Textural features and chemical characteristics of chromites in ultramafic rocks,Chalkidiki Complex (Northeastern Greece)

Summary The lower ultramafic part of the Chalkidiki Ophiolite Complex consists of a successive alternation of chromitite, dunite, and harzburgite, which is repeated periodically in a cyclic manner. This cyclic pattern is consistent with an origin as magmatic cumulates. However, the tectonic fabric superimposed on the cumulate texture of this sequence suggest a solid-state deformation at considerable depth.In the present study, the textural and chemical features of chromite are discussed. The interrelationships between the compositions of chromite, its mode of occurrence as well as the associated silicates are emphasized. It is found that the investigated chromites have properties in common with both podiforn and stratiform chromites. From a geometrical and structural point of view, they are of podiform type. On the other hand, their appreciably high iron content, the strong interdependence between the composition of chromite and the coexisting silicates together with the cryptic layering exhibited by the two mineral groups contrast markedly with podiform chromites.An alternative hypothesis is proposed involving fractional crystallization of an ultrabasic melt introduced to the magma chamber as periodic inflows of essentially the same composition. An upwelling upper-mantle that is subjected to stepwise partial fusion may fulfill these conditions.

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Chemismus und Texturen von Chromiten in ultramafischen Gesteinen des Chalkidiki Komplexes, Nordost-Griechenland

Zusammenfassung Der untere, ultramafische Teil des Chalkidiki Komplexes besteht aus einer wechselnden Abfolge von Chromititen, Duniten und Harzburgiten, die sich zyklisch wiederholt. Diese zyklische Abfolge ist in guter Übereinstimmung mit einer Entstehung als magmatische Kumulate. Das tektonische Gefüge, das auf die Kumulat-Texturen überprägt wurde, weist jedoch auf Deformationen in festem Zustand in beträchtlicher Tiefe hin.Die vorliegende Untersuchung befaßt sich mit dem Chemismus und den Texturen der Chromite. Besonderes Interesse wendet sich dabei den Wechselbeziehungen zwischen den Zusammensetzungen, der Art des Vorkommens und den assoziierten Silikaten zu. Dabei zeigt sich, daß die untersuchten Chromite in vieler Hinsicht Ähnlichkeiten sowohl mit podiformen wie mit stratiformen Vorkommen erkennen lassen. Geometrisch und texturell gesehen gehören sie dem podiformen Typ an. Nicht in Einklang damit stehen jedoch die beträchtlichen Eisengehalte sowie die deutlichen Wechselbeziehungen zwischen der Zusammensetzung von Chromit und der koexistierenden Silikate, und schließlich auch der kryptische Lagenbau, den die zwel Mineralgruppen erkennen lassen. Ein genetisches Modell wird vorgeschlagen, das fraktionierte Kristallisation einer ultrabasischen Schmelze von im wesentlichen gleichmäßiger Zusammensetzung vorsieht, die in periodischen Abständen der Magmenkammer zugeführt wurde. Eine Aufwölbung des oberen Mantels, die schrittweiser, partieller Aufschmelzung ausgesetzt ist, könnte diesen Bedingungen entsprechen.

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With 11 Figures     

Accounting for site effect in probabilistic assessment of seismic hazard for Romania and Bucharest: a case of deep seismicity in Vrancea zone

The paper presents recent achievements in evaluations of site-dependent seismic hazard in Romania and the capital city of Bucharest caused by the Vrancea focal zone (SE-Carpathians). The zone is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume at depths 60–170 km. The database that was used for the hazard evaluation includes parameters of seismicity, ground-motion source scaling and attenuation models (Fourier amplitude spectra), and site-dependent spectral amplification functions. Ground-motion characteristics were evaluated on the basis of several hundred records from more than 120 small magnitude (M 3.5–5) earthquakes occurred in 1996–2001 and a few tens of acceleration records obtained during four large (M 7.4, 7.2, 6.9 and 6.3) earthquakes. The data provide a basis for probabilistic seismic hazard assessment in terms of peak ground acceleration, peak spectral acceleration and MSK intensity using Fourier amplitude spectra for various exceedance probabilities or average return periods. It has been shown that the influence of geological factors plays very important role in distribution of earthquake ground-motion parameters along the territory of Romania.     

Stellar evolution models for Z = 0.0001 to 0.03

We have calculated a grid of empirically well tested evolutionary tracks with masses M between 0.5 and 50 M⊙, spaced by approximately 0.1 in log M , and with metallicities Z  = 0.0001, 0.0003, 0.001, 0.004, 0.01, 0.02 and 0.03. We use a robust and fast evolution code with a self-adaptive non-Lagrangian mesh, which employs the mixing-length theory but treats convective mixing as a diffusion process, solving simultaneously for the structure and the chemical composition. The hydrogen and helium abundances are chosen as functions of the metallicity: X  = 0.76 − 3.0 Z Y  = 0.24 + 2.0 Z .   Two sets of models were computed, one without and one with a certain amount of enhanced mixing or 'overshooting'. This amount has been empirically chosen by means of various sensitive tests for overshooting: (1) the luminosity of core helium burning (blue loop) giants of well-known mass, (2) the width of the main sequence as defined by double-lined eclipsing binaries with well-measured masses and radii, and (3) the shape and implied stellar distribution of isochrones of various open clusters. The first two tests have been the subject of previous papers, the third test is discussed in this paper. On the basis of these tests, we recommend the use of the overshooting models for masses above about 1.5M ⊙.   We describe here the characteristics of the models, the procedure for constructing isochrones for arbitrary age and metallicity from the models, and the performance of these isochrones for several intermediate-age and old open clusters. All original models are available in electronic form and we describe the means by which they may be obtained.     

Ground-motion prediction equations for the intermediate depth Vrancea (Romania) earthquakes

We present the regional ground-motion prediction equations for peak ground acceleration (PGA), peak ground velocity (PGV), pseudo-spectral acceleration (PSA), and seismic intensity (MSK scale) for the Vrancea intermediate depth earthquakes (SE-Carpathians) and territory of Romania. The prediction equations were constructed using the stochastic technique on the basis of the regional Fourier amplitude spectrum (FAS) source scaling and attenuation models and the generalised site amplification functions. Values of considered ground motion parameters are given as the functions of earthquake magnitude, depth and epicentral distance. The developed ground-motion models were tested and calibrated using the available data from the large Vrancea earthquakes. We suggest to use the presented equations for the rapid estimation of seismic effect after strong earthquakes (Shakemap generation) and seismic hazard assessment, both deterministic and probabilistic approaches.     

Diverse contributing sources to chromitite petrogenesis in the Shebenik Ophiolitic Complex, Albania: evidence from new PGE- and Os-isotope data

Summary Chromitites sampled from four different pseudostratigraphic levels of the Mesozoic Shebenik Ophiolite Complex, Albania, have low PGE totals <1 μg/g but show different types of PGE enrichment (Burgath et al., 2003) as well as differing mineral chemistry, PGM mineralogy and Os isotopic signatures. To circumvent analytical problems with low PGE abundances, representative samples were analyzed using HPA-digestion followed by isotope dilution ICP-MS. Osmium isotopes were determined by ICP-QMS and N-TIMS techniques. Podiform chromitites exposed in the mantle (Group I) and tabular chromitites exposed in the upper mantle (Group II) are Os-Ir-Ru-Rh enriched. In the upper mantle to mantle-crust transition zone, schlieren type chromitites (Group III) are enriched in Ru-Rh with low Os-Ir and low Pt-Pd. Within the mantle-crust transition zone disseminated chromitites in dunite are variably enriched in Ru-Rh-Pt, however, Os, Ir and Pd are low. IPGE rich chromites contain abundant small laurite inclusions whilst Rh and Pt are located in sulfarsenides marginally attached to transition zone chromites (see also Burgath et al., 2003). High Cr/Al ratios (>0.75) and low titanium contents of chromites throughout the sample suite are consistent with chromitite petrogenesis in a SSZ environment. Shebenik mantle chromitites with low ¹⁸⁷Re/¹⁸⁸Os ratios have an average, slightly suprachondritic initial osmium isotopic composition of 0.1285 ± 0.0022 (2s). Towards higher pseudostratigraphic levels, ¹⁸⁷Re/¹⁸⁸Os ratios increase and initial Os isotopies are very heterogeneous. Distinctly suprachondritic Os signatures require input of radiogenic source components, whereas subchondritic samples require assimilation of long term Re-depleted PGM.