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31.
Catherine Mevel 《Contributions to Mineralogy and Petrology》1988,100(4):496-509
Gorringe Bank is an anomalously high structure of the eastern part of the north Atlantic, which was known to be composed of mantle-derived peridotites (layer 4) and gabbros (layer 3). During the submersible cruise CYAGOR II in 1981, the contact between layer 4 and layer 3 was observed on Mount Gettysburg and interpreted as tectonic. The overlying series of gabbro was extensively sampled on both mounts composing the bank, Gettysburg and Ormonde. Coarse-grained to pegmatoid clinopyroxene gabbros predominate and are associated with differentiated rocks (ferrogabbros and diorites). Cumulate gabbros are missing. The gabbroic section sampled is therefore interpreted as the upper part of the plutonic section. Most samples were strongly recrystallized during two distinct events. Metamorphism occurred close to the ridge axis, from interaction of a seawater-derived fluid with still hot gabbros. High temperature shear zones favoured fluid circulation, but the water/rock ratio — estimated from the sodium input — was very small in undeformed rocks (<1). The low W/R ratio explains the strong evolution of the fluid phase and therefore some particular compositions of secondary minerals. Low temperature alteration occurred when the gabbros were tectonically emplaced close to the sea bottom. 相似文献
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Isotopic study of the Manaslu granite (Himalaya,Nepal): inferences on the age and source of Himalayan leucogranites 总被引:17,自引:1,他引:17
Catherine Deniel Philippe Vidal Angel Fernandez Patrick Le Fort Jean-Jacques Peucat 《Contributions to Mineralogy and Petrology》1987,96(1):78-92
A detailed isotopic study of the Manaslu leucogranite was carried out. A U-Pb age of 25 Ma and a whole rock Rb-Sr age isochron of 18 Ma were obtained, suggesting that the magmatic activity lasted at least 7 Ma. Initial Sr isotopic ratios are very high (0.740 to 0.760) and initial Nd isotopic ratios are low (
Nd
in
: –13 to –16), and they show the existence of large isotopic variations even at the metre scale. These are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes (e.g. fusion, mixing by convection). These results also support the crustal origin of this leucogranite. The Tibetan slab paragneisses, whose Sr and Nd isotopic ratios are very similar to those of the granite at an age of 20 Ma, are the most probable parental material. Nd model ages for both the leucogranite and the gneisses are in the range 1.5–2 Ga. A model of formation of the Manaslu granite by coalescence of different batches of magma is in agreement with the present data. 相似文献
35.
Keith D. Williams Catherine A. Senior Anthony Slingo John F. B. Mitchell 《Climate Dynamics》2005,24(7-8):701-719
Most of the discrepancies in the climate sensitivity of general circulation models (GCMs) are believed to be due to differences in cloud radiative feedback. Analysis of cloud response to climate change in different ‘regimes’ may offer a more detailed understanding of how the cloud response differs between GCMs. In which case, evaluation of simulated cloud regimes against observations in terms of both their cloud properties and frequency of occurrence will assist in assessing confidence in the cloud response to climate change in a particular GCM. In this study, we use a clustering technique on International Satellite Cloud Climatology Project (ISCCP) data and on ISCCP-like diagnostics from two versions of the Hadley Centre GCM to identify cloud regimes over four different geographical regions. The two versions of the model are evaluated against observational data and their cloud response to climate change compared within the cloud regime framework. It is found that cloud clusters produced by the more recent GCM, HadSM4, compare more favourably with observations than HadSM3. In response to climate change, although the net cloud response over particular regions is often different in the two models, in several instances the same basic processes may be seen to be operating. Overall, both changes in the frequency of occurrence of cloud regimes and changes in the properties (optical depth and cloud top height) of the cloud regimes contribute to the cloud response to climate change. 相似文献
36.
Isabelle Letard Philippe Sainctavit Catherine Deudon 《Physics and Chemistry of Minerals》2007,34(2):113-120
Pyrrhotite (Fe7S8) is a natural iron sulphide that can participate in rock magnetisation. Its electronic structure is not yet surely described.
X-ray magnetic circular dichroism (XMCD) at Fe L2,3 edges on Fe7S8, coupled with multiplet calculations, shows that iron is present only as Fe2+ in this magnetic iron sulphide. It reveals a strong magnetic orbital moment. XMCD at Fe and S K edges shows the quite strong
polarization of both Fe and S in Fe7S8. 相似文献
37.
This study of fossils (pollen, plant macrofossils, stomata and fish) and sediments (lithostratigraphy and geochemistry) from
the Wendel site in North Dakota, USA, emphasizes the importance of considering ground-water hydrology when deciphering paleoclimate
signals from lakes in postglacial landscapes. The Wendel site was a paleolake from about 11,500 14C yr BP to 11,100 14C yr BP. Afterwards, the lake-level lowered until it became a prairie marsh by 9,300 14C yr BP and finally, at 8,500 14C yr BP, an ephemeral wetland as it is today. Meanwhile, the vegetation changed from a white spruce parkland (11,500 to 10,500
14C yr BP) to deciduous parkland, followed by grassland at 9,300 14C yr BP. The pattern and timing of these aquatic and terrestrial changes are similar to coeval kettle lake records from adjacent
uplands, providing a regional aridity signal. However, two local sources of ground water were identified from the fossil and
geochemical data, which mediated atmospheric inputs to the Wendel basin. First, the paleolake received water from the melting
of stagnant ice buried under local till for about 900 years after glacier recession. Later, Holocene droughts probably caused
the lower-elevation Wendel site to capture the ground water of up-gradient lakes. 相似文献
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