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21.
Fluid-present anatexis of metapelites at El Joyazo (SE Spain): constraints from Raman spectroscopy of graphite 总被引:2,自引:0,他引:2
The garnet-biotite-sillimanite anatectic xenoliths in the Neogene dacite dome of El Joyazo (also called Cerro de Hoyazo,
SE Spain) contain four types of graphite (I to IV), distinguished on the basis of grain size and texture. Structural characterization
of graphite by Laser Raman spectroscopy (LRS) shows systematic differences in the degree of ordering among the four types:
only type III is fully consistent with the granulite-facies conditions reached by the xenoliths during partial melting, the
others indicate metamorphic temperatures covering amphibolite-facies conditions, with only a few examples of granulite-grade
crystallinity. All graphite crystallized before or during the anatectic event, indicating that a large fraction of the graphite
did not equilibrate at peak temperatures. The mm-scale coexistence of different types and degrees of ordering in the graphite
suggests different origins, i.e. of biogenic derivation and “fluid-deposited”, and is explained in terms of fluid-melt-graphite
interaction during the anatectic event. Disequilibrium behaviour during high-temperature metamorphism and anatexis is typical
of types I, II and particularly of IV, and is attributed to sluggish kinetics of solid-state graphitization, mainly owing
to the limited time of the process and carbon saturation of the intergranular fluid. The coexisting, well-ordered type III
graphite is the product of melting in the presence of a graphite-saturated fluid, a process that would account for the deposition
of new graphite. The LRS results, together with petrologic observations, suggest that it is possible that high melt fractions
can be generated by fluid-present melting of a metasedimentary protolith also in a closed system. Although this contradicts
the commonly accepted hypothesis that, due to limited rock porosities, extensive fluid-present melting is precluded unless
infiltration occurs, it is a possible end-member model in anatectic settings characterized by rapid heating rates and low-grade
source rocks.
Received: 14 July 1998 / Accepted: 16 November 1998 相似文献
22.
Bernardo?CesareEmail author Sandro?Meli Luca?Nodari Umberto?Russo 《Contributions to Mineralogy and Petrology》2005,149(2):129-140
The Fe3+/Fetot of all Fe-bearing minerals has been analysed by Mössbauer spectroscopy in a suite of biotite-rich to biotite-free graphitic metapelite xenoliths, proxies of an amphibolite-granulite transition through progressive biotite melting. Biotite contains 9 to 16% Fe3+/Fetot, whereas garnet, cordierite and ilmenite are virtually Fe3+ -free (0–1% Fe3+/Fetot) in all samples, regardless of biotite presence. Under relatively reducing conditions (graphite-bearing assemblages), biotite is the only carrier of Fe3+ during high-temperature metamorphism; therefore, its disappearance by melting represents an important event of iron reduction during granulite formation, because haplogranitic melts usually incorporate small amounts of ferric iron. Iron reduction is accompanied by the oxidation of carbon and the production of CO2, according to the redox reaction:
Depending on the nature of the peritectic Fe-Mg mineral produced (garnet, cordierite, orthopyroxene), the CO2 can either be present as a free fluid component, or be completely stored within melt and cordierite. The oxidation of graphite by iron reduction can account for the in situ generation of CO2, implying a consequential rather than causal role of CO2 in some granulites and migmatites. This genetic model is relevant to graphitic rocks more generally and may explain why CO2 is present in some granulites although it is not required for their formation. 相似文献
23.
24.
Ana?C.?L.?SáEmail author José?M.?C.?Pereira Martin?E.?Charlton Bernardo?Mota Paulo?M.?Barbosa A.?Stewart Fotheringham 《Journal of Geographical Systems》2011,13(3):227-248
This study analyses the relationship between fire incidence and some environmental factors, exploring the spatial non-stationarity
of the phenomenon in sub-Saharan Africa. Geographically weighted regression (GWR) was used to study the above relationship.
Environment covariates comprise land cover, anthropogenic and climatic variables. GWR was compared to ordinary least squares,
and the hypothesis that GWR represents no improvement over the global model was tested. Local regression coefficients were
mapped, interpreted and related with fire incidence. GWR revealed local patterns in parameter estimates and also reduced the
spatial autocorrelation of model residuals. All the covariates were non-stationary and in terms of goodness of fit, the model
replicates the data very well (R
2 = 87%). Vegetation has the most significant relationship with fire incidence, with climate variables being more important
than anthropogenic variables in explaining variability of the response. Some coefficient estimates exhibit locally different
signs, which would have gone undetected by a global approach. This study provides an improved understanding of spatial fire–environment
relationships and shows that GWR is a valuable complement to global spatial analysis methods. When studying fire regimes,
effects of spatial non-stationarity need to be incorporated in vegetation-fire modules to have better estimates of burned
areas and to improve continental estimates of biomass burning and atmospheric emissions derived from vegetation fires. 相似文献
25.
Castillo-Sandoval Isaí Fuentes-Cobas Luis E. Pérez-Cazares Bernardo E. Esparza-Ponce Hilda E. Fuentes-Montero María E. Castillo-Michel Hiram Eichert Diane Reyes-Cortes Ignacio Carreño-Márquez Iván J. Napoles-Duarte José M. Montero-Cabrera María E. 《Mineralogy and Petrology》2018,112(6):865-879
Mineralogy and Petrology - The Cave of Swords was discovered in 1910 at Naica, Chihuahua, México. During the last century, human presence has changed the microclimate conditions inside this... 相似文献
26.
27.
This paper reports on the contribution of tourism to the wellbeing of rural residents through the development of economic linkages engaged by community‐based tourism (CBT) in rural Costa Rica. In a qualitative case study of local economic linkages surrounding one such project in Chira Island, economic linkages were assessed at two levels: discourse and practice. The findings indicate that CBT does not involve the collective property of the community, but rather, the collective property of a group of community members organized in a formal association. As a result, a discourse on local economic linkages has been promoted by CBT support organizations in which hopes of wider benefits are placed on small linkages to services and products provided by local community members. However, a field survey suggested that the economic linkages generated by CBT in the community were sporadic and polyvalent and, furthermore, that the linkages with agriculture are negatively affected by scale and seasonality, resulting in leakages out of the community. These findings caution practitioners that CBT may only have small‐scale positive impacts on the local economy. 相似文献
28.
Daiani Kochhann Everton Rodolfo Behr Adilson de Chaves Marcia F. Mesko Valderi L. Dressler Erico M. M. Flores Bernardo Baldisserotto 《Environmental Earth Sciences》2013,70(4):1727-1733
The water quality of the Vacacaí River was assessed at different sites in the period between winter 2005 and autumn 2006. All samples were analyzed for 52 elements (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, Li, Be, Mg, Al, Ca, V, Cr, Mn, Fe, Ni, Co, Cu, Zn, Ga, As, Se, Rb, Sr, Ag, Cd, In, Cs, Ba, Tl, Pb, Bi, U, Na, K, Hg, B, Mo, Sn, Te, Ti), temperature, pH, ammonia, and alkalinity levels. Water from the Vacacaí River ranged from slightly acidic to alkaline. No difference was observed in the chemical composition at different sites of the Vacacaí River. Levels of Ba, Ca, Sr and Mg increase in the dry seasons and reach their highest concentrations in autumn; Be and U decrease in the dry season and reach their highest concentration in spring. Al, Fe, Cr, Ni, Th, U Mn, Ca and Mg are highly positively related, indicating a common origin. Se and Cu are probably from anthropogenic source, from the rice crops of the margins of the river. Waterborne Al and Fe levels were above the desirable level for drinking water at all sites during all seasons. These results demonstrate the need for constant monitoring of water parameters, which is crucial to ensure water quality for the population of this region. 相似文献
29.
The origin and evolution of formation water from Upper Jurassic to Upper Cretaceous mudstone–packstone–dolomite host rocks at the Jujo–Tecominoacán oil reservoir, located onshore in SE-Mexico at a depth from 5200 to 6200 m.b.s.l., have been investigated, using detailed water geochemistry from 12 producer wells and six closed wells, and related host rock mineralogy. Saline waters of Cl–Na type with total dissolved solids from 10 to 23 g/L are chemically distinct from hypersaline Cl–Ca–Na and Cl–Na–Ca type waters with TDS between 181 and 385 g/L. Bromine/Cl and Br/Na ratios suggest the subaerial evaporation of seawater beyond halite precipitation to explain the extreme hypersaline components, while less saline samples were formed by mixing of high salinity end members with surface-derived, low salinity water components. The dissolution of evaporites from adjacent salt domes has little impact on present formation water composition. Geochemical simulations with Harvie-Mφller-Weare and PHRQPITZ thermodynamic data sets suggest secondary fluid enrichment in Ca, HCO3 and Sr by water–rock interaction. The volumetric mass balance between Ca enrichment and Mg depletion confirms dolomitization as the major alteration process. Potassium/Cl ratios below evaporation trajectory are attributed to minor precipitation of K feldspar and illitization without evidence for albitization at the Jujo–Tecominoacán reservoir. The abundance of secondary dolomite, illite and pyrite in drilling cores from reservoir host rock reconfirms the observed water–rock exchange processes. Sulfate concentrations are controlled by anhydrite solubility as indicated by positive SI-values, although anhydrite deposition is limited throughout the lithological reservoir column. The chemical variety of produced water at the Jujo–Tecominoacán oil field is related to a sequence of primary and secondary processes, including infiltration of evaporated seawater and original meteoric fluids, the subsequent mixing of different water types and the formation of secondary minerals by water–rock interaction. A best fit between measured and calculated reservoir temperatures was obtained with the Mg–Li geothermometer for high salinity formation water (TDS > 180 g/L), whereas Na–K, Na–Ka–Ca and quartz geothermometers are partially applicable for less salinite water (TDS < 23 g/L). 相似文献
30.
Madhusoodhan Satish-Kumar Hisayoshi Yurimoto Shoichi Itoh Bernardo Cesare 《Contributions to Mineralogy and Petrology》2011,162(4):821-834
High-spatial resolution carbon isotope analyses of natural graphite using secondary ion mass spectrometry (SIMS), together
with conventional mass spectrometry techniques, demonstrate isotopic heterogeneity within single graphite crystals precipitated
from a partially melted metamorphic rock. SIMS 13C/12C measurements were calibrated using an internal graphite standard previously analyzed by conventional isotope ratio mass
spectrometry, which gave a reproducibility of 0.3‰ (1σ) at a spatial resolution of 2–3 μm. This resolution helped to identify
an unusual carbon isotope distribution in a single graphite crystal from a metapelitic leucosome, showing remarkable core
to rim variations with sharp δ13C steps up to 10‰. The results suggest that the graphite crystal grew from one edge to other forming layers perpendicular
to the c-axis. The sharp isotopic steps indicate the presence of disequilibrium carbon isotope zoning in graphite and points to the
possible existence of carbon isotope sector zoning. Intra-crystalline carbon isotope disequilibrium in graphite is believed
to have resulted from the difference in diffusivity between 12C and 13C in the growth medium to the interface of graphite precipitation in different growth sectors. 相似文献