The lower Triassic anachronistic carbonate facies in space and time

The end-Permian mass extinction greatly affected the sedimentary record, but the sedimentary response was not limited to the Permian-Triassic boundary interval. This transformation extended to sedimentation that spanned the entire Early Triassic. Calcimicrobialites play an important role throughout this time interval, and at least four main events of anomalous carbonate deposition can be shown. A post-extinction calcimicrobial unit occurs above the extensive Permian skeletal carbonate platform exposed in the Taurus Mountains (southern Turkey), in south Armenia, north-west north and Central Iran along the Zagros Mountains. The calcimicrobial unit formed during the flooding of the platform that took place during the earliest Triassic. A similar calcimicrobialite formed during late Griesbachian to Dienerian time atop the shallow Permian skeletal carbonate platform largely exposed in south China. A third event occurred during the Early Olenekian on the first Mesozoic isolated pelagic plateau (Baid seamount, Oman Mountains). Here the change in carbonate sedimentation is reflected in the occurrence of thrombolites and carbonate seafloor fans. Near the end of Early Triassic time, unusual carbonate deposition is recorded both on an isolated pelagic plateau of the Western Tethys (Halstatt limestone of Dobrogea, Romania) and on the eastern Panthalassa margin of the western United States. In the western United States, the event is represented by stromatolites and thrombolites in the Virgin Limestone of the Moenkopi Formation and by seafloor fans in the middle and upper members of the Union Wash Formation. These unusual episodes of anomalous carbonate deposition illustrate a fundamental change in sedimentation that occurred in the aftermath of the end-Permian mass extinction.

Oxygen-isotope thermometry of quartz-calcite veins - Unraveling the thermal-tectonic history of the subgreenschist facies Morcles nappe (Swiss Alps)

Combined structural analysis and oxygen isotope thermometry of syntectonic quartz-calcite fibrous veins can be used to correlate the thermal history of deformed rocks,vith specific structural and tectonic events. Results are presented for the Mercies nappe in the western Helvetic Alps, Switzerland, where mineral parageneses, illite `'crystallinity,'' and fluid inclusion chemistry record an apparent peak metamorphic temperature gradient that increased across the Morcles nappe from anchizonal conditions in the foreland to epizonal conditions in its hinterland root zone. Twenty-seven quartz-calcite veins were analyzed in this study in order to determine the temperatures of veining during formation and deformation of the nappe, Peak metamorphic temperatures ranged from approximate to 260 to 290 degrees C in the shallower, foreland localities and to approximate to 330 to 350 degrees C in the deeper, more hinterland localities at the end of S1-foliation formation, related to large-scale folding. Temperatures gradually decreased throughout the nappe during subsequent development of the S2 foliation and S3 crenulation cleavage, Uplift and erosion of the overlying nappe pile resulted in slow cooling of the Morcles nappe during the waning stages of the Alpine Orogeny. The dominant foliation-forming deformation of the Morcles nappe occurred at elevated temperatures over the course of 10 to 15 Ma. Combined structure-oxygen isotope analyses of quartz-calcite veins yield better temperature and temporal constraints on the thermal histories of subgreenschist vein-bearing tectonites than do other geothermometers.

Conditioning of multiple-point statistics facies simulations to tomographic images

Geophysical tomography captures the spatial distribution of the underlying geophysical property at a relatively high resolution, but the tomographic images tend to be blurred representations of reality and generally fail to reproduce sharp interfaces. Such models may cause significant bias when taken as a basis for predictive flow and transport modeling and are unsuitable for uncertainty assessment. We present a methodology in which tomograms are used to condition multiple-point statistics (MPS) simulations. A large set of geologically reasonable facies realizations and their corresponding synthetically calculated cross-hole radar tomograms are used as a training image. The training image is scanned with a direct sampling algorithm for patterns in the conditioning tomogram, while accounting for the spatially varying resolution of the tomograms. In a post-processing step, only those conditional simulations that predicted the radar traveltimes within the expected data error levels are accepted. The methodology is demonstrated on a two-facies example featuring channels and an aquifer analog of alluvial sedimentary structures with five facies. For both cases, MPS simulations exhibit the sharp interfaces and the geological patterns found in the training image. Compared to unconditioned MPS simulations, the uncertainty in transport predictions is markedly decreased for simulations conditioned to tomograms. As an improvement to other approaches relying on classical smoothness-constrained geophysical tomography, the proposed method allows for: (1) reproduction of sharp interfaces, (2) incorporation of realistic geological constraints and (3) generation of multiple realizations that enables uncertainty assessment.

Modeling of in-situ crystallization processes in the Permian mafic layered intrusion of Mont Collon (Dent Blanche nappe, western Alps)

The Mont Collon mafic complex is one of the best preserved examples of the Early Permian magmatism in the Central Alps, related to the intra-continental collapse of the Variscan belt. It mostly consists (> 95 vol.%) of ol+hy-nonnative plagioclase-wehrlites, olivine- and cpx-gabbros with cumulitic structures, crosscut by acid dikes. Pegmatitic gabbros, troctolites and anorthosites outcrop locally. A well-preserved cumulative, sequence is exposed in the Dents de Bertol area (center of intrusion). PT-calculations indicate that this layered magma chamber emplaced at mid-crustal levels at about 0.5 GPa and 1100 degrees C. The Mont Collon cumulitic rocks record little magmatic differentiation, as illustrated by the restricted range of clinopyroxene mg-number (Mg#(cpx)=83-89). Whole-rock incompatible trace-element contents (e.g. Nb, Zr, Ba) vary largely and without correlation with major-element composition. These features are characteristic of an in-situ crystallization process with variable amounts of interstitial liquid L trapped between the cumulus mineral phases. LA-ICPMS measurements show that trace-element distribution in the latter is homogeneous, pointing to subsolidus re-equilibration between crystals and interstitial melts. A quantitative modeling based on Langmuir's in-situ crystallization equation successfully duplicated the REE concentrations in cumulitic minerals of all rock facies of the intrusion. The calculated amounts of interstitial liquid L vary between 0 and 35% for degrees of differentiation F of 0 to 20%, relative to the least evolved facies of the intrusion. L values are well correlated with the modal proportions of interstitial amphibole and whole-rock incompatible trace-element concentrations (e.g. Zr, Nb) of the tested samples. However, the in-situ crystallization model reaches its limitations with rock containing high modal content of REE-bearing minerals (i.e. zircon), such as pegmatitic gabbros. Dikes of anorthositic composition, locally crosscutting the layered lithologies, evidence that the Mont Collon rocks evolved in open system with mixing of intercumulus liquids of different origins and possibly contrasting compositions. The proposed model is not able to resolve these complex open systems, but migrating liquids could be partly responsible for the observed dispersion of points in some correlation diagrams. Absence of significant differentiation with recurrent lithologies in the cumulitic pile of Dents de Bertol points to an efficiently convective magma chamber, with possible periodic replenishment, (c) 2005 Elsevier B.V. All rights reserved.

Pizgrischite, (Cu,Fe)Cu14PbBi17S35, a new sulfosalt from the Swiss Alps: Description, crystal structure and occurrence

Pizgrischite, (Cu,Fe)Cu14PbBi17S35, is a new mineral species named after the type locality, Piz Grisch Mountain, Val Ferrera, Graubunden, Switzerland. This sulfosalt occurs as thin, striated, metallic lead-grey blades measuring up to I cm in length, embedded in quartz and associated with tetrahedrite, chalcopyrite, pyrite, sphalerite, emplectite and derivatives of the aikinite-bismuthinite series. In plane-polarized light, the new species is brownish grey with no perceptible pleochroism; under crossed nicols in oil immersion, it presents a weak anisotropy with dark brown tints. Minimum and maximum reflectance values (in %) in air are: 40.7-42.15 (470 nm), 41.2-43.1 (546 nm), 41.2-43.35 (589 nm) and 40.7-43.3 (650 nm). Cleavage is perfect along 001 I and well developed on {010}. Abundant polysynthetic twinning is observed on (010). The mean micro-indentation hardness is 190 kg/mm(2) (Mohs hardness 3.3), and the calculated density is 6.58 g/cm(3). Electron-microprobe analyses yield (wt%; mean result of seven analyses): Cu 16.48, Pb 2.10, Fe 0.77, Bi 60.70, Sb 0.35, S 19.16, Se 0.04, total 99.60. The resulting empirical chemical formula is (Cu15.24Fe0.80Pb0.60)(Sigma 16.64)(Bi17.07Sb0.17)(Sigma 17.24)(S35.09Se0.03)(Sigma 35.12), in accordance with the formula derived from the single-crystal refinement of the structure, (Cu,Fe)Cu14PbBi17S35. Pizgrischite is monoclinic, space group C2/m, with the following unit-cell parameters: a 35.054(2), b3.91123(I), c43.192(2) angstrom, beta 96.713(4)degrees, V5881.24 angstrom(3), Z=4. The strongest seven X-ray powder-diffraction lines [d in angstrom (I)(hkl)] are: 5.364(40)((6) over bar 04), 4.080(50)((8) over bar 05), 3.120(40)(118), 3.104(68)((3) over bar 18), 2.759(53) ((9) over bar 11),2.752(44)(910) and 1.956(100)(020). The crystal structure is an expanded monoclinic derivative of kupcikite. Pizgrischite belongs to the cuprobismutite series of bismuth sulfosalts but, sensu stricto, it is not a homologue of cuprobismutite. At the type locality. pizarischite is the result of the Alpine metamorphism under greenschist-facies conditions of pre-Tertiary hydrothermal Cu-Bi mineralization.