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1.
In this paper we investigate two major issues: (1) What are chondrules, and (2) why do they exist? We review the literature pertaining to each question and explore answers. We discuss the diversity of chondrules, especially with regard to their igneous textures and compositions. We review the constraints that have been placed experimentally on the thermal histories of chondrules and list those fundamental properties that all chondrule formation models must explain quantitatively in order to be considered predictive, quantitative models. We provide background on the three major classes of chondrule formation models currently being discussed, and scrutinize each with respect to how well they adhere to the experimental constraints placed on chondrule formation. Finally, we list several unresolved issues that are now or will soon be at the forefront of chondrule research. 相似文献
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Magnetisation measurements were performed on the synthetic analogue of stannite, Cu2FeSnS4, in order to characterise the antiferromagnetic transition at low temperature, evidenced by Bernardini et al. (2000). Temperature and field dependence of the material were checked by means of static magnetisation measurements, carried out scanning the magnetic fields up to 12 T and temperatures in the range 1.4–20 K, while ac susceptibility data were collected at different frequencies ranging from 1.8 to 510 Hz. Both static and dynamic magnetisation data, performed above and below the Néel temperature, 6.1(2) K, confirm stannite to order antiferromagnetically at a long-range scale. Moreover, an increase of both the magnetic anisotropy and the exchange interaction, with respect to the Mn-analogue (Fries et al. 1997), has been observed. 相似文献
5.
A new version of the STRUCTON (2009) computer model is proposed for the simulation of the molecular mass distributions (MMD)
characterizing the diversity of anions in silicate melts depending on their polymerization and temperature. In contrast to
earlier versions, the new version of the model accounts for disproportionation reactions of Q
n
species and makes use of their proportions in the statistical simulations of the origin of real Si-O complexes. The new potentialities
of the STRUCTON program package are illustrated by its application to studying the structural-chemical characteristics of
melts in the Na2O-SiO2 system along its liquidus line, including the points of eutectics and phase transitions at 0.333 ≤ $
N_{SiO_2 }
$
N_{SiO_2 }
< 0.500. This problem is solved with the use of a temperature-composition dependence of polymerization constants K
p
Na in the Toop-Samis approximation. The variations in K
p
Na were proved to be as large as three orders of magnitude due to both the temperature effect at a constant composition and
the composition effect at a constant temperature. The results of the MMD simulations on the liquidus show that the concentration
of the SiO44− ion strongly decreases, and the proportion of chain species increases compared to those at a stochastic distribution. The
concentration of the Si2O76− anion reaches its maximum (∼42%) at 40 mol % in the liquid, i.e., the composition of Na6Si2O7. At $
N_{SiO_2 }
$
N_{SiO_2 }
> 0.40, this ion dominates over the SiO44− monomer. More silicic melts with $
N_{SiO_2 }
$
N_{SiO_2 }
≥ 0.45, are dominated by (Si
n
O3n
)3n− ring species, and the concentrations of these species are related as (Si3O9)6− > (Si4O12)8− > (Si5O15)10−. The maximum concentration of these flat rings also occurs near the composition of stoichiometric metasilicate with Si/O
= 0.333. The comparison of the dependence of the average size of anions i
av and the average number of their species on depolymerization indicates that a change in the proportion of Q
n
species in melt at decreasing temperature results in structural restyling and an increase in the average size of Si-O complexes.
The average number of anion species thereby decreases compared to that in a stochastic MMD. The results presented in this
publication direct the progress in the thermodynamic theory of silicate melts to a new avenue that makes use of the capabilities
and advantages of the ion-polymer model, the theory of associated solutions, spectroscopic data, and the experimental study
of variations in oxide activities depending on composition and temperature. 相似文献
6.
Lucinid bivalves dominate the infauna of tropical seagrass sediments. While the effect of seagrass on lucinids has been studied,
the reverse effect has largely been ignored. Lucinids can alter porewater chemistry (i.e., increase porewater nutrients by
suspension feeding and decrease porewater sulfides by oxygen introduction and bacterial oxidation), which can potentially
change seagrass productivity and growth morphology. To observe correlations between porewater chemistry and lucinid presence,
a field survey and laboratory microcosm experiment were conducted. Survey sampling sites with lucinids had significantly lower
sulfide and higher ammonium concentrations than sampling sites without lucinids. There was no difference in phosphate concentration
among sampling sites. Both lucinid species used in the microcosm experiment (Ctena orbiculata andLucinesca nassula) significantly lowered sulfide concentrations in the sediment porewater. Microcosm and field survey results were incorporated
into a sulfide budget. In seagrass sediments, lucinids remove 2–16% of the total sulfide produced. Sulfide is a major stressor
to both plants and animals in Florida Bay sediments; this removal may be important to maintaining seagrass productivity and
health. Oxygen introduction into sediments byC. orbiculata was estimated in a dye experiment.C. orbiculata were added to small tubes containing sieved mud and incubated in a bath of seawater with a Rhodamine WT. Rhodamine WT accumulation
in the sediment was measured. A first order estimate showed that oxygen introduction can account for less than 5% ofC. orbiculata sulfide removal. 相似文献
7.
The Neogene succession in the western margin of Çank?r? Basin is fragmented by a NNE‐trending tectonic sliver having normal faulted western and thrusted eastern margins. This newly recognized E‐vergent sliver was created by the NW–SE compression due to the North Anatolian and K?r?kkale–Erbaa Fault zones following late Pliocene, accommodating the internal deformation of the Anatolian plate. Determinations of the K?lçak, Kumarta? and Hançili formations on both sides of this tectonic sliver invalidate the stratigraphical, structural and basin evolution models previously proposed by Kaymakç?. 相似文献
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Isotope datings of amphibole-bearing mafics and metamafics in the northern part of the Anadyr-Koryak region allow clarification
of the time of magmatic and metamorphic processes, which are synchronous with certain stages of the geodynamic development
of the northwest segment of the Pacific mobile belt in the Phanerozoic. To define the 40Ar/39Ar age of amphiboles, eight samples of amphibole gabbroids and metamafics were selected during field work from five massifs
representing ophiolites and mafic plutons of the island arc. Rocks from terranes of three foldbelts: 1) Pekulnei (Chukotka
region), 2) Ust-Belaya (West Koryak region), and 3) the Tamvatnei and El’gevayam subterranes of the Mainits terrane (Koryak-Kamchatka
region), were studied. The isotope investigations enabled us to divide the studied amphiboles into two groups varying in rock
petrographic features. The first was represented by gabbroids of the Svetlorechensk massif of the Pekulnei Range and by ophiolites
of the Tamvatnei Mts.; their magmatic amphiboles show the distribution of argon isotopes in the form of clearly distinguished
plateau with an age ranging within 120–129 Ma. The second group includes metamorphic amphiboles of metagabbroids and apogabbro
amphibolites of the Ust-Belaya Mts., Pekulnei and Kenkeren ranges (El’gevayam subterranes). Their age spectra show loss of
argon and do not provide well defined plateaus the datings obtained for them are interpreted as minimum ages. Dates of amphiboles
from the metagabbro of the upper tectonic plate of the Ust-Belaya allochthon points to metamorphism in the suprasubduction
environment in the fragment of Late Neoproterozoic oceanic lithosphere in Middle-Late Devonian time, long before the Uda-Murgal
island arc system was formed. The amphibolite metamorphism in the dunite-clinopyroxenite-metagabbro Pekulnei sequence was
dated to occur at the Permian-Triassic boundary. The age of amphiboles from gabbrodiorites of the Kenkeren Range was dated
to be Early Jurassic that confirmed their assignment to the El’gevayam volcanic-plutonic assemblage. These data are consistent
with geological concepts and make more precise the available age dates. Neocomian-Aptian 40Ar/39Ar age of amphibolites from the Pekulnei and Tamvatnei gabbroids make evident that mafics of these terranes (varying in geodynamic
formation settings and in petrogenesis) were generated in later stages of the development of the West Pekulnei and Mainits-Algan
Middle-Late Jurassic-Early Cretaceous island arc systems, presumably due to breakup of island arcs in the Neocomian. 相似文献
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A numerical model presented here develops a three-dimensional image of alluvial media on an elementary scale significant for groundwater flow modelling. The model was tested on the alluvial plain of the Rhône River (France), on a scale of several kilometres and, from geomorphological observations and dating, reproduced the construction of this alluvial plain from ≈15 000 years BP to the present. The history of the alluvial plain during the Late Glacial and Holocene periods is summarized. Through most of this time, the River Rhône has maintained a braided pattern, with the exception of two incising phases with a meander pattern. The model does not use any physically based equations or water representation. The main processes governing the construction of the plain are modelled by simple rules chosen according to geometrical or empirical laws taken from the literature or as modelling assumptions. Using multi-agent concepts of distribution and interaction of elementary entities, these sedimentary rules are applied to 'sediment' entities or to conceptual 'erosion' entities that simulate local deposition and erosion of sediments. The sedimentation model reproduces the various climatic periods during which the sediments were deposited by simulating genetic periods and associated modelled processes. For each period, the model was constrained by quantitative field data such as altitude of ancient channels and deposits or thickness of sediments. The general geometry of the alluvial deposits was satisfactorily reproduced. During the simulation, characteristic large-scale features emerge despite the use of local rules. The model results are discussed with reference to other approaches, such as geostatistical or Boolean models, and the applicability of the model to other less documented alluvial plains is outlined. 相似文献
12.
The segregation of ten isovalent impurities (Al3+ Cr3+, Eu3+, Gd3+, Ho3+, La3+, Lu3+, Nd3+, Tb3+, Y3+) to the and the (0001) surfaces of haematite (-Fe2O3) have been studied using atomistic simulation where the forces between the atoms are modelled using the Born model of solids. Segregation is found to be energetically favoured in virtually every case. The results for the surface show that the most favourable impurity surface concentration is 33.33%. The (0001) surface has two possible terminations, one terminated by iron atoms and the other by oxygen. No minimum is calculated for the Fe termination of the (0001) surface at low temperatures, but when the effect of raising the temperature is considered, an energy minimum is found, also at 33.33% impurity coverage. In contrast, the O terminated (0001) surface has a minimum in the segregation energy for between 16.67 and 33.33% depending on the cation being considered. 相似文献
13.
In this paper, we explore how ‘peak oil’ anxieties are woven into the spaces and practices of the state in Norway and the consequences of this for environmental justice and the public sphere more widely. We focus in particular on an ongoing struggle over access to hydrocarbon deposits in the Norwegian Arctic, the so-called ‘Battle of the North’. We use this dispute to highlight three wider theoretical points regarding (i) the continuing relevance of the state in the governing of nature-society relations, (ii) the increasingly fragmented and fluid nature of state space, and (iii) the significance of ‘security’ as a term around which social, economic and environmental tensions pivot. The paper concludes by reflecting on current efforts to prevent new oil activities in the north of Norway. 相似文献
14.
The long-term variability of rainfall in the Soummam watershed (NE Algeria) has been analysed over the past 108 years using continuous wavelet method in order to identify the interannual modes controlling the rainfall variability. Statistical analyses of rainfall timeseries have shown its distribution following five periods of time, limited by a series of discontinuities around 1935, 1950, 1970 and 1990. The continuous wavelet transform have demonstrated different low frequency modes: 2–4, 4–8, 8–16 and 16–32 years.The annual band is expanded during the full study period with some pics around 1905, 1920–1935 and 1960; it shows a negative long-term trend, in particular since the period 1970–1990 when a major change has been identified. Then, the relationships between climate patterns of North Atlantic Oscillation (NAO) and Southern Oscillation Index (SOI) and the hydrological variability in the frequency domain have been investigated; they have shown a mean explained variance of 40 and 24 %, respectively. Such variances are less obvious for the annual mode and increase for the interannual frequencies. The coherence suffer from high perturbations since the period 1970–1990 when the NAO (SOI) shifts from negative (positive) phases to positive (negative) ones. Such anomalies are responsible for significant changes of rainfall variability, emphasising the global warming effects. 相似文献
15.
The standard enthalpy of formation of stannite (Cu2FeSnS4) was calculated from the calorimetric measurements of the reactions of its formation from simple synthetic sulfides: Cu2S + FeS2 + SnS → Cu2FeSnS4 and 2CuS + FeS + SnS → Cu2FeSnS4. Using published data for the binary sulfides, the standard enthalpy of formation of stannite from the elements was determined as ΔfH°298.15(Cu2FeSnS4) =–(417.28 ± 2.28) kJ mol–1. 相似文献
16.
Charles Hepworth Holland 《Geological Journal》2000,35(1):25-31
The very rare occurrences of cephalopods in the Přídolí Series of the English and Welsh Silurian are recorded and their taxonomy revised. Apart from one derived form, they constitute a typical relict Ludlow fauna, including Cyrtocycloceras ibex, Leurocycloceras whitcliffense, and Polygrammoceras bullatum. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
17.
Weili Wang Luofu Liu Zhenxue Jiang Ningning Zhong Ying Wang Ping Wang Lin Wu Jianghui Meng Jieli Zhou Yongqiang Guo 《中国地球化学学报》2011,30(4):405-414
The Western Depression of the Liaohe Basin is the major exploration area of the Liaohe Oilfield, and its main source rocks consist of the third and fourth members of the Shahejie Formation (Es3 and Es4). These source rocks are widely distributed in the depression, with semi-deep lake and fan delta as the main sedimentary facies, brown oil shale and black gray-dark gray mudstone as the main rocks, and a total thickness of 270-1450 m. The kerogens are mainly of the types I and IIA, and partly of the type IIB and least of the type III. The Ro values range from 0.4%-0.8%, indicating an evolution stage from immature to mature. The maturity of Es4 source rocks is rela-tively high, reaching the early mature stage, but their distribution and thickness are lower than those of Es3. Besides, according to biomarker analysis, it is thought that the source rocks of Es3 and Es4 are characterized by mixed input, and most of the source rocks were formed in the brackish water-saline and strongly oxygen-free environment. Fur-thermore, the Qingshui, Niuxintuo and Chenjia sags are believed to possess greater potential for hydrocarbon gen-eration and expulsion, for they are source rocks with a larger thickness, have higher organic carbon contents, belong to better organic matter types and possess higher maturities. 相似文献
18.
The creatures that gaze back at us through the binocular microscope are so uniquely well‐preserved and life‐like that had we not known better we would have thought that they were modern. Instead they are half‐a‐billion‐year‐old arthropods extracted from Cambrian rocks of Sweden. The microscopic fossils, colloquially known as the ‘Orsten’ fossils, are now world‐famous and basking in the light of the research catwalk since their discovery in the 1970s. These fossils have provided significant insight into the long lost Cambrian biotas and early animal evolution. 相似文献
19.
Akio Suzuki Eiji Ohtani Hidenori Terasaki Keisuke Nishida Hiromi Hayashi Tatsuya Sakamaki Yuki Shibazaki Takumi Kikegawa 《Physics and Chemistry of Minerals》2011,38(1):59-64
The viscosity of a silicate melt of composition NaAlSi2O6 was measured at pressures from 1.6 to 5.5 GPa and at temperatures from 1,350 to 1,880°C. We employed in situ falling sphere
viscometry using X-ray radiography. We found that the viscosity of the NaAlSi2O6 melt decreased with increasing pressure up to 2 GPa. The pressure dependence of viscosity is diminished above 2 GPa. By using
the relationship between the logarithm of viscosity and the reciprocal temperature, the activation energies for viscous flow
were calculated to be 3.7 ± 0.4 × 102 and 3.7 ± 0.5 × 102 kJ/mol at 2.2 and 2.9 GPa, respectively. 相似文献
20.
Oil ‘futures’: Shell’s Scenarios and the social constitution of the global oil market 总被引：1，自引：0，他引：1
Anna Zalik 《Geoforum》2010,41(4):553-564
This article explores the relationship between the oil industry’s representation of operating conditions in key sites of extraction and the constitution of oil futures markets. An analysis of Shell Oil’s recent Scenarios publications, the ‘Trilemma Scenarios to 2025’ and subsequent ‘Scramble and Blueprints Scenarios to 2050’, provides insight into both the (global) social construction of oil prices and the oil industry’s reaction to social resistance in its operating environment - whether in the form of movements for resource sovereignty or climate change activism. Examining the implications of these two Scenario publications for key sites of Shell investment, the Nigerian Niger Delta and the Canadian Tar Sands, the article demonstrates that understanding the discursive implications of ‘peak oil’ for the petroleum industry requires contextualizing discussions of ‘scarcity’ within business agents role in shaping oil futures markets, and private industry’s interest in the ongoing development of unconventional fossil fuel sources. While the role of deregulated futures trading receives little attention in the Shell Scenarios, speculative trading - and thus perception concerning supply among business agents - is central to shaping global oil prices and thus the social conditions of the oil market. 相似文献