首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5036篇
  免费   1017篇
  国内免费   3244篇
大气科学   25篇
地球物理   832篇
地质学   7809篇
海洋学   319篇
天文学   18篇
综合类   91篇
自然地理   203篇
  2024年   57篇
  2023年   152篇
  2022年   256篇
  2021年   374篇
  2020年   404篇
  2019年   526篇
  2018年   494篇
  2017年   480篇
  2016年   489篇
  2015年   481篇
  2014年   502篇
  2013年   542篇
  2012年   587篇
  2011年   369篇
  2010年   333篇
  2009年   358篇
  2008年   428篇
  2007年   386篇
  2006年   350篇
  2005年   279篇
  2004年   269篇
  2003年   175篇
  2002年   147篇
  2001年   128篇
  2000年   133篇
  1999年   104篇
  1998年   97篇
  1997年   91篇
  1996年   53篇
  1995年   41篇
  1994年   72篇
  1993年   29篇
  1992年   33篇
  1991年   12篇
  1990年   15篇
  1989年   11篇
  1988年   10篇
  1987年   5篇
  1986年   8篇
  1985年   6篇
  1984年   5篇
  1983年   1篇
  1982年   1篇
  1981年   3篇
  1978年   1篇
排序方式: 共有9297条查询结果,搜索用时 31 毫秒
71.
通过对塔东地区古城4井上寒武统和中、上奥陶统碳酸盐岩围岩及充填物的碳、氧、锶同位素地球化学和流体包裹体成分的对比研究表明,充填于中、上奥陶统和上寒武统的流体分属于2个不同来源的流体体系。充填于中、上奥陶统灰岩裂缝中方解石脉的w(87Sr)/w(86Sr)介于0.7084~0.7088,它与早奥陶世海水的w(87Sr)/w(86Sr)相近;流体体系为CH4-H2O体系;充填物与围岩间具有明显的碳、氧同位素差异,表明上部流体体系中的流体来自于奥陶系地层本身。上寒武统白云岩裂缝中方解石脉的w(87Sr)/w(86Sr)为0.7138,明显地高于同时代海水的锶同位素值;流体体系为CO2-H2O体系;下部流体体系中的流体为外来富锶流体。上、下流体体系间互不连通暗示着上寒武统地层具有相对较好的保存条件。  相似文献   
72.
热液锆石U-Pb定年与石英脉型金矿成矿时代:评述与展望   总被引:4,自引:0,他引:4  
石英脉型金矿床是最重要的金矿床类型之一,对其成矿时代的精确测定却一直是一道难题.近年来同位素质谱技术的发展使得通过含金石英脉中热液锆石的U-Pb定年来精确限定石英脉型金矿床成矿时代成为可能.但含金石英脉中的锆石组成通常较复杂,除有热液锆石外,还可能出现从围岩中捕获的岩浆锆石和变质锆石.锆石成因和组成的这种复杂性,经常导致所获得的U-Pb年龄数据难以解释或缺乏明确的地质意义.因此石英脉型金矿床锆石U-Pb定年的关键是有效区分从成矿流体中直接生长的热液锆石和从围岩中捕获的岩浆锆石或变质锆石.通过锆石形貌、结构、微量元素组成(含稀土元素)、矿物或流体包裹体特征等的系统分析和综合研究,可以较好地区分含金石英脉中的不同成因锆石.在此基础上利用先进的SHRIMP或LA-ICPMS锆石U-Pb分析技术对石英脉中的热液锆石进行微区原位定年,可以获得石英脉型金矿床可靠的成矿年龄.  相似文献   
73.
Central Fujian Rift is another new and important volcanogenic massive sulfide Pb-Zn polymetallic metallogenetic belt. In order to find out the material genesis and mineralization period of Meixian-type Pb-Zn-Ag deposits, S and Pb isotope analysis and isotope geochronology of ores and wall rocks for five major deposits are discussed. It is concluded that the composition of sulfur isotope from sulfide ore vary slightly in different deposits and the mean value is close to zero with the 834S ranging from -3.5‰ to +5.6‰ averaging at +2.0‰, which indicates that the sulfur might originate from magma or possibly erupted directly from volcano or was leached from ore-hosted volcanic rock. The lead from ores in most deposits displays radioactive genesis character (206pb/204pb〉18.140, 207Pb/204pb〉15.584, 208pb/204pb〉38.569) and lead isotope values of ores are higher than those of wall rocks, which indicates that the lead was likely leached from the ore-hosted volcanic rocks. Based on isotope data, two significant Pb-Zn metallogenesis are delineated, which are Mid- and Late-Proterozoic sedimentary exhalative metailogenesis (The single zircon U-Pb, Sm-Nd isochronal and Ar-Ar dating ages of ore- hosted wall rocks are calculated to be among 933-1788 Ma.) and Yanshanian magmatic hydrothermal superimposed and alternated metallogenesis (intrusive SHRIMP zircon U-Pb and Rb-Sr isochronal ages between 127-154 Ma).  相似文献   
74.
藏南冈底斯带西段麦拉花岗岩锆石SHRIMP定年及地质意义   总被引:1,自引:0,他引:1  
用SHRIMP测定了冈底斯岩带西段麦拉山口岩体同碰撞黑云母二长花岗岩锆石的U-Pb年龄,黑云母二长花岗岩锆石SHRIMP U-Pb年龄为47.1±1.1Ma。这就为前人提出的雅鲁藏布江地区洋盆闭合、板块开始碰撞的时间为50Ma左右提供了同位素年代学的证据。  相似文献   
75.
中国西秦岭碎屑锆石U-Pb年龄及其构造意义   总被引:5,自引:1,他引:4  
西秦岭是北接华北克拉通、西接祁连与柴达木、南接松潘—甘孜地块的东秦岭造山带的西延。文中研究了该区从前寒武纪到三叠纪的碎屑沉积岩。这些碎屑沉积岩中分离出的锆石由LA-ICPMS(激光剥蚀等离子体质谱)进行了U-Pb定年。全岩Nd亏损地幔模式年龄类似于扬子克拉通年龄,主要分布于1.55~1.98Ga,峰值为1.81Ga,而与华北克拉通主要为古元古代与太古宙的模式年龄形成明显的对比。泥盆系中的碎屑锆石930~730Ma的U-Pb年龄指示其与扬子克拉通具亲缘性。930~730Ma是源区地壳的强烈增长阶段。二叠系—三叠系的碎屑沉积岩主体以含老于1600Ma的碎屑锆石为特征。碎屑锆石U-Pb年龄与Sm-Nd同位素组成指示此时华北克拉通南缘的基底岩石成为二叠系—三叠系碎屑沉积岩的重要物源。扬子克拉通在三叠纪时与华北克拉通拼接。西秦岭二叠系—三叠系碎屑沉积岩含有高达50%的华北克拉通南缘的基底岩石。  相似文献   
76.
Relations between indoor and soil gas radon were experimentally studied in villages of the Baikal region. On the basis of the obtained data, the soil-to-indoor radon ratio was calculated, which can be used for prediction of radon pollution in buildings of the same kind.  相似文献   
77.
王华  张远明  覃嘉铭  李强  杨琰 《地球学报》2008,29(6):725-728
攀枝花市西区龙洞煤矿经过十余年的生产,不但在地下形成了较大范围的采空区,而且通过影响地下水形成了较大范围的疏干,致使矿区内大部分泉点断流.因而,为确定矿山地下水长期疏干对龙洞泉流域的影响程度,笔者通过比较水库水、大气降水、地下水与渗漏水之间的同位素特征,区分出了地下水的补给来源,并为查明本区的水文地质条件及确定矿坑地下水疏干的影响范围提供科学依据.  相似文献   
78.
This paper evaluates the analytical precision, accuracy and long‐term reliability of the U‐Pb age data obtained using inductively coupled plasma–mass spectrometry (ICP‐MS) with a frequency quintupled Nd‐YAG (λ = 213nm) laser ablation system. The U‐Pb age data for seven standard zircons of various ages, from 28 Ma to 2400 Ma (FCT, SL13, 91500, AS3, FC1, QGNG and PMA7) were obtained with an ablation pit size of 30 μm diameter. For 207Pb/206Pb ratio measurement, the mean isotopic ratio obtained on National Institute of Standards and Technology (NIST) SRM610 over 4 months was 0.9105 ± 0.0014 (n = 280, 95% confidence), which agrees well with the published value of 0.9096. The time‐profile of Pb/U ratios during single spot ablation showed no significant difference in shape from NIST SRM610 and 91500 zircon standards. These results encouraged the use of the glass standard as a calibration standard for the Pb/U ratio determination for zircons with shorter wavelength (λ = 213 nm) laser ablation. But 206Pb/238U and 207Pb/235U ages obtained by this method for seven zircon standards are systematically younger than the published U‐Pb ages obtained by both isotope dilution–thermal ionization mass spectrometry (ID‐TIMS) and sensitive high‐resolution ion‐microprobe (SHRIMP). Greater discrepancies (3–4% younger ages) were found for the 206Pb/238U ages for SL13, AS3 and 91500 zircons. The origin of the differences could be heterogeneity in Pb/U ratio on SRM610 between the different disks, but a matrix effect accuracy either in the ICP ion source or in the ablation‐transport processes of the sample aerosols cannot be neglected. When the 206Pb/238U (= 0.2302) newly defined in the present study is used, the measured 206Pb/238U and 207Pb/235U ages for the seven zircon standards are in good agreement with those from ID‐TIMS and SHRIMP within ±2%. This suggests that SRM610 glass standard is suitable for ICP‐MS with laser ablation sampling (LA‐ICP‐MS) zircon analysis, but it is necessary to determine the correction factor for 206Pb/238U by measuring several zircon standards in individual laboratories.  相似文献   
79.
The EC funded Geochemical Seismic Zonation program (EEC GSZ Project 1996–1998) chose Sardinia as a low-seismicity site, in which the relationships between fluid geochemistry and seismo-tectonics had to be investigated and results compared with outcomes from other selected high-seismicity sites. A first article, examining the role of fault segmentation and seismic quiescence on the geochemical composition of groundwaters and gases, has already been presented (Angelone et al. 2005). This article deals with environmental isotopes which, together with selected hydrochemical data, give hints on tectonically-related fluid circulations. Four water-dominated hydrothermal systems were considered, all located along regional fault systems and discharging groundwaters belonging to the Na–HCO3 and Na–Cl facies. In the considered systems, groundwater circulation takes place, principally, in the Palaeozoic Crystalline Basement (PCB), with the exception of the Logudoro system, where hydrological circuits develop in the Mesozoic Carbonate Platform (MCP). The high CO2 contents, the non-attainment of fluid-rock equilibrium and the large lithological variability prevent the construction of a unique hydrogeological–geochemical conceptual model. In this case, stable isotopes provide a useful tool to describe the origin of fluids and their subterranean movements. Stable isotopes of water, integrated with hydrochemical data, indicate that fluids are derived from three main end members. The dominant component is a relatively recent local meteoric water; the second one is marine water; and the third one is a fossil freshwater, depleted in heavy isotopes with respect to modern rains. The latter end member entered the aquifer system in the past, when climatic conditions were greatly different from today. At least two circulation systems can be recognised, namely a shallow cold system and a deep hydrothermal system, as well as two distinct hydrological processes: (1) gravity-controlled descent of cold water towards greater depths and (2) convection linked to a thermal gradient, causing deep fluids to rise up from the hydrothermal reservoir towards the surface. The highly variable δ13CTDIC values suggest the presence of two distinct CO2 sources, namely, a biogenic one and a thermogenic one. The relation between the isotopic compositions of CO2 and He indicates an increased mantle signature in uprising CO2-rich fluids.  相似文献   
80.
Environmental tracers sampled from the carbonate Madison aquifer on the eastern flank of the Black Hills, South Dakota, USA indicated the approximate locations of four major karst conduits. Contamination issues are a major concern because these conduits are characterized by direct connections to sinking streams, high groundwater velocities, and proximity to public water supplies. Objectives of the study were to estimate approximate conduit locations and assess possible anthropogenic influences associated with conduits. Anomalies of young groundwater based on chlorofluorocarbons (CFCs), tritium, and electrical conductivity (EC) indicated fast moving, focused flow and thus the likely presence of conduits. δ18O was useful for determining sources of recharge for each conduit, and nitrate was a useful tracer for assessing flow paths for anthropogenic influences. Two of the four conduits terminate at or near a large spring complex. CFC apparent ages ranged from 15 years near conduits to >50 years in other areas. Nitrate-N concentrations >0.4 mg/L in groundwater were associated with each of the four conduits compared with concentrations ranging from <0.1 to 0.4 mg/L in other areas. These higher nitrate-N concentrations probably do not result from sinking streams but rather from other areas of infiltration.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号