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51.
The Effect of Postcumulus Reactions on Composition of Chrome-spinels from the Jimberlana Intrusion 总被引:2,自引:2,他引:2
Chromites in olivine adcumulates, mesocumulates and orthocumulatesfrom drill core of the Jimberlana intrusion have been analysedand related to the cumulate type and to the nature of the surroundingsilicate mineral. Chromites in adcumulates and mesocumulatesshow a restricted range of composition and are high in Mg, Aland Cr. The orthocumulate chromites vary in composition fromthat found in adcumulates to chromites which are much higherin Fe and Ti and with a higher Fe3?: Fe2? ratio. The chromitesin orthocumulates vary in composition depending upon the natureof the enclosing silicate mineral. This is believed to reflectthe ability of the enclosing mineral to protect the originalcumulus chromite from reaction with the intercumulus liquid.Thus chromite within early bronzite oikocrysts was protectedfrom reaction whereas that in plagioclase and phlogopite wasprotected at a much later stage and has a higher Fe and Ti contentChromite within olivine appears to have been able to equilibratewith intercumulus liquid until late in the magmatic historyexcept where the olivine enclosing chromite has itself beensurrounded by bronzite. It is suggested that chromite can exchangeelements with intercumulus liquid through the olivine. Thereare two possibilities; either elements such as Cr, Al, Ti andFe3 ? were able to diffuse through the olivine structure orthe apparently enclosed chromite crystals were able to maintaindirect contact with the melt along fine fractures produced bythe differential thermal contraction of olivine and chromite. The average diameters of chromite crystals within orthocumulatebronzite and olivine are 28 and 20 microns respectively whereaschromites in plagioclase and phlogopite have average diametersof 48 and 56 microns. There is no obvious correlation betweenthe size of the chromite and their composition for grains foundwithin a particular silicate. Chromites of every size have beenable to equilibrate with the liquid unless they were protectedfrom reaction. Nucleation of reaction products played an important role indetermining the final composition of any particular chromitecrystal. The significance of reaction and nucleation on a localscale of millimetres is considered with respect to the majorsilicates and to the location of the last liquid. It is suggestedthat the last liquid tended to concentrate in pockets of reactantcrystals, where product crystals failed to nucleate until latein the magmatic history. It is estimated that in rocks withan orthocumulate texture, the intercumulus liquid crystallizedover a temperature range as large as 300 ?C and that it becamesignificantly more oxidizing near the solidus temperature. 相似文献
52.
Mineralogy, Chemistry, and Genesis of the Boninite Series Volcanics, Chichijima, Bonin Islands, Japan 总被引:12,自引:1,他引:11
TAYLOR REX. N.; NESBITT ROBERT W.; VIDAL PHILLIPE; HARMON RUSSELL S.; AUVRAY BERNARD; CROUDACE IAN W. 《Journal of Petrology》1994,35(3):577-617
The Bonin archipelago represents an uplifted fore-arc terrainwhich exposes the products of Eocene supra-subduction zone magmatism.Chichijima, at the centre of the chain, represents the typelocality for the high-Mg andesitic lava termed boninite. Therange of extrusives which constitute the boninite series volcanicsare present on Chichijima, and are disposed in the sequenceboninite-andesite-dacite with increasing height in the volcano-stratigraphy.Progression to evolved compositions within the Chichijima boniniteseries is controlled by crystal fractionation from a boniniteparental magma containing 15% MgO. Olivine and clinoenstatiteare the initial liquidus phases, but extraction of enstatiticorthopyroxene, followed by clinopyroxene and plagioclase, isresponsible for the general evolution from boninite, throughandesite, to dacite. Some andesites within the overlying MikazukiyamaFormation are petrographically distinct from the main boniniteseries in containing magnetite phenocrysts and a high proportionof plagioclase. As such, these andesites have affinities withthe calc-alkaline series. Major and trace element data for 74 boninitic series rocks fromChichijima are presented. Although major element variation isdominantly controlled by high-level crystal fractionation, thelarge variations in incompatiable trace element concentrationsat high MgO compositions cannot be explained by this mechanism.Nd, Pb, and Sr isotopic data indicate the following: (1) a strongoverprint on 87Sr/86Sr by seawater alteration; (2) Pb isotopeslie above the northern hemisphere reference line (NHRL) andare thus similar to the <30-Ma are and basin lavas of theIzuBonin system, and (3) Nd(40 Ma) ranges between 2.8and 6.8 within the boninite series volcanics. Differences inrare-earth elements (REE), Zr, Ti, and 143Nd/144Nd at similardegrees of fractionation can be explained by the addition ofa component of fixed composition from the down-going oceaniccrustal slab to a variably depleted source region within theoverlying wedge. Data presented for Sm/Zr and Ti/Zr indicatethat boninite series volcanics are characterized by low valuesfor both of these ratios. In particular, boninites appear tohave uniquely low Sm/Zr ratios. These characteristics may bethe result of slab melting in the presence of residual amphibole;the resultant melt could combine with typical slab dehydrationfluids and infiltrate the overlying mantle wedge. Such a fluidmeltcomponent could mix either with shallow mantle or directly withprimitive melts from depleted mantle. Trace elements, REE, andisotope data thus point to a model for boninite genesis whichrequires tightly constrained pressuretemperature conditionsin the slab combined with melting of a variably depleted sourcein the overlying wedge. Such constraints are rarely met exceptduring the subduction of juvenile oceanic crust beneath a young,hot overriding plate. 相似文献
53.
High-Temperature Fluid-Rock Interaction and Scapolitization in an Extension-Related Hydrothermal System, Mary Kathleen, Australia 总被引:1,自引:0,他引:1
OLIVER NICHOLAS H. S.; RAWLING TIMOTHY J.; CARTWRIGHT IAN; PEARSON PAUL J. 《Journal of Petrology》1994,35(6):1455-1491
In the polymetamorphic Mary Kathleen Fold Belt, at the centreof the Proterozoic Mount Isa Inlier, Queensland, an early phaseof extension (17601730 Ma) resulted in intrusion of voluminousgranitic and doleritic magmas into the carbonate-evaporite-dominatedCorella Formation at 510 km depths. Widespread high-temperaturemetasomatism ensued, involving scapolitization in dolerite,formation of albite-scapolite shear zones in granite, exo- andendoskarn formation, and a zone of KNaCa alterationin the lowermost Corella Formation. Granites and dolerites werealtered to an unusual NaCa-rich bulk composition, reflectinghigh-temperature infiltration of highly saline, chemically reactiveexternally derived fluid. The alteration products and theirdistribution suggest not only reaction of magmatic/aqueous fluidswith the country rocks but also extensive halite dissolutionand recirculation of saline fluids back into the intrusive bodies.The bulk of fluid flow occurred at high temperatures (500700C), and major element and isotopic fronts were generally notsmoothed out by the effects of temperature gradients, with theexception of one part of the system which shows evidence forup-temperature fluid flow. Analysis of oxygen isotopic dataand the position of isotopic and geochemical fronts revealstime-integrated fluid fluxes of up to 2 104 m3/m2 for themetasomatism. Although very high salinities (up to 50 mol% NaCl)were attained by evaporite dissolution, 18O values of most alterationproducts are in the range 712%, reflecting a predominanceof oxygen derived from an igneous fluid. The position and interrelationshipsof metasomatic and isotopic fronts indicate an earlier stageof infiltration dominated by fluid released from crystallizinggranite (with 18O 1012%), and a later stage (18O 79%)in which fluid had already interacted with halite and a mixedmafic-felsic igneous source or was repeatedly circulated betweenthese rock types during alteration. The data reflect only aminimal contribution from fluids produced by devolatilizationof the abundant carbonate-bearing rocks in the Corella Formation,and there are substantial areas of Corella Formation rocks thathave escaped metasomatism during this phase of intrusion-relatedhydrothermal activity and during the subsequent regional metamorphicoverprint. Along with the requirement that the fluids dissolvedlarge amounts of halite from the same sequence, and the structuralobservations, we favour a model where fluid was preferentiallychannelled along specific permeable conduits, including formerevaporite layers, before interaction with the now exposed alteredrocks. Fluid was probably driven by both convective circulationand dilatancy-related deformation accompanying emplacement ofmagmas into a major crustal extensional dcollement. 相似文献
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Polar bear hunting success on ringed seals in subnivean lairs and basking on the fast ice, was studied in Hornsund, Svalbard. A few bears were observed while actually hunting, but in most cases bear tracks were followed by snowmobile to determine the outcome of hunts. Seal carcasses were classified from size and tooth annuli as pups, juveniles or adults. A total of 62 subnivean structures were entered by bears, with six seals killed. One of ten charges on basking seals resulted in a kill. Success rates of bears in Hornsund arc compared with results of studies in Canada and Svalbard. OHunting success, polar bear, predation, ringed seal 相似文献
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The South Auckland Volcanic Field is a Pleistocene (1·590·51Ma) basaltic intraplate, monogenetic field situated south ofAuckland City, North Island, New Zealand. Two groups of basaltsare distinguished based on mineralogy and geochemical compositions,but no temporal or spatial patterns exist in the distributionof various lava types forming each group within the field: GroupA basalts are silica-undersaturated transitional to quartz-tholeiiticbasalts with relatively low total alkalis (3·04·6wt %), Nb (729 ppm), and (La/Yb)N (3·47·6);Group B basalts are strongly silica-undersaturated basanitesto nepheline-hawaiites with high total alkalis (3·37·9wt %), Nb (32102 ppm), and (La/Yb)N (1247). GroupA has slightly higher 87Sr/86Sr, similar Nd, and lower 206Pb/204Pbvalues compared with Group B. Contrasting geochemical trendsand incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) areconsistent with separate evolution of Groups A and B from dissimilarparental magmas derived from distinct sub-continental lithosphericmantle sources. Differentiation within each group was controlledby olivine and clinopyroxene fractionation. Group B magmas weregenerated by <8% melting of an ocean island basalt (OIB)-likegarnet peridotite source with high 238U/204Pb mantle (HIMU)and enriched mantle (EMII) characteristics possibly inheritedfrom recycled oceanic crust. Group A magmas were generated by<12% melting of a spinel peridotite source also with HIMUand EMII signatures. This source type may have resulted fromsubduction-related metasomatism of the sub-continental lithospheremodified by a HIMU plume. These events were associated withMesozoic or earlier subduction- and plume-related magmatismwhen New Zealand was at the eastern margin of the Gondwana supercontinent. KEY WORDS: continental intraplate basalts; geochemistry; HIMU, EMII; Sr, Nd, and Pb isotopes; South Auckland; sub-continental lithospheric sources 相似文献
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