全文获取类型
收费全文 | 206篇 |
免费 | 10篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 4篇 |
大气科学 | 17篇 |
地球物理 | 41篇 |
地质学 | 108篇 |
海洋学 | 15篇 |
天文学 | 27篇 |
自然地理 | 8篇 |
出版年
2020年 | 4篇 |
2019年 | 2篇 |
2018年 | 4篇 |
2017年 | 8篇 |
2016年 | 7篇 |
2015年 | 5篇 |
2014年 | 5篇 |
2013年 | 14篇 |
2012年 | 19篇 |
2011年 | 8篇 |
2010年 | 10篇 |
2009年 | 10篇 |
2008年 | 10篇 |
2007年 | 2篇 |
2006年 | 5篇 |
2005年 | 5篇 |
2004年 | 3篇 |
2003年 | 11篇 |
2002年 | 12篇 |
2001年 | 10篇 |
2000年 | 3篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1973年 | 2篇 |
1970年 | 1篇 |
1902年 | 1篇 |
排序方式: 共有220条查询结果,搜索用时 15 毫秒
151.
We performed nitrogen and argon isotopic analyses in single 200-μm-sized ilmenite grains of lunar regolith samples 71501, 79035 and 79135. Cosmogenic and trapped components were discriminated using stepwise heating with a power-controlled CO2 laser. Cosmogenic 15N and 38Ar correlate among different ilmenite grains, yielding a mean 15Nc/38Arc production ratio of 14.4±1.0 atoms/atom. This yields a 15N production rate in bulk lunar samples of 3.8-5.6 pg (g rock)−1 Ma−1, which agrees well with previous estimates. The trapped δ15N values show large variations (up to 300‰) among different grains of a given soil, reflecting complex histories of mixing between different end-members. The 36Ar/14N ratio, which is expected to increase with increasing contribution of solar ions, varies from 0.007 to 0.44 times the solar abundance ratio. The trapped δ15N values correlate roughly with the 36Ar/14N ratios from a non-solar end-member characterized by a 36Ar/14N ratio close to 0 and variable but generally positive δ15N values, to lower δ15N values accompanied by increasing 36Ar/14N ratios, supporting the claim of Hashizume et al. (2000) that solar nitrogen is largely depleted in 15N relative to meteoritic or terrestrial nitrogen. Nevertheless, the 36Ar/14N ratio of the 15N-depleted (solar) end-member is lower than the solar abundance ratio by a factor of 2.5-5. We explain this by a reprocessing of implanted solar wind atoms, during which part of the chemically inert rare gases were lost. We estimate that the flux of non-solar N necessary to account for the observed δ15N values is comparable to the flux of micrometeorites and interplanetary dust particles estimated for the Earth. Hence we propose that the variations in δ15N values observed in lunar regolith can be simply explained by mixing between solar wind contributions and micrometeoritic ones infalling on the Moon. Temporal variations of δ15N values among samples of different antiquities could be due to changes in the micrometeoritic flux through time, in which case such flux has increased by up to an order of magnitude during the last 0.5 Ga. 相似文献
152.
Rare earth element diffusion in diopside: influence of temperature, pressure, and ionic radius, and an elastic model for diffusion in silicates 总被引:9,自引:2,他引:9
James A. Van Orman Timothy L. Grove Nobumichi Shimizu 《Contributions to Mineralogy and Petrology》2001,141(6):687-703
Volume diffusion rates for five rare earth elements (La, Ce, Nd, Dy, and Yb) have been measured in single crystals of natural diopside at pressures of 0.1 MPa to 2.5 GPa and temperatures of 1,050 to1,450 °C. Polished, pre-annealed crystals were coated with a thin film of rare earth element oxides, then held at constant temperature and pressure for times ranging from 20 to 882 h. Diffusion profiles in quenched samples were measured by SIMS (secondary ion mass spectrometry) depth profiling. At 1 atm pressure, with the oxygen fugacity controlled near the quartz-fayalite-magnetite buffer, the following Arrhenius relations were obtained for diffusion normal to (001) (diffusion coefficient D in m2/s): log10DYb=(-4.64ǂ.42)-(411ᆠ kJ/mol/2.303RT); log10DDy=(-3.31ǃ.44)-(461ᆽ kJ/mol/2.303RT); log10DNd=(-2.95DŽ.64)-(496ᇡ kJ/mol/2.303RT); log10DCe=(-4.10ǃ.08)-(463ᆳ kJ/mol/2.303RT); log10DLu=(-4.22DŽ.66)-(466ᇢ kJ/mol/2.303RT). Diffusion rates decrease significantly with increasing ionic radius, with La a factor of ~35 slower than Yb. The relationship between diffusivity and ionic radius is consistent with a model in which elastic strain plays a critical role in governing the motion of an ion through the crystal lattice. Activation volumes for Yb and Ce diffusion, at constant temperature and oxygen fugacity, are 9.0DŽ.0 cm3/mol and 8.9Dž.2 cm3/mol, respectively, corresponding to an order of magnitude decrease in diffusivity as pressure is increased from 0 to 3 GPa at 1,200 °C. Diffusion of Nd is such that grain-scale isotopic equilibrium in the mantle can be achieved in ~1 My under conditions near the peridotite solidus (~1,450 °C at 2.5 GPa). The equilibration time is much longer under P, T conditions of the lithospheric mantle or at the eclogite solidus (~1 Gy at 1.5 GPa and 1,150 °C). Because of the relatively strong decrease in diffusivity with pressure (two orders of magnitude between 2.5 and 15 GPa along an adiabatic temperature gradient), Nd transport in clinopyroxene will be effectively frozen at pressures approaching the transition zone, on time scales less than 100 My. Rare earth element diffusion rates are slow enough that significant disequilibrium uptake of REE by growing clinopyroxene phenocrysts may be preserved under natural conditions of basalt crystallization. The relative abundances and spatial distributions of REE in such crystals may provide a sensitive record of the cooling and crystallization history of the host lava. 相似文献
153.
We have analysed volatiles (H2O, He, Ar, CO2) in differentiated (basaltic andesite, dacite) volcanic glasses dredged at a depth of ca. 2000 m in the eastern part of the Manus Basin between 151°20′ and 152°10′ E. These samples have Sr–O–B isotopic ratios that show that they most likely represent lavas evolved from a common magma source. Since these glasses are very fresh, they provide a unique opportunity to study the behaviour of magmatic volatiles during assimilation–fractional crystallisation–degassing (AFCD). The samples are highly vesicular (up to 18%) and the volatiles trapped in vesicles consist predominantly of H2O with minor amounts of CO2, and the concentration of water in the glasses indicates that H2O saturation was attained. Rare gases except helium are atmospheric in origin, and the 3He/4He ratios and the CO2/3He ratios are respectively lower and higher than those typical of Mid-Ocean Ridge Basalt (MORB), and appear to correlate with the degree of differentiation. AFCD allows efficient degassing of mantle-derived volatiles and contribution of crust-derived and atmosphere-derived volatiles. Given the widespread occurrence of differentiated magmatism at arcs, we suggest that AFCD is responsible for large-scale occurrence of 3He-rich crustal fluids and of atmospheric-like rare gases in arc emanations, and that most of the volatiles are lost continuously during fractional crystallisation, rather than catastrophically during eruptions. 相似文献
154.
青藏东缘新生代高钾岩浆活动的热年代学制约 总被引:3,自引:0,他引:3
热年代学研究表明 ,青藏东缘存在两期新生代高钾岩浆活动 ,其活动时限分别为 40~ 2 8Ma和 16~ 0Ma。两期高钾岩石在地质背景、岩石类型和地球化学特征上明显不同 ,表明早期高钾岩石是在转换压缩过程中产生的 ,起源于富集地幔的部分熔融 ,而晚期高钾岩石则与东亚和青藏地区构造伸展有关 ,起源于新近交代富集的亏损地幔。 相似文献
155.
156.
Hornblende gabbro sill complex at Onion Valley,California, and a mixing origin for the Sierra Nevada batholith 总被引:9,自引:0,他引:9
T. W. Sisson T. L. Grove D. S. Coleman 《Contributions to Mineralogy and Petrology》1996,126(1-2):81-108
The steep crest of the Sierra Nevada, California, near Onion Valley, exposes natural cross sections through a mafic intrusive
complex that formed as part of the Mesozoic Sierra Nevada batholith. Sheeted sills of hornblende gabbro to hornblende diorite,
individually as thick as 1.5 m, form the upper 200 to 300 m of the complex. Thicker, multiply-injected sills, as well as mafic
stocks, lie underneath at elevations below 3600 m. Lens-shaped cumulate bodies, as thick as 200 m and more than 700 m broad,
lie near the base of the sheeted sill suite. Cumulates are flat-lying, modally layered hornblende gabbro with subsidiary ultramafic
olivine hornblendite, plagioclase hornblendite, and late-mobile hornblende-plagioclase pegmatite. Fine grain size, scarce
phenocrysts and xenocrysts, and quench mineral textures are evidence that hornblende gabbro sills injected in a largely liquid
state and preserve basaltic melt compositions. Most sills reached volatile saturation, as shown by tiny miarolitic cavities
that are also widespread in cumulates. Although some sills chilled directly against others, most chilled against septa, millimeters
to a few centimeters thick, of medium-grained diorite to granodiorite. Mutually crosscutting relations, as well as chilling,
show that the septa were partly molten at the time the sills injected and likely formed the lower portions of an overlying
more silicic magma chamber that has since been removed by erosion. Sill compositions range from evolved high-alumina basalt
to aluminous andesite with major and trace element abundances similar to those of modern arc magmas. Experimental phase equilibria
indicate dissolved water contents near 6 wt% (Sisson and Grove 1993a). The sills show unequivocally that hydrous arc basaltic
magmas reached shallow levels in the crust during formation of the largely granodioritic Sierra Nevada batholith. The basaltic
magmas appear to have been produced from an enriched mantle source with 87Sr/86Sr ∼0.7065, ɛNd ∼−4.3, 206Pb/204Pb ∼18.6, 207Pb/204Pb ∼15.6, 208Pb/204Pb ∼38.6. Although crystal fractionation contributed to forming the sill suite and the associated cumulates, nearly constant
concentrations of Na2O, P2O5, Nb, Zr, and light rare earth elements in the sills indicate that mixing between sill basaltic and more evolved septa magmas
was important for producing sills with andesitic compositions. Average Sierran granodiorite major and trace element concentrations
are readily reproduced by a simple mixture of average basaltic sill from Onion Valley and average Sierran low-silica granite.
This result supports the inference that Sierran granitoids formed chiefly by mixing between crustal and mantle-derived magmas,
although in some cases these crustal melts may have been derived by refusion of earlier mafic intrusions near the base of
the crust. The common mafic inclusions (enclaves) in Sierran granodiorites bear a superficial resemblance to Onion Valley
mafic sills; however, high concentrations of lithophile elements in the inclusions point to extensive chemical exchange between
inclusions and their host magmas. The prevalence of hornblende-rich mafic intrusive rocks at Onion Valley, elsewhere in the
Sierra Nevada, and in other shallow subduction batholiths stems from two effects of high melt water concentrations (∼4–6 wt%
H2O). The hydrous parent basaltic and basaltic andesite magmas had low liquidus temperatures, compared to nearly dry basaltic
melts, and thus were chilled less during ascent through the crust and were more capable of ascent as liquids. More importantly,
their high water concentrations led to low melt densities, higher than granitoid liquids, but comparable to or less dense
than partly solidified granitoid magmas. Thus, the hydrous basaltic and basaltic andesite magmas were neutrally or positively
buoyant and were capable of penetrating and rising through partly crystallized granitoids and their partly molten source regions
to reach upper crustal emplacement levels. Drier basaltic magmas were probably abundant at depth and contributed heat and
mass to granite generation, but were insufficiently buoyant to ascend to shallow levels.
Received: 2 August 1995 / Accepted: 26 June 1996 相似文献
157.
Distribution of sterol and fatty alcohol biomarkers in particulate matter from the frontal structure of the Alboran Sea (S.W. Mediterranean Sea) 总被引:1,自引:0,他引:1
Imma Tolosa Nathalie LeBlond Claire Copin-Montgut Jean-Claude Marty Stephen de Mora Louis Prieur 《Marine Chemistry》2003,82(3-4):161-183
Sterol and fatty alcohol biomarkers were analyzed in suspended and sinking particles from the water column (20–300 m) of the Almeria–Oran frontal zone to characterize the biogenic sources and biogeochemical processes. Diatom- and haptophyte-related sterols were predominant at all sites and vertical distributions of sterol, and fatty alcohol biomarkers in sinking particles were markedly different from suspended particles. In contrast to the relatively fresh sinking particles with elevated concentrations of phyto- and zooplanktonic sterols, suspended particles were extensively degraded with increasing depth and exhibited a more terrestrial and zooplanktonic signature with depth.Sterol and alcohol biomarkers distributions and δ13C values from the jet core and the associated gyre of Atlantic waters showed a decoupling between the sinking particles of 100- and 300-m depth, demonstrating the influence of lateral advection in the frontal zone. In contrast, vertical transport of the particulate organic matter in Mediterranean waters was interpreted from the similar isotopic and molecular composition at both depths. The high abundance of phytosterols and phytol below the euphotic zone at 100 m signified that downwelling of biomass occurred on the downstream side of the gyre. The high concentrations of phytosterols and POC, in combination with the high phytosterols/phytol ratio, indicated the accumulation of detrital plant material in the oligotrophic Mediterranean waters near the frontal zone.A higher contribution of phytol in the sinking particles collected during the night at the surface of the jet and at the upstream side of the gyre provided evidence of diel vertical zooplankton migration and important grazing by herbivorous zooplankton.Carbon isotope ratios of sterols confirmed that the 24-ethylcholest-5-en-3β-ol, commonly associated with terrestrial sources, was a substantial constituent of the phytoplankton in this area. However, the more δ13C depleted values obtained for this compound in suspended particles suggested that there was some terrestrial contribution that only becomes evident after degradation of the more labile marine organic matter. 相似文献
158.
As part of an investigation into the role of East African mangrove creeks as fish spawning and nursery areas, nocturnal ichthyoplankton sampling was carried out at five stations in Tudor Creek, Mombasa, Kenya, from September to November 1985. Throughout the creek, the salinity was close to that of seawater at about 35‰, but from the mouth to the upper creek there were gradients of increasing temperature and turbidity, and decreasing depth and dissolved oxygen concentration. Pelagic eggs and larvae of fish considered to be creek residents and creek non-residents were distinguished. Gradients of decreasing egg and larval abundance (particularly of non-residents), and of decreasing diversity of recognized types, were apparent from the mouth towards the upper creek. Such a distribution suggests that the mangrove system is not an important spawning or development area for the earliest life-history stages of non-resident species. The existence of the gradients also has implications both for sampling at fixed stations in tidal waters, and for investigating the existence of circatidal and semilunar spawning cycles in fish. 相似文献
159.
R. Hekinian J. Francheteau V. Renard R. D. Ballard P. Choukroune J. L. Cheminee F. Albarede J. F. Minster J. L. Charlou J. C. Marty J. Boulegue 《Marine Geophysical Researches》1983,6(1):1-14
A submersible study of a fast spreading (12 cm yr-1) Mid-Ocean ridge segment (East Pacific Rise near 12°50′ N) led to the discovery of intense hydrothermal activity. Twenty four sites with active vents and sixty inactive hydrothermal deposits were found within a narrow graben averaging about 300 m in width along a 20 km long segment of the ridge crest. The graben is floored with fresh basaltic sheet flows including collapsed pits or lava lake structures. From both deep towed camera stations and manned submersible observations, it is estimated that the average spacing between the ridge axis hydrothermal deposits averaging in size 10–50 m in diameter lies between 100 to 200 m. The hydrothermal deposits found in the central graben are believed to have formed rapidly (on the order of a few decades). Detailed investigations of one active site have enabled us to witness the growth of an active chimney which increased its height by 40 cm in 5 days i.e. 8 cm per day. Extensive hydrothermal deposits were discovered on an off-axis seamount located 6 km East of the ridge axis. The hydrothermal deposits found on both the ridge axis and on the seamount are similar in composition and consist essentially of zinc, copper and iron mineral phases. Chemical studies conducted on the venting fluids (320°C) showed that the concentration of the dissolved major metal ions (Fe, Mn, and Zn) is about 0.10–0.13 gl-1. If we accept an average flow rate of 10ls-1 based on visual observations the mass of metallic products spewing out from an active chimney must be about 100 kg per day. 相似文献
160.