现代海底超镁铁质岩系热液系统与地质意义

现代海底热液循环与洋中脊地质过程一直是国际洋中脊计划研究的热点.海底热液系统多数都与海底玄武岩及其水-岩反应直接相关,而一类与深海橄榄岩的产出及其蛇纹石化作用有关的海底热液系统——超镁铁质岩系热液系统,以具有高浓度H2和CH4异常而低SiO2浓度为显著特征,主要分布在慢速扩张大西洋中脊和超慢速扩张北冰洋Gakkel洋脊和西南印度洋中脊.超镁铁质岩系热液系统在流体组成、构造背景和硫化物成矿方面与玄武岩热液系统有很大差异,主要表现在地幔来源超镁铁质岩石的普遍出露、喷口流体高的H2和CH4异常以及硫化物中高Co/Ni比值.超镁铁质岩系热液系统的发现丰富了全球洋中脊热液系统的研究内容,对洋中脊地质过程、海底热液活动及其成矿作用研究具有重要意义.     

西南印度洋脊49°39′E热液活动区IODP钻探计划的科学意义

自20世纪70年代以来,现代海底热液活动调查研究一直是地球科学领域研究的热点和焦点.中国大洋19航次第Ⅰ、Ⅱ航段于2007年3月在西南印度洋脊首次发现的海底热液活动区,为IODP在超慢速扩张洋脊热液活动区进一步的调查和研究创造了条件.为了能够更深入地开展该热液区的地质地球物理、矿产资源和生物多样性调查研究,笔者建议应在该区域开展IODP钻探研究,以便对热液活动区硫化物矿体、基岩及深部物质的三维分布和结构物质组成,热液流体与岩石、沉积物相互作用关系,以及热液循环、热及物质通量和成矿机制、成矿物质来源演化关系等方面得到更深入的认识,进而探索地球深部极端环境下的微生物生命现象.     

西南印度洋中脊超慢速扩张的构造和岩浆活动特征

西南印度洋中脊是当前研究超慢速扩张中脊岩石圈增生模式和热液硫化物矿产资源的热点区域.本研究从西南印度洋中脊的扩张速率变化与全球地质历史事件的关系、岩浆活动强度及其不同尺度上的运移规律、热点-中脊交互作用的地质现象和地球物理特征等方面,系统总结了有关西南印度洋中脊的最新认识.针对以上内容,提出了进一步工作建议,如调查航次的区域与方法、岩浆-构造活动与岩石圈状态的相互关系以及热点-中脊交互作用的动力学机制.     

西南印度洋脊49°39′E热液活动区IODP钻探计划的科学意义

自20世纪70年代以来,现代海底热液活动调查研究一直是地球科学领域研究的热点和焦点。中国大洋19航次第Ⅰ、Ⅱ航段于2007年3月在西南印度洋脊首次发现的海底热液活动区,为IODP在超慢速扩张洋脊热液活动区进一步的调查和研究创造了条件。为了能够更深入地开展该热液区的地质地球物理、矿产资源和生物多样性调查研究,笔者建议应在该区域开展IODP钻探研究,以便对热液活动区硫化物矿体、基岩及深部物质的三维分布和结构物质组成,热液流体与岩石、沉积物相互作用关系,以及热液循环、热及物质通量和成矿机制、成矿物质来源演化关系等方面得到更深入的认识,进而探索地球深部极端环境下的微生物生命现象。     

西南印度洋中脊51°E(#26洋脊段)地质特征及其对海底热液活动的指示

在海底多金属硫化物调查过程中，对于已知热液区需要精细地质填图工作，而这种填图往往存在覆盖面积较小、所包含的地质要素较少以及与区域构造作用和岩浆活动联系不足等问题。基于历年大洋航次在西南印度洋中脊#26洋脊段（51°E区域）所获得的深海光学拖曳系统的资料，结合高精度多波束水深数据，提出一种系统的底质热液异常划分原则，识别出热液蚀变岩石或角砾、疑似热液沉积物、热液生物及其遗骸富集和胶结碳酸盐4种底质热液异常类型。高分辨率海底地质填图结果表明，本区存在4处热液活动的异常区。#26洋脊段热液活动频率值为2～10，至少是全球海底热液活动频率经验公式的1.8倍以上。我们认为在超慢速扩张脊局部熔融异常或者发育非转换不连续带的洋脊段，可能存在更多的海底热液活动，也具有形成大型多金属硫化物矿床的潜力。     

我国海底热液活动研究的阶段性进展与展望

海底热液活动调查研究是深海进入、深海探测和深海开发的切入点之一。近十年来,中国在西太平洋弧后盆地、东太平洋海隆、大西洋洋中脊和印度洋脊,发现了一批新的海底热液活动区,围绕着热液活动区的硫化物、流体、热液柱、生物等热液产物开展了调查研究,构建了海底热液地质学,提出了热液活动、冷泉及天然气水合物的同源异汇假说,出版了《海底热液地质学》、《现代海底热液硫化物成矿地质学》、《现代海底热液活动》、《东太平洋海隆热液地质》专著,获得了一批调查研究成果。未来,聚焦海底热液活动的深部过程及其资源环境效应关键问题,发展海底热液活动探测技术,拓展极地海底热液活动调查研究新领域,围绕烟囱体、热液柱、含金属沉积物、流体以及热液区生物等热液产物,开展深入、系统的调查研究工作,无疑将推动海底热液地质学取得新的进展。     

热液喷口探测化学传感器的研制及应用

海底热液喷口周围的水体具有显著的浊度和化学组分异常,是寻找热液喷口的重要标志。文章提出了一种海底热液喷口的探测技术方法,设计了低功耗化学传感器。该化学传感器可搭载在相关平台,实时探测水体的Eh、H_2S、pH及CO_3~(2-)等电位值,结合浊度异常,可以推断热液喷口的位置。在西南印度洋中脊海试结果表明,该化学传感器可有效探测由热液活动产生的水体异常,是一种探测海底热液喷口的有效技术。     

西南印度洋中脊岩石地球化学特征及其岩浆作用研究

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。     

热液喷口探测化学传感器的研制及应用*

海底热液喷口周围的水体具有显著的浊度和化学组分异常, 是寻找热液喷口的重要标志。文章提出了一种海底热液喷口的探测技术方法, 设计了低功耗化学传感器。该化学传感器可搭载在相关平台, 实时探测水体的Eh、H₂S、pH及CO₃^2-等电位值, 结合浊度异常, 可以推断热液喷口的位置。在西南印度洋中脊海试结果表明, 该化学传感器可有效探测由热液活动产生的水体异常, 是一种探测海底热液喷口的有效技术。     

现代海底热液硫化物的成矿序列和指示意义——以印度洋中脊为例

与快速扩张的洋中脊相比,主要由超慢速-慢速扩张洋脊组成的印度洋中脊具有独特的热液硫化物成矿模式.运用高精度矿相显微镜、XRD、电子探针和ICP-AES/MS等测试手段,对印度洋中脊的热液硫化物矿床样品开展了矿物成分、结构构造、地球化学等各方面分析.结果表明,来自中印度洋脊(CIR)艾德蒙德(Edmond)热液区的硫化物A主要由黄铁矿、白铁矿以及黄铜矿构成,其成矿期次可划分为白铁矿-黄铁矿阶段(Ⅰ)、闪锌矿-黄铜矿阶段(Ⅱ)以及后期石英阶段(Ⅲ),成矿流体温度经历了低-高-低的变化;同样来自于艾德蒙德热液区的硫化物B主要矿物成分为黄铁矿、白铁矿和硬石膏,成矿期次划分为硬石膏-白铁矿-黄铁矿阶段(Ⅰ)和胶状黄铁矿-石英(Ⅱ) 2个阶段,流体温度经历了低-高的变化;与之相比,来自西南印度洋脊(SWIR)龙旂热液区的硫化物C主要由纤铁矿、黄铜矿、黄铁矿和白铁矿组成,成矿期次划分为纤铁矿-白铁矿-黄铁矿阶段(Ⅰ)和闪锌矿-黄铜矿(Ⅱ)阶段,后期闪锌矿、黄铜矿的出现反映热液流体温度发生了升高.地球化学特征表明,印度洋中脊的热液硫化物总体为富Fe型,并相对富集Co和Ni元素,而Zn和Cu元素的含量相对较低.此外,取自艾德蒙德热液区的硫化物与EPR 21°N热液硫化物组成非常相似,而与慢速扩张脊TAG相比,Pb、Zn、Ag和Sr元素含量较高,Cu和Fe元素含量则较低.     

北冰洋中脊海底热水活动研究及展望

海底热水活动是当前海洋地质研究的热点,北极是全球变化研究的理想地区,发生在北极地区的海底热水活动不仅对大洋矿产资源研究有着现实意义,而且对全球变化研究也有重要意义。在介绍北冰洋中脊的地质背景和北冰洋Gakkel海岭的海底热水活动特征的基础上,从地球物理、海洋矿产、海洋化学、海洋热结构、海洋生物研究等角度提出了一些研究展望,认为北极地区热水活动的发现和研究必将提升对海底热水活动研究的认识,而且对北冰洋乃至全球的热结构、气候变化以及极端环境下的生命活动的研究起到重要作用。     

Geochemistry of pyrite and chalcopyrite from an active black smoker in 49.6°E Southwest Indian Ridge

Active hydrothermal chimneys, as the product of submarine hydrothermal activity, can be used to determine the fluid evolution and formation process of potential volcanic-hosted massive sulfide deposits. A hard-won specimen from an active hydrothermal chimney was collected in the 49.6°E ultraslow-spreading Southwest Indian Ridge (SWIR) field through a television-guided grab. A geochemical study of prominent sulfide (e.g., pyrite and chalcopyrite) included in this sample was performed using laser ablation inductively coupled plasma mass spectroscopy. The early sulfides produced at low temperature are of disseminated fine-grained anhedral morphology, whereas the late ones with massive, coarse euhedral features precipitated in a high-temperature setting. The systematic variations in the contents of minor and trace elements are apparently related to the crystallization sequence, as well as to texture. Micro-disseminated anhedral sulfides rich in Pb, As, Ni, Ba, Mn, Mo, U, and V were formed during the initial chimney wall growth, whereas those rich in Sn, Se, and Co with massive, coarse euhedral morphology were formed within the late metallogenic stage. The hydrothermal fluid composition has experienced a great change during the chimney growth. Such a conclusion is consistent with that indicated by using principal component analysis, which is a powerful statistical analysis method widely used to project multidimensional datasets (e.g., element contents in different mineral phases) into a few directions. This distribution pattern points to crystallographic controls on minor and trace element uptake during chimney growth, occurring with concomitant variations in the fluid composition evolutionary history. In this pyrite-chalcopyrite-bearing active hydrothermal chimney at the SWIR, the metal concentration and precipitation of sulfides largely occurred at the seafloor as a result of mixing between the upwelling hot hydrothermal fluid and cold seawater. Over the course of mixing, significant variations in metal solubility were caused by changes in temperature, pH, and redox conditions in the parental fluid phase.     

Newly discovered hydrothermal fields along the ultraslow-spreading Southwest Indian Ridge around 63°E

The ultraslow-spreading Southwest Indian Ridge(SWIR) to the east of the Melville fracture zone is characterized by very low melt supply and intensive tectonic activity. Due to its weak thermal budget and extremely slow spreading rate, the easternmost SWIR was considered to be devoid of hydrothermal activity until the discovery of the inactive Mt. Jourdanne hydrothermal field(27°51′S, 63°56′E) in 1998. During the COMRA DY115-20 cruise in2009, two additional hydrothermal fields(i.e., the Tiancheng(27°51′S, 63°55′E) and Tianzuo(27°57′S, 63°32′E)fields) were discovered. Further detailed investigations of these two hydrothermal sites were conducted by Chinese manned submersible Jiaolong in 2014–2015. The Tiancheng filed can be characterized as a lowtemperature(up to 13.2°C) diffuse flow hydrothermal field, and is hosted by fractured basalts with hydrothermal fauna widespread on the seafloor. The Tianzuo hydrothermal field is an inactive sulfide field, which is hosted by ultramafic rocks and controlled by detachment fault. The discovery of the three hydrothermal fields around Segment #11 which receives more melt than the regional average, provided evidence for local enhanced magmatism providing heat source to drive hydrothermal circulation. We further imply that hydrothermal activity and sulfide deposits may be rather promising along the easternmost SWIR.     

Petrophysical characteristics of rocks and sulfides from the SWIR hydrothermal field

Study of petrophysical properties of rocks in seafioor hydrothermal fields has great significance for inves- tigation of seafloor hydrothermal activities, especially for polymetallic sulfides prospecting. In the present study, based on the current experimental conditions, we conducted systematic experiments to measure the magnetic susceptibility, electrical resistivity, porosity, density, as well as acoustic wave velocity of seafloor rocks and sulfides. Subsequently, we measured the physical characteristics of hydrothermal sulfides, basalts and peridotites which were collected from newly discovered seafloor hydrothermal fields at 49.6°E, 50.5°E, 5 1°E, 63.5°E, and 63.9°E of the Southwest Indian Ridge （SWIR）. Previously available and newly collected data were combined to characterize the physical differences between polymetallic sulfides and rocks. We also discussed the impact of hydrothermal alteration on the bedrock and demonstrated how these petrophysical properties of rocks can help in geophysical prospecting of seafloor hydrothermal fields as indicators.     

Geological characteristics of the Qiaoyue Seamount and associated ultramafic-hosted seafloor hydrothermal system (～52.1°E,Southwest Indian Ridge)

Hydrothermal vent incidence was once thought to be proportional to the spreading rate of the mid-ocean ridges (MORs). However, more and more studies have shown that the ultraslow-spreading ridges (e.g., Southwest Indian Ridge (SWIR)) have a relatively higher incidence of hydrothermal venting fields. The Qiaoyue Seamount (52.1°E) is located at the southern side of segment #25 of the SWIR, to the west of the Gallieni transform fault. The Chinese Dayang cruises conducted eight preliminary deep-towed surveys of hydrothermal activity in the area during 2009 and 2018. Here, through comprehensive analyses of the video and photos obtained by the deep-towed platforms, rock samples, and water column turbidity anomalies, a high-temperature, ultramafic-hosted hydrothermal system is predicted on the northern flank of the Qiaoyue Seamount. We propose that this hydrothermal system is most likely to be driven by gabboric intrusions. Efficient hydrothermal circulation channels appear against a backdrop of high rock permeability related to the detachment fault.     

The potential hydrothermal systems unexplored in the Southwest Indian Ocean

Deep-sea hydrothermal vents possess complex ecosystems and abundant metallic mineral deposits valuable to human being. On-axial vents along tectonic plate boundaries have achieved prominent results and obtained huge resources, while nearly 90% of the global mid-ocean ridge and the majority of the off-axial vents buried by thick oceanic sediments within plates remain as relatively undiscovered domains. Based on previous detailed investigations, hydrothermal vents have been mapped along five sections along the Southwest Indian Ridge (SWIR) with different bathymetry, spreading rates, and gravity features, two at the western end (10°–16°E Section B and 16°–25°E Section C) and three at the eastern end (49°–52°E Section D, 52°–61°E Section E and 61°–70°E Section F). Hydrothermal vents along the Sections B, C, E and F with thin oceanic crust are hosted by ultramafic rocks under tectonic-controlled magmatic-starved settings, and hydrothermal vents along the Section D are associated with exceed magmatism. Limited coverage of investigations is provided along the 35°–47°E SWIR (between Marion and Indomed fracture zones) and a lot of research has been done around the Bouvet Island, while no hydrothermal vents has been reported. Analyzing bathymetry, gravity and geochemical data, magmatism settings are favourable for the occurrence of hydrothermal systems along these two sections. An off-axial hydrothermal system in the southern flank of the SWIR that exhibits ultra-thin oceanic crust associated with an oceanic continental transition is postulated to exist along the 100-Ma slow-spreading isochron in the Enderby Basin. A discrete, denser enriched or less depleted mantle beneath the Antarctic Plate is an alternative explanation for the large scale thin oceanic crust concentrated on the southern flank of the SWIR.     

慢速扩张洋中脊热液成矿的典型实例——青藏高原北部德尔尼铜矿地质对比研究

青海德尔尼铜矿是发育在青藏高原北部的块状硫化物矿床,其赋矿围岩为阿尼玛卿蛇绿岩套中的超基性岩,代表了古特提斯洋的残骸。通过对德尔尼铜矿的详细地质解剖,认为其保留了大量海底热液喷流的地质记录,包括:矿体上部普遍发育的薄层喷流岩;多孔状硫化物中保留下来的胶状结构、草莓结构和角砾结构;矿石的主要矿物组成;与黄铁矿碎屑同期形成的方解石、长英质胶结物;与大西洋Rainbow、TAG热液硫化物矿床相似的矿体分带性。根据洋脊玄武岩中TiO2全岩含量估算德尔尼蛇绿岩所代表的这一段古特提斯洋洋中脊为慢速-超慢速扩张洋中脊,半扩张速率为1.1~2.5cm/a。类比现今洋中脊热液硫化物成矿过程,认为德尔尼铜矿经历了海底喷流、冷却保存和俯冲侵位等3个阶段,其中海底喷流阶段可能与洋底核杂岩具有成因联系,俯冲侵位过程中的矿体和超基性岩、玄武岩受逆冲断层控制。与世界上其他陆地上保存的类似矿床相比,德尔尼铜矿时代年轻(石炭纪),矿床结构和构造及其围岩蚀变特征保存完整,是慢速-超慢速洋脊超基性围岩热液成矿作用的典型实例,可以称之为德尔尼型成矿作用,研究其成矿过程,对研究和理解现今洋中脊超基性岩系统热液成矿作用(尤其是深部成矿作用)具有重要的意义。     

海底热液活动的环境与产物

近23年的调查研究,使我们认识到分布于洋中脊、弧后盆地、岛弧和热点等环境的海底热液活动发育在多种围岩类型之上,包括超基性岩石、基性岩石、中性岩石、酸性岩石和沉积物。海底热液活动经历了岩浆去气作用、流体-岩石/沉积物相互作用和流体-海水混合,获取了岩浆、岩石、海水和沉积物的物质,构成了热液循环,产生了高温、低氧、高或低pH值、富含Fe、Mn、Cu、Zn、Pb、Hg、As等元素以及气体组分（甲烷、氢等）的喷口流体,影响了海水、沉积、岩石和生物环境,形成了热液柱、硫化物、含金属沉积物和蚀变岩石等热液产物,组成了海底热液系统。未来,促进海底热液活动探测技术和热液产物测试方法的发展,对海底热液区的岩石、喷口流体、热液柱、硫化物、含金属沉积物以及热液循环、生物活动的持续观测与研究,无疑将为人类探知海底地质过程及生命活动、保护海底热液环境和合理开发利用海底资源提供有力的工作支撑。