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351.
Xenolith evidence for lithospheric melting above anomalously hot mantle under the northern Canadian Cordillera 总被引:2,自引:0,他引:2
Lang Shi Don Francis John Ludden Andrew Frederiksen Michael Bostock 《Contributions to Mineralogy and Petrology》1998,131(1):39-53
A comparison of mantle xenolith suites along the northern Canadian Cordillera reveals that the xenoliths from three suites
exhibit bimodal populations whereas the xenoliths from the other four suites display unimodal populations. The bimodal suites
contain both fertile lherzolite and refractory harzburgite, while the unimodal suites are dominated by fertile lherzolite
xenoliths. The location of the three bimodal xenolith suites correlates with a newly discovered P-wave slowness anomaly in
the upper mantle that is 200 km in width and extends to depths of 400–500 km (Frederiksen AW, Bostock MG, Van Decar JC, Cassidy
J, submitted to Tectonophysics). This correlation suggests that the bimodal xenolith suites may either contain fragments of
the anomalously hot asthenospheric mantle or that the lithospheric upper mantle has been affected by the anomalously hot mantle.
The lherzolite xenoliths in the bimodal suites display similar major element compositions and trace element patterns to the
lherzolite xenoliths in the unimodal suites, suggesting that the lherzolites represent the regional lithospheric upper mantle.
In contrast, the harzburgite xenoliths are highly depleted in terms of major element composition, but their clinopyroxenes
[Cpx] have much higher incompatible trace element contents than those in the lherzolite xenoliths. The major element and mildly
incompatible trace element systematics of the harzburgite and lherzolite xenoliths indicate that they could be related by
a partial melting process. The lack of textural and geochemical evidence for the former existence of garnet argues against
the harzburgite xenoliths representing actual fragments of the deeper anomalous asthenospheric mantle. Furthermore, the calculated
P-wave velocity difference between harzburgite and lherzolite end-members is only 0.8%, with the harzburgites having higher
P-wave velocities. Therefore the 3% P-wave velocity difference detected teleseismically cannot be produced by the compositional
difference between the lherzolite and harzburgite xenoliths. If temperature is responsible for the observed 3% P-wave velocity
perturbation, the anomalous mantle is likely to be at least 200 °C higher than the surrounding mantle. Taken together these
data indicate that the refractory harzburgite xenoliths represent the residue of 20–25% partial melting of a lherzolite lithospheric
mantle. The incompatible trace element enrichment of the harzburgites suggests that this melting was accompanied by the ingress
of fluids. The association of the bimodal xenolith suites with the mantle anomaly detected teleseismically suggests that anomalously
hot asthenospheric mantle provided both the heat and volatiles responsible for the localized melting and enrichment of the
lithospheric mantle.
Received: 16 May 1997 / Accepted: 25 October 1997 相似文献
352.
Serpentinization of cumulate ultramafic rocks from the North Arm Mountain massif of the Bay of Islands ophiolite 总被引:1,自引:0,他引:1
Partially serpentinized dunites and wehrlites comprise the bulk of the cumulate ultramafic unit at the North Arm Mountain massif of the Bay of Islands ophiolite complex, Newfoundland. In a suite of 59 dunites and werhlites from the base of the unit, the serpentinized portions consist of lizardite + chrysotile + brucite + (accessory) magnetite. The ratio of . Petrographic observations show that most serpentinization occurred at the expense of olivine; only limited amounts of clinopyroxene were serpentized. An estimated volume increase of 32% accompanied serpentinization of the peridotites. Reconstructions of the primary modal proportions of wehrlites (made taking this volume increase into account) contain an average of 6% more clinopyroxene and 6% less olivine than do modal reconstructions that ignore the volume increase. Mass balance calculations provide no clear evidence for appreciable metasomatism of Al2O3, CaO, FeO, MgO, or SiO2 during Serpentinization. The presence of brucite, the evidence that most serpentinization occurred at the expense of olivine, and the lack of appreciable metasomatism, suggest that the primary reaction that controlled serpentinization of the peridotites is: . olivine added serpentine brucite 相似文献
353.
Don Francis 《Contributions to Mineralogy and Petrology》1985,89(2-3):144-154
The Baffin Bay picrites have been the focal point of a controversy concerning the MgO content of primary magmas derived from the upper mantle. A sample population of 48 lava chilled margins collected across the Baffin Bay volcanic succession at the northeastern tip of Padloping Island exhibits a prominent compositional mode between 14 and 16 weight percent MgO (19–22 Mg, cation units = Mg/100 cations). The petrography of these samples, however, requires that the Padloping magmas were mixtures of olivine crystals and liquid at their eruption. Olivine phenocrysts constituted 15 to 30 volume percent of these magmas and retain compositions requiring coexisting liquid compositions with only 10 to 13.5 weight percent MgO (14–18.5 Mg). However, highly magnesian, olivine xenocrysts (up to Fo 93) found in the most magnesian lavas require the former existence of liquids with at least 18 weight percent MgO (24 Mg). If these xenocrysts represent early cumulates, then the primary liquids of the Padloping suite must have been at least this MgO rich with temperatures greater than 1,425° C. Such primary liquids could have evolved by olivine crystallization to a steady state, equilibrated crystal — liquid mixtures in a shallow reservoir system prior to eruption. The compositions of the liquids of these mixtures appear to have been perched at the point of plagioclase saturation at approximately 1,275° C.Despite the complications of mechanical sorting of olivine crystals, the virtual compositional reciprocity of olivine addition and olivine fractionation requires that the bulk compositions of picritic lavas provide compositional analogues of their primary magmas. A comparison of Phanerozoic picrite suites indicates that the Fe contents of terrestrial primary magmas of tholeiitic affinity have a restricted range from 6–9 Fe. Primary magmas associated with intra-plate volcanism appear to be distinctly more Fe-rich than those associated with inter-plate volcanism. The Al/Si ratios of Phanerozoic picrite suites could suggest that the primary magmas of MORB volcanism have equilibrated with relatively Fe-poor source regions at deeper levels in the Earth's mantle than those of other tholeiitic primary magmas. 相似文献
354.
Sixteen basic and intermediate composition igneous glacial erratics from Anglian (pre-423,000 years) deposits in Hertfordshire and Buckinghamshire, southern Britain, were selected for chemical and petrographic analysis in order to determine their original source outcrops. Major and trace element compositions suggest that seven samples (plus two uncertain) originated in the Lower Carboniferous volcanics of the Scottish Midland Valley (SMV), four came from the Upper Carboniferous quartz dolerite association which crops out in Scotland, northern England (Whin Sill) and extends to Norway, and one came from the northern England Cleveland Dyke. One sample of altered dolerite is ambiguous but has some similarity to the Old Red Sandstone (Devonian) age lavas of the SMV, and one meta-basalt sample may be from southwest Scotland or Scandinavia. These results identify specific outcrops which provided glacial erratics within currently accepted ice trails in the United Kingdom, and provide the first supporting evidence based on geochemistry, rather than petrography, for these ice movements. The distribution and provenance of glacial erratics are of importance in archaeological studies, because erratics provided a potential source of raw material for stone implement production. There is a marked geographical correlation between the distribution of prehistoric stone implements of quartz dolerite in the United Kingdom, and directions of ice movements from quartz dolerite outcrops within Britain. This correlation lends support to the hypothesis that prehistoric man made extensive use of glacial erratics for implement manufacture, as an alternative to quarrying at outcrops and subsequent long-distance trade. © 1999 John Wiley & Sons, Inc. 相似文献