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   22258篇
  免费   3755篇
  国内免费   2984篇
测绘学   379篇
大气科学   287篇
地球物理   8769篇
地质学   10252篇
海洋学   1614篇
天文学   5144篇
综合类   979篇
自然地理   1573篇
  2024年   31篇
  2023年   156篇
  2022年   468篇
  2021年   573篇
  2020年   684篇
  2019年   800篇
  2018年   635篇
  2017年   611篇
  2016年   634篇
  2015年   753篇
  2014年   1100篇
  2013年   1202篇
  2012年   1272篇
  2011年   1455篇
  2010年   1384篇
  2009年   1728篇
  2008年   1660篇
  2007年   1728篇
  2006年   1706篇
  2005年   1455篇
  2004年   1328篇
  2003年   1139篇
  2002年   967篇
  2001年   809篇
  2000年   812篇
  1999年   717篇
  1998年   553篇
  1997年   411篇
  1996年   451篇
  1995年   358篇
  1994年   336篇
  1993年   287篇
  1992年   162篇
  1991年   119篇
  1990年   97篇
  1989年   95篇
  1988年   74篇
  1987年   49篇
  1986年   39篇
  1985年   30篇
  1984年   20篇
  1983年   16篇
  1982年   15篇
  1981年   14篇
  1980年   12篇
  1979年   19篇
  1978年   5篇
  1977年   15篇
  1954年   8篇
  1875年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
93.
94.
Photoelectric radial-velocity measurements show that HD 118670 is a double-lined spectroscopic binary in an orbit which is not quite circular and whose period is about 48 days. Spectral types of K0 V and K7 V would satisfy the photometry and the mass ratio; the mass function would then suggest the possibility of eclipses. However, actual spectral classification indicates a luminosity somewhat above the main sequence  相似文献   
95.
Abstract Large calcite veins and pods in the Proterozoic Corella Formation of the Mount Isa Inlier provide evidence for kilometre-scale fluid transport during amphibolite facies metamorphism. These 10- to 100-m-scale podiform veins and their surrounding alteration zones have similar oxygen and carbon isotopic ratios throughout the 200 × 10-km Mary Kathleen Fold Belt, despite the isotopic heterogeneity of the surrounding wallrocks. The fluids that formed the pods and veins were not in isotopic equilibrium with the immediately adjacent rocks. The pods have δ13Ccalcite values of –2 to –7% and δ18Ocalcite values of 10.5 to 12.5%. Away from the pods, metadolerite wallrocks have δ18Owhole-rock values of 3.5 to 7%. and unaltered banded calc-silicate and marble wallrocks have δ13Ccalcite of –1.6 to –0.6%, and δ18Ocalcite of 18 to 21%. In the alteration zones adjacent to the pods, the δ18O values of both metadolerite and calc-silicate rocks approach those of the pods. Large calcite pods hosted entirely in calc-silicates show little difference in isotopic composition from pods hosted entirely in metadolerite. Thus, 100- to 500-m-scale isotopic exchange with the surrounding metadolerites and calc-silicates does not explain the observation that the δ18O values of the pods are intermediate between these two rock types. Pods hosted in felsic metavolcanics and metasiltstones are also isotopically indistinguishable from those hosted in the dominant metadolerites and calc-silicates. These data suggest the veins are the product of infiltration of isotopically homogeneous fluids that were not derived from within the Corella Formation at the presently exposed crustal level, although some of the spread in the data may be due to a relatively small contribution from devolatilization reactions in the calc-silicates, or thermal fluctuations attending deformation and metamorphism. The overall L-shaped trend of the data on plots of δ13C vs. δ18O is most consistent with mixing of large volumes of externally derived fluids with small volumes of locally derived fluid produced by devolatilization of calc-silicate rocks. Localization of the vein systems in dilatant sites around metadolerite/calc-silicate boundaries indicates a strong structural control on fluid flow, and the stable isotope data suggest fluid migration must have occurred at scales greater than at least 1 km. The ultimate source for the external fluid is uncertain, but is probably fluid released from crystallizing melts derived from the lower crust or upper mantle. Intrusion of magmas below the exposed crustal level would also explain the high geothermal gradient calculated for the regional metamorphism.  相似文献   
96.
97.
98.
99.
100.
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