Geologic setting of the Snake Pit hydrothermal site: An active vent field on the Mid-Atlantic Ridge

TheSnake Pit Hydrothermal Site lies on the axis of the Mid-Atlantic Ridge at 23°22′ N latitude, about 30 km south of the Kane Transform Intersection. Active ‘black smoker’ vents and a surrounding field of hydrothermal sediment occur at the crest of a laterally extensive neovolcanic ridge. It is one of the first active hydrothermal vent fields to be found on a slow-spreading ridge axis and despite significant differences in its geologic setting from those of the East Pacific Rise, has many similarities to its fast-spreading counterparts. Although preliminary reports have documented many interesting aspects of these vents and their surroundings, new data collected from the manned submersible ALVIN and the deep-towed ANGUS camera system define the regional tectonic setting as well as the local geologic environment of this fascinating area. The Snake Pit vents are located on a local peak of a volcanic constructional ridge at a depth of 3450 m, 700–800 m deeper than vents known from the East Pacific Rise, Galapagos, or Juan de Fuca spreading centers. The vent field is at least 600 m long and up to 200 m wide and is covered by a thick blanket of greenish to yellow-orange hydrothermal sediment. Both active and extinct vents are perched along the crests of steep-sided sulfide mounds that reach heights of over 40 m. High-temperature (350° C) fluids are vented from black smoker chimneys and low-temperature (226° C) fluids seep from sulphide domes and subordinate anhydrite constructions. Water temperatures, flow rates, fluid chemistries, and mineralization are strikingly similar to vents of faster spreading ridge crests; however, a somewhat distinct fauna inhabit the area.     

Large Amplitude Magnetic Anomalies in the Northern Sector of the Powell Basin,NE Antarctic Peninsula

Magnetic profiles obtained during the Hesant 92/93 cruise with the R/V Hesperides show large amplitude anomalies (up to 1000 nT) along a 100 km wide band in the northern margin of the Powell Basin. The anomalies, which are also locally identified in the eastern and western margins, are attributed to the continuation of the two branches of the Antarctic Peninsula Pacific Margin Anomaly (PMA). Interactive modelling of two-dimensional bodies in four profiles oriented NNW-SSE allows us to determine the main features of the magnetic source bodies within the continental crust. These are elongated in a N60/degE trend, and their base is located at a depth exceeding 15 km. Equivalent magnetic susceptibilities mostly between 0.07 and 0.1 (SI) are obtained. These values are consistent with the hypothesis that remanent magnetisation of the magnetic source bodies is sub-parallel to the present geomagnetic field (norÍmally magnetised). The general trends of the bathymetry a nd the geometry of the acoustic basement on multichannel seismic profiles are consistent with the upper surface of magnetic bodies. In order to match the observed anomalies it is also necessary to consider a second tabular shaped body with induced magnetisation in almost all the profiles, which could represent layers 2 and 3 of the oceanic crust of the Powell Basin. Three different geometries of connection between the anomalies in the Powell Basin margins and the PMA branches are discussed. The most plausible one is the occurrence of two branches, although they are closer together than in the Bransfield Strait. The northern branch would continue along the fragments of continental crust of the South Scotia Ridge located at the northern boundary of the Powell Basin, whereas the southern branch would be located only in the eastern and western passive margins of the Powell Basin. The apparent splitting of the southern branch of the anomalous body indicates that it was emplaced before Oligo cene times, when the opening of this basin occurred, and that it was subsequently fragmented during the Cenozoic. A possible time of formation of the PMA body would be during the long Cretaceous normal polarity interval, which also coincides with a peak in magmatic activity along the Antarctic Peninsula.     

东太平洋水合物海岭沉积物中黏土矿物与水合物饱和度相关性研究

为了探讨水合物储层中不同黏土矿物与水合物饱和度的关系,选取位于东太平洋水合物海岭国际大洋钻探计划204航次中的3个钻孔(1245B孔、1244C孔和1251B),开展储层沉积物黏土矿物的测试和综合分析。结果表明,蒙脱石、伊利石、绿泥石为主要黏土矿物(平均含量分别为40.3%、33.4%、21.4%);高岭石为次要黏土矿物(平均4.9%)。伊利石、绿泥石、高岭石三种矿物含量的垂向变化规律相似,但它们与蒙脱石的垂向变化趋势相反。该区水合物饱和度与蒙脱石含量呈正相关,且正相关程度在“细粒岩性”层段较高(R=0.55~0.97);水合物饱和度与伊利石、绿泥石、高岭石则显示负相关。3个钻孔中水合物储层厚度、水合物饱和度在各钻孔中分布的差异性表明水合物饱和度的受控因素复杂,首先是气源和流体迁移与供给,其次为沉积物岩性;而“细粒岩性”层段较高含量蒙脱石与水合物饱和度正相关关系可能代表了更次一级的沉积影响因素。为检验其它海区是否也存在黏土矿物对水合物饱和度的影响,将上述3个钻孔与印度外海Krishna-Godavari盆地17-07P钻孔记录进行对比。结果显示,17-07P孔的砂含量高,在粗砂层蒙脱石含量与水合物饱和度呈正相关,反映两个海域岩性对水合物饱和度控制机理不同。上述“细粒岩性”层段蒙脱石含量和伊利石、绿泥石、高岭石3种黏土矿物含量与水合物饱和度之间相关性的差异,可能因为蒙脱石具有特殊层状结构和表面化学性质(层间表面带负电荷、具有可交换的层间水合阳离子等),有利于促进水合物的形成和富集,因而表现为正相关;而伊利石、绿泥石、高岭石自身属性与蒙脱石不同(吸水膨胀能力、对甲烷的吸附能力较蒙脱石弱),不利于水合物的聚集,表现为负相关。     

跳出南海看南海——新特提斯洋闭合与南海的形成演化

本文总结了笔者参与基金委重大研究计划"南海深海过程演变"的研究成果。我们发现南海和青藏高原都是新特提斯洋闭合的产物,而非前人所说的南海是由青藏高原碰撞导致的中南半岛逃逸所形成。与青藏高原碰撞隆升机制不同,南海是新特提斯闭合后期弧后拉张的结果。新特提斯洋位于北边的欧亚大陆与南面的非洲、印度和澳大利亚板块之间,呈东宽西窄的喇叭型。在西部,新特提斯洋向北的俯冲可能在侏罗纪就开始了,局部形成了弧后盆。约130Ma前,由于凯尔盖朗等大火成岩省的喷发,新特提斯洋脊也开始向北漂移。由于新特提斯洋东部宽度较大,弧后拉张明显,形成了古南海。新特提斯洋闭合过程中一个重大事件是洋脊俯冲:从菲律宾经福建及两广到青藏高原,均有100Ma左右的埃达克岩产出,是洋脊俯冲的产物。其中,菲律宾、福建、广东埃达克岩形成了斑岩铜金矿床;而在青藏高原,埃达克岩虽有矿化,但没有形成大规模的斑岩铜金矿床。同时期,华南出现了一次短暂的大规模挤压事件,与洋脊俯冲契合。这次挤压事件可能导致了古南海闭合的开始。与此同时,青藏高原冈底斯出现高温岩石——埃达克质紫苏花岗岩;其北面有～110Ma短时间内发生的大规模花岗岩事件。考虑到板块重建的结果,这些埃达克岩和华南短时间挤压事件的时空分布显示新特提斯洋脊在约100～110Ma,近似平行于俯冲带俯冲到了欧亚大陆之下;其前片下沉,扰动软流圈,形成大规模岩浆活动;后片则缓慢后撤,于～80Ma形成了A-型花岗岩。这些A-型花岗岩多属于A2型,受到了还原性板块俯冲的影响而普遍含锡,形成了全球60%的锡矿。俯冲板片的后撤,导致了拉张,可以合理解释南海北缘的"神狐运动"。随着俯冲板片后撤,俯冲角度加大,形成新的弧后拉张,于～33Ma出现洋壳,形成了南海。青藏高原碰撞引起的物质向东、南、北等各方向逃逸,对东亚大陆的构造格局也产生了重要的影响,但是并非南海拉张的主要控制因素。到～23Ma时,东经九十度海岭的俯冲阻挡了青藏高原下方地幔物质向东南方向逃逸,改变了东亚构造格局。同时,由于该海岭俯冲产生的向北东方向的挤压,造成印支半岛向西南挠曲,导致南海洋脊产生向南的跃迁。     

Paleocene on-spreading-axis hotspot volcanism along the Ninetyeast Ridge: An interaction between the Kerguelen hotspot and the Wharton spreading center

Investigations of three plausible tectonic settings of the Kerguelen hotspot relative to the Wharton spreading center evoke the on-spreading-axis hotspot volcanism of Paleocene (60-54 Ma) age along the Ninetyeast Ridge. The hypothesis is consistent with magnetic lineations and abandoned spreading centers of the eastern Indian Ocean and seismic structure and radiometric dates of the Ninetyeast Ridge. Furthermore, it is supported by the occurrence of oceanic andesites at Deep Sea Drilling Project (DSDP) Site 214, isotopically heterogeneous basalts at Ocean Drilling Program (ODP) Site 757 of approximately the same age (59-58 Ma) at both sites. Intermix basalts generated by plume-mid-ocean ridge (MOR) interaction, exist between 11° and 17°S along the Ninetyeast Ridge. A comparison of age profile along the Ninetyeast Ridge between ODP Sites 758 (82 Ma) and 756 (43 Ma) with similarly aged oceanic crust in the Central Indian Basin and Wharton Basin reveals the existence of extra oceanic crust spanning 11° latitude beneath the Ninetyeast Ridge. The extra crust is attributed to the transfer of lithospheric blocks from the Antarctic plate to the Indian plate through a series of southward ridge jumps at about 65, 54 and 42 Ma. Emplacement of volcanic rocks on the extra crust resulted from rapid northward motion (absolute) of the Indian plate. The Ninetyeast Ridge was originated when the spreading centers of the Wharton Ridge were absolutely moving northward with respect to a relatively stationary Kerguelen hotspot with multiple southward ridge jumps. In the process, the spreading center coincided with the Kerguelen hotspot and took place on-spreading-axis volcanism along the Ninetyeast Ridge.     

大西洋中脊TAG热液活动区中热液沉积物的稀土元素地球化学特征

用ＩＣＰ－ＭＳ对ＴＡＧ热液活动区表层热液沉积物中６个块状硫化物样品进行了稀土元素分析。所有样品均表现出正Ｅｕ异常（ＥｕＮ／Ｅｕ＊Ｎ＝１．２７～２．６８）和ＬＲＥＥ相对富集（ＬａＮ／ＹｂＮ＝１５．７９～４７．６２）的球粒陨石标准化配分模式,热液沉积物样品中稀土元素组成的变化是由于海底热液循环体系中热液流体和海水不同程度混合作用的结果,并且,海水与热液流体的混合作用是在热液沉积物形成以前发生的,与黑烟囱流体的稀土元素配分模式对比,表明热液沉积物的ＲＥＥ部分来自下伏的玄武岩基底,在热液沉积物的形成过程中,ＬＲＥＥ得到了较高程度的富集,同时,海底热液沉积物的稀土元素中Ｅｕ的变化可以一定程度的反映出热液流体的演化特征。     

新疆主要地震区pp回归综合预报模型研究

应用基于ｐｐ回归理论的数值型地震综合预报软件系统,选择了ｂ值、Ｃｂ值、Ａｂ值等１５个测震学参量,建立了它们与响应变量（未来任意时段内最大地震震级）的ｐｐ回归模型．对新疆４个主要地震区分别给出了各区的建模岭函数及综合预报模型．回顾检验及实际预报结果均表明该模型的预报效果是较为理想的．     

水合物层下伏游离气渗漏过程的数值模拟及实例分析

海洋环境中天然气水合物层是理想的毛细管封闭层,游离气被抑制在水合物层下,游离气层的气体压力随气体聚集和气层厚度的增加而升高,当气压超过封闭层的毛细管力时,游离气会克服毛细管进入压力、刺入上伏封闭层孔隙空间,毛细管封闭作用随之消失,从而形成水合物下伏游离气向海底的渗漏.通过对该过程进行的数值模拟计算表明：渗漏气体是以活塞式驱动上伏沉积层中的孔隙水向海底排出,水合物稳定带内流体渗漏速度随水流柱高度的减小而增加,当水流阻抗大于相应沉积层段的静岩压力时,沉积层将转变为流沙,流沙沉积被海流移除后便在海底留下凹陷麻坑.麻坑形成后流体运移通道演化为气体通道,气体快速排放.麻坑深度主要取决于游离气层的厚度和水合物封闭层（底界）的深度,而与沉积层的渗透率无关.麻坑深度一定程度上指示了渗漏前水合物层下伏游离气层的资源量.对布莱克海台海底麻坑深度的数值模拟计算表明,形成4 m深的海底麻坑需要至少22 m厚的游离气层.     

Mantle heterogeneity beneath the Antarctic–Phoenix Ridge off Antarctic Peninsula

Abstract   We determined the Sr, Nd and Pb isotopic compositions of basalts recovered from the Antarctic–Phoenix Ridge (APR), a fossil spreading center in the Drake Passage, Antarctic Ocean, in order to understand the nature of the subridge mantle source. There are no known hotspots in close proximity to the site. We observe that small-scale isotopic heterogeneity exists at a shallow level in the subaxial mantle of the APR. Enriched (E-type) mid-ocean ridge basalts (MORB) coexist with normal (N-type) MORB in this region. The E-type basalts are: (i) relatively young compared to the N-type samples; (ii) were erupted after the extinction of the APR; and (iii) have been generated by low-degree partial melting of an enriched mantle source. Extinction of the APR likely caused the extent of partial melting in this region to decrease. We interpret that the geochemically enriched materials dispersed in the ambient depleted mantle were the first fraction to melt to form the E-type MORB.