Microtextures and crystal chemistry in P21/c pigeonites     

M. Tribaudino  D. Pasqual  G. Molin  L. Secco 《Mineralogy and Petrology》2003,77(3-4):161-176

Summary ?A single-crystal X-ray investigation was performed on crystals of P2₁/c natural pigeonite with varying Ca and Fe^* ( = Fe²⁺ + Mn²⁺) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae. The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated. The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples. In particular, it is shown that Ca and Fe^* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and decrease the difference between the M2–O3 distances. Received October 18, 2001; revised version accepted February 15, 2002  相似文献   

The Nordic geological surveys： Geology for society in practice   总被引：1，自引：0，他引：1  

Morten Smelror Andreas AhlstrФm Lena Ekelund Jens Morten Hansen Keijo Nenonen Anette K. Mortensen 《《幕》》2008,31(1):193-200

Since the mid-nineteenth century., when the first of the Nordic Geological Surveys were established, they have generated a substantial amount of information on the Earth＇s crust, its natural resources, its processes, and on the geological history of Nordic areas. The collective mission of the geological surveys is to carry out ＂Geology for Society＂, by doing research and providing services, and by making geological information and data easily accessible to all the varied end users in industry, government agencies, government institutes, public administrations, technical offices, academia and research institutes, as well as for private individuals. The present paper gives a brief overview of a few, selected, research areas and projects currently undertaken by the Nordic geological surveys. These serve as practical examples of how the Nordic geological surveys address important societal problems and challenges that require geological input for their solution.  相似文献   

Effect of wave-directionality on second-order loads induced by the set-down     

B. Molin  V. Fauveau 《Applied Ocean Research》1984,6(2):66-72

At first-order of approximation a sea-state may be considered as an infinite sum of Airy components with angular frequencies ω_i and wave-number vectors k_i. A second-order analysis shows the co-existence of long waves appearing at the difference frequencies ω_i—ω_j with wave-number vectors k_i—k_j. In shallow water they become appreciable in amplitudes and may induce slow-drift motion of moored structures.For small values of ω_i—ω_j,k_i—k_j may take all kinds of directions for an angular-spread wave system. Then it may be questioned how the in-line and transverse second-order accelerations compare to those obtained for a mono-directional wave-system.This analysis is carried out here by relating the spectra of the second-order horizontal accelerations to the directional wave-spectrum. Numerical applications are first performed for deep water. They show that at low frequencies, even for very narrowly spread wave systems, the transverse component is larger than the in-line component. In shallow water both components are dratically reduced as compared to the mono-directional case. As a consequence one may question the validity of model-testings or numerical models which take no account of the directionality of the wave-system.  相似文献   

Crystal-chemistry of clinopyroxene from Sunda volcanic arc     

G. Salviulo  G. M. Molin 《Mineralogy and Petrology》1993,49(3-4):233-248

Summary The crystal-chemistry of clinopyroxene from a suite of Sunda arc volcanic rocks was investigated by X-ray structure refinement and microprobe analysis. Relationships with clinopyroxene from volcanic rocks of the intraplate environment were highlighted.Sunda clinopyroxenes were distinguished into two groups. The first consists of clinopyroxene from SiO₂-saturated rock types and crystal rim from SiO₂-undersaturated rock types, the second of crystal core from SiO₂-undersaturated rock types. The crystal chemical behaviour of the two groups differs significantly, mainly in the geometrical relationships between M2 and T sites. While in the first group M2-O3 and T-O3 bond lengths shorten and lengthen respectively due to Ca and Al^IV increase, in the second group in which M2 site is almost fully coordinated by Ca, -O3 bond lengths are forced to lengthen in spite of low Al^IV content, in order to mantain the local charge balance around the O3 oxygens.It is noteworthy that Sunda clinopyroxene shows strong analogies with that from basaltic rocks from the Ethiopian plateau and K-rich lavas from the Roman Province respectively, reflecting similarities between the composition and mineralogical assemblages of the host rocks, although they all came from different tectonic environments. A relationship between intracrystalline Mg-Fe 2+ disorder and the explosive character of the volcanism is demonstrated.

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Kristallchemie von Klinopyroxenen aus Gesteinen des Vulkanbogens der Sunda-Inseln

Zusammenfassung Die Kristallchemie von Klinopyroxenen aus einer Serie von vulkanischen Gesteinen des Sunda-Bogens wurde mit Röntgen-Strukturverfeinerungen und Mikrosonden-analysen untersucht. Beziehungen zu Klinopyroxenen vulkanischer Gesteine aus intra-Plattenpositionen werden beleuchtet. Die Sunda-Klinopyroxene wurden in zwei Gruppen eingeteilt. Die erste umfaßt Klinopyroxene SiO₂-gesättigter Gesteinstypen und Kristallsäume SiO₂-untersättigter Gesteinstypen, die zweite Kristallkerne SiO₂-untersättigter Gesteinstypen. Das kristallchemische Verhalten der beiden Gruppen unterscheidet sich beträchtlich, hauptsächlich in den geometrischen Beziehungen zwischen den M2 und T Positionen. Während in der ersten Gruppe die M2-O3 und T-O3 Bindungsldngen durch Zunahme von Ca und Al^IV kleiner bzw. größer werden, sind in der zweiten Gruppe die T-O3 Bindungslängen trotz niedrigen Al^IV Gehaltes zu einer Vergrößerung gezwungen, damit um die O3 Atome der lokale Ladungsausgleich erhalten bleibt.Es ist bemerkenswert, daß die Sunda-Klinopyroxene große Analogien zu denen aus basaltischen Gesteinen des Abessinischen Plateaus und aus K-reichen Laven sowohl der Römischen Provinz als auch des Westafrikanischen Grabens zeigen. Dies spiegelt Ähnlichkeiten zwischen Zusammensetzung und Mineralbestand der Muttergesteine wider, obwohl sie all aus tektonisch unterschiedlichen Positionen kommen.Eine Beziehung zwischen intrakristalliner Mg-Fe²⁺ Unordnung und dem explosiven Charakter des Vulkanismus wird aufgezeigt.

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Palaeoclimatic studies in South Shetland Islands,Antarctica, based on numerous stratigraphic variables in lake sediments     

Svante Björck  Hannelore Håkansson  Siv Olsson  Lena Barnekow  Jan Janssens 《Journal of Paleolimnology》1993,8(3):233-272

The hitherto longest found lake sediment sequence on Byers Peninsula, Livingston Island, South Shetland Islands, was analysed with respect to lithology, chronology, diatoms, Pediastrum, pollen and spores, mosses, mineralogy, and sediment chemistry. During the ca. 5000 year long development the sediments were influenced by frequent tephra fall-outs. This volcanic impact played a major role in the lake's history during two periods, 4700–4600 and 2800–2500 BP, but was of importance during the lake's entire history with considerable influence on many of the palaeoenvironmentally significant indicators. The large and complex data set was analysed and zonated with different types of multivariate analysis. This resulted in a subdivision of the sequence into 8 time periods and 21 variables. Redundancy analysis (RDA) of this data set, both without and with the tephra periods, and with 4–6 of the variables as explanatory environmental variables, reveal that climatic/environmental signals are detectable. The palaeoclimatic picture that emerged out of the tephra noise suggests that the first 100 years were characterized by mild, humid conditions. This was followed by a less mild and humid climate until ca. 4000 BP when a gradual warming seems to have started, coupled with increased humidity. These mild and humid conditions seem to have reached an optimum slightly after 3000 BP. At ca. 2500 BP a distinct climatic deterioration occurred with colder and drier conditions and long seasons with ice cover. This arid, cold phase probably reached its optimum conditions at ca. 1500 BP, when slightly warmer conditions might have prevailed for a while. Except for the modern sample with rather mild climate, the last 1400 years seem to have been fairly arid and cold, and the effects of the frequent volcanic activity during this period is only vaguely seen in the records.  相似文献   

A comparison of chironomid biostratigraphy from Lake Vuolep Njakajaure with vegetation, lake-level, and climate changes in Abisko National Park, Sweden     

Markus Heinrichs  Lena Barnekow  Sandra Rosenberg 《Journal of Paleolimnology》2006,36(2):119-131

Chironomid remains from the sediment of Lake Vuolep Njakajaure reflect limnological conditions resulting from changing climate and vegetation throughout the Holocene, but do not strictly follow accepted climate trends or the vegetation history based on regional pollen and macrofossil analyses. Chironomid community changes appear to be influenced by organic nutrient input from the surrounding catchment vegetation and lake hydrology, both of which are indirectly responding to some combination of climate change, hypolimnetic oxygen concentration, and changes in basin morphology. The chironomid-based quantitative mean July air-temperature reconstruction differs from other regional quantitative records; this discrepancy is likely related to limnological conditions particular to Lake Vuolep Njakajaure. Comparison of a northern Swedish temperature transfer function and one from western Canada reveals differences in the mean July air-temperature optima of several common taxa, suggesting that the existing conservative estimates of Holocene climate change in northern Sweden may be underestimated due to the limited temperature gradient captured by the Swedish training set.  相似文献   

Detailed geomorphological survey of a small mountain drainage area, Abisko, northern Swedish Lapland     

Lena Rubensdotter 《Geografiska Annaler: Series A, Physical Geography》2002,84(3&4):267-273

A thorough geomorphological survey of a small (2km²) drainage area has been conducted using digital magnification of aerial photographs in conjunction with field visits. The result presented is a geomorphological map where individual geomorphological features down to metre size can be identified. The study was done in connection with a project focusing on the relationship between geomorphological processes and lacustrine sedimentation. Good knowledge of the geomorphological setting and the related process activity in the catchment is important in studies using lacustrine sediments as environmental archives.
The survey reveals a small–scale geomorphology dominated by a number of different periglacial upfreezing forms together with bedrock–controlled slope processes. Three different geographically separated geomorphological assemblages were recognised with few sediment transportation pathways connecting them. Composition of substrate, soil water content and vegetation cover combined with different slope angles are probably the most important factors controlling the distribution of the geomorphological features.  相似文献   

Uraninite recrystallization and Pb loss in the Oklo and Bangombé natural fission reactors, Gabon   总被引：1，自引：0，他引：1  

Lena Z. Evins  Keld A. Jensen  Rodney C. Ewing 《Geochimica et cosmochimica acta》2005,69(6):1589-1606

The Oklo and Bangombé natural fossil fission reactors formed ca. 2 Ga ago in the Franceville basin, Gabon. The response of uraninite in the natural reactors to different geological conditions has implications for the disposal of the UO₂ in spent nuclear fuel. Uraninite and galena from two reactor zones, RZ16 at Oklo and RZB at Bangombé, were studied to clarify the chronology and effect of alteration events on the reactor zones. In addition, ion microprobe U-Pb analysis of zircons from a dolerite dyke in the Oklo deposit were completed to better constrain the age of the dyke, and thereby testing the link between the dyke and an important alteration event in the reactor zones.The analyzed uraninite from RZ16 and RZB contains ca. 6 wt% PbO, indicating a substantial loss of radiogenic Pb. Transmission electron microscopy showed that microscopic uraninite grains in the reactor zones consist of mainly defect-free nanocrystalline to microcrystalline aggregates. However, the nanocrystalline regions have elevated Si contents and lower Pb contents than coarser uraninite crystallites. Single stage model ages of large, millimeter-sized galena grains at both RZ16 and RZB correlate well with the age of the Oklo dolerite dyke, 860 ± 39 Ma (2σ). Thus, the first major Pb loss from uraninite occurred at both Oklo and Bangombé during regional extension and the intrusion of a dyke swarm in the Franceville basin, ∼860-890 Ma ago. Uraninite Pb isotopes from RZ16 and RZB give lower ages of ca. 500 Ma. These ages agree with the “chemical” ages of the uraninite, and show that an ancient Pb loss occurred after the intrusion of the dolerite dykes. The presence of nanocrystallites in the reactor uraninite indicates internal recrystallization, which may have occurred around 500 Ma, resulting in the 6wt% PbO uraninite. It is suggested that leaching by fluid interaction triggered by the Pan-African orogeny was important during this second Pb-loss event. Thus, there are indications that uraninite at both the Oklo and Bangombé natural reactors has experienced at least two ancient episodes of Pb loss associated with internal recrystallization. These recrystallization events have occurred without significantly depleting the 2 Ga fission products compatible with the uraninite structure.  相似文献   

The capacity of European Higher Educational Institutions to address threats imposed by natural hazards     

Skevi Perdikou  Jiri Horak  Lena Halounová  Roshani Palliyaguru  Andrew Lees 《Natural Hazards》2016,81(3):1447-1466

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Simulating seismic chimney structures as potential vertical migration pathways for CO<Subscript>2</Subscript> in the Snøhvit area,SW Barents Sea: model challenges and outcomes     

Alexandros Tasianas  Lena Mahl  Melanie Darcis  Stefan Buenz  Holger Class 《Environmental Earth Sciences》2016,75(6):504

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