天然气水合物成藏条件与富集控制因素

天然气水合物是由烃类气体(主要是甲烷)和水在一定温度、压力条件下形成的一种固态似冰状笼形化合物。与常规油气藏系统不同,天然气水合物成藏的关键因素主要包括天然气水合物稳定条件、水源条件、气源条件、流体运移条件和储集空间条件。天然气水合物的成藏要素决定了天然气水合物稳定带内天然气水合物的产出既非连续也非随机,不同地质背景下的天然气水合物有着不同的分布范围和地质特征。天然气水合物成藏特征和富集控制因素体现在以下几方面:天然气水合物形成与分布受温度和压力条件控制,在天然气水合物稳定带内动态成藏;天然气水合物主要赋存在晚中新世以来松散沉积物中,埋藏深度较浅,通常位于海底0~500m;天然气水合物资源丰度普遍较低,大面积分布、局部富集,存在"甜点"核心区;天然气水合物形成气兼具微生物成因和热成因特征,天然气水合物规模化成藏富集有赖于流体运移;天然气水合物以固态形式赋存,生长和赋存模式多样,存在构造型、地层型和复合型天然气水合物藏。

海南岛高峰、保城地区花岗岩年代学、地球化学特征及构造意义

海南岛南部地区遍布晚中生代花岗岩,高峰、保城岩体即为其中的典型代表。应用同位素地质年代学和岩石地球化学分析方法,测得两岩体的年龄分别为(105. 4±3. 7) Ma和(105. 8±2. 4) Ma,岩石硅、碱含量中等,铝饱和指数分别为0. 95～1. 03和1. 05～1. 30,轻稀土富集,重稀土亏损,轻重稀土分馏程度较高,高场强元素亏损,大离子亲石元素富集。斜长石含量> 19%,角闪石含量> 3%,石英含量﹤32%,P2O5与Si O2呈负相关,表明高峰、保城花岗岩体属于Ⅰ型花岗岩。海南岛南部晚中生代花岗岩的大量形成并非孤立现象,该地区早白垩世安山岩-流纹岩组合和白垩纪沉积序列与花岗岩一起构建起较为完整的安第斯型大陆边缘弧体系,而且这一活动陆缘体系在岩石组合、展布和演化特征上与时代大体相当的浙闽地区活动陆缘体系存在明显区别,在区域构造意义的研究中值得给予特别关注。

粤桂地区白垩纪中期安山岩年代学、地球化学特征及其构造意义

华南沿海地区存在丰富的白垩纪岩浆活动记录。粤桂地区作为华南沿海的重要组成部分,其白垩纪安山岩的出露数量虽然有限,但对于揭示晚中生代大陆边缘的构造性质具有特殊意义。选取广西玉林、广东连平的安山岩作为研究对象,结合前人对广东仁化地区样品的研究成果,应用岩相学、同位素年代学、地球化学等多种分析方法,对上述岩石开展系统研究。结果显示:(1)安山岩具斑状结构,斑晶主要为角闪石、辉石和斜长石,基质主要为斜长石微晶;(2)玉林安山岩U-Pb谐和年龄为(93.38±0.83)Ma,略晚于仁化的105 Ma,白垩纪中期粤桂地区有众多安山岩形成;(3)安山岩高MgO含量和Mg^(#)值,FeO^(T)/MgO值较低,富集轻稀土元素和LILEs,但亏损HFSEs。研究区安山岩由俯冲沉积物部分熔融形成的硅质熔体与地幔橄榄岩平衡反应所形成。高镁安山岩与邻近区域的埃达克质岩石基本同时产出,显示与受太平洋域控制的浙闽地区在构造环境上存在明显差别。据此推测,南海北部陆缘在晚中生代时可能受到来自新特提斯域洋脊俯冲的影响。

洋中脊富钛铁氧化物辉长岩成因的一个新思路:以西南印度洋脊ODP 735B岩心为例

西南印度洋脊的ODP 735B岩心中拥有大量富含钛铁氧化物的含氧化物辉长岩,此现象在岩性单元Ⅳ中表现尤为显著。同时,岩性单元Ⅳ中主要硅酸盐矿物亦具有独特的成分变化趋势。以前研究将造成岩性单元Ⅳ中氧化物辉长岩成因和主要造岩矿物成分变化趋势归因于具浮力的、高度分异的富钛铁熔体的作用,这些熔体应形成于735B钻孔之下或其附近,沿剪切带运移至剖面中,并与原岩矿物发生反应。通过对岩性单元Ⅳ氧化物辉长岩进行岩相学特征分析和结构可控的电子探针矿物成分测试,发现当某些因外来物质加入改造而成分进化的硅酸盐矿物形成之时,液相体系中钛铁氧化物并未处于饱和状态。此外,在单斜辉石结晶过程中,单斜辉石内部成分具有从普通辉石—易变辉石—普通辉石的变化,而且,在与某些后期生成的矿物(如钛铁氧化物或斜方辉石等)接触处,单斜辉石则表现出明显进化组成特征。根据岩相学特征和矿物电子探针数据,并结合对前人提出的富钛铁熔体的物理性质的讨论,本文认为在岩性单元Ⅳ形成过程中,更有可能是源自地幔的外来含水流体/含水流体与熔浆的混合体,而非仅仅是极度分异的玄武质残余熔体,参与了岩性单元Ⅳ中氧化物辉长岩的形成。从理论上讲,这些外来含水流体可能与玄武质熔体并非来自同一源区,这意味着其运移方式可能不仅有垂向运移,而且有可能为远距离侧向运移。岩性单元Ⅳ中的氧化物辉长岩可能具有相当独特的同位素特征,这可能会成为判别岩性单元Ⅳ乃至是整个735B岩心中氧化物辉长岩成因的重要线索。

Natural thermoluminescence of fossil foraminiferals as a potential proxy for deep-sea temperature changes

Foraminiferal shells from two piston cores separately located at the Ninetyeast Ridge and the Bengal Fan of the Indian Ocean were selected and purified for measurements of natural thermoluminescence （NTL） intensity by a high precision thermoluminescence meter （RGD-3）. Variations of the NTL intensity along the two core sequences both spanning the last two glacial--interglacial cycles displayed a strong, identical signal of the global ice volume cycles, which matched well with their corresponding oxygen isotope data. As higher NTL intensity occurred within interglacial periods and changes in an NTL signal were most likely influenced by the temperature of ambient seawater in which the planktonic foraminiferal shells long existed, the NTL signal could be considered as a potential proxy for orbital scale temperature changes of bottom seawater in the tropical Indian Ocean.

Geochemical variation of volcanic rocks from the South China Sea and neighboring land: Implication for magmatic process and mantle structure

Geochemical study of Kon Tum（KT） plateau, Sanshui（SS） basin and Daimao（DM） seamount volcanic rocks provides new insight into magmatic processes and characteristics of the mantle source beneath the South China Sea（SCS） basin and its surroundings. Geological signature of basaltic lavas from KT, SS and DM indicate rather than a deep-rooted plume derived from the core-mantle boundary, a shallower mantle domain, such as subcontinental lithosphere mantle or asthenospheric mantle, is more likely to be the mantle source region beneath the SCS basin and its adjacent areas. The mantle source beneath the SCS basin has been shown to be more depleted relative to the source regions of the SS basin and the KT plateau, indicating that the magmatism in the SS basin has been interrupted due to the SCS spreading and then recovered in the KT plateau area. The mantle heterogeneity has been sampled and an enriched component that is geochemically comparable to EM2 endmember has been identified in the mantle source region of the SCS basin and its surroundings. This EM2 component was formed by the recycling of Mesozoic subducted proto-SCS slab along with terrigenous sediments.

山东黑山岛近海风电场海底地形地貌及沉积特征

山东海域海上风资源丰富,风电规模化开发条件好。为了解海底地形地貌及沉积物分布特征,以顺利推进近海风电场的开发,选取黑山岛附近海域作为研究对象,在研究海域进行了综合地质地球物理调查。对调查数据的分析表明,研究区域海底地形平缓,整体水体较浅,最浅10 m,最深25 m,平均17.5 m。研究区内沉积物主要呈青灰色、灰色、浅褐色。沉积物类型主要粉砂,粉砂含量均在75%以上,全区分布。沉积物峰态主要表现为平坦、中等、尖锐3种。研究区西区海底表层沉积物分选系数为1.35~1.8,东区海底表层沉积物分选系数为0.9~1.75。整个研究区海域表层沉积物平均粒径在4.3Φ~5.9Φ之间变化,沉积物颗粒较细,以粉砂为主。分析表明,这种地形地貌及沉积物特点反映出研究海域属于离岸海域,水动力作用较弱,适于进行海上风电开发等工程建设。

Paleoenvironmental implications of the calcium isotope characteristics in the MD81349 from the Nintyeast Ridge in the Indian Ocean

A δ^（44） Ca curve from shells of the planktonic foraminifera Globigerinoides sacculifer in calcareous biogenic oozes has been extracted from the Nintyeast Ridge in the Indian Ocean since 300 ka. By combining terrigenous inputs（e.g., grain size, magnetic susceptibility, and turbidite frequency） with the oceanic productivity（e.g., biogenic content and Neogloboquadrina dutertrei content）, it is found that the curve＇s variations are closely related to the historical evolution of the oceanic calcium cycle. The δ^（44）Ca value is in lower tendency and has small oscillation during Marine Isotope Stage（MIS） 6, when the supply of terrigenous detrital is highest. In contrast, during MIS 3,5 and 7, the δ^（44） Ca values are in higher tendency, and their fluctuations are consistent with the variations of the productivity proxies. These results suggest that the calcium isotopes are mainly influenced by the input of the Himalayan erosion products to the northern Indian Ocean. In addition, the developmental stages of calcareous planktons may have a secondary impact on the fluctuations of the calcium isotope ratio of sea water.

多金属结壳Os同位素组成区域性和阶段性偏高异常

Os同位素地层法可以得到包含生长间断期的分辨率较高的结壳年代框架,相对传统定年方法具有更优的发展前景.在比对多金属结壳分层Os同位素组成曲线与大洋海水标准曲线从而为多金属结壳定年的同时,研究标准曲线的区域性特征及其与壳源、幔源物质供应的关系.依据比值和趋势贴合原则,得到的多金属结壳生长期包括80~75 Ma、70~65 Ma、60~50 Ma、42~40 Ma、35~28 Ma、15~10 Ma、8~0 Ma.在中、西太平洋地区,麦哲伦和马绍尔海山区结壳Os同位素组成测定值与标准曲线形态和取值贴合良好,而马尔库斯威克和莱恩海山区结壳测定值虽与标准曲线趋势较为贴合,但分别在7 Ma以来和33 Ma以前相对标准曲线取值异常偏高.这种异常偏高现象的出现与2个海山区进、出盛行西风带和赤道信风带的过程有关,结壳在处于风带时接收到风力输运的美洲和亚洲陆源风尘物质,使得其记录的Os同位素组成相对标准曲线偏高.

Os isotope dating and growth hiatuses of Co-rich crust from central Pacific

Up to now, accurate determination of the growth age and hiatuses of the Co-rich crust is still a difficult work, which constrains the researches on the genesis, growth process, controlling factors, regional tectonics, paleo-oceanographic background, etc. of the Co-rich crust. This paper describes our work in determining the initial growth age of the Co-rich crust to be of the late Cretaceous Campanian Stage (about 75-80 Ma), by selecting the Co-rich crust with clear multi-layer structures in a central Pacific seamount for layer-by-layer sample analysis and using a number of chronological methods, such as Co flux dating, dating by correlation with 187Os/188Os evolution curves of seawater, and stratigraphic divi- sion by calcareous nannofossils. We have also discovered growth hiatuses with different time intervals in the early Paleocene, middle Eocene, late Eocene and early-middle Miocene, respectively. These re- sults have provided an important age background for further researches on the Co-rich crust growth process and the paleo-oceanographic environment evolution thereby revealed in the said region.

Determination on ^(87)Sr/^(86)Sr ratio and stratigraphic dating of single-grain foraminifera

The Sr isotopic composition of foraminifera has been used in studies of chronstratigraphy and sedi- mentary geochemistry. However, it is difficult to sample foraminifera without overprinting of later diagenesis, which contains enough amount of Sr for isotopic measurement. Analysis of single-grain foraminifera was successively conducted by a combination of low Sr procedural blank and high precision determination of 87Sr/86Sr ratio. This combi- nation of single-grain foraminifera can be significant for the study and application of Sr isotopic composition of fo- raminifera. Measured foraminifera of this study, having a size between 250 μm and 400 μm, were sampled from the carbonate basement of a ferromanganese crust in western Pacific. Mean value of 87Sr/86Sr ratios of the analyzed sin- gle-grain foraminifera is 0.709150 ±0.000013 with internal precision between 4 ppm and 8 ppm for each measurement run. Average value of total procedural blank of Sr is 14 pg. It is significant but difficult to date foraminifera in carbonate basements of ferromanganese crusts. Based on the ob- tained value of 87Sr/86Sr ratio, the analyzed foraminifera can be dated at 0.91 +0.33/?0.39 Ma.

Growth of a polymetallic nodule from northwestern continental margin of the South China Sea and its response to changes in paleoceanographical environment of the late Cenozoic

In the northern South China Sea,the accumulation of enormous quantities of terrigenous sediment during Cenozoic rendered well-developed polymetallic nodules very rare.In this study,we analyzed a polymetallic nodule from the northwestern continental margin of the South China Sea using microscopic mineralogical observation,electron probes,X-ray diffraction(XRD),ICP-MS,and Be isotope dating.We found the nodule's shell layers rich in different types of microstructures,including columnar,laminar,stack-like,petal-like,and porphyritic structures.The major mineral components of the nodule are MnO2.Unlike nodules from the eastern Pacific basin,this nodule has high contents in Fe,Si,Al,and REEs but low contents in Mn,Cu,Co,and Ni.The Mn/Fe ratio is also low and the average REEs content is 1370.4 ppm.There is a strong positive anomaly of Ce;and the Be(beryllium) isotope dating shows the initial time of growth of the nodule to be about 3.29 Ma.The inner compact layer formed from 3.29 Ma to about 1.83 Ma.The laminar and stack-like structures and the low contents of the terrigenous elements such as Fe,Si,REE,and Al indicate the paleoceanographical environment with weak undersea currents and favorable oxidizing conditions.From 1.83 Ma to 0.73 Ma,the growth rate of the nodule increased by about 3%;the microstructures formed during this period are stack-like and columnar.The contents of Si and Al are increased by nearly 10%,indicating an increase of terrigenous sediment input in the northern South China Sea.The content of Ce is decreased by about 16%,indicating a significant weakening of the oxidizing conditions at the seabed.From 0.73 Ma to 0.69 Ma,the growth rate of the nodule rapidly rose up to 8.27 times that of the nodule's average growth rate,and the contents of Fe,Al,and REEs in the layer also increased,forming a loose layer characterized by oolitic,granular,porphyritic,and petal-like structures,indicating the paleoceanographical environment with a high sedimemtation rate and abundant supply of terrigenous sediment in the northern South China Sea.From 0.69 Ma to 0.22 Ma,the growth rate of the nodule suddenly slowed and the outer compact layer formed.Contents of Fe,Si,REE,Al,Mn,Cu,Co,and Ni in this layer were significantly lower than in other layers.The main structures of the layer are laminar and fissure filling structures.These reflect the paleoceanographical environment with stable undersea currents,poor oxidizing conditions,and other conditions not conducive to nodule growth.The growth process of nodule S04-1DG-1 was found to respond sensitively to the changes of the paleoceanographical environment of the northern South China Sea during the late Cenozoic.

The relationship between the growth process of the ferromanganese crusts in the Pacific seamount and Cenozoic ocean evolvement

Base on the Os isotope stratigraphy together with the empirical growth rate models using Co concentrations, the growth ages of the ferromanganese crusts MHD79 and MP3D10 distributed in the seamount of Pacific are confirmed. Through the contrast and research on the previous achievements including ODP Leg 144 and the crusts CD29-2, N5E-06 and N1-15 of the seamount of the Central Pacific, the uniform five growth and growth hiatus periods of them are found, and closely related to the Cenozoic ocean evolvement process. In the Paleocene Carbon Isotope Maximum (PCIM), the rise of the global ocean productivity promoted the growth of the seamount crust; the first growth hiatus (I) of the ferromanganese crust finished. In the Paleocene-Eocene Thermal Maximum (PETM), though the vertical exchange of seawater was weakened, the strong terrestrial chemical weathering led to the input of a great amount of the terrigenous nutrients, which made the bioproductivity rise, so there were no crust hiatuses. During 52-50 Ma, the Early Eocene Optimum Climate (EECO), the two poles were warm, the latitudinal temperature gradient was small, the wind-driven sea circulation and upwelling activity were weak, the terrestrial weathering was also weakened, the open ocean bioproductivity decreased, and the ferromanganese crust had growth hiatus again (II). From early Middle Eocene-Late Eocene, Oligocene, it was a long-term gradually cooling process, the strengthening of the sea circulation and upwelling led to a rise of bioproductivity, and increase of the content of the hydrogenous element Fe, Mn and Co and the biogenous element Cu, Zn, so that was the most favorable stage for the growth of ferromanganese crust (growth periods III and IV) in the studied area. The hiatus III corresponded with the Eocene-Oligocene boundary, is inferred to relate with the global climate transformation, celestial body impact event in the Eocene-Oligocene transition. From the early to the middle Miocene, a large-scale growth hiatus (hiatus period IV) of the ferromanganese crust in the studied area is inferred to relate with temporary warm up climate and ephemeral withdrawal of Antarctic bottom water in the early Miocene. After that, the Antarctic ice sheets extended, the bottom water circumfluence strengthened, the ocean fertility increased, and the once interrupted crust continued to grow in the late Miocene (growth period V).