全文获取类型
收费全文 | 501篇 |
免费 | 65篇 |
国内免费 | 186篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 60篇 |
地球物理 | 56篇 |
地质学 | 507篇 |
海洋学 | 83篇 |
天文学 | 11篇 |
综合类 | 19篇 |
自然地理 | 13篇 |
出版年
2024年 | 1篇 |
2023年 | 4篇 |
2022年 | 13篇 |
2021年 | 10篇 |
2020年 | 12篇 |
2019年 | 17篇 |
2018年 | 12篇 |
2017年 | 7篇 |
2016年 | 15篇 |
2015年 | 11篇 |
2014年 | 26篇 |
2013年 | 22篇 |
2012年 | 22篇 |
2011年 | 23篇 |
2010年 | 23篇 |
2009年 | 41篇 |
2008年 | 38篇 |
2007年 | 40篇 |
2006年 | 38篇 |
2005年 | 35篇 |
2004年 | 24篇 |
2003年 | 23篇 |
2002年 | 31篇 |
2001年 | 27篇 |
2000年 | 21篇 |
1999年 | 36篇 |
1998年 | 21篇 |
1997年 | 21篇 |
1996年 | 32篇 |
1995年 | 21篇 |
1994年 | 21篇 |
1993年 | 13篇 |
1992年 | 12篇 |
1991年 | 9篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1986年 | 5篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有752条查询结果,搜索用时 46 毫秒
101.
The fast neutron capture process (the r-process) occurs in the neutron-rich circumstance. However its concrete physical environment is not very clear. With recent progress in observations, many extremely metal-poor halo stars have been discovered. They have two characteristics: one is the overabundance of fast neutron elements with the relative abundance consistent with that of the sun; the other is that fast neutron element contents in stars at the same metal abundance have a very large dispersion. This provides a particular way to study the origin of the r-process. Simulation was used to study the galaxy's evolution process and the resulting dispersion of fast neutron nuclide contents in stars. The model of galaxy evolution obtained in this way not only contains spontaneous star formation in the gas region, but also includes the star formation excited by the supernova explosion. It is shown from our results that the supernovae at the low mass end should be the place producing the fast neutron nuclides. In addition, it is also shown that the non-uniformity of the galaxy evolution caused by the supernova explosion is not enough to explain the observed dispersion of fast neutron element contents in halo stars. This problem should be further studied. 相似文献
102.
In high-Mg, Al metapelites, monophase sapphirine corona occur around spinel–corundum aggregates in monomineralic cordierite
layers, and bi-phase orthopyroxene–sillimanite aggregates replace locally warped sapphirine in polygonized cordierite aggregates.
P–T phase topologies computed (Perple_X software) using compositions of cordierite-rich layers that host the reaction textures
did not match the assemblages for the discontinuous reactions spinel + corundum + cordierite → sapphirine and sapphirine + cordierite → orthopyroxene + sillimanite.
Instead, the reaction assemblages were reproduced using P–T pseudosection analysis for micro-domain reaction volumes estimated from compositions of product phases in the volume proportion
they occur. The results are consistent with known phase relations deduced using Schreinmakers P–T grids. Apparently, the compositions of cordierite-rich layers that hosted the reaction textures were inadequate chemical
proxies for determining P–T–X relations of phase-boundary controlled reactions influenced by compositions of the nearest-neighbor minerals in the proportion
they react (effective composition), and not in the proportion they existed in the layer/bulk rock. In other words, P–T–X phase topologies and reconstructed P–T paths in dry and aluminous rocks may be best understood by thermodynamic modeling of reactions using effective reaction volume
compositions rather than the bulk composition of the rock or the mineralogical layer that host the reaction textures. 相似文献
103.
Intracrystal microtextures formed by a process of mutual replacement in alkali feldspars record fluid–rock reactions that
have affected large volumes of the Earth’s crust. Regular, ≤1 μm-scale ‘strain-controlled’ perthitic microtextures coarsen,
by up to 103, by a dissolution–reprecipitation process, producing microporous patch or vein perthites on scales >100 μm. We have developed
earlier studies of such reactions in alkali feldspar cm-scale primocrysts in layered syenites from the Klokken intrusion,
South Greenland. We present new hyperspectral CL, SEM images, and laser ICPMS analytical data, and discuss the mechanism of
such replacement reactions. The feldspars grew as homogeneous sodic sanidines which unmixed and ordered by volume diffusion
during cooling into the microcline field at ~450°C, giving regular, fully coherent ‘braid’ cryptoperthite. At ≤450°C the crystals
reacted with a circulating post-magmatic aqueous fluid. The braid perthite behaved as a single reactant ‘phase’ which was
replaced by two product phases, incoherent subgrains of low albite and microcline, with micropores at their boundaries. The
driving force for the reactions was coherency strain energy, which was greater than the surface energy in the subgrain mosaic.
The external euhedral crystal shapes and bulk major element composition of the primocrysts were unchanged but they became
largely pseudomorphs composed of subgrains usually with the ‘pericline’ and ‘adularia’ habits (dominant {110} and subordinate
{010} morphology) characteristic of low T growth. The subgrains have an epitactic relationship with parent braid perthite. Individual subgrains show oscillatory zoning
in CL intensity, mainly at blue wavelengths, which correlates with tetrahedral Ti. Regular zoning is sometimes truncated by
irregular, discordant surfaces suggesting dissolution, followed by resumption of growth giving regular zoning. Zones can be
traced through touching subgrains, of both albite and microcline, for distances up to ~500 μm. At ≤340°C, the microcline subgrains
underwent a third stage of unmixing to give straight lamellar film perthites with periodicities of ~1 μm, which with further
cooling became semicoherent by the development of spaced misfit dislocations. Sub-grain growth occurred in fluid films that
advanced through the elastically strained braid perthite crystals, which dissolved irreversibly. Braid perthite was more soluble
than the strain-free subgrain mosaics which precipitated from the supersaturated solution. Some volumes of braid texture have
sharp surfaces that suggest rapid dissolution along planes with low surface energies. Others have complex, diffuse boundaries
that indicate a phase of coherent lamellar straightening by volume diffusion in response to strain relief close to a slowly
advancing interface. Nucleation of strain-free subgrains was the overall rate-limiting step. To minimise surface energy subgrains
grew with low energy morphologies and coarsened by grain growth, in fluid films whose trace element load (reflected in the
oscillatory zoning) was dictated by the competitive advance of subgrains over a range of a few tens of mm. The cross-cutting
dissolution surfaces suggest influxes of fresh fluid. Removal of feldspar to give 2 vol% porosity would require a feldspar:fluid
ratio of ~1:26 (by wt). The late reversion to strain-controlled exsolution in microcline subgrains is consistent with loss
of fluid above 340°C following depressurization of the intrusion. A second paper (Part II) describes trace element partitioning
between the albite and microcline subgrains, and discusses the potential of trace elements as a low-T geothermometer.
This paper and the Part II are dedicated in memory of J.V. Smith and W.L. Brown, both of whom died in 2007, in acknowledgement
of their unrivalled contributions to the study of the feldspar minerals over more than half a century. 相似文献
104.
Interplay between equilibrium and kinetics in prograde metamorphism of pelites: an example from the Nelson aureole, British Columbia 总被引:2,自引:0,他引:2
The distribution of metapelitic mineral assemblages in the Nelson aureole, British Columbia, generally conforms to what is predicted from phase equilibria. However, in detail, the sequence and spacing of isograds, mineral textures and mineral compositions and mineral chemical zoning do not. Two of the main disequilibrium features in the aureole are: (i) delay in the onset and progress of several reactions, i.e. overstepping in temperature; and (ii) unreactivity of staurolite and especially garnet porphyroblasts when they are reactants in prograde reactions. The thermal overstepping is ascribed to difficulty of nucleation of the product porphyroblasts and sluggishness of dissolution of porphyroblasts when they are reactants. The extent to which these kinetic barriers delay the onset of reaction is related to the reaction affinity of each reaction, defined herein as the Gibbs free‐energy difference between the thermodynamically stable, but not‐yet‐crystallized, products and the metastable reactants. For oversteps in temperature (ΔT), reaction affinity is, in turn, related to the difference in entropy (ΔS) between these two states through the relation A = ΔT * ΔS. Mineral reactions which release large quantities of H2O, such as chlorite‐consuming reactions, have a higher entropy change per unit of temperature overstep, and therefore a higher reaction affinity, than those which release little or no H2O, such as the chlorite‐free staurolite‐consuming reaction. Thermal overstepping is consequently expected to be less for the former than for the latter, as was estimated in the aureole where 0 to 30 °C overstepping was required for garnet, staurolite and andalusite growth from a muscovite + chlorite‐bearing precursor rock and ~70 °C overstepping was required for the growth of Al2SiO5 from a staurolite‐bearing, chlorite‐free precursor. In all cases, reaction progress was strongly influenced by the presence or absence of fluid, with presence of fluid lowering kinetic barriers to nucleation and growth and therefore the degree of thermal overstepping. Textural features of rocks from the nearly coincident garnet, staurolite and andalusite isograds are suggestive of a fluid‐catalysed ‘cascade effect’ in which reaction took place in a narrow temperature interval; several competing muscovite + chlorite‐consuming reactions, some metastable, appear to have occurred in parallel. Metamorphic reaction, fluid release and possibly fluid presence in general in the aureole were episodic rather than continuous, and in several cases well removed from equilibrium conditions. The extent to which these findings apply to regional metamorphism depends on several factors, a major one being enhanced deformation, which is expected to lower kinetic barriers to nucleation and growth. 相似文献
105.
Hironobu Harada Tatsuki Tsujimori Keitaro Kunugiza Katsuyuki Yamashita Shogo Aoki Kazumasa Aoki Hideko Takayanagi Yasufumi Iryu 《Island Arc》2021,30(1):e12389
Marble has a great potential to understand a history of various geological events occurring during tectonic processes. In order to decode metamorphic–metasomatic records on C–O isotope compositions of marble at mid-crustal conditions, we conducted a C–O–Sr isotope study on upper amphibolite-facies marbles and a carbonate–silicate rock from the Hida Belt, which was once a part of the crustal basement of the East Asian continental margin. Carbon and oxygen isotope analyses of calcite from marbles (Kamioka area) and a carbonate–silicate rock (Wadagawa area) show a large variation of δ13C [VPDB] and δ18O [VSMOW] values (from −4.4 to +4.2 ‰ and +1.6 to +20.8 ‰, respectively). The low δ13C values of calcites from the carbonate–silicate rock (from −4.4 to −2.9 ‰) can be explained by decarbonation (CO2 releasing) reactions; carbon–oxygen isotope modeling suggests that a decrease of δ13C strongly depends on the amount of silicate reacting with carbonates. The occurrence of metamorphic clinopyroxene in marbles indicates that all samples have been affected by decarbonation reactions. All δ18O values of calcites are remarkably lower than the marine-carbonate values. The large δ18O variation can be explained by the isotope exchange via interactions between marble, external fluids, and/or silicates. Remarkably low δ18O values of marbles that are lower than mantle value (~+5 ‰) suggest the interaction with meteoric water at a later stage. Sr isotope ratios (87Sr/86Sr = 0.707255–0.708220) might be close to their protolith values. One zircon associated with wollastonite in a marble thin-section yields a U–Pb age of 222 ± 3 Ma, which represents the timing of the recrystallization of marble, triggered by H2O-rich fluid infiltration at a relatively high-temperature condition. Our isotope study implies that the upper amphibolite-facies condition, like the Hida Belt, might be appropriate to cause decarbonation reactions which can modify original isotope compositions of marble if carbonates react with silicates. 相似文献
106.
白垩系砾岩及泥岩地层在长春地区普遍存在,但由于砾岩地层现场钻探难以取样进行室内土工试验,泥岩地层由于存在水稳性、膨胀性、崩解性等因素影响,砾岩和泥岩的承载力值、抗剪强度参数指标、渗透系数以及基床系数等参数多以经验数据评估方法为主,在合理准确确定相关参数的问题上存在较大分歧。通过对长春地区白垩系砾岩及泥岩进行原位平板载荷试验、浸水载荷试验以及现场原位剪切试验,获取砾岩、泥岩的承载力值、抗剪强度指标、渗透系数以及现场基床系数值等,为地铁建设工程地质问题分析及设计提供充足的地质依据。 相似文献
107.
撞击事件诱发核反应热力学过程及其在事件地质学研究中的意义 总被引:1,自引:0,他引:1
摘要:从大规模撞击事件的基本动力学和热力学公式推导出发,讨论了白垩纪-第三纪界线(K/T界线)事件能量随时间的释放过程,并对发生热核聚变反应的条件进行了分析。最后,还对其可能的事件地质学意义进行了讨论. 相似文献
108.
109.
110.
A study was conducted to examine the OH-initiated degradation products of the four title compounds in the presence of sub-part-per-million levels of NOx. The oxidation was conducted in a dynamic reactor to minimize the conversion of the aromatic compounds. The experiments were designed to represent reaction pathways that occur in the atmosphere at ambient NO2 concentrations. A wide range of ring-retaining and ring-cleavage products having widely varying yields were measured during the study. For m-xylene, the major primary products observed (with molar yields) were methyl glyoxal (0.40), 4-oxo-2-pentenal (0.12), glyoxal (0.079), and m-tolualdehyde (0.049). For p-xylene, the major primary products were p-tolualdehyde (0.103), 2,5-dimethylphenol (0.13), cis-3-hexene-2,5-dione (0.176), trans-3-hexene-2,5-dione (0.045), 2-methyl-butenedial (0.071), glyoxal (0.394), and methylglyoxal (0.217). Several other reaction products were measured at yields less than 3%. The primary products for OH + 1,2,4-trimethylbenzene were found as follows: methylglyoxal (0.44), glyoxal (0.066), cis-3-hexene-2,5-dione (0.13), trans-3-hexene-2,5-dione (0.031), biacetyl (0.114), 3-methyl-3-hexene-2,5-dione (0.079), and 2-methyl-butenedial (0.045). Six other (ring retaining) products were measured at yields less than 3%. The primary products for OH + 1,3,5-trimethylbenzene were methylglyoxal (0.90), 3-methyl-5-methylidene-5(2H)-furanone (0.1), 3,5-dimethyl-3(2H)-2-furanone (0.1), 3,5-dimethyl-5(2H)-2-furanone biacetyl (0.08), and 2-methyl-4-oxo-2- pentenal (0.05). Three other products were detected at molar yields less than 5%. In some cases, the yields for the ring fragmentation products could only be based on calibrations from surrogate compounds. Yields for several of the unsaturated dicarbonyl compounds have not been reported previously while yields for methylglyoxal, glyoxal, and biacetyl are largely consistent with previous reports. Some of the primary furanone products are the identical to those reported as secondary products in aromatic systems. 相似文献