东海陆架平北地区残留沉积物磁性特征及其油气指示意义

利用1994～1995年在东海陆架平北地区获取的370个柱状样，对该区残留沉积物作磁性测试和磁性矿物鉴定，以进行陆架残留沉积物磁性特征与深部油气渗漏关系的研究。磁性测试显示本区存在两种磁性异常，即磁化率异常和饱和等温剩磁异常。根据磁性参数特征和多种矿物的物理学鉴定结果，确定导致本区磁化率异常的矿物为陆源碎屑磁铁矿，导致剩磁异常的矿物为自生或次生铁硫化物胶黄铁矿和磁黄铁矿。证据显示本区剩磁异常是油气渗漏的产物；而磁化率异常则主要指示古滨海砂的分布。由此提出，在本区的油气化探中剩磁参数是有效的化探磁性指标。     

罗布泊自生胶黄铁矿对磁性参数χARM/SIRM指示意义的影响

沉积物中磁性矿物的颗粒大小具有重要环境指示意义,是环境磁学研究的重要内容。磁性参数非磁滞剩磁磁化率/饱和等温剩磁（χ_ARM/SIRM）常被用于指示磁性颗粒大小,当沉积物磁性特征为较粗的多畴（MD）颗粒主导时表现为低值,而为较细的单畴（SD）颗粒主导时表现为高值。本研究对罗布泊盐湖LB剖面的沉积物进行了系统环境磁学测试,结果发现剖面中含有5个χ_ARM/SIRM值低于上覆层和下伏层的层位,指示其磁性颗粒较粗。但磁滞参数和扫描电镜结果显示这5个层位磁性颗粒明显细于上覆层和下伏层沉积物,显示χ_ARM/SIRM在这5个层位的指示意义失真。矿物鉴定结果显示这5个层位的主导磁性矿物为早期成岩成因的SD胶黄铁矿,赋存形态主要为颗粒团聚体、单颗粒或颗粒团聚体附着于硅酸盐矿物表面以及分布于薄片状硅酸盐内,这种赋存形态导致其具有较强的磁相互作用,并限制了χ_ARM的增加,最终造成χ_ARM/SIRM下降,指示意义失真。本研究显示当沉积物中存在胶黄铁矿等自生矿物时,利用比值参数χ_ARM/SIRM判断磁性矿物颗粒特征需谨慎。     

冲绳海槽现代沉积物磁学性质分布特征与环境的关系

选取东海陆架至冲绳海槽槽底平原的110个表层沉积样品,通过磁化率、等温剩磁、非磁滞剩磁和饱和等温剩磁等多种磁学参数测量试验手段,分析探讨了沉积物磁性与物质来源及环境的内在联系,指出不同沉积环境中,各磁性参数表现出不同的磁性特征。冲绳海槽东槽底平原沉积物因含有较多的火山物质,磁学特征与其它沉积环境有明显差异,各磁性参数特征值明显较强。CaCO3含量与磁性参数没有明显的相关性,但在高含量区可能对沉积物磁性矿物的含量具有稀释作用,使沉积物表现出相对减弱的磁性特征。     

东海外陆架浅孔沉积物的岩性和磁性特征及成因讨论?

利用东海外陆架的10根柱状样，进行磁性、粒度、氯离子含量和粘土矿物分析。结果表明：在东海外陆架扬子浅滩前缘，浅部沉积地层自上而下可分为两层。上层为砂泥混合物，含大量贝壳碎片，磁化率与饱和等温剩磁较强，主要是磁铁矿富集所致，反映了冰后期海侵对晚更新世沉积物的强烈改造。研究还发现，现代海底表层沉积物中存在铁硫化物，它是浅海还原环境中铁磁性矿物次生变化的产物。下层为坚硬致密的黄褐色泥质粉砂和粉砂质泥（硬土层），磁化率与饱和等温剩磁较弱，但局部存在剩磁异常高，从矫顽力等磁性特征证明它是硫复铁矿的富集所致。另外，在硫复铁矿富集层中绿泥石和氯离子含量明显富集，反映该硬土层是早期海侵沉积物在末次冰期干冷气候条件下的风化产物。     

黄东海陆架及朝鲜海峡泥质沉积物的磁学特征比较研究

黄东海陆架和朝鲜海峡等5个泥质沉积区全新世中期以来(自约6kaB.P.至今)的细粒沉积物主要是黄河、长江和朝鲜半岛的河流入海物质在复杂的海洋动力系统中的沉积产物.对取自山东半岛北部近岸泥质区和长江口以南的闽浙沿岸泥质区(舟山群岛近岸海区)的表层细粒沉积物(粘土质粉砂和粉砂质粘土)开展了详细的岩石磁学测试,包括磁化率、等温剩磁和样品磁化率随温度的变化等分析,并将实验数据与以往研究过的济州岛西南陆架区、南黄海东南部陆架区和朝鲜海峡西岸等3个泥质沉积区细粒沉积物的磁学特征进行比较研究.结果表明,上述5处细粒沉积物的磁性矿物主要组分为磁铁矿、磁赤铁矿和赤铁矿,其中低矫顽力磁性矿物为主,平均粒度为准单畴(PSD).根据磁性矿物的粒度和低矫顽力组分的相对含量,这5处细粒沉积物可以划分为3种类型:第1种类型是朝鲜海峡西岸细粒沉积物,其磁性矿物的粒度最粗,低矫顽力组分的相对含量为中等;第2种类型是南黄海东南部陆架细粒沉积物,其磁性矿物的粒度为中等,低矫顽力组分的相对含量最大;第3种类型是山东半岛北部近岸、闽浙沿岸(舟山群岛近岸)和济州岛西南陆架区的细粒沉积物,其磁性矿物的粒度最细,低矫顽力组分的相对含量最低.在这5处细粒沉积物中,闽浙沿岸(舟山群岛近岸)细粒沉积物的磁性矿物含量最高,其他4处细粒沉积物的磁性矿物含量都较低且大体相当.这种磁学特征的差异反映了物源的不同.从磁学特征来分析,济州岛西南陆架区的泥质沉积物主要来自黄河和长江;南黄海东南部泥质区的主要物源是朝鲜半岛进入黄海的河流,黄河和长江对其没有重要贡献.     

南海东北部岩芯沉积物磁性特征及对甲烷事件的指示

在甲烷渗漏海域,沉积物磁化率通常表现出异常的低值特征,这与硫酸盐-甲烷转换带（SMTZ）内甲烷厌氧氧化反应（AOM）的发育而导致的自生矿物的形成作用有关。通过测定南海东北部Site DH-CL11、Site 973-2、Site 973-4三个站位400个岩芯沉积物样品的磁化率,并结合三个站位自生黄铁矿丰度和硫同位素等数据探讨了南海北部天然气水合物潜在区沉积物磁化率的变化特征及其对甲烷渗漏事件的指示意义。结果表明:在甲烷异常渗漏海域,上涌甲烷与下渗硫酸盐在SMTZ内发生AOM反应生成了大量的HS-,造成亚铁磁性矿物大量溶解,同时生成大量顺磁性自生黄铁矿,导致沉积物磁化率的异常降低;但是,在HS-不足时,铁硫化物黄铁矿化不充分,会优先生成胶黄铁矿,进而出现二次磁信号。在天然气水合物潜在海域,沉积物磁化率的异常特征可以反映下部甲烷通量的变化,从而指示下伏天然气水合物藏演化,因此能够成为探测天然气水合物藏的一种间接有效的手段,将有助于我国南海北部海域天然气水合物的勘探。     

三门峡会兴沟剖面黄土—古土壤序列的岩石磁学研究

岩石磁学是古地磁学和环境磁学研究的基础,是鉴定岩石和沉积物中磁性矿物种类、粒度和含量的有效途径。对黄土高原东南部三门峡盆地的水沟—会兴沟旧石器遗址会兴沟剖面黄土—古土壤序列(S_0~S_8)进行系统的岩石磁学研究表明:本剖面沉积物的主要载磁矿物为磁铁矿、磁赤铁矿和赤铁矿,显示准单畴(PSD)磁性颗粒特征。所有磁学参数曲线均表现出基本一致的变化特征,与深海氧同位素曲线能够很好的对应,反映了第四纪以来的冰期—间冰期旋回中,东亚季风影响下的风尘黄土堆积中磁性矿物种类、粒度和含量的周期性变化特征。黄土中高矫顽力磁性矿物的相对含量要高于古土壤中的,而随着成土作用的加强,在古土壤中细粒的低矫顽力磁性矿物显著增加的同时,其中高矫顽力磁性矿物的绝对含量也相应增加。质量磁化率(χ)与非磁滞剩磁磁化率(χ_(ARM))和饱和等温剩磁(SIRM)及磁粒度参数χ_(ARM)/SIRM和χ_(ARM)/χ均呈明显的正相关关系,表明由成土作用产生的单畴(SD)颗粒和较小PSD颗粒对磁化率增强有显著的贡献。     

渭河流域典型黄土剖面的环境磁学特征及其古气候意义

对黄土高原南部耀县全新世黄土地层的形成及保存环境进行了研究.结果表明:地层中磁性矿物主要为软磁性矿物,硬磁性矿物对地层的磁学性质贡献小;随着成壤强度的增加,硬磁性矿物的相对含量在减少.本区全新世地层的形成和保存环境为氧化环境,适宜用磁学手段反演全新世古气候.对磁化率和频率磁化率、无磁滞剩磁磁化率的对比分析,可以看出:成壤强度较低时,细粒磁性矿物相对于磁性矿物总量的增加速率相对较快;成壤强度较高时,细粒磁性矿物的相对含量会达到或趋近饱和,相对于磁性矿物总量的增加速率相对较慢.研究表明,在黄土地层中,χfd%和χARM/χ等指示细粒磁性矿物的磁学参数可以显示更细微的气候变化;在古土壤中,χlf的气候指示意义相对更好.     

渭河流域典型黄土剖面的环境磁学特征及其古气候意义

对黄土高原南部耀县全新世黄土地层的形成及保存环境进行了研究. 结果表明: 地层中磁性矿物主要为软磁性矿物, 硬磁性矿物对地层的磁学性质贡献小; 随着成壤强度的增加, 硬磁性矿物的相对含量在减少. 本区全新世地层的形成和保存环境为氧化环境, 适宜用磁学手段反演全新世古气候. 对磁化率和频率磁化率、无磁滞剩磁磁化率的对比分析, 可以看出: 成壤强度较低时, 细粒磁性矿物相对于磁性矿物总量的增加速率相对较快; 成壤强度较高时, 细粒磁性矿物的相对含量会达到或趋近饱和, 相对于磁性矿物总量的增加速率相对较慢. 研究表明, 在黄土地层中, χfd%和χARM/χ等指示细粒磁性矿物的磁学参数可以显示更细微的气候变化; 在古土壤中, χlf的气候指示意义相对更好.     

长江口水下三角洲Y7柱样磁性特征及其影响因素

以长江口水下三角洲380cm长的Y7柱样为对象,在磁性测量、地球化学和粒度分析的基础上,探讨了近百年以来长江口水下三角洲沉积物磁性特征变化及其影响因素.研究结果表明,磁铁矿是主导Y7柱样磁性特征的主要矿物,相比常量元素和粒度的均一组成,磁性特征存在显著的垂向变化.220cm以下,频率磁化率(χfd)、非磁滞剩磁磁化率(χARM)、磁化率(χ)、饱和等温剩磁(SIRM)、χARM/χ以及χARM/SIRM随深度增加而下降,而退磁参数S-100mT和S-300mT则呈相反变化趋势;220cm以上,χfd,χ,χARM/χ,χARM/SIRM以及S-100mT和S-300mT随深度变化不大,而SIRM和χARM则随深度增加呈现下降趋势.Y7柱样中硬剩磁(HIRM)随深度变化不大.结合Mn/Fe比值的变化,研究认为Y7柱样300cm以下层位,随深度增加沉积物磁性减弱和超顺磁颗粒急剧减少,还原成岩作用是重要因素之一,不完整反铁磁性矿物因相对稳定而导致其在磁性矿物组合中的比例上升.此外,长江中上游主要支流泥沙贡献率的变化可能会对Y7柱样磁性特征有一定影响,但尚需进一步的工作支持.     

Carbon–sulfur–iron relationships in sedimentary rocks from southwestern Taiwan: influence of geochemical environment on greigite and pyrrhotite formation

The importance of the magnetic iron sulfide minerals, greigite (Fe₃S₄) and pyrrhotite (Fe₇S₈), is often underappreciated in geochemical studies because they are metastable with respect to pyrite (FeS₂). Based on magnetic properties and X-ray diffraction analysis, previous studies have reported widespread occurrences of these magnetic minerals along with magnetite (Fe₃O₄) in two thick Plio-Pleistocene marine sedimentary sequences from southwestern Taiwan. Different stratigraphic zones were classified according to the dominant magnetic mineral assemblages (greigite-, pyrrhotite-, and magnetite-dominated zones). Greigite and pyrrhotite are intimately associated with fine-grained sediments, whereas magnetite is more abundant in coarse-grained sediments. We measured total organic carbon (TOC), total sulfur (TS), total iron (Fe_T), 1N HCl extractable iron (Fe_A), and bulk sediment grain size for different stratigraphic zones in order to understand the factors governing the formation and preservation of the two magnetic iron sulfide minerals. The studied sediments have low TS/Fe_A weight ratios (0.03–0.2), far below that of pyrite (1.15), which indicates that an excess of reactive iron was available for pyritization. Observed low TS (0.05–0.27%) is attributed to the low organic carbon contents (TOC=0.25–0.55%), which resulted from dilution by rapid terrigenous sedimentation. The fine-grained sediments also have the highest Fe_T and Fe_A values. We suggest that under conditions of low organic carbon provision, the high iron activity in the fine-grained sediments may have removed reduced sulfur so effectively that pyritization was arrested or retarded, which, in turn, favored preservation of the intermediate magnetic iron sulfides. The relative abundances of reactive iron and labile organic carbon appear to have controlled the transformation pathway of amorphous FeS into greigite or into pyrrhotite. Compared to pyrrhotite-dominated sediments, greigite-dominated sediments are finer-grained and have higher Fe_A but lower TS. We suggest that diagenetic environments with higher supply of reactive iron, lower supply of labile organic matter, and, consequently, lower sulfide concentration result in relatively high Eh conditions, which favor formation of greigite relative to pyrrhotite.     

Magnetic properties of light and dark sediment layers from the japan sea: Diagenetic and paleoclimatic implications

Rock magnetic/paleoclimatic/diagenetic relationships of sediments spanning the last 0.78 Ma have been investigated using samples collected from light and dark layers recovered at ODP Sites 794 (Yamato Basin) and 795 (Japan Basin). Rock-magnetic parameters (K, K_fd, ARM, SIRM, S-ratio) are shown to reflect diagenetic processes and climate-related variations in the concentration, mineralogy and grain-size of the magnetic minerals contained within the sediments. The magnetic mineralogy is dominated by ferrimagnetic (magnetite-type) minerals with a small contribution made by hematite and iron sulphides such as pyrrhotite and/or greigite. Magnetic mineral concentration and grain size vary between light and dark layers with the former characterized by a higher magnetic content and a finer magnetic grain size. Magnetite dissolution, related to sulfate reduction due to bacterial degradation of organic matter, is the process responsible for the magnetic characteristics observed in the dark layers, testifying to the reducing conditions in the basin. Variations in the rock magnetic properties of the sediments are strongly correlated with global oxygen isotope fluctuations, with glacial stages characterized by a lower magnetic mineral content and a coarser magnetic grain size relative to interglacial stages. Major downcore changes in the magnetic properties observed at Site 794 can be related to changes in the oceanographic conditions of the basin associated with the flow of the warm Tsushima Current into the Japan Sea at about 0.35–0.40 Ma ago.     

长江口潮滩植物对沉积物铁的地球化学循环及磁性特征的影响

通过长江河口高潮滩芦苇带沉积物柱状样CYLWA 1活性铁、有机质、粒度分析及磁性测量,探讨潮滩植物对沉积物铁的地球化学循环及磁性特征变化的影响。结果显示,沉积物柱状样总体上以Fe2 + 占优势,且含量随深度逐渐增加。但在亚表层 (10～ 2 0cm),由于根系的释氧作用,存在着以Fe3 + 占优势的氧化层。在 10～ 2 0cm这一氧化层,指示细颗粒磁性矿物的参数 (χ_fd、χ_ARM)明显减小,而反映磁性矿物类型的参数 (B_CR、SIRM/ χ)显著增大,指示了亚铁磁性矿物颗粒变粗,不完整反铁磁性矿物明显富集,这一现象反映了氧化层不完整反铁磁性矿物的相对富集     

中亚地区现代表土磁学特征及其古环境意义*

在中国西北地区主要沙漠、绿洲和戈壁及蒙古国南部戈壁等亚洲中部粉尘源区采集了表土(包括沙漠和土壤)样品,系统分析了样品的磁学性质(包括磁化率、无磁滞磁化率、等温剩磁、磁滞回线和热磁曲线)。结果表明,中亚地区表土样品中主要磁性矿物是铁磁矿,含有赤铁矿和磁赤铁矿,并伴有少量顺磁性矿物;磁性矿物粒度主要是准单畴(PSD)和多畴(MD),超顺磁性颗粒含量较低,接近于中国西部黄土地层含量。中亚地区表土磁性矿物含量总体都较低,磁性矿物含量在空间上与降雨量成正比。研究结果指示西部地区(准噶尔盆地和南部塔里木盆地)表土样品较北部地区(鄂尔多斯高原、阿拉善高原以及蒙古高原)表土样品,其粗颗粒磁性矿物含量高,而细颗粒磁性矿物含量低。本研究结果为黄土地层古土壤磁性增强源自成土过程提供了最直接的证据。     

Integrated Geophysical Works on the Kozlovka Anomaly (Kaluga Region)

This work presents integrated geophysical studies that were carried out to identify the nature of the intense magnetic anomaly detected by the results of land surveying on the right bank of the Slushka River in the territory of the Yukhnovskii district of Kaluga Region. The comprehensive interpretation of the geophysical data and the laboratory measurements of the core made it possible to determine the nature and parameters of the anomaly-forming object that is located in the upper part of the section; it consists of loams that contain such magnetic minerals as greigite, pyrrhotite, and magnetite.     

Fe–S mineralization in the Jurassic biogenic remains (Częstochowa,Poland)

Various Fe–S minerals of the mackinawite–greigite–pyrite association, ubiquitous in biogenic remains from Jurassic mudstones, have been described in detail in an SEM–EDS study. Two diagenetic stages of Fe sulphide formation and preservation in the Jurassic organic skeletons are identified. In the first stage, pyrite formed as euhedra and framboids shortly after deposition, mainly in the interiors of the skeletons which still contained labile organic matter. The second stage of iron sulphide formation was related to the later stages of diagenesis, when the influence of the surrounding sediment was more dominant, although some organic matter was still present in the biogenic skeletons. A Fe-rich carbonate–aluminosilicate cement was then introduced between the earliest iron sulphides and later subsequently sulphidized, to form a metastable iron monosulphide of mackinawite composition and then greigite.     

Magnetic Susceptibility in Surface Sediments in the Southern South China Sea and Its Implication for Sub-sea Methane Venting

In order to understand the characteristics of magnetic variability and their possible implication for sub-sea methane venting,magnetic susceptibility (MS) of 145 surface sediment samples from the southern South China Sea (SCS) was investigated.Magnetic particles extracted from 20 representative samples were also examined for their mineral,chemical compositions and micromorphology.Results indicate that MS values range between -7.73×10-8 and 45.06x10-8 m3/kg.The high MS zones occur at some hydrecarbon-bearing basins and along main tectonic zones,and low ones are distributed mainly within the river delta or along continental shelves.Iron concretions and manganese concretions are not main contributors for high MS values in sediments,while authigenic iron sulphide minerals are possibly responsible for the MS enhancement.This phenomenon is suspected to be produced by the reducing environment where the high upward venting methane beneath the seafloor reacts with seawater sulfate,resulting in seep precipitation of highly susceptible intermediate mineral pyrrhotite,greigite and paramagnetic pyrite.It suggests that MS variability is possibly one of the geochemical indicators for mapping sub-sea zones of methane venting in the southern SCS.     

The distribution of hydrocarbon minerals in the Welsh borderlands and adjacent areas

Solid hydrocarbon minerals occur in small quantities in the Lower Palaeozoic Welsh Basin, where Ordovician igneous intrusions mobilized them from local organic-rich source rocks. Hydrocarbon minerals are widespread in the Wenlockian and Carboniferous Limestones, and at least in the Carboniferous Limestone they show a close and probably genetic relationship with lead-zinc mineralization. The association of hydrocarbon minerals with lead-zinc-copper ores in Ordovician and Longmydian rocks' in the West Shropshire mining region is however largely coincidental. The hydrocarbon minerals in that region are residual from Carboniferous reservoir hydrocarbons. Reservoir hydrocarbon minerals in a breccia at Row Brook include crystallites of iron sulphides and manganese oxides. Hydrocarbon minerals in siderite nodules in the Coal Measures are spatially related to metal sulphides.