Diagenetic fluid evolution and water-rock interaction model of carbonate cements in sandstone: An example from the reservoir sandstone of the Fourth Member of the Xujiahe Formation of the Xiaoquan-Fenggu area,Sichuan Province,China

Carbonate cement is the most abundant cement type in the Fourth Member of the Xujiahe Formation in the Xiaoquan-Fenggu area of the West Sichuan Depression. Here we use a systematic analysis of carbonate cement petrology, mineralogy, carbon and oxygen isotope ratios and enclosure homogenization temperatures to study the precipitation mechanism, pore fluid evolution, and distribution of different types of carbonate cement in reservoir sand in the study area. Crystalline calcite has relatively heavy carbon and oxygen isotope ratios(δ13C = 2.14‰, δ18O = -5.77‰), and was precipitated early. It was precipitated directly from supersaturated alkaline fluid under normal temperature and pressure conditions. At the time of precipitation, the fluid oxygen isotope ratio was very light, mainly showing the characteristics of a mixed meteoric water-seawater fluid(δ18O = -3‰), which shows that the fluid during precipitation was influenced by both meteoric water and seawater. The calcite cement that fills in the secondary pores has relatively lighter carbon and oxygen isotope ratios(δ13C = -2.36‰, δ18O = -15.68‰). This cement was precipitated late, mainly during the Middle and Late Jurassic. An important material source for this carbonate cement was the feldspar corrosion process that involved organic matter. The Ca2+, Fe3+ and Mg2+ ions released by the clay mineral transformation process were also important source materials. Because of water-rock interactions during the burial process, the oxygen isotope ratio of the fluid significantly increased during precipitation, by about 3‰. The dolomite cements in calcarenaceous sandstone that was precipitated during the Middle Jurassic have heavier carbon and oxygen isotope ratios, which are similar to those of carbonate debris in the sandstone(δ13C = 1.93‰, δ18O = -6.11‰), demonstrating that the two are from the same source that had a heavier oxygen isotope ratio(δ18O of about 2.2‰). The differences in fluid oxygen isotope ratios during cement precipitation reflect the influences of different water-rock interaction systems or different water-rock interaction strengths. This is the main reason why the sandstone containing many rigid particles(lithic quartz sandstone) has a relatively negative carbon isotope ratio and why the precipitation fluid in calcarenaceous sandstone has a relatively heavier oxygen isotope ratio.     

地表岩石侵蚀速率对宇生核素暴露测年影响的研究

利用原地生宇生核素测定暴露年代时,通常会假设地貌体侵蚀速率为0。研究表明,该假设会低估地貌体的真实暴露年代。搜集2009~2012年全球不同区域56个岩石样品的宇生核素¹⁰Be测年数据,探讨侵蚀速率为0对于侵蚀速率为0.5、1以及2 mm/ka的样品,在不同暴露尺度上对暴露年代计算的影响幅度。结果表明,对于1×10⁴a尺度的样品暴露年代可能低估约0.5%,1%,2%;对于10×10⁴a尺度的样品可能低估约5%,7%,20%;对于50×10⁴a尺度的样品可能低估约40%,70%甚至100%以上。     

中天山天湖东铁钼矿含矿片麻状花岗岩年代学、地球化学和锆石Hf同位素-对于中天山早古生代构造演化的启示

以中天山东段的天湖东铁钼矿含矿花岗岩为例,在LA-ICP-MS锆石U-Pb年代学测定其为早古生代花岗岩（（445.3±4.6） Ma）基础上,通过岩相学、地球化学及锆石原位Hf同位素组成等多方面研究,探讨该岩体的成岩作用及其构造背景。天湖东含矿片麻状花岗岩的主要矿物为斜长石、石英、钾长石,并含少量黑云母和角闪石等。全岩地球化学分析结果表明,该片麻状花岗岩高硅、弱富铝、富钙、富钠而贫钾,ASI值为0.68~0.82,属于准铝质钙碱性花岗岩,总体上富集大离子亲石元素（LILE）Rb、Ba等和轻稀土元素La、Ce、Nd等,而亏损高场强元素Nb、Ta、Ti、Yb等,轻重稀土分异明显,轻稀土分异较为明显,而重稀土分异不明显,表现出典型岛弧岩浆岩的地球化学特征。锆石的ε_Hf(445 Ma)值为-6.31~-1.77,二阶段Hf模式年龄（T_DM2）为1.538~1.825 Ga,表明该花岗岩的源区主要为壳源物质。综合分析上述资料,认为天湖东铁钼矿片麻状花岗岩是由俯冲过程中地壳物质重熔的产物。结合前人的研究和本课题组的新近研究成果认为,在早古生代时,中天山为岩浆弧构造环境,形成一系列的钙碱性岩浆岩,而该岩浆弧的形成可能是受到介于吐哈陆块和塔里木板块之间的古天山大洋在早古生代时期向南俯冲而形成的。     

热液成因有色金属矿多维异常体系:以马头斑岩型钼铜矿为例

从深部矿地球化学勘查实际需求出发,根据近年来20余个矿床研究结果,总结提出了热液成因有色金属矿床矿致异常规律--多维异常体系。多维异常体系是指产出在特定成矿地质时期地质体中,空间有序共存、形成机理各异、成矿指向递进的多属性地球化学异常体系,将在斑岩型、矽卡岩型、热液型以及沉积改造型等与热液作用有关的有色金属矿产深部矿预测和评价中发挥重要作用。以安徽马头斑岩型钼铜矿为例,对该矿床多维异常体系进行了探讨。结果表明: 在该矿床中,存在着以Na₂O为代表的负异常体系、以S为代表的矿化剂元素异常体系、矿化剂元素S与Fe和成矿元素间协同平衡体系、成矿及其伴生元素异常体系等,这些异常体系对成矿的指示作用是递进的,从而证实了多维异常体系的存在。对应用多维异常体系思路预测和评价深部矿进行了说明。     

渤海湾盆地古近纪海侵问题研究进展及展望

有关渤海湾盆地古近纪是否发生过海侵的问题争论了二十多年,至今尚无定论。尽管“海侵”论者提供了矿物学、岩石学、古生态学以及地球化学等多种标志,但是因为这些标志与标准海相标志比较,都有一定差别,而且至今没有发现确实可靠的海侵通道和明显的海相性递增现象,所以又出现了“海泛”论、“海啸”论和“陆相”论。几种观点长期并存,笔者认为要解决这一问题,可以从四方面入手①深入研究渤海湾盆地的构造背景,推断海侵通道存在的可能性;②进一步厘定已有的海相标志的准确性;③寻求新的、唯一的能区分海陆相成因的证据;④搜集国外在相关层位有类似沉积的地区,进行对比研究,并探索其成因。     

大别造山带大陆深俯冲和折返过程中壳-幔相互作用信息--来自大理岩铅同位素的证据

对大别造山带不同产状大理岩的铅同位素研究表明,红安群王家店和双河大理岩分别经历过高压和超高压变质作用,具有较高的铅同位素变化范围和高的放射性成因铅,反映了上地壳铅同位素的特征;新店大理岩的铅同位素比值和变化范围很小,具有EM1型地幔铅同位素特征,表明有地幔铅同位素特征的记录;而位于剪切带的方高坪大理岩的铅同位素除了n(206Pb)/n(204Pb)值更高外,与新店大理岩的铅同位素特征相似,记录了放射性铅同位素的大量加入或丢失,导致本来具有与双河大理岩类似的铅同位素特征变得不明显.这些结果均表明大陆深俯冲及其折返过程对大理岩铅同位素组成的改造是相当复杂的.     

Extreme Enrichment of Tellurium in Deep-Sea Sediments

Tellurium is a sort of scattered rare element on the earth. Its concentration is very low in earth＇s crust, only 1.0 ng/g. However, it has extremely high abundance in Co-rich crusts, marine polymetallic nodules, deep-sea sediments and aerolites. To find out the origin of tellurium enrichment in deep-sea sediments, we analyzed and compared tellurium concentrations and helium isotope compositions in the magnetic parts and those in the bulk parts of deep-sea sediments. The result indicates that the helium content, 3He/4He ratio and tellurium concentration are obviously higher in the magnetic parts than those in the bulk parts. The 3He abundance varies synchronously with the tellurium concentration. 3He and Te have a distinct positive correlation with each other. It is the first time that the paper brings forward that the extreme enrichment of tellurium in deep-sea sediments, like helium isotope anomalies, probably results from the input of interplanetary dust particles （IDPs）. Similarly, the extreme enrichment of tellurium in marine polymetallic nodules and Co-rich crusts is possibly related to IDPs.     

Organic Geochemistry of the Early Jurassic Oil Shale from the Shuanghu Area in Northern Tibet and the Early Toarcian Oceanic Anoxic Event

This paper presents new geological and geochemical data from the Shuanghu area in northern Tibet, which recorded the Early Toarcian Oceanic Anoxic Event. The stratigraphic succession in the Shuanghu area consists mostly of grey to dark-colored alternating oil shales, marls and mudstones. Ammonite beds are found at the top of the Shuanghu oil shale section, which are principally of early Toarcian age, roughly within the Harplocearasfalciferrum Zone. Therefore,the oil shale strata at Shuanghu can be correlated with early Toarcian black shales distributing extensively in the European epicontinental seas that contain the records of an Oceanic Anoxic Event. Sedimentary organic matter of laminated shale anomalously rich in organic carbon across the Shuanghu area is characterized by high organic carbon contents, ranging from 1.8% to 26.1%. The carbon isotope curve displays the δ^13C values of the kerogen (δ^13Ckerogen) fluctuating from -26.22 to -23.53‰ PDB with a positive excursion close to 2.17‰, which, albeit significantly smaller, may also have been associated with other Early Toarcian Oceanic Anoxic Events (OAEs) in Europe. The organic atomic C/N ratios range between 6 and 43, and the curve of C/N ratios is consistent with that of the δ^13Ckerogen values. The biological assemblage,characterized by scarcity of benthic organisms and bloom of calcareous nannofossils (coccoliths), reveals high biological productivity in the surface water and an unfavorable environment for the benthic fauna in the bottom water during the Oceanic Anoxic Event. On the basis of organic geochemistry and characteristics of the biological assemblage, this study suggests that the carbon-isotope excursion is caused by the changes of sea level and productivity, and that the black shale deposition, especially oil shales, is related to the bloom and high productivity of coccoliths.     

松辽盆地白垩纪青山口阶的同位素地层标志及其与海相Cenomanian阶的对比

本研究利用大庆油田在松辽盆地中部的岩心样品进行了青山口组有机碳稳定同位素的测试,通过化学地层和生物地层研究提出了青山口阶的顶底标志。在青山口组底部同位素值最低,为- 32 .2‰,与介形虫Cypridea elliptica的消失界面吻合。同位素值随后迅速上升,形成底界之上的第一个峰值,为- 2 8.1‰。随后的4个峰值自下而上分别是- 2 8‰、- 2 7.8‰、- 2 7.3‰和- 2 6 .6‰,相间的低峰值一般处于- 30‰～- 2 9‰左右。最为重要的是,有机碳稳定同位素值在青山口组顶部出现一个明显的异常,由- 2 9.5‰急剧升为- 2 3.1‰,随后又迅速降为- 30 .12‰,形成一个明显的正向峰值。这一正向偏移的位置与介形类Cypridea panda和Triangulicyprisfusiformis的消失界面基本吻合,并与全球Cenom anian- Turonian界线稳定同位素事件表现一致。据此,将这一稳定同位素正向偏移确定为青山口阶顶界的化学标志。通过这一标志可以将青山口阶的顶界与海相Cenom anian阶顶界进行对比。研究结果表明,化学地层标志是陆相与海相地层的对比的渠道。青山口阶的有机碳稳定同位素值具有全球一致的意义,与我国西藏南部海相地层,乃至全球重要地区Cenomanian- Turonian阶稳定同位素界线均具有可比性。