全文获取类型
收费全文 | 102篇 |
免费 | 0篇 |
专业分类
大气科学 | 1篇 |
地球物理 | 21篇 |
地质学 | 24篇 |
海洋学 | 11篇 |
天文学 | 43篇 |
自然地理 | 2篇 |
出版年
2014年 | 1篇 |
2013年 | 4篇 |
2012年 | 1篇 |
2011年 | 2篇 |
2010年 | 2篇 |
2009年 | 10篇 |
2008年 | 5篇 |
2007年 | 11篇 |
2006年 | 11篇 |
2005年 | 4篇 |
2004年 | 8篇 |
2003年 | 4篇 |
2002年 | 2篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有102条查询结果,搜索用时 31 毫秒
21.
Late Pleistocene or Recent lavas from San Quintin, Baja California are basanitoids and alkali basalts. The surface quench temperatures of the lavas average 1 005° C with log
=–11.4, as deduced from the groundmass Fe-Ti oxides. Spinel lherzolite xenoliths and megacrysts of augite and andesine have been found in lava flows and cinder deposits. Using analytical data on the rocks and minerals and simple thermodynamic expressions, the pressures and temperatures of equilibration of lavas and xenoliths, megacrysts and phenocrysts have been calculated. The lavas could have been in equilibrium with lherzolite at 1 330–1 410° C and 27.5–31.6 kb, the more silica-poor liquid having the higher values. The basanitoid could have equilibrated with the megacrysts at about 10.5 kb and with phenocrysts at about 1.4 kb and 1130° C. The variation in composition of the lavas may be explained by a rising zone of melting within the mantle, the most silica-poor liquid having the deepest source. The source of the San Quintin basalts is probably related to spreading of the ocean floor in the Gulf of California. 相似文献
22.
210Pb/226Ra and 210Po/210Pb disequilibria in seawater and suspended particulate matter 总被引:1,自引:0,他引:1
The distribution of210Po and210Po in dissolved (<0.4 μm) and particulate (>0.4 μm) phases has been measured at ten stations in the tropical and eastern North Atlantic and at two stations in the Pacific. Both radionuclides occur principally in the dissolved phase. Unsupported210Pb activities, maintained by flux from the atmosphere, are present in the surface mixed layer and penetrate into the thermocline to depths of about 500 m. Dissolved210Po is ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter is enriched in210Po, with210Po/210Pb activity ratios greater than 1.0, similar to those reported for phytoplankton. Box-model calculations yield a 2.5-year residence time for210Pb and a 0.6-year residence time for210Po in the mixed layer. These residence times are considerably longer than the time calculated for turnover of particles in the mixed layer (about 0.1 year). At depths of 100–300 m,210Po maxima occur and unsupported210Po is frequently present. Calculations indicate that at least 50% of the210Po removed from the mixed layer is recycled within the thermocline. Similar calculations for210Pb suggest much lower recycling efficiencies.Comparison of the210Pb distribution with the reported distribution of226Ra at nearby GEOSECS stations has confirmed the widespread existence of a210Pb/226Ra disequilibrium in the deep sea. Vertical profiles of particulate210Pb were used to test the hypothesis that210Pb is removed from deep water by in-situ scavenging. With the exception of one profile taken near the Mid-Atlantic Ridge, significant vertical gradients in particulate210Pb concentration were not observed, and it is necessary to invoke exceptionally high particle sinking velocities to account for the inferred210Pb flux. It is proposed instead that an additional sink for210Pb in the deep sea must be sought. Estimates of the dissolved210Pb/226Ra activity ratio at depths greater than 1000 m range from 0.2 to 0.8 and reveal a systematic increase, in both vertical and horizontal directions, with increasing distance from the sea floor. This observation implies rapid scavenging of210Pb at the sediment-water interface and is consistent with a horizontal eddy diffusivity of 3?6 × 107 cm2/sec. The more reactive element Po, on the other hand, shows evidence of rapid in-situ scavenging. In filtered seawater,210Po is deficient, on the average, by ca. 10% relative to210Pb; a corresponding enrichment is found in the particulate phase. Total inventories of210Pb and210Po over the entire water column, however, show no significant departure from secular equilibrium. 相似文献
23.
New investigations of the geology of Crater Lake National Park necessitate a reinterpretation of the eruptive history of Mount Mazama and of the formation of Crater Lake caldera. Mount Mazama consisted of a glaciated complex of overlapping shields and stratovolcanoes, each of which was probably active for a comparatively short interval. All the Mazama magmas apparently evolved within thermally and compositionally zoned crustal magma reservoirs, which reached their maximum volume and degree of differentiation in the climactic magma chamber 7000 yr B.P.The history displayed in the caldera walls begins with construction of the andesitic Phantom Cone 400,000 yr B.P. Subsequently, at least 6 major centers erupted combinations of mafic andesite, andesite, or dacite before initiation of the Wisconsin Glaciation 75,000 yr B.P. Eruption of andesitic and dacitic lavas from 5 or more discrete centers, as well as an episode of dacitic pyroclastic activity, occurred until 50,000 yr B.P.; by that time, intermediate lava had been erupted at several short-lived vents. Concurrently, and probably during much of the Pleistocene, basaltic to mafic andesitic monogenetic vents built cinder cones and erupted local lava flows low on the flanks of Mount Mazama. Basaltic magma from one of these vents, Forgotten Crater, intercepted the margin of the zoned intermediate to silicic magmatic system and caused eruption of commingled andesitic and dacitic lava along a radial trend sometime between 22,000 and 30,000 yr B.P. Dacitic deposits between 22,000 and 50,000 yr old appear to record emplacement of domes high on the south slope. A line of silicic domes that may be between 22,000 and 30,000 yr old, northeast of and radial to the caldera, and a single dome on the north wall were probably fed by the same developing magma chamber as the dacitic lavas of the Forgotten Crater complex. The dacitic Palisade flow on the northeast wall is 25,000 yr old. These relatively silicic lavas commonly contain traces of hornblende and record early stages in the development of the climatic magma chamber.Some 15,000 to 40,000 yr were apparently needed for development of the climactic magma chamber, which had begun to leak rhyodacitic magma by 7015 ± 45 yr B.P. Four rhyodacitic lava flows and associated tephras were emplaced from an arcuate array of vents north of the summit of Mount Mazama, during a period of 200 yr before the climactic eruption. The climactic eruption began 6845 ± 50 yr B.P. with voluminous airfall deposition from a high column, perhaps because ejection of 4−12 km3 of magma to form the lava flows and tephras depressurized the top of the system to the point where vesiculation at depth could sustain a Plinian column. Ejecta of this phase issued from a single vent north of the main Mazama edifice but within the area in which the caldera later formed. The Wineglass Welded Tuff of Williams (1942) is the proximal featheredge of thicker ash-flow deposits downslope to the north, northeast, and east of Mount Mazama and was deposited during the single-vent phase, after collapse of the high column, by ash flows that followed topographic depressions. Approximately 30 km3 of rhyodacitic magma were expelled before collapse of the roof of the magma chamber and inception of caldera formation ended the single-vent phase. Ash flows of the ensuing ring-vent phase erupted from multiple vents as the caldera collapsed. These ash flows surmounted virtually all topographic barriers, caused significant erosion, and produced voluminous deposits zoned from rhyodacite to mafic andesite. The entire climactic eruption and caldera formation were over before the youngest rhyodacitic lava flow had cooled completely, because all the climactic deposits are cut by fumaroles that originated within the underlying lava, and part of the flow oozed down the caldera wall.A total of 51−59 km3 of magma was ejected in the precursory and climactic eruptions, and 40−52 km3 of Mount Mazama was lost by caldera formation. The spectacular compositional zonation shown by the climactic ejecta — rhyodacite followed by subordinate andesite and mafic andesite — reflects partial emptying of a zoned system, halted when the crystal-rich magma became too viscous for explosive fragmentation. This zonation was probably brought about by convective separation of low-density, evolved magma from underlying mafic magma. Confinement of postclimactic eruptive activity to the caldera attests to continuing existence of the Mazama magmatic system. 相似文献
24.
Concentrations of230Th and231Pa were measured in particulate matter collected by sediment traps deployed in the Sargasso Sea (Site S2), the north equatorial Atlantic (site E), and the north equatorial Pacific (Site P) as well as in particles collected by in situ filtration at Site E. Concentrations of dissolved Th and Pa were determined by extraction onto manganese dioxide adsorbers at Site P and at a second site in the Sargasso Sea (site D).Dissolved230Th/231Pa activity ratios were 3–6 at Sites P and D. In contrast, for all sediment trap samples from greater than 2000 m, unsupported230Th/231Pa ratios were 22–35 (average 29.7). Ratios were lower in particulate matter sampled at shallower depths. Particles filtered at 3600 m and 5000 m at Site E had ratios of 50 and 40. Results show that suspended particulate matter in the open ocean preferentially scavenges Th relative to Pa. Most of the230Th produced by decay of234U in the open ocean is removed by adsorption to settling particulate matter. In contrast, less than 50% of the231Pa produced by decay of235U is removed from the water column by this mechanism. Mixing processes transport the remainder to other sinks. 相似文献
25.
Uranium, thorium and protactinium isotopes were measured in particulate matter collected by sediment traps deployed in the Panama Basin and by in-situ filtration of large volumes of seawater in the Panama and Guatemala Basins. Concentrations of dissolved Th and Pa isotopes were determined by extraction onto MnO2 adsorbers placed in line behind the filters in the in-situ pumping systems.Concentrations of dissolved 230Th and 231Pa in the Panama and Guatemala Basins are lower than in the open ocean, whereas dissolved 230Th/231Pa ratios are equal to, or slightly greater than, ratios in the open ocean. Particulate 230Th/231Pa ratios in the sediment trap samples ranged from 4 to 8, in contrast to ratios of 30 or more at the open ocean sites previously studied. Particles collected by filtration in the Panama Basin and nearest to the continental margin in the Guatemala Basin contained 230Th/231Pa ratios similar to the ratios in the sediment trap samples. The ratios increased with distance away from the continent.Suspended particles near the margin show no preference for adsorption of Th or Pa and therefore must be chemically different from particles in the open ocean, which show a strong preference for adsorption of Th. Ocean margins, as typified by the Panama and Guatemala Basins, are preferential sinks for 231Pa relative to 230Th. Furthermore, the margins are sinks for 230Th and, to a greater extent, 231Pa transported by horizontal mixing from the open ocean. 相似文献
26.
J.K. Cochran M.P. Bacon S. Krishnaswami K.K. Turekian 《Earth and Planetary Science Letters》1983,65(2):433-452
Disequilibrium between210Po and210Pb and between210Pb and226Ra has been mapped in the eastern and central Indian Ocean based on stations from Legs 3 and 4 of the GEOSECS Indian Ocean expedition.210Po/210Pb activity ratios are less than 1.0 in the surface mixed layer and indicate a residence time for Po of 0.6 years.210Po and210Pb are generally in radioactive equilibrium elsewhere in the water column except at depths of 100–500 m, where Po may be returned to solution after removal from the surface water, and in samples taken near the bottom at a few stations.210Pb excesses relative to226Ra are observed in the surface water but these excesses are not as pronounced as in the North Pacific and North Atlantic. The difference is attributable to a lower flux of210Pb from the atmosphere to the Indian Ocean. Below the main thermocline,210Pb activities increase with depth to a broad maximum before decreasing to lower values near the bottom. Departures from this pattern are especially evident at stations taken in the Bay of Bengal (where210Pb/226Ra activity ratios as low as 0.16 are observed) and near the Mid-Indian Ridge. The data suggest that removal of210Pb at oceanic boundaries, coupled with eddy diffusion along isopycnals, can explain gradients in210Pb near the boundary. Application of a simple model including isopycnal diffusion, chemical removal, production and radioactive decay produces fits the observed210Pb/226Ra gradients for eddy diffusion coeffients of ~ 107 cm2/s. High productivity in surface waters of the Bay of Bengal makes this region a sink for reactive nuclides in the northern Indian Ocean. 相似文献
27.
A 150-m-long, wedge-shaped unit of folded and faulted marly siltstone crops out between undeformed sedimentary rocks on the north flank of the Coso Range, California. The several-meter-thick blunt end of this wedge abuts the north margin of a basaltic sill of comparable thickness. Chaotically deformed siltstone crops out locally at the margin of this sill, and at one locality breccia pipes about one meter in diameter crosscut the sill. The sill extends about 1 km south up the paleoslope, where it merges through continuous outcrop with a lava flow that in turn extends 1.4 km to a vent area marked by more than 100 m of agglutinate and scoria. Apparently, lava extruded at this vent flowed onto unconsolidated sediments, burrowed into them, and fed a sill at about 40 m depth within the sedimentary sequence. The sill initially propagated by wedging between sedimentary beds, but eventually began to push some beds ahead of itself, forming a remarkable train of folds in the process. The sediments apparently were wet at the time of sill emplacement, because hydrothermal alteration is common near the contact between the two rock types and because the breccia pipes that crosscut the sill apparently resulted from phreatic explosions of pore water heated at the base of the cooling sill. Comparison of deformation of the host material at the Coso locality with that reportedly caused by emplacement of sills elsewhere indicates that the character of deformation differs greatly among the various localities. The specific response of host material depends upon such parameters as initial properties of magma and host material, rate of sill growth and attendant rate of strain of host material, and depth of sill emplacement. Some properties may change considerably during an intrusive-deformational episode, thus complicating accurate reconstruction of such an event. 相似文献
28.
Evolution of the magma chamber at Mount Mazama involved repeated recharge by two types of andesite (high-Sr and low-Sr), crystal
fractionation, crystal accumulation, assimilation, and magma mixing (Bacon and Druitt 1988). This paper addresses the modal
compositions, textures, mineral chemistry and magmatic temperatures of (i) products of the 6845±50 BP climactic eruption,
(ii) blocks of partially fused granitoid wallrock found in the ejecta, and (iii) preclimactic rhyodacitic lavas leaked from
the chamber in late Pleistocene and early Holocene time. Immediately prior to the climactic eruption the chamber contained
≳ 40 km3 of rhyodacite (10 vol% plag + opx + aug + hb + mt + ilm, ∼880° C) overlying high-Sr andesite and cumulus-crystal mush (28–51
vol% plag + hb ± opx ± aug + mt ± ilm, 880° to ≥950° C), which in turn overlay low-Sr crystal mush (50–66 vol% plag + opx
+ aug ± hb ± ol + mt + ilm, 890° to ≥950† C). Despite the well known compositional gap in the ejecta, no thermal discontinuity
existed in the chamber. Pre-eruptive water contents of pore liquids in most high-Sr and low-Sr mushes were 4–6 wt%, but on
average the high-Sr mushes were slightly richer in water. Although parental magmas of the crystal mushes were andesitic, xenocrysts
of bytownite and Ni-rich magnesian olivine in some scoriae record the one-time injection of basalt into the chamber. Textures
in ol-bearing scoriae preserve evidence for the reactions ol + liq = opx and ol + aug + liq(+ plag?) = hb, which occurred
in andesitic liquids at Mount Mazama. Strontium abundances in plagioclase phenocrysts constrain the petrogenesis of preclimactic
and climactic rhyodacites. Phenocryst cores derived from high-Sr and low-Sr magmas have different Sr contents which can be
resolved by microprobe. Partition coefficients for plagioclase in andesitic to rhyolitic glasses range from 2 to 7, and increase
as glass %SiO2 increases. Evolved Pleistocene rhyodacites (∼30–25,000 BP) and rhyodacites of the Holocene Llao Rock center (7015±45 BP)
contain Sr-poor plagioclase and are derivatives from low-Sr magma. Rhyodacites of the Pleistocene Sharp Peak domes, Holocene
Cleetwood flow (∼6850 BP), and climactic ejecta contain discrete Sr-rich and Sr-poor plagioclase phenocryst populations and
are hybrids produced by mixing low-Sr rhyodacite (containing Sr-poor plag + opx + aug) with a more mafic high-Sr magma (with
Sr-rich plag [ + hb?]). The data reinforce the conclusions of crystal-liquid mixing calculations (Bacon and Druitt 1988),
and suggest some important refinements to the magma chamber model. 相似文献
29.
Hein J.W. De Baar Peter G. Brewer Michael P. Bacon 《Geochimica et cosmochimica acta》1985,49(9):1961-1969
We have measured profiles of the rare earth elements (REE) in Atlantic and Pacific Ocean waters. The data, normalized versus shales, exhibit a pronounced anomaly in Gd relative to its neighbors Eu and Tb in the REE series such that the Gd concentrations are high by 30–50%. Closer inspection reveals that the anomaly is made up of both elevated Gd and depressed Tb concentrations, likely associated with solution chemistry shifts in the transition from an exactly half filled 4f electron shell. Anomalies in Gd and Tb solution complexation are also indicated by the Turner-Whitfield-Dickson speciation model. The overall trend of heavy REE(III) enrichment in seawater and the anomaly described here tend to support scavenging as an important removal mechanism for the REE from seawater. 相似文献
30.
Michael P. Bacon 《Chemical Geology》1984,46(2):97-111
The cause of the climatically controlled fluctuations in the carbonate content of deep-sea sediments remains the subject of uncertainty and debate. Three variables are involved: supply of biogenic carbonate, loss by dissolution, and dilution by non-carbonate phases. It is suggested that 230Th, which is produced in the ocean at a constant rate provides a reliable reference for measuring variations in rate of sedimentation on a regional scale. Results of a preliminary analysis based on published data indicate that, for depths at and above the lysocline, the carbonate fluctuations observed in cores from the North Atlantic Ocean are due primarily to variations in the terrigenous clay input, which was 2–5 times higher during glacials than during interglacials. Carbonate deposition appears to have been somewhat reduced during glacials, but probably not by more than a factor of 2. From published profiles it appears that the South Atlantic Ocean also received increased inputs of terrigenous clay during glacial periods. 相似文献