Connexion porphyre cuprifère–épithermaux de type low-sulfidation : analyse structurale et représentation 3D du secteur de Bolcana,monts Apuseni,Roumanie

The AuPbZn low-sulfidation epithermal ore deposits of Troita, Trestia, and Magura (Apuseni Mountains, Romania) are spatially related to the Bolcana Cu-porphyry. In an attempt to demonstrate the connection between these mineralizations, a geometric study was made based on structural measurements and GOCAD^© geomodeller 3D representation of deposits. This study indicates that a specific spatial distribution of the different Au and PbZn veins of the epithermal deposits occurs around the Cu-porphyry, which cannot result from telescoped systems. To cite this article: O. Cardon et al., C. R. Geoscience 337 (2005).     

Observations sur le mode de gisement et les conditions de mise en place des coulées volcaniques spilitisées de Peyrebrune (Région de Realmont) Tarn/France

In the district of Peyrebrune, several lava-flows are interstratified in a Cambrosilurian aged Schisto-Sandstone Series. The relationships between sediments and lavas, specified by a cut in detail, are the subject of a petrographic and sedimentologic study. According to this study each volcanic eruption is divided in two flows separated by a little amount of “mictites” (mixture of sediments which were rearranged during the eruption, and elements of volcanic origin). The upper flow is much than the lower one and is sometimes thicker cut in pillow-lavas. The mineralogical study shows that the center of the principal flow is made of albitic dolerite with preserved pyroxene and albite and that the boundaries up to one meter and also the lower flow have a spilitic composition. In one flow, taken for example, measurements of density show that the density is 2,99 in the center of the flow, and 2,81 in the boundaries. The chemical analyses indicate that the center has a hyperaluminous basaltic composition and that the more spilitified parts are considerably modified by:

• -Addition of Na (around 60 Kg by m³)
• -loss of Ca and Mg (corresponding to the possible formation of 350 Kg of magnesium calcarous by m³ of spilitified stony mass).

     

Portales Valley: Petrology of a metallic‐melt meteorite breccia

Abstract— Portales Valley (PV) is an unusual metal‐veined meteorite that has been classified as an H6 chondrite. It has been regarded either as an annealed impact melt breccia, as a primitive achondrite, or as a meteorite with affinities to silicated iron meteorites. We studied the petrology of PV using a variety of geochemical‐mineralogical techniques. Our results suggest that PV is the first well‐documented metallic‐melt meteorite breccia. Mineral‐chemical and other data suggest that the protolith to PV was an H chondrite. The composition of FeNi metal in PV is somewhat fractionated compared to H chondrites and varies between coarse vein and silicate‐rich portions. It is best modeled as having formed by partial melting at temperatures of ?940–1150 °C, with incomplete separation of solid from liquid metal. Solid metal concentrated in the coarse vein areas and S‐bearing liquid metal concentrated in the silicate‐rich areas, possibly as a result of a surface energy effect. Both carbon and phosphorus must have been scavenged from large volumes and concentrated in metallic liquid. Graphite nodules formed by crystallization from this liquid, whereas phosphate formed by reaction between P‐bearing metal and clinopyroxene components, depleting clinopyroxene throughout much of the meteorite and growing coarse phosphate at metal‐silicate interfaces. Some phosphate probably crystallized from P‐bearing liquids, but most probably formed by solid‐state reaction at ?975‐725 °C. Phosphate‐forming and FeO‐reduction reactions were widespread in PV and entailed a change in the mineralogy of the stony portion on a large scale. Portales Valley experienced protracted annealing from supersolidus to subsolidus temperatures, probably by cooling at depth within its parent body, but the main differences between PV and H chondrites arose because maximum temperatures were higher in PV. A combination of a relatively weak shock event and elevated pre‐shock temperatures probably produced the vein‐and‐breccia texture, with endogenic heating being the main heat source for melting, and with stress waves from an impact event being an essential trigger for mobilizing metal. Portales Valley is best classified as an H7 metallic‐melt breccia of shock stage S1. The meteorite is transitional between more primitive (chondritic) and evolved (achondrite, iron) meteorite types and offers clues as to how differentiation could have occurred in some asteroidal bodies.     

Length scales of mantle heterogeneities and their relationship to ocean island basalt geochemistry

The upper mantle is widely considered to be heterogeneous, possibly comprising a “marble-cake” mixture of heterogeneous domains in a relatively well-mixed matrix. The extent to which such domains are capable of producing and expelling melts with characteristic geochemical signatures upon partial melting, rather than equilibrating diffusively with surrounding peridotite, is a critical question for the origin of ocean island basalts (OIB) and mantle heterogeneity, but is poorly constrained. Central to this problem is the characteristic length scale of heterogeneous domains. If radiogenic osmium signatures in OIB are derived from discrete domains, then sub-linear correlations between Os isotopes and other geochemical indices, suggesting melt-melt mixing, may be used to constrain the length scales of these domains. These constraints arise because partial melts of geochemically distinct domains must segregate from their sources without significant equilibration with surrounding peridotite. Segregation of partial melts from such domains in upwelling mantle is promoted by compaction of the domain mineral matrix, and must occur faster than diffusive equilibration between the domain and its surroundings. Our calculations show that the diffusive equilibration time depends on the ratios of partition and diffusion coefficients of the partial melt and surrounding peridotite. Comparison of time scales between diffusion and melt segregation shows that segregation is more rapid than diffusive equilibration for Os, Sr, Pb, and Nd isotopes if the body widths are greater than tens of centimeter to several meters, depending on the aspect ratio of the bodies, on the melt fraction at which melt becomes interconnected in the bodies, and on the diffusivity in the solid. However, because Fe-Mg exchange occurs significantly more rapidly than equilibration of these isotopes under solid-state and partially molten conditions, it is possible that some domains can produce melts with Fe/Mg ratios reflecting that of the surrounding mantle but retaining isotopic signatures of heterogeneous domains. Although more refined estimates on the rates of, and controls on, Os mobility are needed, our preliminary analysis shows that heterogeneous domains large enough to remain compositionally distinct in the mantle (as solids) for ∼10⁹ yr in a marble-cake mantle, can produce and expel partial melts faster than they equilibrate with surrounding peridotite.     

Constraining diamond metasomatic growth using C- and N-stable isotopes: examples from Namibia

The present paper provides C- and N-stable isotope characteristics, N-contents and N-aggregation states for alluvial diamonds of known paragenesis from placers along the Namibian coast. The sample set includes diamonds with typical peridotitic and eclogitic inclusions and the recently reported “undetermined” suite of Leost et al. [Contrib. Mineral. Petrol. 145 (2003) 15] which resulted from infiltration of high temperature, carbonate-rich melts. δ¹³C-values range from −20.3‰ to −0.5‰ (n=48) for peridotitic diamonds and from −38.5‰ to −1.6‰ (n=45) for eclogitic diamonds. Diamonds belonging to the “undetermined” suite span a narrower range in δ¹³C from −8.5‰ to −2.7‰ (n=13). When compared with previous studies, diamonds from Namibia are characterised by unusually low proportions of N-free (i.e. Type II) peridotitic and eclogitic diamonds (3% and 2%, respectively) and an unprecedented high proportion of N-rich diamonds (15% and 73%, respectively, have N-contents >600 ppm). δ¹⁵N-values for diamonds of the peridotitic, eclogitic and “undetermined” suites range from −10‰ to +13‰ without correlations with either N-content or δ¹³C. The similarity in N-isotopic composition and the N-rich character of diamonds belonging to the eclogitic, peridotitic and “undetermined” suites is striking and suggests a close genetic relationship. We propose that a large part of the diamonds mined in Namibia formed during metasomatic events of similar style that introduced carbon and nitrogen into a range of different host lithologies.     

The Atmospheric Boundary-Layer Structure Within A Cold Air Outbreak: Comparison Of In Situ, Lidar And Satellite Measurements With Three-Dimensional Simulations

A cold-air outbreak over the Mediterranean, associated with a Tramontane event, has been simulated with the atmospheric non-hydrostatic model Meso-NH using a horizontal resolution of 2 km. Results are compared with in situ aircraft, airborne lidar and satellite measurements. On average, the mean and turbulent parameters simulated in the surface layer and mixed layer compared well with in situ measurements. The model was able to reproduce accurately the Foehn effect in the wake of Cape Creus, as well as the occurence of rolls in the coastal region in connection with cloud streets observed with AVHRR. Over the sea, the threshold value of turbulent kinetic energy defining the height of the atmospheric boundary-layer top in the model (defined as 25% of the maximum turbulent kinetic energy in the profile) enables the simulated atmospheric boundary-layer height to match the one retrieved from lidar measurements. Nevertheless, the model did not handle very well the abrupt gradients of all meteorological parameters observed at the top of the atmospheric boundary-layer. Reasons for this are investigated.     

Atmospheric stability effect on subgrid-scale physics for large-eddy simulation

Field measurements in the atmospheric boundary layer were carried out to identify the effect of atmospheric stability on subgrid-scale physics for large-eddy simulation. The basic instrumentation setup consisted of 12 three-dimensional sonic anemometers arranged in two parallel horizontal arrays (seven sensors in the lower array and five sensors in the upper array). Data from this setup are used to compute the subgrid-scale (SGS) heat fluxes and SGS dissipation of the temperature variance under stable and unstable stability conditions. The relative contribution of the SGS vertical flux to the total turbulent flux increases when going from unstable to stable conditions. The relative importance of negative SGS dissipation (backscatter) events becomes larger under stable conditions. The model coefficients for two well-known SGS models (eddy-viscosity and non-linear) are computed. Model coefficients are found to depend strongly on stability. Under both stable and unstable conditions, large negative SGS dissipation is associated with the onset of ejection events while large positive SGS dissipation tends to occur during the onset of sweep events. These findings are also supported by conditionally sampled 2D velocity and temperature fields obtained using the 12 anemometers placed in a vertical array.     

Evaluation of combination rules for maximum response calculation in multicomponent seismic analysis

The existing rules for combining peak response to individual components of ground motion are evaluated. The response values r_e to two horizontal components of ground motion estimated by four multicomponent combination rules—SRSS‐, 30%‐, 40%‐ and simplified‐SRSS‐rules—are compared with the critical response, r_cr, obtained by the CQC3‐rule, which takes into account the direction of the principal ground components with respect to the structural axes and provides the largest response over all possible seismic incident angles. The following results are obtained in the first part of the paper and are valid for any elastic structure and any earthquake design response spectrum: For realistic values of the ratio γ of the design spectra for the two principal components of ground motion the SRSS‐rule estimate lies between 0.79r_cr and 1.00r_cr, the Simplified‐SRSS‐rule estimate lies between 1.00r_cr and 1.26r_cr, the 40%‐rule estimate lies between 0.99r_cr and 1.25r_cr, and the 30%‐rule estimate lies between 0.92r_cr and 1.16r_cr. None of the multicomponent combination rules account for the increase in response of systems if the vibration periods of the two modes that contribute most to the response to the x‐ and y‐components of ground motion are close to each other. Evaluated in the second part of the paper is the accuracy of the multicomponent combination rules in estimating the response of a range of one‐storey systems with (a) symmetrical plan and (b) unsymmetrical plan, and of two multistorey buildings. The SRSS‐rule underestimates the response by up to 16% and the other three rules overestimate it by up to 18%. Although these errors appear to be smaller than the many approximations inherent in structural design, they can be eliminated with very little additional computation by using an explicit formula for the critical response based on the CQC3 rule. Copyright © 2001 John Wiley & Sons, Ltd.     

Comparing response of SDF systems to near‐fault and far‐fault earthquake motions in the context of spectral regions

In spite of important differences in structural response to near‐fault and far‐fault ground motions, this paper aims at extending well‐known concepts and results, based on elastic and inelastic response spectra for far‐fault motions, to near‐fault motions. Compared are certain aspects of the response of elastic and inelastic SDF systems to the two types of motions in the context of the acceleration‐, velocity‐, and displacement‐sensitive regions of the response spectrum, leading to the following conclusions. (1) The velocity‐sensitive region for near‐fault motions is much narrower, and the acceleration‐sensitive and displacement‐sensitive regions are much wider, compared to far‐fault motions; the narrower velocity‐sensitive region is shifted to longer periods. (2) Although, for the same ductility factor, near‐fault ground motions impose a larger strength demand than far‐fault motions—both demands expressed as a fraction of their respective elastic demands—the strength reduction factors R_y for the two types of motions are similar over corresponding spectral regions. (3) Similarly, the ratio u_m/u₀ of deformations of inelastic and elastic systems are similar for the two types of motions over corresponding spectral regions. (4) Design equations for R_y (and for u_m/u₀) should explicitly recognize spectral regions so that the same equations apply to various classes of ground motions as long as the appropriate values of T_a, T_b and T_c are used. (5) The Veletsos–Newmark design equations with T_a=0.04 s, T_b=0.35 s, and T_c=0.79 s are equally valid for the fault‐normal component of near‐fault ground motions. Copyright © 2001 John Wiley & Sons, Ltd.