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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周期阶坎是海底陆坡常见的地貌单元，以连续的波状底形为特征，是深水沉积体系的重要场所，是现今海洋学研究的热点。周期阶坎一般发育环境分为海底和陆上两类，其中在海底陆坡和坡折位置处向上游迁移的长波状底形最为常见。国外学者Paker和他的助手首先在明渠模拟实验中观察到一系列向上游迁移的台阶状底形，并正式提出“周期阶坎（cyclic steps）”一词[1]。国内钟广法等[2-3]最早发现并报道了南海东北部陆坡区海底峡谷谷底、越岸区和出口部位存在大量超临界流成因的大型沉积物波，将其解释为“周期阶坎”。大多学者对海底周期阶坎进行了研究[2-5]，但对于琼东南地区现今海底周期阶坎的研究颇少。在琼东南地区，以往学者更加关注于海底峡谷的研究，并发现在海底峡谷或水道中常伴生一种类似台阶状的底形，且都有规律地朝一个方向运动，最初认为这种底形为海底沉积物波[6]。然而关于琼东南陵水凹陷现今海底是否为周期阶坎还有待考究，研究区域内阶坎底形和其形成机制有助于理解海底流体活动，并为其他区域海底地貌单元识别提供参考。
本文基于琼东南盆地陵水凹陷浅层285 km2三维地震数据，对周期阶坎的构型和形成原因进行了分析。南海陆坡周期阶坎研究对加深海底地貌单元以及重力流沉积发育控制因素具有重要的意义。
1. 地质概况
琼东南盆地位于南海北部大陆边缘，地质构造复杂多变，属于陆架较窄和陆坡较陡的非典型被动大陆边缘性盆地[7]。物源主要来自越南和海南岛的双物源供给，发育滑塌体、峡谷和海底扇等沉积体系[7-8]。陆坡自西北向东南坡度整体逐渐变缓，陆坡海底发育大量的水道和大规模的重力流沉积[9]。
琼东南盆地陆坡区自西向东按照陆坡的宽度、有无明显的坡折带和陆坡倾角大小，分为盆地西部、盆地中部和盆地东部。陵水凹陷研究区位于琼东南盆地中部、水深700～1500 m的上陆坡区，坡度大约为2°～16°（图1a）。
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L1测线号6053，研究区最西侧；L2、L3测线号分别为6353、6653，依次靠近水道左侧区域；L4测线号6953，左侧水道壁附近；L5测线号7253，水道内部；L6测线号7553，右侧水道壁附近；L7和L8测线号分别为7854、8147，研究区最东侧。L1 inline 6053, on the westernmost side of the study area; L2, L3 inline 6353 and 6653, respectively, close to the left side of the channel; L4 inline 6953, near the left side of the channel wall; L5 inline 7253, channel Inside; L6 inline 7553, on the right side of the channel wall; L7 and L8 inline are 7854 and 8147 respectively, the easternmost side of the study area.2. 数据和方法
本研究所使用的数据主要源于中国海洋石油有限公司从琼东南盆地获得并拥有的约300 km2的三维地震数据。研究区三维地震数据面元大小为12.5 m×12.5 m（Inline×Crossline），采样率为2 ms，频带宽度为6～90 Hz，主频约45 Hz，最大垂向分辨率（λ/4）约15 m。
利用GeoFrame软件对研究区现今海底浅层目的层同相轴进行解释，解释的网格精度为10×10（CDP），并将解释的层位进行时深转换，提取地震属性倾角、方位角和均方根（图2a为倾角属性平面图）等。利用解释的三维地震层位数据结合Surfer软件绘制了现今海底地形图（图1b）。研究区海底地形图揭示现今海底发育阶梯状地貌，陆坡上存在多条小水道和一条清晰的大型水道（由两支小水道复合而成即水道复合体），水道壁附近台阶形态杂乱（图1b））。
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图 2 琼东南盆地现今海底阶梯地形分析图a. 倾角属性平面图（红色线是地震剖面所在位置图），b. 典型地震剖面图，c. 阶梯几何构型图。Figure 2. Present submarine cyclic step topography of the Qiongdongnan Basina. dip_ attribute map, b. typical seismic profile, c. stepped geometric configuration diagram.3. 周期阶坎底形的识别及特征描述
周期阶坎的识别依据主要基于三点：① 周期阶坎经常发育在高坡度和坡折带区域[10-16]；② 当坡度超过0.6°时，浊流可能达到超临界流状态[16-18]；③ 周期阶坎发育的波长较长，多为链状，不对称，向上游迁移，形态似台阶状[16-17, 19]。研究区发育的底形所处位置、坡度以及形态特征与形成“周期阶坎”的条件吻合。
NW-SE向剖面上周期阶坎（图2b）表现为波状亚平行结构地震相，多组同相轴互相平行并呈波浪状起伏，波形起伏较小，不对称，波脊逆坡迁移。波状底形是由波脊和波谷相间构成，似阶梯状，高度为6～10 m。周期阶坎类型为长波形、不对称、似正弦曲线多数向上游迁移，部分向下游迁移的新月形。
由于海水与地层之间的密度差，上下地层的振幅反射特征发生了明显变化，波阻抗系数增大，因此地震同相轴连续性较好，利于研究周期阶坎的具体形态特征。选取8条测线（图1b中L1—L8）计算和分析每一测线上周期阶坎的几何构型（图2c）。使用几何构型参数（波长（L）、倾角（θ）、波高（H）、迎流面长度（Lstep）、背流面长度（Llee）、迎流面夹角（α）、背流面夹角（β）以及迎流面与背流面长度的比值（R）），刻画海底阶梯形态。
数据统计结果表明：单个周期阶坎波高4～10 m，波长20～150 m，倾角2°～14°，波长/波高为4～30。研究区自西向东（自L1到L8）阶坎波长随着坡度变缓依次增加（图3），即区域内周期阶坎之间的间距不等。研究区水道内部的阶坎波长（L5）较水道外部波长变化曲线趋势更加明显。水道壁附近（L4、L6）波长随深度变化数据有跳跃变化。单个周期阶坎的迎流面与背流面随深度变化：① 长度：迎流面长20～140 m，背流面长10～40 m。迎流面长度波动范围更广，背流面长度变化则更加集中（图4）；② 角度：迎流面角度0.1°～0.15°，背流面角度0.2°～0.8°。整个曲线趋势总体是背流面角度远大于迎流面，极少数迎流面角度大于背流面角度。随深度变化整个8条测线上的周期阶坎：① 迎流面和背流面长度均逐渐增加，但迎流面长度增长趋势远大于背流面；② 迎流面角度变化集中，背流面角度波动范围更广。
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4. 讨论
琼东南盆地陵水凹陷现今海底开放陆坡上分布成片大面积的周期阶坎底形是如何形成的？有哪些有利条件导致了这些底形的形成？针对这些问题，在前人的研究基础之上综合研究区所处地质环境对形成区域周期阶坎底形的形成过程和成因机制进行了详细的探究。陵水凹陷现今海底发育的周期阶坎底形分布范围广、数量多、类型多而集中。
4.1 周期阶坎形成过程
周期阶坎的形成与流体动力学有关。区域内发育的周期阶坎底形是由于浊流携带的陆源碎屑物质通过海底峡谷或水道重力流流体通道从陆架途径陆坡向下游搬运，在搬运途中随流体动力学参数弗劳德数（
$Fr = U/\sqrt {RCgD} $ ，其中，R：浊流中沉积物折算密度，C：浊流中平均体积浓度，D：浊流的厚度）变化而形成的一种阶梯状底形[20]。研究区成片大面积分布的周期阶坎底形足够说明水流携带的泥沙等碎屑物质能量强。靠近上陆坡区，浊流能量强，单位时间内形成的周期阶坎底形数目多，在立体图上显示出它们之间排列更加紧凑（图1b）。靠近下陆坡区，坡度减小，水动力减弱，沉积物沉积厚度变薄，单位时间形成的周期阶坎数目减小，故形成的周期阶坎底形在下陆坡间距增大（图1b）。单独一条测线上形成的周期阶坎形态类同，但大小不同。一个完整的周期阶坎形成，需要两种水流流态：① 水流携带的泥沙等碎屑物质从阶梯底部向顶部运动过程中，由于水流流速不断地减小，流体从上一个阶段的超高速超临界流结束不断地向亚临界流转化，一部分水流动能被紊流消散，剩余的动能转化为位能，会导致液面升高，即水流产生的惯性小于流体自身的重力，形成亚临界流，此时Fr＜1[4, 21]。② 当Fr＞1时，水流向下游流动的惯性大于向上游传播的波速，产生超临界流，此时不可能有向上游移动的波[22]，从而形成长波长的迎流面。理想状态下，流体从第一个阶坎底部向顶部水力跳跃完成第一个周期阶坎，接下来将剩余的能量用来完成第二个周期阶坎，依次类推，直到水跃能量消失，底形将不存在。周期阶坎的形成是浊流从超临界流到亚临界流过程中水力跃变形成的底形。周期阶坎由一系列连续的陡峭背流面和平缓迎流面组成，并且经常在背流面与迎流面转换区域形成冲沟或深坑[20]。依据侵蚀作用的强弱，分为三种类型：① 沉积型周期阶坎。水流中携带的沉积物很难被带走从而在迎流面卸载，当超临界流流经每个阶梯底部时，向亚临界流转化并产生水跃。水流继续沿着迎流面上倾方向流动，流速逐渐减缓，水流能态进一步降低，水流侵蚀作用被削弱因而以沉积作用为主[18, 23]。② 过渡型周期阶坎。水流能态不足以达到超临界流形成水跃，沉积作用与侵蚀作用相当。③ 侵蚀型周期阶坎。当水流经过阶梯顶点后位能最大，位能沿背流面下倾方向不断加速转化为动能，即水流流速逐渐增大使得浊流携带的碎屑物质很难被保留下来，其水流流态从亚临界流向超临界流转化，因此以侵蚀作用为主，从而形成的阶坎底形背流面陡峭[24]。长波状周期阶坎的形成，迎流面以沉积作用为主，背流面以侵蚀作用为主。
研究区周期阶坎迎流面长度主要大于背流面长度，在规模上属于中型沉积型周期阶坎。随深度的增加，沉积物在平缓的一侧不断加积，使得周期阶坎迎流面一侧长度不断变长。周期阶坎平缓的迎流面和陡峭的背流面，使得沉积物发生了沉积/侵蚀差异作用，最终沉积波向浊流上游方向迁移逐渐形成月牙状[4, 23, 25-26]。另外，迎流面长度与背流面长度比值大于数值1，即周期阶坎底形呈不对称性。周期阶坎迎流面主要是以亚临界流沿上倾方向加速沉积形成的平缓状斜坡，而背流面是超临界流沿下倾方向水跃侵蚀作用形成的陡峭坡[18, 23]，说明迎流面沉积粒度比背流面粗。周期阶坎呈现的有规律的向下游排列的线性构造底形指示沉积物向下游搬运，其沉积波上游方向粒度最粗，向下游方向粒度逐渐变细。
4.2 周期阶坎的成因机制
近年来，众多学者研究认为周期阶坎在海底广泛存在[4, 16, 18]。Cattane等[20]研究认为周期阶坎的形成可能是浊流与原先存在的不规则地形之间相互作用，但Kubo等质疑Cattane等只是简单地夸大了地形对形成周期阶坎的影响，并没有考虑到浊流水力跃变输送的能量。之后，Spinewine等[27]实验结果和Kostic等[24]数值模拟结果均一致表明持续的浊流易于形成周期阶坎，与预先存在的地形没有直接的联系。王海荣等[28]认为浊流形成的沉积波具有迁移特征，周期阶坎的形成是多种成因共同的结果。在前人的研究基础之上，笔者从浊流和坡度两方面对研究区的周期阶坎进行了分析。
4.2.1 浊流
浊流是重力流的一种表现形式，浊流的流体动力学影响了深水沉积体系结构单元的演化，可以分为超临界流和亚临界流两种。浊流内部携带的碎屑物质属于高密度流体，流速一般很大[29]。研究区沉积波的地震剖面显示下切水道十分发育，周期阶坎的形成是浊流作用的结果。
周期阶坎与浊流有关的因素有：① 与浊流中高密度流体密切联系。陵水凹陷现今海底陆坡区靠近物源，地形高差大，偶然的事件沉积（异重流）引起泥砂混杂持续性、密度高的沉积物容易形成周期阶坎底形，且Cartigny数值模拟实验[12]证实，沉积物浓度是发生水力跃变的有力因素。水跃往往发生在沉积物浓度确定的范围内，浓度越大，形成周期阶坎的个数越多，规模越大[14]。② 与浊流粒度和沉积速率有关。丰沛的碎屑物质从“源”系统途经研究区汇入到深海盆地，沉积物沉积速率逐渐降低，沉积物粒度由粗变细。整个周期阶坎的迎流面长度自L1到L8不断地变长，其中沉积物的沉积厚度迎流面比背流面厚，形成的周期阶坎底形往往不稳定。研究区西部沉积物粒度比东部粗，西部沉积物的沉积速率比东部高。高的沉积速率易于波状底形的完整保存，而且容易形成不对称底形[14]。③ 与浊流的流量和流速有关。随流速的增加底形会依次出现：无颗粒运动的平坦床沙、沙纹、沙浪、沙丘、过渡丘（或低角度沙丘）、平坦床沙、周期阶坎、流槽和凹坑[22, 30-31]。随浊流流量的增加，超临界流的侵蚀能力逐渐增强，形成水力跃变的可能性越高，更容易形成周期阶坎底形。在前人理论研究基础之上，笔者认为研究区浊流的流速高而稳定，因此形成了大范围的周期阶坎底形。
周期阶坎的形成、演化过程与发育在陆架边缘斜坡和峡谷水道中的浊流体系密不可分[32]。到目前为止，部分学者已经证实了浊流形成的沉积波能够以周期阶坎底形出现[2, 19, 21, 33]。周期阶坎底形甚至在高山流水、冰川中都可以出现。
4.2.2 坡度
地形坡度是控制超临界流体发生水跃形成周期阶坎底形的关键因素。地形坡度较大时，浊流携带的沉积物位能高，其沿斜坡分力较大，侵蚀作用明显，因此形成侵蚀型和粗粒度的沉积型周期阶坎；随研究区域坡度的变缓，浊流所携带的沉积物能量逐渐递减，流体的搬运能力相应地也逐渐减弱，水力跳跃后在阶梯顶部需要的能量更大，其缓冲距离加大，大量的沉积物开始堆积，粒度较细的沉积物开始沿着上倾斜坡不断地堆积形成长高比不断增大的沉积型周期阶坎[23, 34-35]。鲁勇的水槽实验表明，在坡道转换处会发生水跃，其水跃强度随坡度的增加而增加。在坡度、水流量和体积比浓度相等的情况下，坡上的平均沉积厚度小于坡下平均沉积厚度，即迎流面沉积速率与背流面的沉积速率不同[34]。一般情况下，陡峭的坡度和高密度的弗劳德数有利于周期阶坎的形成[36]。然而，坡度超过0.92°形成周期阶坎底形可能性会降低。坡度越陡，流体重力沿斜坡的分量增大促使流体不断加速，从而抑制流体内产生水跃的能量且增大了转化为亚临界流的可能性[18]。
坡度是超临界流水形成周期阶坎底形的前提要素[3, 18]。在一定的条件下，由陡峭的斜坡过渡到相对平缓的斜坡可能会导致浊流发生水力跃变[37]。
5. 结论
（1）陵水凹陷现今陆坡海底具备的坡度较陡、近物源等为周期阶坎底形的发育提供了有利条件，周期阶坎底形是构成深水沉积体系的重要沉积单元。
（2）陵水凹陷现今海底发育一条宽约6.5 km具有明显侵蚀特征的大型水道，是构成“源-汇”系统的重要通道。水道内外发育的周期阶坎底形在地震剖面上表现为连续波状亚平行强振幅地震相。周期阶坎的发育指示现今海底浊流流速急剧，流量大。
（3）通过对周期阶坎底形形成机制进行分析，认为浊流和坡度是形成周期阶坎的最主要的因素。周期阶坎在坡度适中、浊流发育的地带容易形成。
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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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