind by the former melting events. The latest research about the Longqi hydrothermal area (～ 49.7° E) suggests that the hydrothermal circulation is closely related to the development of detachment faults, and the maximum depth of hydrothermal circulation may reach 6 km below the Moho boundary. Therefore, it is suggested that the future study be strengthened in such issues as the mantle heterogeneity in different spatial scales, the tectonic-magmatic processes in the ridge system, ridge-plume interaction, and the seafloor hydrothermal activity and deposits.
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渗流指流体在孔隙介质中的流动[1]，陆地沉积层水体渗流对地下水资源开发[2]、水利与岩土工程安全有重要影响。除此之外，海床渗流也普遍存在，水槽试验[3-5]和现场观测[6]均发现，波浪等外部荷载作用下海床内部产生超孔隙水压力，与自由排水海床面之间形成渗流压力梯度，引起海床内部孔隙水和细颗粒向床面渗流[7]。海床孔隙水和细颗粒渗流影响海水与沉积层之间的物质交换[8]，在铁板砂[9]、塌陷凹坑[10]等海底动力地貌演化过程中扮演重要角色，还能导致海床承载力下降[11]、管线弯折[12]及平台失稳[13]。因此，准确描述海床细颗粒渗流发展过程并精确测定其渗流量，对于科学认知海底动力沉积过程、合理评价海床稳定性至关重要。海床内部孔隙水及细颗粒渗流是涉及水动力作用下床面、床内耦合的动态响应过程[9]，现场观测受复杂环境变量影响，渗流过程无法捕捉、细颗粒渗流量难以精确测定[7]，相比而言，室内模拟试验[5]具有便于控制试验变量、易于观察试验现象及方便开展分析测试等优势。
同位素示踪、温度示踪、染色示踪及化学示踪等示踪方法是研究物质迁移的常用方法。Burnett和Dulaiova [14]通过连续观测孔隙水中天然同位素222Rn浓度研究了海底地下水排泄及其动态变化过程。陈建生等[15]通过在单孔水中均匀标记人工同位素131I溶液并测定其浓度变化，进而得到流速、流向等水力学参数，分析大坝渗漏通道。孙晓宇等[16]利用温度示踪法反演库水-地下水的垂向交换速率。Cascarano等[17]运用荧光染色示踪方法研究海床沉积物-水界面水体和保守溶质运移。上述方法中，稳定同位素示踪剂种类少、价格昂贵、测定复杂，温度示踪和染色示踪主要用于研究大范围水体及溶质渗流、并且以定性描述为主，无法描述沉积物孔隙中的细颗粒渗流，化学示踪一般选取遇水呈现惰性、易于分离和测定的化学物质作为示踪剂[18]，研究流体及固体颗粒的渗流和运移。因此，本文基于化学示踪的思路，优选出氢氧化镁粉末作为示踪剂，提出了一种海床细颗粒渗流定量测量方法，并通过室内模拟试验初步验证了该方法的可行性。
1. 海床细颗粒渗流测量方法
1.1 试验用土
海床细颗粒渗流需满足以下条件： ① 海床土新近沉积且级配良好，以黏粒为主的细颗粒可在大颗粒骨架之间能够形成渗流通道；② 有波浪等外部荷载作用在海床上，引起海床内部超孔隙水压力及垂直床面向上的渗流压力梯度。现代黄河三角洲由以粉土为代表的粉质类土新近沉积而成，波浪及风暴事件引起的海床渗流现象多发[19-21]。因此，本文选择黄河三角洲原始沉积粉土为试验用土，粒径累积曲线如图1（a）所示。
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图 1 粒径累积曲线a. 粉土，b. 氢氧化镁粉末。Figure 1. Particle size accumulation curvea. Silt, b. magnesium hydroxide powder.1.2 氢氧化镁粉末示踪剂
经多次测试，本文最终优选出氢氧化镁粉末作为化学示踪剂，原因如下：
（1）氢氧化镁粉末可用于近似替代黏粒。氢氧化镁粉末难溶于水，18 ℃时溶解度仅为 0.0009 g/100 g，在海水环境下与粉土各矿物成分不发生化学反应、不产生明显吸附，具有良好的物理和化学稳定性，并且氢氧化镁粉末中值粒径为0.006 mm（图1b），颗粒密度为2.49 g/cm3，然而，黏粒颗粒密度为2.6～2.68 g/cm3，粒径小于0.005 mm [22]，氢氧化镁粉末与黏粒性质接近，可用于近似替代黏粒。
（2）氢氧化镁粉末可以很好地指示海床细颗粒渗流过程。如图2所示，在圆柱形有机玻璃筒中自下而上依次铺设10 cm饱和粉土、1 cm氢氧化镁粉末、10 cm饱和粉土。静置并达到稳定后，以质量为1 kg的圆盘自距离床面0.5 m处自由下落作为动荷载，重复施加300次。从图2中可以看出，白色氢氧化镁粉末与土体颜色明显不同，很好地指示了动荷载作用下细颗粒在粉土海床中的渗流过程及其形成的不同规模、不同形状的渗流通道。
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图 2 氢氧化镁粉末向上渗流及形成的渗流通道a. 土柱正面，b. 土柱反面。Figure 2. Upward seepage of magnesium hydroxide powder and the formed seepage channela. Front view, b. reverse side of soil column: rear view.1.3 示踪测量方法
基于氢氧化镁粉末示踪的海床细颗粒渗流测量步骤如下：① 将试样称重得到质量m1, 乘以土中背景镁离子浓度p，得到背景镁元素含量m2= m1p；② 在试样中加过量盐酸充分反应后过滤，得到体积V的滤液，使用离子色谱仪测试得到镁离子浓度c，从而得到总镁元素质量m3=cV；③ 计算渗流镁元素含量m4=m3−m2，根据Mg(OH)2分子式计算渗流氢氧化镁质量m5=58 m4/24；④ 对海床分层取样，重复上述步骤即可得到不同土层氢氧化镁粉末质量，进而实现对海床细颗粒垂向渗流过程的定量描述。
为验证测量方法定量测定土体镁元素的准确性，取10 g粉土样品并加入氢氧化镁粉末0.68 g，根据镁离子背景浓度0.26%及氢氧化镁分子式计算得到镁元素理论值为10 g×0.26 g+0.68×24/58 = 0.307 g；按照本文步骤得到镁元素测量值为0.305 g，二者偏差仅为0.65%。
2. 渗流模拟试验
进一步开展动荷载作用下粉土海床渗流模拟试验并分层测定细颗粒渗流量，用以验证本文方法的有效性。
2.1 试验装置
如图3所示，试验装置由水槽、孔隙水压力测量系统以及荷载施加装置组成。水槽为有机玻璃制作，尺寸为55 cm×40 cm×30 cm。孔隙水压力测量系统由3个孔隙水压力传感器（西安微正电子科技有限公司，CYY2型，精度0.1 kPa）以5 cm的垂向间隔安装于指向不同方位的支架水平杆外缘，线路通过支架水平杆和立柱统一输出海床，以保证孔隙水压力传感器之间及与海床表面不发生水力联系。荷载施加装置由一圆柱形砝码固定于圆形铁片上，圆形铁片直径20 cm，总重75 N，用于施加外部荷载。
2.2 试验步骤
（1）海床制备及示踪粉末的埋置。将取自黄河三角洲潮滩的土样风干，加水搅拌成均匀，流塑状态的泥浆。在水槽中央预先放置带有孔隙水压力传感器的支架，将泥浆沿侧壁缓慢注入水槽至高度10 cm，在其上铺设总质量500 g、厚约0.1 cm的氢氧化镁粉末层，之后继续铺设厚度5 cm的粉土泥浆，在海床表面加入2 cm水，确保海床始终处于饱和状态。
（2）动荷载施加、孔隙水压力与渗流量测定。将海床静置直至监测到的孔隙水压力等于该深度土体有效自重，即海床完成自重固结。将海床表面水排干，在距离水槽右侧壁10 cm处插入一塑料隔板，将海床分为试验区和对照区（图3）。在对照区取样，测试得到海床土体物理参数见表1。在试验区人工施加动荷载，将荷载施加装置由距床面50 cm高度处自由下落，重复500次、持续时间约0.5 h。试验过程中实时监测海床孔隙水压力并观察海床细颗粒渗流现象。待孔隙水压力保持稳定且不再有新的渗流现象产生，将试验区海床自床面向下每1 cm为1层进行取样，按照1.3节方法测定每层海床中的氢氧化镁含量，得到自床面以下5 cm示踪层至床面的逐层示踪物渗流量。
表 1 海床土体物理参数Table 1. Physical parameters of seabed soil容重γ/(kN·m−2) 含水率w/% 孔隙比e 密度Gs 粒径d50/mm 19 30 0.7 2.7 0.035 2.3 试验结果
图4给出了试验过程中孔隙水压力变化情况，可以看出，施加动荷载前各深度处孔隙水压力与静水压力值一致，显示出粉土海床处于正常固结状态。施加动荷载后，各深度的孔隙水压力均快速累积升高，5 cm处孔隙水压力峰值达到1.09 kPa、超过上覆土体自重0.95 kPa，海床发生液化，10 cm处孔隙水压力最大值为1.88 kPa、接近上覆自重1.9 kPa，海床接近发生液化。结束动荷载后，各深度处的孔隙水压力在逐步消散。海床内部孔隙水压力累积升高，及其引起的指向床面的压力梯度，是引起海床内部孔隙水和细颗粒垂向渗流的驱动力。
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图 4 海床表面以下不同深度处孔隙水压力变化情况Figure 4. Variation of pore water pressure at different depths beneath seabed surface与海床内部孔隙水压力累积升高相对应，在动荷载开始施加约10 min后，从水槽侧壁观察到位于床面以下5 cm的细颗粒示踪粉末开始脱离土骨架向上渗流，形成多个规模、形状各异的渗流通道。如图5所示，白色氢氧化镁示踪粉末清晰显示了渗流通道形态及细颗粒渗流发展过程，各渗流通道直径呈现出自下而上逐渐减小的趋势，部分渗流通道（如7号）贯通至海床表面。与此同时，未施加荷载的对照区示踪粉末保持原来位置不动、未观察到渗流现象。循环荷载停止后，孔隙水压力逐渐消散（图4）、作为驱动力的渗流压力梯度逐渐减小直至消失，导致部分粉末自示踪层发生向上渗流但未及到达床面；此外，形成的渗流通道的形态、规模各异，其携带示踪粉末量、向上输运速度不同，所以，观察到示踪粉末渗流量呈现出自示踪层向上逐渐较少、仅部分示踪粉末到达床面的现象。
取样测试得到的每层海床氢氧化镁粉末含量如图6所示，渗出到海床表面的示踪物为2.216 g，仅占示踪物总质量的0.4%，示踪物含量自床面向下逐层递增，自示踪层向上渗流的细颗粒共129.48 g，占示踪物总质量的25.896%。以上结果与试验中观察到的示踪物大部分留在原示踪层位置、部分示踪物沿渗流通道向上运移、渗流通道规模自下向上逐渐减小、仅个别渗流通道连通到床面的现象一致。
3. 讨论
稳定性、指示性和可测性决定了示踪剂能否发挥其效能。本文采用的氢氧化镁粉末与孔隙水及沉积物矿物成分不发生化学反应，与沉积物颗粒不发生明显吸附，在海床沉积及渗流环境下具有较好的物理和化学稳定性，避免了稀土元素[23-24]、天然同位素及人工同位素[25-28]渗流示踪过程中由于示踪剂损失造成的误差。试验结果显示，氢氧化镁示踪粉末清晰指示细颗粒物质在动荷载及其引起的超孔隙水压力和向上的渗流压力梯度作用下自示踪层开始向上运移，形成规模和形状各异的渗流通道、部分到达海床表面的渗流发展过程。与水槽试验中通过孔隙水和黏粒迁移观察渗流现象、以床面黏粒含量升高判断细颗粒物质垂向渗流 [29]相比，本文白色氢氧化镁示踪粉末与周围海床土形成明显对比，更加直观、准确指示出了渗流通道形成发展及细颗粒物质向上渗流的过程。此外，与天然及人工同位素示踪方法相比，氢氧化镁粉末廉价易获取，可按照需求加工成不同粒径满足不同层次的细颗粒渗流示踪需求，只需盐酸和离子色谱仪等常见化学分析试剂和仪器，测量方法安全、简便。
传统的同位素、温度、染色等示踪方法主要用于定性及定量描述水体渗流情况，本文方法则在室内模拟试验层面实现了对海床细颗粒渗流的定量描述。以波浪为外部荷载、粉土海床为作用对象、细颗粒渗流现象明显的水槽试验[30]为例，其人工海床尺寸为3.8 m×0.5 m×0.6 m，从海床内部渗流至床面的细颗粒质量为9.5 kg，假设不同深度海床对细颗粒渗流的贡献均等，计算出厚度1 mm海床渗流到单位面积（1 m2）渗流量为8.3 g；本文海床尺寸0.55 m×0.4 m×0.15 m，渗流到床面的细颗粒为2.2 g，可计算出1 mm示踪层渗流到床面的细颗粒为10.1 g，二者处于一个数量级且较为接近，说明本文定量测量的合理性。另外，水槽试验现有方法只能确定整个海床渗流至海床表面的细颗粒质量，本文方法可得到海床不同深度土层渗流量，实现了从源头至海床面细颗粒渗流的定量描述，更加细致地刻画了海床内部渗流过程，对于从实验室定量角度明确波浪作用下海床内部细颗粒渗流过程及分布规律、进一步理清波浪作用下粉土海床液化、渗流、再悬浮机理并开展相应的定量评价有重要意义。
4. 结论
（1）基于氢氧化镁化学示踪方法具有以下特征，决定了其可用于海床细颗粒渗流测量。首先，氢氧化镁示踪剂的颗粒密度、粒径与海床黏粒接近，难溶于水、不与海床土发生反应、物理化学性质稳定。其次，示踪剂为白色，与海床土颜色形成鲜明对比，能够清晰指示渗流通道。再者，将混有示踪剂的土样与盐酸充分反应，经离子色谱仪测定溶液镁离子，即可得到氢氧化镁示踪剂含量，测定方法简便、精确。
（2）动荷载作用下粉土海床内部超孔隙水压力及垂直床面向上的压力梯度是细颗粒渗流的驱动力，本文化学示踪方法清晰指示了细颗粒自示踪层向上渗流输运及其发展过程，测量得到的海床内部至床面细颗粒渗流量逐层递减的趋势，以及示踪物大部分留在原示踪层位置、渗流通道规模自下而上逐渐减小、仅个别渗流通道连通到床面的试验现象，从实验室层面实现了海床细颗粒垂向渗流的定量解释，有望为进一步揭示波浪作用下粉土海床液化、渗流、再悬浮机理并实现定量评价提供新的方法支持。
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图 1 西南印度洋地理位置及地形图
a. 西南印度洋地理位置图，b. 西南印度洋脊地形图和岩石类型分布图，c. 西南印度洋自由空气大地水准面图（改编自文献[10]）。EGM96：地球引力模型1996；红色箭头指示洋脊半扩张速率，分别是7.1和7.3 mm/a。
Figure 1. Geographical location and topographic map of Southwest Indian Ocean
a. Geographical location map of Southwest Indian Ocean, b. Topographic map and rock type distribution map of Southwest Indian Ridge, c. Free air geoid map of the Southwest Indian Ocean （modified from reference [10]）. EGM96: Earth Gravitational Model 1996. Half spreading rate marked by red arrows, which is 7.1 and 7.3 mm / a, respectively.
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图 2 西南印度洋中脊玄武岩沿洋脊延伸方向同位素比值变化图
a. 横向红色虚线代表MORB参考线87Sr/86Sr =0.7028，b. 横向红色虚线代表MORB参考线143Nd/144Nd =0.5129，c. 横向红色虚线代表MORB参考线206Pb/204Pb =18.4，e. 横向红色虚线代表MORB参考线3He/4He =8RA（RA =空气中的3He/4He）。Sr-Nd-Pb同位素数据来自文献[27]，其余来自PetDB数据库（http://www.earthchem.org/petdb/）。He同位素数据来自文献[30-32]。图中灰色实线为断裂带位置；大写字母表示断裂带名称（简写）。图中右侧色棒分别代表罗德里格斯三联点、中印度洋脊、东南印度洋脊玄武岩对应的同位素变化范围。
Figure 2. Variations in isotopic ratios of MORBs along SWIR
a. The horizontal red dotted line represents MORB reference line of 87Sr/86Sr=0.7028; b. The horizontal red dotted line represents MORB reference line of 143Nd/144Nd=0.5129; c. The horizontal red dotted line represents MORB reference line of 206Pb/204Pb=18.4; e. The horizontal red dotted line represents the MORB reference line of 3He/4He=8RA （RA=3He/4He in air）. Sr-Nd-Pb isotopic data are from reference [27], and others are from petdb database （http://www.earthchem.org/petdb/）. He isotope data are from reference [30-32]. The gray solid line in the figure shows the location of the fault zone; Capital letters indicate the name of the fault zone （abbreviation）.Solid bars on right-hand side represent the range of isotope variations reported for Rodrigues Triple Junction (RTJ), Central Indian Ridge (CIR), and Southeast Indian Ridge (SEIR) MORB, respectively.
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图 3 ODP 735B孔辉长岩随深度变化剖面
a. 全岩Mg# 随深度变化剖面（改编自文献[37]），不同符号代表不同类型辉长岩；b-d. 全岩87Sr/86Sr-143Nd/144Nd-206Pb/204Pb比值随深度变化剖面；e. 长石δ18O值随深度变化剖面。Sr-Nd-Pb同位素数据来自文献[40-41]；氧同位素数据来自文献[39, 42]。
Figure 3. Geochemical characteristics for gabbros versus depth from Hole ODP 735B
a. whole rock Mg# versus depth profile （modified from reference [37]）, different symbols represent different types of gabbros; b-d. whole rock 87Sr/86Sr-143Nd/144Nd-206Pb/204Pb profile with depth; e. feldspar δ18O profile with depth. Sr-Nd-Pb isotope data are from references [40-41]; Oxygen isotope data are from references [39, 42].
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图 4 西南印度洋中脊橄榄岩和对应的洋中脊玄武岩Nd同位素组成沿洋中脊延伸方向变化特征[48]
绿色星号橄榄岩数据来自文献[47]，绿色加号橄榄岩数据来自文献[50]，其余橄榄岩数据来自文献[44]。玄武岩数据来自PetDB数据库（http://www.earthchem.org/petdb/）。
Figure 4. Variations in Nd isotopic compositions of peridotites and corresponding MORBs along SWIR[48]
Green stars peridotite data are from reference [47], green plusses peridotite data are from reference [50], and other peridotite data are from reference [44]. Basalt data is from petDB database （http://www.earthchem.org/petDB/）.
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图 5 西南印度洋中脊不同脊段的岩浆供应及地壳增生模式
a. 9°～25°E岩浆脊段及“有效脊段”。红色箭头代表每个岩浆脊段下方熔体集中的方向和岩石圈底部的坡度；灰色区域代表每个脊段熔岩相对富集的程度（改编自文献[18]）。b. 在慢速扩张脊，主要的岩浆脊段与二级构造脊段对应（图中标注数字“2”的直线）。熔体的高度集中（小的水平和垂直箭头）导致脊段内地壳厚度的变化，短周期的岩浆体（地壳内的椭圆填充）在洋中脊中段形成，并由此形成了较厚的洋壳（改编自文献[8]）。
Figure 5. The model of magma supply and crustal accretion in different segments of SWIR
a. 9°～25°E magmatic segmentation and associated “effective segmentation”. Red arrows indicate simplified direction of melt focusing beneath each magmatic segment and theoretical slope of lithospheric base; the shade of gray reflects the relative enrichment of lavas for each segment （modified from reference [18]）. b. At slow-spreading ridges, principal magmatic segments coincide with second-order tectonic segments （numbered vertical lines indicate discontinuities of orders 1～2）. Strong melt focusing （indicated with small subhorizontal to subvertical arrows） results in large variations in crustal thickness within segments, short-lived crustal magma bodies （filled ellipses in crust） can be formed in mid-segment where thicker crust typically emplaced （modified from reference [8]）.
断裂名称（缩写） 走向 断距/km 活动时期/MaBP 东经 Bouvet (BO) NE 65° 240 0～50 1°55′ Islas Orcadas (IO) NE 65° 100 0～70 6°03′ Shaka (SH） NE 60° 180 0～70 9°30′ DuToit (DT) NE 35° 160 0～70 25°25′ Andrew Bain (AB) NE 40° 720 0～＞120 32°18′ Marion (MA) NE 30° 125 0～＞120 33°40′ Prince Edward (PE) NE 25° 155 0～＞120 35°30′ Eric Simpson (ES) NE 18° 100 0～60 39°20′ Discovery Ⅰ(D Ⅰ) NE 10° 320 0～60 41°50′ Discovery Ⅱ(D Ⅱ) NE 10° 320 0～60 42°30′ Indomed (IN) NE 15° 135 0～60 46°00′ Gallieni (GA) NE 10° 90 0～60 52°20′ Atlantis Ⅱ(A Ⅱ) NE 5° 190 0～50 57°00′ Melville (MEL) NE 5° 125 0～50 60°45′ 
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