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81.
Chemical composition of the phases occurring as corona structures in the Thessaloniki gabbros are subjected to algebraic method of treatment following Korzhinskii [1]. The analysis reveals that the mineralogical reactions responsible for the growth of the corona structures were possibly allochemical in nature in regard to the non-volatile components. The chemical similarity of the pyroxenes occurring within and beyond the corona structures in the gabbros appears compatible with such a model. 相似文献
82.
Stability of manganocordierite and related phase equilibria in part of the system MnO-Al2O3-SiO2-H2O
The pressure temperature stability of the phase Mn-cordierite hitherto not recorded as a mineral has been determined at temperatures ranging from 400° C up to the melting mainly using standard hydrothermal techniques at the oxygen fugacities provided by the buffering power of the bomb walls. Manganocordierite is a pronounced low-pressure phase with a maximum pressure stability of about 1 kb near 400° C and decreasing pressure limits at higher temperatures. Throughout the temperature range investigated the stable high-pressure breakdown assemblage of Mn-cordierite is spessartine, an Al-silicate, and a SiO2-polymorph. Due to the variable water contents of Mn-cordierite and spessartine there is a pronounced curvature in the negative dP/dT-slope of the requisite upper pressure breakdown curve of Mn-cordierite. Only theoretical deductions were possible concerning the stable hydrous low-temperature breakdown assemblage of Mn-cordierite below about 400° C.The manganocordierites synthesized are orthorhombic low-cordierites with distortion indices increasing with temperature, water pressure, and duration of heating. Their mean refractive indices increase with rising contents of absorbed water in the structural channels. Based on experiments with natural material the upper temperature stability limit of the mineral carpholite must lie at temperatures below about 400° C for water pressures up to 2.5 kb.The absence of Mn-cordierite from natural rocks studied thus far cannot be explained on chemical grounds, but must be due to its narrow pressure temperature stability range. The phase may yet be discovered as a mineral in manganiferous metasediments formed by lowpressure contact metamorphism. 相似文献
83.
Prabir Dasgupta 《Journal of Earth System Science》2005,114(3):287-302
In the Lower Gondwana succession of the Jharia basin of eastern India, the Barren Measures Formation is characterized by the
cyclic disposition of fine-grained lacustrine deposits and relatively coarse-grained fluvial deposits. The cyclic variation
in the rate of coarse clastic input is attributed to the sedimentary response to basin tectonics. The sandstone-shale alternations
of the Barren Measures succession can be correlated with the tectonic cyclothems developed on the hangingwall dip-slope and
adjoining trough in a continental half-graben setting. Enhancement of the gradient of the hangingwall dip-slope during reactivation
of the basin margin faults led to progradation of the existing fluvial system towards the half-graben trough and deposition
of the coarser clastics on the fine-grained lacustrine deposits of the trough. Peneplanation of the hangingwall slope and
slow increase in the lake level caused lacustrine transgression and retrogration of the fluvial system on the hangingwall
block. The fluvial sediments were onlapped by the fine-grained lacustrine deposits. Episodic rejuvenation of the basin margin
faults thus caused development of tectonic cyclothem on the hangingwall block. The paleocurrent pattern indicates that a persistent
northward paleoslope was maintained during Barren Measures sedimentation. The inferred depositional settings were much more
extensive than the present limit of the outcrop. The faults, presently defining the northern limit of the Barren Measures
Formation, were possibly emplaced after Barren Measures sedimentation. The final movement along these fault planes caused
preservation of the downthrown hangingwall block and the Barren Measures sediments on the footwall block were eroded during
subsequent denudation. The Southern Boundary Fault came into existence after the deposition of the Barren Measures sediments. 相似文献
84.
Somnath Dasgupta Jürgen Ehl Michael M. Raith Pulak Sengupta Pranesh Sengupta 《Contributions to Mineralogy and Petrology》1997,129(2-3):182-197
Pelitic rocks were thermally metamorphosed at the contact of the Chimakurthy mafic-ultramafic igneous complex, Eastern Ghats
Belt, India. The rocks show progressive change in mineralogy from biotite-sillimanite-quartz-garnet-K-feldspar (association
I, 150 m from the intrusive contact) to garnet-spinel-cordierite-K-feldspar-sillimanite (association II, 20–30 m from the
intrusive contact) to cordierite-K-feldspar-(cordierite-orthopyroxene-K-feldspar symplectite after osumilite)-spinel-FeTiAl
oxides with/without garnet (associations III and IV, 5 m from the intrusive contact), and finally to spinel-orthopyroxene-cordierite-K-feldspar
(association V, xenoliths). Oxide mineral clots in associations III and IV resemble emery-type rocks. Initial mineral reactions
involved biotite-dehydration melting with partial segregation of the melt. Down-temperature mineral reactions were largely
diffusion controlled and preservation of symplectitic and coronitic textures in microdomains is common. Interpretation of
reaction textures in relevant petrogenetic grids for the sytems KFMASH and FMAS and combined with geothermobarometry suggest
that the pelitic rocks were thermally metamorphosed at c. 6 kbar pressure along a heating-cooling trajectory within the temperature interval between c. 750 °C and c. 1000 °C.
Received: 20 October 1996 / Accepted: 17 June 1997 相似文献
85.
Mixing behavior in quaternary garnet solid solution and an extended Ellis and Green garnet-clinopyroxene geothermometer 总被引:7,自引:0,他引:7
Pulak Sengupta Somnath Dasgupta P. K. Bhattacharya Y. Hariya 《Contributions to Mineralogy and Petrology》1989,103(2):223-227
Applications of the different formulations of garnet-clinopyroxene geothermometers to crustal granulites and particularly to metamorphosed manganese formations show a wide scatter of estimates. This is primarily related to the uncertainty associated with the formulation of the activity-composition relationship in garnet solid solution besides that in the clinopyroxene solid solution. The largest uncertainty appears to have resulted from the adopted Mg-Mn interaction parameter in garnet which is yet to be experimentally determined. A fresh statistical regression of eleven pairs of garnet and clinopyroxene equilibrated at identical P-T conditions from a manganese formation in India yields W
Mg-Mn
Gt
(=WMn-Mg) value of 1600±500 cal/mols. An extension of the Ellis and Green (1979) geothermometer, known to work well in normal granulites, incorporating terms associated with the activity coefficients of the exchangeable cations, now yields consistent temperature estimation from normal granulites as well as from metamorphosed manganese formations. The geothermometer, however, will not work well for garnet having XFe0.65 and clinopyroxenes containing appreciable Na, Al and/or Fe3+. 相似文献
86.
P. K. Bhattacharyya Somnath Dasgupta M. Fukuoka Supriya Roy 《Contributions to Mineralogy and Petrology》1984,87(1):65-71
In the metamorphosed manganese oxide ores of India, braunite is ubiquitous in all assemblages from chlorite to sillimanite grades. Chemical analyses of braunite from different prograde assemblages confirm the presence of a fixed R2+ (=Mn2++Mg+Ca) SiO3 molecule in the mineral. Element partitioning between coexisting braunite and bixbyite indicates a near-ideal mixing of Fe+3/ -Mn+3 in the phases. This also indicates that braunite became relatively ferrian while equilibrating with associated phases such as bixbyite, hollandite and jacobsite during prograde reactions. Petrogenetic studies show that as a general trend, prograde lower oxide phases appeared by deoxidation of higher oxide phases. But braunite, a more reduced phase than bixbyite, appeared early from deoxidation of pyrolusite in presence of quartz. Bixbyite could appear later from the reacting pyrolusite-braunite-quartz assemblage. Inferred mineral reaction paths and the general trend of pro-grade deoxidation reactions suggest that the composition of ambient fluid phase was internally buffered during metamorphism. 相似文献
87.
The ultimate bearing capacity of two closely spaced strip footings, placed on a cohesionless medium and loaded simultaneously
to failure at the same magnitude of failure load, was determined by using an upper bound limit analysis. A logarithmic spiral
radial shear zone, comprising of a number of triangular rigid blocks, was assumed to exist around each footing edge. The equations
of the logarithmic spiral arcs were based on angles φL and φR rather than soil friction angle φ; the values of φL and φR were gradually varied in between 0 and φ. The ultimate bearing capacity was found to become maximum corresponding to a certain
critical spacing between the footings. For spacing greater than the critical, the bearing capacity was found to decrease continuously
with increase in the spacing. The extent of the spacing corresponding to which the ultimate bearing capacity becomes either
maximum or equal to that of a single isolated footing increases with increase in φ. The results compare reasonably well with
the available theoretical and experimental data. 相似文献
88.
The sandwich technique for determining the composition of partial melts in equilibrium with mantle lithologies may be a particularly
powerful method for determining melt compositions at the onset of melting if the method is applied iteratively. However, conventional
iterative sandwich experiments, in which the liquid from a preceding experiment is used as the “meat” of the sandwich in the
following experiment, may require many iterations before the melts produced can be directly relatable to the melting relations
of the target bulk rock composition. A modified iterative sandwich experimental (MISE) technique is proposed that may circumvent
many of the problems of more conventional techniques. Consideration of experimental uncertainties, including both random and
systematic errors in determination of partial melt compositions as well as the influence of errors in estimates of the solidus
temperature of the rock of interest, suggests that the MISE technique may produce robust results even when melt composition
errors are significant and that errors in estimation of the solidus location are detectable and therefore avoidable. 相似文献
89.
90.
Partial Melting Experiments of Peridotite + CO2 at 3 GPa and Genesis of Alkalic Ocean Island Basalts
We document compositions of minerals and melts from 3 GPa partialmelting experiments on two carbonate-bearing natural lherzolitebulk compositions (PERC: MixKLB-1 + 2·5 wt% CO2; PERC3:MixKLB-1 + 1 wt% CO2) and discuss the compositions of partialmelts in relation to the genesis of alkalic to highly alkalicocean island basalts (OIB). Near-solidus (PERC: 1075–1105°C;PERC3: 1050°C) carbonatitic partial melts with <10 wt%SiO2 and 40 wt% CO2 evolve continuously to carbonated silicatemelts with >25 wt% SiO2 and <25 wt% CO2 between 1325 and1350°C in the presence of residual olivine, orthopyroxene,clinopyroxene, and garnet. The first appearance of CO2-bearingsilicate melt at 3 GPa is 150°C cooler than the solidusof CO2-free peridotite. The compositions of carbonated silicatepartial melts between 1350 and 1600°C vary in the rangeof 28–46 wt% SiO2, 1·6–0·5 wt% TiO2,12–10 wt% FeO*, and 19–29 wt% MgO for PERC, and42–48 wt% SiO2, 1·9–0·5 wt% TiO2,10·5–8·4 wt% FeO*, and 15–26 wt% MgOfor PERC3. The CaO/Al2O3 weight ratio of silicate melts rangesfrom 2·7 to 1·1 for PERC and from 1·7 to1·0 for PERC3. The SiO2 contents of carbonated silicatemelts in equilibrium with residual peridotite diminish significantlywith increasing dissolved CO2 in the melt, whereas the CaO contentsincrease markedly. Equilibrium constants for Fe*–Mg exchangebetween carbonated silicate liquid and olivine span a rangesimilar to those for CO2-free liquids at 3 GPa, but diminishslightly with increasing dissolved CO2 in the melt. The carbonatedsilicate partial melts of PERC3 at <20% melting and partialmelts of PERC at 15–33% melting have SiO2 and Al2O3 contents,and CaO/Al2O3 values, similar to those of melilititic to basaniticalkali OIB, but compared with the natural lavas they are moreenriched in CaO and they lack the strong enrichments in TiO2characteristic of highly alkalic OIB. If a primitive mantlesource is assumed, the TiO2 contents of alkalic OIB, combinedwith bulk peridotite/melt partition coefficients of TiO2 determinedin this study and in volatile-free studies of peridotite partialmelting, can be used to estimate that melilitites, nephelinites,and basanites from oceanic islands are produced from 0–6%partial melting. The SiO2 and CaO contents of such small-degreepartial melts of peridotite with small amounts of total CO2can be estimated from the SiO2–CO2 and CaO–CO2 correlationsobserved in our higher-degree partial melting experiments. Thesesuggest that many compositional features of highly alkalic OIBmay be produced by 1–5% partial melting of a fertile peridotitesource with 0·1–0·25 wt% CO2. Owing to verydeep solidi of carbonated mantle lithologies, generation ofcarbonated silicate melts in OIB source regions probably happensby reaction between peridotite and/or eclogite and migratingcarbonatitic melts produced at greater depths. KEY WORDS: alkali basalts; carbonated peridotite; experimental petrology; ocean island basalts; partial melting 相似文献