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排序方式: 共有239条查询结果,搜索用时 15 毫秒
51.
Davide Lenaz Henrik Skogby Francesco Princivalle Ulf Hålenius 《Physics and Chemistry of Minerals》2006,33(7):465-474
The influence on the spinel structure of Fe3+ → Cr substitution was studied in flux-grown synthetic single crystals of the magnesiochromite–magnesioferrite (MgCr2O4–MgFe2O4) solid solution series. Samples were analysed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption and Mössbauer spectroscopy. With the exception of iron-poor samples (3–12 mol-% MgFe2O4), optical absorption and Mössbauer spectra show that iron occurs almost exclusively as trivalent Fe in the present samples. A very intense and broad absorption band at ca 7,800 cm?1 dominates the optical absorption spectra of samples with higher Fe-contents. The appearance of this band is related to a distinct structural disorder of Fe3+ and a development of magnetic ordering as demonstrated by Mössbauer spectra. Profound composition-related changes are observed in the Mössbauer spectra, which are magnetically unsplit in the range 2–41 mol-% magnesioferrite, but become magnetically split in the range 59–100 mol-% magnesioferrite. Structural parameters a 0 and M–O increase with magnesioferrite content and inversion degree, while u and T–O decrease. Our study confirms the previously reported (Lavina et al. 2002) influence of Fe3+ at the M site on T–O bond lengths in the spinel structure. 相似文献
52.
Long-term summer temperature variations in the Pyrenees 总被引:4,自引:0,他引:4
Two hundred and sixty one newly measured tree-ring width and density series from living and dry-dead conifers from two timberline
sites in the Spanish Pyrenees were compiled. Application of the regional curve standardization method for tree-ring detrending
allowed the preservation of inter-annual to multi-centennial scale variability. The new density record correlates at 0.53
(0.68 in the higher frequency domain) with May–September maximum temperatures over the 1944–2005 period. Reconstructed warmth
in the fourteenth to fifteenth and twentieth century is separated by a prolonged cooling from ∼1450 to 1850. Six of the ten
warmest decades fall into the twentieth century, whereas the remaining four are reconstructed for the 1360–1440 interval.
Comparison with novel density-based summer temperature reconstructions from the Swiss Alps and northern Sweden indicates decadal
to longer-term similarity between the Pyrenees and Alps, but disagreement with northern Sweden. Spatial field correlations
with instrumental data support the regional differentiation of the proxy records. While twentieth century warmth is evident
in the Alps and Pyrenees, recent temperatures in Scandinavia are relatively cold in comparison to earlier warmth centered
around medieval times, ∼1450, and the late eighteenth century. While coldest summers in the Alps and Pyrenees were in-phase
with the Maunder and Dalton solar minima, lowest temperatures in Scandinavia occurred later at the onset of the twentieth
century. However, fairly cold summers at the end of the fifteenth century, between ∼1600–1700, and ∼1820 were synchronized
over Europe, and larger areas of the Northern Hemisphere. 相似文献
53.
Dmitry P. Gladkochub Sergei A. Pisarevsky Tatiana V. Donskaya Richard E. Ernst Michael T.D. Wingate Ulf Söderlund Anatoliy M. Mazukabzov Eugene V. Sklyarov Michael A. Hamilton John A. Hanes 《Precambrian Research》2010
We present a summary of late Paleoproterozoic to Neoproterozoic mafic magmatism in the Siberian craton, including recently published U–Pb and 40Ar–39Ar dates. These new precise ages suggest that at least some of the previously published K–Ar ages of Siberian mafic bodies should be ignored. The time–space geochronological chart, or the ‘barcode’ of mafic magmatic events shows significant differences between northern and southern Siberia. Both are characterized by ∼1900–1700 Ma magmatic events, but then there was an almost 1 Ga mafic magmatic ‘pause’ in south Siberia until ∼800 Ma. Meanwhile there are indications of multiple mafic magmatic events in North Siberia (Anabar shield and Olenek uplift) between ∼1600 and 1000 Ma. A series of magmatic events probably related to the breakup of Rodinia occurred in southern Siberia after ∼800 Ma. So far, there are no indications of late Neoproterozoic mafic magmatism in North Siberia. Ca. 1000–950 Ma mafic sills were reported from Meso- to Neo-Proterozoic sedimentary successions in the Sette-Daban area on the east side of the Siberian craton, but their tectonic setting is debated. Recent Ar–Ar dates of ∼1750 Ma for NW-trending dykes in the Aldan and Anabar shields, together with similar-age NNE-trending Baikal uplift dykes in south-eastern Siberia suggest the existence of a giant radial dyke swarm possibly related to a mantle plume centred in the Vilyui River area. 相似文献
54.
Coordination of boron in nominally boron-free rock forming silicates: Evidence for incorporation of BO3 groups in clinopyroxene 总被引:1,自引:0,他引:1
Ulf Hålenius Henrik Skogby Sabrina Nazzareni Jeppa Resmark 《Geochimica et cosmochimica acta》2010,74(19):5672-1594
To explore mechanisms of B-incorporation in common chain silicates we have investigated synthetic diopside samples produced under boron-saturated conditions by 11B and 29Si magic-angle spinning (MAS) NMR and single-crystal NRA, FTIR, EMP and XRD/SREF techniques. Our samples contain 0.14-0.65 wt.% B2O3. NMR reveals that B is predominantly present in trigonal coordination in the clinopyroxene structure. This observation is supported by vibrational bands characteristic for B-O stretching in BO3 groups in the range 1250-1400 cm−1 in polarised single crystal FTIR-spectra. Single crystal structure refinements suggest that boron replaces Si at the T site. Combined, these results suggest that boron replacement for Si at the T-site leads to disruption of one of the T-O bonds of the nominal clinopyroxene structure resulting in replacement of SiO4 tetrahedra by BO3 groups. Our results show that high concentrations of boron can be incorporated in the nominally boron-free diopside. Elevated B-concentrations in the present calcic clinopyroxenes are accompanied by modifications of the diopside crystal structure involving the breaking of one T-O bond and simultaneous formation of vacancies at the octahedral M2 site. These structural modifications destabilize the structure and constitute thereby limiting factors for incorporating higher boron concentrations in diopside. 相似文献
55.
Soil-precipitation feedbacks during the South American Monsoon as simulated by a regional climate model 总被引:1,自引:0,他引:1
Anna A. Sörensson Claudio G. Menéndez Patrick Samuelsson Ulrika Willén Ulf Hansson 《Climatic change》2010,98(3-4):429-447
We summarize the recent progress in regional climate modeling in South America with the Rossby Centre regional atmospheric climate model (RCA3-E), with emphasis on soil moisture processes. A series of climatological integrations using a continental scale domain nested in reanalysis data were carried out for the initial and mature stages of the South American Monsoon System (SAMS) of 1993–92 and were analyzed on seasonal and monthly timescales. The role of including a spatially varying soil depth, which extends to 8 m in tropical forest, was evaluated against the standard constant soil depth of the model of about 2 m, through two five member ensemble simulations. The influence of the soil depth was relatively weak, with both beneficial and detrimental effects on the simulation of the seasonal mean rainfall. Secondly, two ensembles that differ in their initial state of soil moisture were prepared to study the influence of anomalously dry and wet soil moisture initial conditions on the intraseasonal development of the SAMS. In these simulations the austral winter soil moisture initial condition has a strong influence on wet season rainfall over feed back upon the monsoon, not only over the Amazon region but in subtropical South America as well. Finally, we calculated the soil moisture–precipitation coupling strength through comparing a ten member ensemble forced by the same space–time series of soil moisture fields with an ensemble with interactive soil moisture. Coupling strength is defined as the degree to which the prescribed boundary conditions affect some atmospheric quantity in a climate model, in this context a quantification of the fraction of atmospheric variability that can be ascribed to soil moisture anomalies. La Plata Basin appears as a region where the precipitation is partly controlled by soil moisture, especially in November and January. The continental convective monsoon regions and subtropical South America appears as a region with relatively high coupling strength during the mature phase of monsoon development. 相似文献
56.
Ohne Zusammenfassung
English Summary: Lake trout and its prey fauna in Swiss mountain lakes)
Die Nahrungsfauna wurde vonGrim⇘s, Nahrungs?kologie und Wachstum der Fische vonNilsson bearbeitet. 相似文献
57.
Guoyin Shen Yingwei Fei Ulf Hålenius Yanbin Wang 《Physics and Chemistry of Minerals》1994,20(7):478-482
Electronic absorption spectra have been measured at room temperature and pressure for polycrystalline samples of (Mg, Fe)SiO3 silicate perovskites synthesized by multi-anvil device. One strong near-infrared band at about 7000 cm-1 and several weak bands in the visible region were found. The near-infrared band at 7000 cm-1 is assigned to a spin-allowed transition of Fe2+ at the 8–12 coordinated site in perovskite. However, definite assignments of the weak bands in the visible region are difficult because of their low intensities and the scattering effect at the gain boundaries. Crystal field calculations for Fe2+ at different sites in perovskite have been carried out based on the crystal structure data. The results agree with the assignment of Fe2+ to the 8–12 coordinated site in perovskite. Crystal field stabilization energy of Fe2+ with coordination number of 8 in perovskite is 3332 cm-1 which is small compared to the octahedral site of magnesiowüstite (4320 cm-1), another important lower-mantle mineral. 相似文献
58.
Mn-activated cathodoluminescence can be used in several fields of carbonate petrography. It may, for instance, be possible to recognize
- cement sequences and their correlation (Tab. 1, Figs. 1, 2, 4; Tab. 2, Fig. 1)
- growth fabrics of skeletons (Tab. 2, Figs. 2, 3, 4; Tab. 3, Figs. 1, 2)
- dolomitisation processes and problems (Tab. 1, Figs. 1, 2; Tab. 4, Fig. 2)
- transformation paths from Mg-calcite to calcite and from aragonite to calcite (Tab. 2, Fig. 1; Tab. 3, Figs. 3, 4; Tab. 4, Fig. 1)
- growth structures in certain types of ooids (Tab. 1, Fig. 4; Tab. 3, Fig. 3; Tab. 4, Fig. 1)
- reworked skeletal particles (Tab. 3, Fig. 4)
- phantom grains and fossil-outlines in a micro- or macrocrystalline groundmass (Tab. 4, Figs. 2, 3)
- healed fissures crossing micro- or macrocrystalline carbonate rocks (Tab. 4, Fig. 4).
59.
Ulf Högström Roger Taesler Stig Karlsson Leif Enger Anns Ofi Smedman Högström 《Boundary-Layer Meteorology》1978,15(1):69-80
An extensive urban meteorology project in Uppsala, Sweden, is described. The city itself is considered suitable as a kind of model city, being almost circular, having sharp urban/rural boundaries and being situated in a relatively flat area. The project incorporates numerous measurements. A main station with a 100-m mast is situated at the NE urban/rural border. Its instrumentation consists of a slow-response (profile) system and a turbulence system. The profile instrumentation is described in detail. In addition to the main station, the project comprises: measurements from two 14-m masts, one on the top of a centrally located building, the other mobile; pilot balloon ascents; tethered balloon soundings, car-borne temperature traverses etc. Some preliminary results are presented: analyses of wind profiles from the mast and three-dimensional heat island studies. 相似文献
60.
Marc Daignieres Vincent Courtillot Roger Bayer Paul Tapponnier 《Earth and Planetary Science Letters》1975,26(2):222-232
In Iceland tectonic activity in the neovolcanic zone leads to the formation of three kinds of parallel structures: open fissures, emissive fissures, and normal faults.This observation is used to built a kinematic model which is based on the superposition of lava flows generated from an active central belt of finite width. The results are in good agreement with the recent results in magnetism and tectonic observations of both subaerial and underwater active ridges. 相似文献