Based upon a re-interpretation of previous data and a new field campaign, a structural evolution is proposed for the early history of Piton de la Fournaise volcano from 500,000 to 50,000 years. Conceptually, it is shown that the formation of a caldera in which lava flows are contained inside the caldera depression, gives time for erosion to excavate deep canyons on the external slopes of the volcano, for example, the Rivière des Remparts, the Rivière Langevin and the Rivière de l'Est canyons on Piton de la Fournaise volcano. These canyons are infilled when lavas, filling the caldera and overflowing its rim, are able again to flow on the external slopes of the volcano. In the past, this excavating/infilling process has occurred twice following the formation of the Rivière des Remparts and Morne Langevin calderas. The formation of the third caldera, the Plaine des Sables caldera, was followed by the excavation of the current canyons. In addition to this process, two large landslides have been documented in the field. The first, which happened about 300,000 years ago, is apparently the first episode of the break up of Piton de la Fournaise volcano, predating the formation of the four large calderas. The second landslide, which occurred 150,000 years ago and is considered to be less extensive, has carried away the entire southern flank of the Rivière des Remparts caldera. 相似文献
Trace element and isotopic characteristics of late Carboniferous to early Permian minettes and kersantites have been determined. These lamprophyres have been sampled throughout the Western European Hercynian orogen, from Brittany to the west to Schwarzwald to the east. In spite of sharp petrological differences reflected by mineralogy and major element geochemistry, minettes and kersantites exhibit close identity with respect to trace element and isotopic features. These features comprise enrichment in incompatible elements, highCs/Rb and lowCe/Pb ratios, Ta and Ti relative depletion, high abundance in transition elements and highNi/Mg ratios. Pb isotope ratios are undistinguishable from those measured on Hercynian continental crust. Initial143Nd/144Nd ratios are between0.5120 (εi −5) and0.5122 (εi −1) for minettes and kersantites whereas initial87Sr/86Sr ratios vary between 0.7055–0.710 for minettes and 0.707–0.708 for kersantites. No simple mixing relations are visible on RbSr and SmNd isochron diagrams. The exceptional homogeneity of these geochemical characteristics along a 1000 km traverse does not allow for an hypothesis of enrichment through upper level assimilation and thus leads to propose that these rocks originated through melting of a mantle enriched by recycling of crustal material. 相似文献
We examined particle size distributions of suspended particulate matter (SPM); physical and environmental influences on the observed distributions; and relationships between particle size and geochemical partitioning of metals, over the fall and winter period in a small urban river (Don River, Toronto, Ontario, Canada). For this dataset, the majority of particles (80%) in suspension were less than 10 µm in size. In addition, while total SPM concentrations showed a positive trend with increasing discharge (Q); the proportions of particles found within given size classes were independent of both SPM concentration and Q. Temperature was the only measured environmental variable related to the particle concentrations within size classes. As water temperature increased, the concentration of particles in the smallest size class (1–4 µm) decreased, while the concentration of silt and/or algae sized particles (10–50 µm) increased. Increasing water temperatures may promote bacterial attachment to particles and their subsequent flocculation into larger sized particles. Decreasing concentrations of leachable (most labile) Cd, Zn and Mn were associated with increasing concentrations of the largest particles (70–150 µm) in suspension. In contrast, higher reducible (oxides) associated concentrations of Cd, Zn and Mn occurred with increasing concentrations of smaller particles (1–10 µm) in suspension. Both of these trends are speculated to reflect the importance of particle surface area for metal sorption reactions. 相似文献
A bifurcation phenomenon with relevance to atmospheric chemistry is discussed. The gasphase reactions in the troposphere exhibit two types of temporal evolution which are controlled by the strength of the source,Q, of nitric oxide, NO, via the nonlinear chemical coupling between the hydrogen oxides and nitrogen oxides chemistry. IfQ remains below a threshold value, all short-lived species, including NO, approach steady-state concentrations, while above the threshold bifurcation to another state with increasing (nonstationary) NO concentrations accompanied by a depletion of the OH and HO2 abundances takes place. 相似文献
During the last decade, it has been shown that accurate analyses of calcite twinning in thin section may allow computation of the paleo-deviatoric stress tensor associated with this deformation. Following from the work of Larroque and Laurent (1988), and Lacombe et al. (1989), this is a new application of the Etchecopar inverse method. Weakly deformed limestones from about 150 to 200 km north of the Pyrenean chain (South France) were studied. The Etchecopar inverse method, which can be used on a microcomputer, is comparable to the method of analysis of striated fault planes proposed by Etchecopar et al. in 1981, as it determines the principal stress orientations and the stress ellipsoid shape ratio φ = (σ2 − σ3)/(σ1 − σ3). The Etchecopar method using calcite twins has been selected for this study because it is suitable for the determination of polyphase deformation in coarse-grained limestones (Lacombe et al. 1989).
Ten samples of oolitic limestones, cemented with sparry calcite, from the Bajocian (170 Ma) and the Oligocene (30 Ma) were studied. For each of them, only one or two stress tensors were determined: the first is characterized by a principal compressive stress axis σ1 which is horizontal and approximately N-S. Local departures from this orientation have been observed and are interpreted as local stress perturbations. For the second, which was defined in the Oligocene, but occurred only locally in the Jurassic limestones, the position of σ1 is nearly vertical; the direction of extension σ3 is either NE-SW or undefined into the horizontal plane (σ2 = σ3); this radial extension is interpreted as the result of two or more superposed extensive deformation phases with vertical σ1. The N-S compression was never found in the Oligocene limestones, and from its orientation, it is assumed to correspond to the Pyrenean deformation. These results are compared to the more complex deformation pattern obtained from classical microtectonical studies (Arthaud and Choukroune, 1972; Bonijoly and Blès, 1983; Granier and Blès, 1988; Blès et al., 1989). Only the major tectonic events are recorded by calcite twinning. Finally, the suitability of calcite twin analyses for the determination of complex polyphase deformation is discussed. 相似文献
The40Ar-39Ar degassing spectra of white micas and amphiboles from three tectonic units of the central Tauern Window (Pennine basement and cover in the Eastern Alps) have been measured. White micas are classified as (1) pre-Alpine low-Si relic micas with an age value of 292 Ma, variously disturbed by the Alpine metamorphism; (2) Alpine phengitic micas of variable composition with an age between 32 and 36 Ma; (3) Alpine low-Si micas with a maximum age of 27 Ma. We attribute the higher Alpine ages to a blueschist facies event, whereas the lower age reflects the late cooling of the nappe pile. Blueschist facies phengites from the basement (Lower Schieferhülle) and the tectonic cover (Upper Schieferhülle) crystallized at a temperature below the closure temperature (Tc) for argon diffusion in white mica and record ages of 32 to 36 Ma. At the same time a thin, eclogite facies unit (Eclogite Zone) was thrust between the Lower and the Upper Schieferhülle and cooled from eclogite facies conditions at about 600°C at 20 kbar to blueschist facies conditions at 450°C or even 300°C at >10 kbar. Eclogite facies phengites closed for argon diffusion and record cooling ages, coinciding with the crystallization ages in the hanging and the footwall unit. Amphibole age spectra (actinolite, glaucophane, barroisite) are not interpretable in terms of geologically meaningful ages because of excess argon. 相似文献
A climatology of the stratosphere is determined from a 20-year integration with the stratospheric version of the Atmospheric
General Circulation Model LMDz. The model has an upper boundary at near 65 km, uses a Doppler spread non-orographic gravity
waves drag parameterization and a subgrid-scale orography parameterization. It also has a Rayleigh damping layer for resolved
waves only (not the zonal mean flow) over the top 5 km. This paper describes the basic features of the model and some aspects
of its radiative-dynamical climatology. Standard first order diagnostics are presented but some emphasis is given to the model’s
ability to reproduce the low frequency variability of the stratosphere in the winter northern hemisphere. In this model, the
stratospheric variability is dominated at each altitudes by patterns which have some similarities with the arctic oscillation
(AO). For those patterns, the signal sometimes descends from the stratosphere to the troposphere. In an experiment where the
parameterized orographic gravity waves that reach the stratosphere are exaggerated, the model stratosphere in the NH presents
much less variability. Although the stratospheric variability is still dominated by patterns that resemble to the AO, the
downward influence of the stratosphere along these patterns is near entirely lost. In the same time, the persistence of the
surface AO decreases, which is consistent with the picture that this persistence is linked to the descent of the AO signal
from the stratosphere to the troposphere. A comparison between the stratospheric version of the model, and its routinely used
tropospheric version is also done. It shows that the introduction of the stratosphere in a model that already has a realistic
AO persistence can lead to overestimate the actual influence of the stratospheric dynamics onto the surface AO. Although this
result is certainly model dependent, it suggests that the introduction of the stratosphere in a GCM also call for a new adjustment
of the model parameters that affect the tropospheric variability. 相似文献
The Moselle river flows in the north-east of France, from Vosges Mountains to neighboring countries Luxembourg and Germany.
One of its tributaries, the Fensch river, drains a highly industrialized watershed, strongly impacted by mining, smelting
and surfacing activities. The objective of this work, part of a general research program on Moselle watershed (Zone Atelier
Moselle) was to assess the impact of the polluted Fensch river on the global quality of the Moselle river. For that purpose,
water, sediments and suspended particulate matter were sampled in both rivers, upstream and downstream the junction. Four
main sampling campaigns were carried out, in winter during a flood event and in spring at low water level. On a first step,
mineralogical analyses (XRD and FTIR) and chemical analyses (ICP-MS, ICP-AES), were performed on sediments, suspended particulate
matters and filtered waters. Major and trace elements concentrations were obtained on two different granulometric fractions
(0–2 mm and 0–50 μm) revealing the enrichment in heavy metals of fine particles. From one collecting campaign to another,
seasonal variations could be evidenced on suspended matter composition even though major minerals (calcite, quartz and kaolinite)
were always present. Furthermore, spatial variations were evidenced for Fensch and Moselle downstream sediments. Thus, very
fine-grained sediments, poorly crystallized, displaying at the same time higher metal concentrations and higher organic matter
content than in Fensch river material, were collected downstream, in a low hydrodynamic conditions zone, assumed as a preferential
sedimentary zone or even as a placer. Strong correlations could be revealed between iron content and contaminant concentrations,
confirming the origin of polluted material. 相似文献