全文获取类型
收费全文 | 24417篇 |
免费 | 172篇 |
国内免费 | 916篇 |
专业分类
测绘学 | 1410篇 |
大气科学 | 1977篇 |
地球物理 | 4504篇 |
地质学 | 11598篇 |
海洋学 | 1003篇 |
天文学 | 1633篇 |
综合类 | 2161篇 |
自然地理 | 1219篇 |
出版年
2023年 | 1篇 |
2021年 | 1篇 |
2020年 | 1篇 |
2018年 | 4761篇 |
2017年 | 4037篇 |
2016年 | 2577篇 |
2015年 | 233篇 |
2014年 | 82篇 |
2013年 | 30篇 |
2012年 | 989篇 |
2011年 | 2730篇 |
2010年 | 2015篇 |
2009年 | 2311篇 |
2008年 | 1888篇 |
2007年 | 2360篇 |
2006年 | 52篇 |
2005年 | 197篇 |
2004年 | 402篇 |
2003年 | 409篇 |
2002年 | 249篇 |
2001年 | 48篇 |
2000年 | 51篇 |
1999年 | 13篇 |
1998年 | 22篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1976年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 32 毫秒
991.
Hugues Raimbourg Bruno Goffé Laurent Jolivet 《Contributions to Mineralogy and Petrology》2007,153(1):1-28
Caledonian eclogite-facies metamorphism partially reworking Grenvillian granulite-facies anorthosite allows us to study the processes of garnet reequilibration at high pressure and to reconstruct the evolution of the unit near metamorphic peak conditions. Our results indicate that eclogite-facies metamorphism happened in two successive phases: first, inherited granulitic garnet was fractured and reequilibrated from their boundaries (crystal or fracture rims); then eclogite-facies minerals were crystallised in the fractures as overgrowths on inherited garnets. The reequilibration of inherited garnets is achieved through Fe2+Mg−1 exchange, whereas eclogite-facies garnets crystallised during the subsequent phase are notably richer in Ca than un- and re-equilibrated granulitic garnet. Pseudosection construction shows that this lack in Ca reequilibration cannot be related to variations in thermodynamic conditions (a
H2O, reacting system composition) between the two phases. From the compilation of the available data, the reequilibration of granulitic garnet seems to be controlled by the inefficient intra- and inter-granular transport properties of Ca compared to Fe2+ and Mg. While these kinetic factors confine garnet reequilibration to Fe2+Mg−1 exchange, the extent of reequilibration along this exchange vector is controlled by partitioning with adjacent omphacite. On the contrary to the diffusional reequilibration of granulitic garnet that lasted for several My according to our modelling of the diffusional relaxation, the strong compositional gradients between eclogite-facies and reequilibrated garnets, which are almost unaffected by diffusional reequilibration, provide evidence that rapid exhumation followed the crystallisation of eclogite-facies minerals. We propose that the movement reversal itself, from burial to exhumation, and associated deformation and fluid flow, triggered this crystallisation event. The resulting evolution near metamorphic peak conditions is therefore strongly asymmetrical: on the one hand, the prograde diffusional relaxation profiles indicate slow movement during the last stages of burial, whereas the unaffected retrograde overgrowth indicates fast exhumation rates. 相似文献
992.
993.
Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe,western Alps 总被引:1,自引:0,他引:1
Thomas J. Lapen Clark M. Johnson Lukas P. Baumgartner Giorgio V. Dal Piaz Susanne Skora Brian L. Beard 《Contributions to Mineralogy and Petrology》2007,153(2):139-157
High precision U–Pb geochronology of rutile from quartz–carbonate–white mica–rutile veins that are hosted within eclogite
and schist of the Monte Rosa nappe, western Alps, Italy, indicate that the Monte Rosa nappe was at eclogite-facies metamorphic
conditions at 42.6 ± 0.6 Ma. The sample area [Indren glacier, Furgg zone; Dal Piaz (2001) Geology of the Monte Rosa massif: historical review and personal comments. SMPM] consists of eclogite boudins that are exposed
inside a south-plunging overturned synform within micaceous schist. Associated with the eclogite and schist are quartz–carbonate–white
mica–rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist.
Intrusion of the veins at about 42.6 Ma occurred at eclogite-facies metamorphic conditions (480–570°C, >1.3–1.4 GPa) based
on textural relations, oxygen isotope thermometry, and geothermobarometry. The timing of eclogite-facies metamorphism in the
Monte Rosa nappe determined in this study is identical to that of the Gran Paradiso nappe [Meffan-Main et al. (2004) J Metamorphic Geol 22:261–281], confirming that these two units have shared the same Alpine metamorphic history. Furthermore,
the Gran Paradiso and Monte Rosa nappes underwent eclogite-facies metamorphism within the same time interval as the structurally
overlying Zermatt-Saas ophiolite [∼50–40 Ma; e.g., Amato et al. (1999) Earth Planet Sci Lett 171:425–438; Mayer et al. (1999) Eur Union Geosci 10:809 (abstract); Lapen et al. (2003) Earth Planet Sci Lett 215:57–72]. The nearly identical P–T–t histories of the Gran Paradiso, Monte Rosa, and Zermatt-Saas units suggest that these units shared a common Alpine tectonic
and metamorphic history. The close spatial and temporal associations between high pressure (HP) ophiolite and continental
crust during Alpine orogeny indicates that the HP internal basement nappes in the western Alps may have played a key role
in exhumation and preservation of the ophiolitic rocks through buoyancy-driven uplift. Coupling of oceanic and continental
crust may therefore be critical in preventing permanent loss of oceanic crust to the mantle. 相似文献
994.
We report on the use of X-ray diffractometry under controlled conditions of temperature and relative humidity (RH-XRD) for
the investigation of NaCl deliquescence in the pore space of glass filter frits, which were used as model substrates. The
study confirms that RH-XRD is an appropriate experimental technique for the in situ observation of phase transformation in
porous materials. It is used for an investigation of both the deliquescence kinetics and the deliquescence humidity within
pores of different median pore diameter. Several major influences affecting deliquescence rates in the pore space close to
the surface of a porous material are discussed. It appears that quite short-term variation of ambient relative humidity, e.g.,
typical daily fluctuations, might induce damaging deliquescence–crystallization cycles within the pore space of building materials.
In agreement with theoretical considerations it was found that confinement of NaCl crystals in pores with median diameters
in the range 1.4–70 μm does not affect the deliquescence humidity of the salt. 相似文献
995.
Julien Berger Olivier Féménias Nicolas Coussaert Jean-Claude C. Mercier Daniel Demaiffe 《Contributions to Mineralogy and Petrology》2007,153(5):557-575
Ultramafic and mafic xenoliths of magmatic origin, sampled in the Beaunit vent (northern French Massif Central), derive from
the Permian (257 Ma) Beaunit layered complex (BLC) that was emplaced at the crust-mantle transition zone (∼1 GPa). These plutonic
xenoliths are linked to a single fractional crystallisation process in four steps: peridotitic cumulates; websteritic cumulates;
Al-rich mafic cumulates (plagioclase, pyroxenes, garnet, amphibole and spinel) and finally low-Al mafic cumulates. This sequence
of cumulates can be related to the compositional evolution of hydrous Mg basaltic magma that evolved to high-Al basalt and
finally to andesitic basalt. Sr and Nd isotopic compositions confirm the co-genetic character of the various magmatic xenoliths
and argue for an enriched upper mantle source comparable to present mantle wedges above subduction zones. LILE, LREE and Pb
enrichment are a common feature of all xenoliths and argue for an enriched sub-alkaline transitional parental magma. The existence
of a Permian magma chamber at 30 km depth suggests that the low-velocity zone observed locally beneath the Moho probably does
not represent an anomalous mantle but rather a sequence of mafic/ultramafic cumulates with densities close to those of mantle
rocks. 相似文献
996.
W. N. Mussel E. Murad J. D. Fabris W. S. Moreira J. B. S. Barbosa C. C. Murta W. P. Abrahão J. W. V. De Mello V. K. Garg 《Physics and Chemistry of Minerals》2007,34(6):383-387
We studied a chalcopyrite from a Cu ore deposit in Rio Grande do Sul, Brazil, by Mössbauer spectroscopy at room temperature and 110 K. Supporting methods to check for sample purity and to characterize further sample properties were slow-scanning X-ray powder diffraction and optical and microprobe analyses of polished sections of selected grains. Chemical analyses obtained using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer showed the sample to consist of homogeneous and essentially stoichiometric chalcopyrite (CuFeS2). Mössbauer spectra taken at both the above temperatures consist of asymmetric magnetically ordered patterns with unequal intensities of the line pairs 1–6 and 2–5, pointing to the existence of non-equivalent or multiple Fe sites. Least-squares fittings evidenced that the resonance intensity ratio of subspectrum with lower quadrupole shift (indicative of a more symmetric environment) to that with higher shift is 69:31, at room temperature, and 68:32, at 110 K. Rietveld refinement of the XRD data indicates the existence of tetragonal [a = 0.52855(1) and c = 1.0412(1) nm] and cubic [a 0 = 0.5273(2) nm] modifications in a proportion of 74:26, in good agreement with the Mössbauer data. The saturation magnetization of the sample was 32.7 J/(T kg), confirming the oxidation state of Fe as trivalent and pointing to little to no spin canting. 相似文献
997.
998.
Pakama Queenscious Siyongwana 《GeoJournal》2009,74(4):293-310
The demise of South African Apartheid Planning in 1994 and subsequent lost of Umtata’s capital status when the Transkei was
subsumed into the new Eastern Cape Province resulted in the major political transformation of the Transkei state. Central
to the post-apartheid transformation was restructuring of Transkei bureaucracy which at the time of South Africa’s independence
in 1994 displayed abnormalities. This paper documents the restructuring of the Transkei bureaucracy focusing on Umtata since
1994. The study has brought to the forefront the following facts: Firstly, that at the time of the Transkei merger into South
Africa, employment in the government was ‘booming’ and to greater extent it was affected by Umtata’s role—being the capital
city of the pseudo-Transkei state. Secondly, the post 1994-political transformation of the Transkei Bantustan impacted negatively
on Umtata’s employment notably the civil service sector by ‘squeezing’ it during the early years of democracy (1994–2000).
Thirdly, since 2001, with Umtata serving as the major urban centre of both King Sabata Local Municipality and OR Tambo District
Municipality, employment in the civil service and municipality has been revitalized. 相似文献
999.
Transparent exopolymer particle (TEP) distributions were examined in North Carolina’s Neuse River Estuary (NRE) over a 1-year
period, and experiments were conducted to examine controls upon TEP formation. TEP-carbon averaged 16% of the total organic
carbon pool in the NRE. From May to early August 2007, TEP ranged from 805 to 1,801 μg xanthan gum (XG) L−1. A large phytoplankton bloom peaked in early August and then subsequently declined. Within 2 weeks of the bloom’s peak, TEP
concentrations increased to >3,500 μg XG L−1 and remained elevated through mid-December. Decreasing water temperatures and enhanced retention due to drought conditions
in Fall 2007 likely aided in the persistence of TEP concentrations during this timeframe. Thereafter, TEP decreased coincident
with increased river flow and flushing of the estuary that began in January 2008, and TEP remained low (ranging from 991 to
1,712 μg XG L−1) until the end of April 2008. TEP was positively correlated with salinity, suggesting that cation availability (which co-varies
with salinity and stabilizes the structural integrity of TEP) may play a role in estuarine TEP formation. In two cation addition
experiments using water from the mesohaline region, TEP concentrations tended to be higher in treatments with vs. without
cations added. Statistically significant relationships were also found between in situ TEP concentrations and pH, mixing,
and temperature, suggesting that a complex suite of environmental parameters affect TEP formation and its distribution in
estuaries. 相似文献
1000.
Long records of monthly salinity observations along the axis of Chesapeake Bay, Delaware Bay, and Long Island Sound are used
to test a simple advection–dispersion model of the salt distribution in linearly tapered estuaries developed in a previous
paper. We subdivide each estuary into three to five segments, each with linear taper allowing a distributed input of fresh
water, and evaluate the dispersion in each segment. While Delaware Bay has weak dispersion and a classical sigmoidal salinity
structure, Long Island Sound and Chesapeake Bay are more dispersive and have relatively small gradients in the central stretches.
Long Island Sound is distinguished by having a net volume and salt flux out of its low-salinity end resulting in a smaller
range of salinity and increasing axial gradients at its head rather than the usual asymptotic approach to zero salinity. Estimates
of residence times based on model transport coefficients show that Long Island Sound has the most rapid response to fresh-water
flux variations. It also has the largest amplitude cycle in river discharge fluctuation. In combination, these cause the large
seasonal variation in the salinity structure relative to interannual variability in Long Island Sound as compared with Chesapeake
Bay and Delaware Bay. 相似文献