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41.
Udo Harms 《Chemie der Erde / Geochemistry》2010,70(2):175-184
Gersdorffite from two mineralization types (post-Variscan vein deposits, strata-bound mineralization) was investigated in the Niederberg area Rhenish Massif. In the ternary Ni–Co–Fe space gersdorffite from post-Variscan vein deposits displays a tight cluster with the highest Ni-contents ranging from 0.825 to 0.962 atoms per formula unit (a.p.f.u.). As/S ratios comprise a narrow range from 0.875 to 1.012. In contrast gersdorffite from the strata-bound mineralization displays a substitutional trend. Co and Fe substitute for Ni in a ± fixed ratio. Ni ranges between 0.494 and 0.836 a.p.f.u. As/S ratios (1.025–1.211) display a wider range and indicate higher As-contents relative to gersdorffite from post-Variscan vein deposits. Based on these results, two different hydrothermal fluid systems can be identified in the Niederberg area forming gersdorffite in both mineralization types. The hydrothermal fluids circulating in the post-Variscan vein deposits were homogeneous (high Ni-activities, lower As fugacities) and mixing occurred far away from the site of deposition whereas the fluids of the strata-bound mineralization were more heterogeneous (decreasing Ni-activities) with moderate elevated As fugacities. With respect to the post-Variscan vein deposits in the Niederberg area the results are compatible with earlier findings.Comparison with available gersdorffite analyses from adjacent areas (borehole Viersen, Ramsbeck deposit) reveal that gersdorffite compositions provide a reliable tool in distinguishing between different hydrothermal systems on a regional scale in the northern Rhenish Massif. However, gersdorffite compositions cannot be used to discriminate between Variscan and post-Variscan deposits with confidence.The country rocks in the Niederberg area are possible sources for Ni, Co and Fe during gersdorffite formation of the strata-bound mineralization. However, due to the remarkable homogeneity of gersdorffite compositions of the post-Variscan vein deposits irrespective of age and composition of the immediate adjacent host-rocks it is assumed that these host-rocks are not the source of the metals. Reduced Zechstein sulfate is assumed to be the source of sulfur. The As source remains unknown.Due to conflicting experimental data concerning the gersdorffite solid solution field it is not possible to derive reliable formation temperatures for the strata-bound mineralization. However, gersdorffite compositions of the post-Variscan vein deposits are compatible with low formation temperatures (<300 °C) in accordance with earlier findings. 相似文献
42.
Friedrich Lucassen Peter Dulski Rainer Abart Gerhard Franz Dieter Rhede Rolf L. Romer 《Contributions to Mineralogy and Petrology》2010,160(2):279-295
Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry. 相似文献
43.
Friedrich Lucassen Michael Wiedicke Gerhard Franz 《International Journal of Earth Sciences》2010,99(3):687-701
Marine Quaternary trench and slope sediments were sampled along the margin of the Southern Andes, Chile between 36° and 40°S.
Major and trace element contents indicate only minor influence of weathering and transport fractionation. The whole rock composition
of the sediments is similar to the average rock of the Cretaceous to Holocene magmatic arc of this section of the southern
volcanic zone. Sr, Nd, and Pb isotope composition of the sediments also resembles closely the average composition of the magmatic
arc. The contribution of compositionally distinct Palaeozoic crust, which makes up most of the volume of the forearc, is ~0–20%
crustal Sr, Nd, and Pb according to the isotope record of the trench and slope sediments. Input of sediments from the continent
into the subduction system was dominated by detritus from the magmatic arc at least for the last 20 My on the basis of the
Oligocene to Holocene exhumation history of the margin. 相似文献
44.
Udo Zimmermann Heinrich Bahlburg Klaus Mezger Jasper Berndt Suzanne Mahlburg Kay 《International Journal of Earth Sciences》2014,103(4):1023-1036
Ultramafic rocks and gabbros are exposed in the southern Puna (NW Argentina) in tectonic association with continental arc-related Ordovician (volcano) sedimentary successions and granitoids. The origin of this mafic rock suite has been debated for three decades as either representing an Ordovician terrane suture, primitive Ordovician arc-related rocks or relics of the pre-Ordovician basement in tectonic contact with the Ordovician retro-arc basin successions. We present the first U–Pb ages of primary and inherited zircon from gabbros of this mafic–ultramafic assemblage. LA-ICP-MS analyses on cores and rims of these zircon grains yielded a concordia age of 543.4 ± 7.2 Ma for the gabbroic rocks. Other analysed zircons have Mesoproterozoic, and Early Ediacaran core and rim ages indicating that the magmas also assimilated Meso- and Neoproterozoic crustal material prior to final crystallization. The mafic rocks witnessed higher metamorphic grade than associated Ordovician rocks, which are unmetamorphosed or only affected by anchimetamorphism. The gabbros are mostly tholeiitic and enriched in Zr, Th, as well as other incompatible elements and have εNd t=540Ma ranging from 1.3 to 7.4 with most of the values between 5 and 7. 147Sm/144Nd ratios show evidence of weak crustal contamination. The mafic rocks do not reveal any affinity to mid-ocean ridge basalts in their geochemistry but point instead to an emplacement in an active plate margin arc environment. Chromites from ultramafic rocks show typical Ti, Al, Cr#, Fe3+ abundances found in magmatic arc rocks. The formation of the gabbros and the associated ultramafic rocks in the southern Argentine Puna is related to the evolution of the margin of the Pampia terrane, including the Puncoviscana basin, during the Late Neoproterozoic and earliest Cambrian. In contrast to previous interpretations, the rocks predate the Ordovician evolution of the Central proto-Andean active margin. Consequently, interpretations assuming these rocks to represent an oceanic terrane suture of Ordovician age have to be dismissed as much as all palaeotectonic models that define Ordovician terranes in the Central Andes based on assumption that the ultramafic rocks and gabbros exposed in the southern Puna mark plate boundaries. 相似文献
45.
L. C. Gallo I. Lehmann W. Pietsch Th. Boller W. Brinkmann P. Friedrich D. Grupe 《Monthly notices of the Royal Astronomical Society》2006,365(2):688-698
We present a sample of 21 ROSAT bright active galactic nuclei (AGNs), representing a range of spectral classes, and selected for follow-up snapshot observations with XMM–Newton . The typical exposure was between 5 and 10 ks. The objects were primarily selected on the bases of X-ray brightness and not on hardness ratio; thus the sample cannot be strictly defined as a 'soft'sample. One of the main outcomes from the XMM–Newton observations was that all of the AGN, including 11 type 1.8–2 objects, required low levels of intrinsic absorption ( N H ≲ 1021 cm−2 ) . The low absorption in type 2 systems is a challenge to account for in the standard orientation-based unification model, and we discuss possible physical and geometrical models which could elucidate the problem. Moreover, there does not appear to be any relation between the strength and shape of the soft excess, and the spectral classification of the AGN in this sample. We further identify a number of AGN which deserve deeper observations or further analysis: for example, the low-ionization nuclear emission regions (LINERs) NGC 5005 and NGC 7331, where optically thin thermal and extended emission is detected, and the narrow-line Seyfert 1 II Zw 177, which shows a broad emission feature at ∼ 5.8 keV. 相似文献
46.
Acta Geotechnica - A coupled bio-chemo-hydro-mechanical model (BCHM) is developed to investigate the permeability reduction and stiffness improvement in soil by microbially induced calcite... 相似文献
47.
48.
Abstract— Several experimentally and naturally shocked silicate samples were analyzed for noble gas contents to further characterize the phenomenon by which ambient gases can be strongly implanted into silicates by shock and to evaluate the possible importance of this process in capturing planetary atmospheres in naturally shocked samples. Gas implantation efficiency is apparently mineral independent, as mono-mineralic powders of oligoclase, labradorite, and diopside and a powdered basalt shocked to 20 GPa show similar efficiencies. The retentivity of shock-implanted gas during stepwise heating in the laboratory is defined in terms of two parameters: activation energy for diffusion as determined from Arrhenius plots, and the extraction temperature at which 50% of the gas is released, both of which correlate with shock pressure. These gas diffusion parameters are essentially identical for radiogenic 40Ar and shock-implanted 40Ar in oligoclase and labradorite shocked to 20 GPa, suggesting that the two 40Ar components occupy analogous lattice sites. Our experiments indicate that gas implantation occurs through an increasing production of microcracks/defects in the lattice with increasing shock pressure. The ease of diffusive loss of implanted gas is controlled by the degree of annealing of these microcracks/defects. Identification of a shock-implanted component requires relatively large concentrations of implanted gas which is strongly retained (i.e., moderate activation energy) in order to separate implanted gas from surface adsorbed gases. Literature data on shocked terrestrial samples indicate only weak evidence for shock-implanted gases, with an upper limit for 40Ar of ~ 10?6 cm3STP/g. New analyses of shocked samples from the Wabar Crater indicate the presence of shock-implanted Ar, having concentrations (~ 10?4 cm3STP/g) and activation energies for diffusive loss which are essentially that expected from experimental studies. Lack of sufficient target porosity or the presence of ground water may explain the sparse evidence for shock-implanted gas at other terrestrial craters. Although Wabar Crater may represent an unusually favorable environment on Earth for shock-implanting gases, surfaces of other planetary bodies, such as Mars, may frequently provide such environments. Analyses of returned samples from old Martian terraines may document temporal changes in earlier atmospheric composition. 相似文献
49.
Atmospheric composition in the turbopause regime was determined by four rocket-borne mass spectrometers, which employed shock-freezing cryo-ion sources. Number densities of N2, O2, O, Ar and CO2 are presented for these experiments. The results are compared with those of other rocket experiments taken from the literature. Ar/N2 ratios are analyzed with respect to atmospheric turbulence. Magnitude and variability of atomic oxygen layer maximum density and layer content are discussed. Variations of O densities and simultaneous Ar/N2 ratio changes are compared. Six CO2 measurements are discussed in terms of CO2/N2 ratios. 相似文献
50.