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281.
Temperature-dependent isotopic fractionation of lithium between clinopyroxene and high-pressure hydrous fluids 总被引:2,自引:1,他引:2
Bernd Wunder Anette Meixner Rolf L. Romer Wilhelm Heinrich 《Contributions to Mineralogy and Petrology》2006,151(1):112-120
The fractionation of lithium isotopes between synthetic spodumene as representative of Li-bearing clinopyroxene and Cl- and
OH-bearing aqueous fluids was experimentally determined between 500 and 900°C at 2.0 GPa. In all the experiments, 7Li was preferentially partitioned into the fluid. The fractionation is temperature dependent and approximated by the equation
Δ7Li(clinopyroxene–fluid)=−4.61×(1,000/T [K]) + 2.48; R
2=0.86. Significant Li isotopic fractionation of about 1.0‰ exists even at high temperatures of 900°C. Using neutral and weakly
basic fluids revealed that the amount of fractionation is not different. The Li isotopic fractionation between altered basalt
and hot spring water (350°C) in natural samples is in good agreement with our experimentally determined fractionation curve.
The data confirm earlier speculations drawn from the Li isotopic record of dehydrated metamorphic rocks that fluids expelled
from a dehydrating slab carry heavier Li into the mantle wedge, and that a light Li component is introduced into the deeper
mantle. Li and Li isotopes are redistributed among wedge minerals as fluids travel across the wedge into hotter regions of
arc magma production. This modifies the Li isotopic characteristics of slab-derived fluids erasing their source memory, and
explains the absence of cross-arc variations of Li isotopes in arc basalts. 相似文献
282.
283.
Jingwen Mao Yitian Wang Bernd Lehmann Jinjie Yu Andao Du Yanxiong Mei Yongfeng Li Wenshuan Zang Holly J. Stein Taofa Zhou 《Ore Geology Reviews》2006,29(3-4):307-324
The area of the Middle–Lower Yangtze River valley, Eastern China, extending from Wuhan (Hubei province) to western Zhenjiang (Jiangsu province), hosts an important belt of Cu–Au–Mo and Fe deposits. There are two styles of mineralization, i.e., skarn/porphyry/stratabound Cu–Au–Mo–(Fe) deposits and magnetite porphyry deposits in several NNE-trending Cretaceous fault-bound volcanic basins. The origin of both deposit systems is much debated. We dated 11 molybdenite samples from five skarn/porphyry Cu–Au–Mo deposits and 5 molybdenite samples from the Datuanshan stratabound Cu–Au–Mo deposit by ICP-MS Re–Os isotope analysis. Nine samples from the same set were additionally analyzed by NTIMS on Re–Os. Results from the two methods are almost identical. The Re–Os model ages of 16 molybdenite samples range from 134.7 ± 2.3 to 143.7 ± 1.6 Ma (2σ). The model ages of the five samples from the Datuanshan stratabound deposit vary from 138.0 ± 3.2 to 140.8 ± 2.0 Ma, with a mean of 139.3 ± 2.6 Ma; their isochron age is 139.1 ± 2.7 Ma with an initial Os ratio of 0.7 ± 8.1 (MSWD = 0.29). These data indicate that the porphyry/skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system. Albite 40Ar/39Ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 Ma, i.e., 10–20 Ma later. Both mineralization styles characterize transitional geodynamic regimes, i.e., the period around 140 Ma when the main NS-trending compressional regime changed to an EW-trending lithospheric extensional regime, and the period of 125–115 Ma of dramatic EW-trending lithospheric extension. 相似文献
284.
Mobility and retention of trace elements in hardpan-cemented cassiterite tailings, north Queensland, Australia 总被引:1,自引:0,他引:1
This study reports on the mobility and retention of trace elements in cassiterite tailings at the inactive Jumna mill, tropical north Queensland. Since the 1980s, the uncapped tailings have developed laterally discontinuous Fe-rich hardpans, which are located in the higher parts of gently sloping tailings masses and at the top (<50 cm) of the tailings piles. Hardpan-cemented tailings comprise thin layers (typically ∼0.2–2 mm thick) of HFO (hydrous ferric oxides) and sulfate efflorescences cementing tailings grains. In comparison to the tailings, the hardpan-cemented tailings contain significantly higher median As, Ca, Cd, Co, Cu, Fe, In, Mn, Mo, Stotal, Th, U, Y and Zn values. Partial leaching studies of tailings and pond water analyses indicate that wetting and acidification of Fe-cemented tailings removes significant proportions of trace elements into pore and surface waters. Tin shows no mobility due to the presence of weathering-resistant cassiterite (SnO2) and, As and Pb display limited mobility possibly due to their coprecipitation with jarosite-type phases and HFO materials at the top of the tailings profile. By contrast, the trace elements Cd, Ce, Cu, La, Ni, Pb, U and Zn display the greatest mobility, possibly due to their incorporation in soluble sulfate efflorescences and sorption onto mineral and HFO surfaces. Hence, the Fe-rich hardpans do not protect the sulfidic tailings from further oxidation nor do they cause permanent sequestration of trace elements. 相似文献
285.
A system capable of oceanic 226Ra measurements with a precision of ±1% is described, which represents an improvement of approximately a factor of three over existing techniques. 222Rn grown-in from 226Ra decay in 14-l seawater samples is quantitatively transferred to, and measured in, proportional gas counters. Errors other than counting statistics are estimated not to exceed ±0.5%, which is consistent with repeated 226Ra measurements on the same samples. A NE Atlantic 226Ra depth profile (2000–5000 m) is reported as an example. It is found that with the precision reported here, certain hitherto unresolved features of the 226Ra distribution in deep water become apparent. 相似文献
286.
David L. Dettman Karl W. Flessa Bernd R. Schöne 《Geochimica et cosmochimica acta》2004,68(6):1253-1263
We describe a new method for the calculation of river flow that uses the oxygen isotope composition of bivalve mollusk shells that grew in the river-water/seawater mixing zone of the Colorado River estuary. Sclerochronological techniques are used to identify tidally-induced, fortnight-scale bundles of daily growth increments within shell cross-sections. These fortnightly markers are used to establish a chronology for samples taken for δ18O analysis. A composite seasonal δ18O profile derived from five shells that grew in the absence of river-water flow is used as a baseline against which profiles of river-influenced shells are compared. Because this comparison is between matched fortnights within a year, the temperature of shell growth is likely to be very similar. The difference in δ18O between the river-influenced shell and the “no-flow” composite shell therefore represents the change in the δ18O of the water due to the presence of river water in the mixing zone. The river water end-member is also determined within a fortnightly context so that the change in the δ18O of mixing-zone water can be used to calculate the relative proportions of seawater and fresh-water. The fresh-water end-member is calculated from the δ18O of bivalves alive prior to the emplacement of dams and water diversions on the Colorado River. The marine end-member is based on direct measurements of the δ18O of northern Gulf of California water during times of no Colorado River flow. The system has been calibrated to absolute flow amounts using recent releases of known volume and rate. 相似文献
287.
Metamictisation of natural zircon: accumulation versus thermal annealing of radioactivity-induced damage 总被引:12,自引:0,他引:12
Lutz Nasdala Marita Wenzel Gerhard Vavra Gert Irmer Thomas Wenzel Bernd Kober 《Contributions to Mineralogy and Petrology》2001,141(2):125-144
Annealing of the radiation damage generated in !-decay events is a common phenomenon in natural zircons. We have studied relationships between radiation damage, age and radionuclide content of natural zircons on a micro-scale. The degree of metamictisation was estimated by means of confocal laser-Raman spectroscopic measurements. Raman band broadening in well-ordered to moderately metamict zircons that have not experienced significant healing of the radiation damage since the time of their growth, shows a nearly linear dependence on the !-fluence. This dependence is proposed as a calibration line for examining unknown zircons in order to consider complete or incomplete damage storage, which may contribute to the understanding of the low-T thermal history of their host rocks. Seven examples for the application are presented. Radiation damage may anneal heterogeneously, with preferred re-ordering in more damaged micro-areas. Results suggest that single point defects do not anneal continuously over geologic periods of time but are, in contrast, a comparably stable type of radiation damage. In contrast to recrystallisation processes, structural recovery due to simple thermal annealing is not necessarily connected with loss of radiogenic Pb. The most prominent example for this are concordant zircons from Sri Lanka. Although they are excellent standards for U-Pb dating, Sri Lankan zircons should not be used as standards for structural radiation damage. The experimental work is completed by Monte Carlo simulations of !-decay events in zircon. 相似文献
288.
289.
Jaroslava Plomerová Libuše Ruprechtová Bernd Tittel Joachim Weyrauch Kurt Wylegalla Reviewer A. Zátopek 《Studia Geophysica et Geodaetica》1979,23(2):189-196
n¶rt; auau uu ¶rt; mnu¶rt; n, n, g, Sg, m anmam anauu ¶rt; numa amu 2.6° ¶rt; 4.7° m aa amu uu (mau). aa, m m auum Sg(D) mam mua ¶rt; m n¶rt;u aum¶rt; uma a uu mu. 相似文献
290.
Bernd R. Schöne Nicholas A. Page David L. Rodland Jens Fiebig Sven Baier Samuli O. Helama Wolfgang Oschmann 《International Journal of Earth Sciences》2007,96(3):525-540
We present the results of sclerochronologically calibrated growth and stable isotope analyses of the freshwater bivalve Margaritifera falcata collected from an agricultural, suburban setting near Vancouver, BC. The oxygen isotope range of shell aragonite can be explained by the temperature range during the growing season, assuming the water δ 18O composition remained constant. However, shell growth is strongly influenced by local summer precipitation and potentially runoff of nutrient-rich stormwater. About 44% of the variability of annual shell growth can be explained by amounts of local summer (June–September) rainfall. Local winter precipitation and El Niño–Southern Oscillation (ENSO) strength during the preceding year exert a weak, but significant control on shell growth. In combination, summer and winter precipitation can explain up to 50% of the variability in annual shell growth. Spectral analyses substantiate the effect of precipitation on shell growth and demonstrate that shell growth and ENSO are coupled by precipitation. Common spectral density was found at periods of 6.5–9 years, particularly between 1985 and 2004. Higher frequency oscillation corresponding to periods of 3–5 years occurred during the early 1970s, early to mid 1980s, and later 1990s. These results suggest that skeletal records of bivalve mollusks provide suitable archives of ENSO-coupled precipitation in areas where other climate proxies such as tree-rings and speleothems may not be available. 相似文献