全文获取类型
收费全文 | 3605篇 |
免费 | 181篇 |
国内免费 | 69篇 |
专业分类
测绘学 | 142篇 |
大气科学 | 279篇 |
地球物理 | 1062篇 |
地质学 | 1261篇 |
海洋学 | 269篇 |
天文学 | 546篇 |
综合类 | 18篇 |
自然地理 | 278篇 |
出版年
2023年 | 17篇 |
2022年 | 22篇 |
2021年 | 56篇 |
2020年 | 89篇 |
2019年 | 73篇 |
2018年 | 141篇 |
2017年 | 118篇 |
2016年 | 156篇 |
2015年 | 144篇 |
2014年 | 146篇 |
2013年 | 226篇 |
2012年 | 155篇 |
2011年 | 214篇 |
2010年 | 178篇 |
2009年 | 212篇 |
2008年 | 187篇 |
2007年 | 161篇 |
2006年 | 142篇 |
2005年 | 104篇 |
2004年 | 105篇 |
2003年 | 100篇 |
2002年 | 78篇 |
2001年 | 84篇 |
2000年 | 75篇 |
1999年 | 70篇 |
1998年 | 51篇 |
1997年 | 52篇 |
1996年 | 41篇 |
1995年 | 30篇 |
1994年 | 29篇 |
1993年 | 23篇 |
1992年 | 33篇 |
1991年 | 17篇 |
1990年 | 27篇 |
1989年 | 22篇 |
1988年 | 25篇 |
1987年 | 19篇 |
1986年 | 30篇 |
1985年 | 35篇 |
1984年 | 25篇 |
1983年 | 35篇 |
1982年 | 26篇 |
1981年 | 25篇 |
1980年 | 22篇 |
1979年 | 26篇 |
1978年 | 20篇 |
1977年 | 19篇 |
1976年 | 18篇 |
1975年 | 20篇 |
1973年 | 17篇 |
排序方式: 共有3855条查询结果,搜索用时 31 毫秒
101.
Origin of low-Ca pyroxene in amoeboid olivine aggregates: Evidence from oxygen isotopic compositions
Alexander N. Krot Timothy J. Fagan Kazuhide Nagashima Hisayoshi Yurimoto 《Geochimica et cosmochimica acta》2005,69(7):1873-1881
Amoeboid olivine aggregates (AOAs) in primitive carbonaceous chondrites consist of forsterite (Fa<2), Fe,Ni-metal, spinel, Al-diopside, anorthite, and rare gehlenitic melilite (Åk<15). ∼10% of AOAs contain low-Ca pyroxene (Fs1-3Wo1-5) that is in corrosion relationship with forsterite and is found in three major textural occurrences: (i) thin (<15 μm) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) 5-10-μm-thick haloes and subhedral grains around Fe,Ni-metal nodules in AOA peripheries, and (iii) shells of variable thickness (up to 70 μm), commonly with abundant tiny (3-5 μm) inclusions of Fe,Ni-metal grains, around AOAs. AOAs with the low-Ca pyroxene shells are compact and contain euhedral grains of Al-diopside surrounded by anorthite, suggesting small (10%-20%) degree of melting. AOAs with other textural occurrences of low-Ca pyroxene are rather porous. Forsterite grains in AOAs with low-Ca pyroxene have generally 16O-rich isotopic compositions (Δ17O < −20‰). Low-Ca pyroxenes of the textural occurrences (i) and (ii) are 16O-enriched (Δ17O < −20‰), whereas those of (iii) are 16O-depleted (Δ17O = −6‰ to −4‰). One of the extensively melted (>50%) objects is texturally and mineralogically intermediate between AOAs and Al-rich chondrules. It consists of euhedral forsterite grains, pigeonite, augite, anorthitic mesostasis, abundant anhedral spinel grains, and minor Fe,Ni-metal; it is surrounded by a coarse-grained igneous rim largely composed of low-Ca pyroxene with abundant Fe,Ni-metal-sulfide nodules. The mineralogical observations suggest that only spinel grains in this igneous object were not melted. The spinel is 16O-rich (Δ17O ∼ −22‰), whereas the neighboring plagioclase mesostasis is 16O-depleted (Δ17O ∼ −11‰).We conclude that AOAs are aggregates of solar nebular condensates (forsterite, Fe,Ni-metal, and CAIs composed of Al-diopside, anorthite, spinel, and ±melilite) formed in an 16O-rich gaseous reservoir, probably CAI-forming region(s). Solid or incipiently melted forsterite in some AOAs reacted with gaseous SiO in the same nebular region to form low-Ca pyroxene. Some other AOAs appear to have accreted 16O-poor pyroxene-normative dust and experienced varying degrees of melting, most likely in chondrule-forming region(s). The most extensively melted AOAs experienced oxygen isotope exchange with 16O-poor nebular gas and may have been transformed into chondrules. The original 16O-rich signature of the precursor materials of such chondrules is preserved only in incompletely melted grains. 相似文献
102.
Alexander Likholyot 《Geochimica et cosmochimica acta》2005,69(12):2949-2958
We have studied thermochemistry of the first hydration steps for Cl−, Br−, and I− in the gas phase both experimentally using high-pressure mass spectrometry (HPMS) and theoretically using density functional theory (DFT) calculations. The highest hydration steps measured experimentally were n = 8 for Cl−, n = 7 for Br−, and n = 5 for I−, all of them being higher than previously reported. Both experimental and theoretical stepwise enthalpies and entropies of hydration for these halides exhibited non-monotonic behavior for successive hydration steps that was not reported in previous HPMS investigations of these reactions. This behavior can be successfully interpreted using halide water cluster geometries obtained from DFT calculations by considering the number of additional hydrogen bonds formed at each hydration step and simultaneous weakening of ion-solvent interaction with increasing cluster size. Results of DFT calculations for surface cluster geometries agree better with experimental results than do the results for interior cluster geometries. We conclude that predominantly surface clusters were observed in our experiments and that small surface clusters have larger number of possible isomers than the interior clusters of the same size. The results for enthalpies of hydration for the studied halide ions lead to the conclusion that ion-solvent interaction is stronger than solvent-solvent interaction for chloride-water clusters. The difference between the two types of interaction diminishes with increasing anion size. The ion-solvent and solvent-solvent interactions are of nearly equal magnitude for iodide. 相似文献
103.
Chronology and stratigraphy of the Magdalen Islands archipelago from the last glaciation to the early Holocene: new insights into the glacial and sea‐level history of eastern Canada
下载免费PDF全文
![点击此处可从《Boreas: An International Journal of Quaternary Research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
104.
105.
Diamond in metasedimentary crustal rocks from Pohorje,Eastern Alps: a window to deep continental subduction
下载免费PDF全文
![点击此处可从《Journal of Metamorphic Geology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
M. Janák N. Froitzheim K. Yoshida V. Sasinková M. Nosko T. Kobayashi T. Hirajima M. Vrabec 《Journal of Metamorphic Geology》2015,33(5):495-512
We report the first finding of diamond and moissanite in metasedimentary crustal rocks of Pohorje Mountains (Slovenia) in the Austroalpine ultrahigh‐pressure (UHP) metamorphic terrane of the Eastern Alps. Microscopic observations and Raman spectroscopy show that diamond occurs in situ as inclusions in garnet, being heterogeneously distributed. Under the optical microscope, diamond‐bearing inclusions are of cuboidal to rounded shape and of pinkish, yellow to brownish colour. The Raman spectra of the investigated diamond show a sharp, first order peak of sp3‐bonded carbon, in most cases centred between 1332 and 1330 cm?1, with a full width at half maximum between 3 and 5 cm?1. Several spectra show Raman bands typical for disordered graphitic (sp2‐bonded) carbon. Detailed observations show that diamond occurs either as a monomineralic, single‐crystal inclusion or it is associated with SiC (moissanite), CO2 and CH4 in polyphase inclusions. This rare record of diamond occurring with moissanite as fluid‐inclusion daughter minerals implies the crystallization of diamond and moissanite from a supercritical fluid at reducing conditions. Thermodynamic modelling suggests that diamond‐bearing gneisses attained P–T conditions of ≥3.5 GPa and 800–850 °C, similar to eclogites and garnet peridotites. We argue that diamond formed when carbonaceous sediment underwent UHP metamorphism at mantle depth exceeding 100 km during continental subduction in the Late Cretaceous (c. 95–92 Ma). The finding of diamond confirms UHP metamorphism in the Pohorje Mountains, the most deeply subducted part of Austroalpine units. 相似文献
106.
Vadim S. Kamenetsky Alexander Belousov Victor V. Sharygin Liudmila M. Zhitova Kathy Ehrig Michael E. Zelenski Ilya Chaplygin Marina A. Yudovskaya Pavel N. Nesterenko Sergey M. Zakharov 《地学学报》2019,31(6):511-517
Subvolcanic environments in supra‐subduction zones are renowned for hosting epithermal deposits that often contain electrum and native gold, including bonanza examples. This study examined mineral assemblages and processes occurring in shallow‐crust volcanic settings using recent eruption (2012–2013) of the basaltic Tolbachik volcano in the Kamchatka arc. The Tolbachik eruptive system is characterized by an extensive system of lava tubes. After cessation of magma input, the tubes maintained the flow of hot oxidized gases that episodically interacted with the lava surfaces and sulphate‐chloride precipitates from volcanic gases on these surfaces. The gas‐rock interaction had strong pyrometamorphic effects that resulted in the formation of molten salt, oxidized (tenorite, hematite, Cu‐rich magnesioferrite) and skarn‐like silicate mineral assemblages. By analogy with experimental studies, we propose that a combination of these processes was responsible for extraction of metals from the basaltic wall rocks and deposition of Cu‐, Fe‐ and Cu‐Fe‐oxides and native gold. 相似文献
107.
Primary aragonite and high‐Mg calcite in the late Cambrian (Furongian). Potential evidence from marine carbonates in Oman
下载免费PDF全文
![点击此处可从《地学学报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Transient aragonite seas occurred in the early Cambrian but several models suggest the late Cambrian was a time of calcite seas. Here, evidence is presented from the Andam Group, Huqf High, Oman (Gondwana) that suggests a transient Furongian (late Cambrian) aragonite sea, characterized by the precipitation of aragonite and high‐Mg calcite ooids and aragonite isopachous, fibrous, cements. Stable carbon isotope data suggest that precipitation occurred just before and during the SPICE (Steptoean Positive Carbonate Isotope Excursion). Aragonite and high‐Mg calcite precipitation can be accounted for if mMg:Ca ratios were around 1.2 given the very high atmospheric CO2 at that time and if precipitation occurred in warm waters associated with the SPICE. This, together with reported occurrences of early Furongian aragonite ooids from various locations in North America (Laurentia), suggests that aragonite and high‐Mg calcite precipitation from seawater may have been more than just a local phenomenon. 相似文献
108.
Arctic river basins are amongst the most vulnerable to climate change. However, there is currently limited knowledge of the hydrological processes that govern flow dynamics in Arctic river basins. We address this research gap using natural hydrochemical and isotopic tracers to identify water sources that contributed to runoff in river basins spanning a gradient of glacierization (0–61%) in Svalbard during summer 2010 and 2011. Spatially distinct hydrological processes operating over diurnal, weekly and seasonal timescales were characterized by river hydrochemistry and isotopic composition. Two conceptual water sources (‘meltwater’ and ‘groundwater’) were identified and used as a basis for end‐member mixing analyses to assess seasonal and year‐to‐year variability in water source dynamics. In glacier‐fed rivers, meltwater dominated flows at all sites (typically >80%) with the highest contributions observed at the beginning of each study period in early July when snow cover was most extensive. Rivers in non‐glacierized basins were sourced initially from snowmelt but became increasingly dependent on groundwater inputs (up to 100% of total flow volume) by late summer. These hydrological changes were attributed to the depletion of snowpacks and enhanced soil water storage capacity as the active layer expanded throughout each melt season. These findings provide insight into the processes that underpin water source dynamics in Arctic river systems and potential future changes in Arctic hydrology that might be expected under a changing climate. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
109.
110.