Magmatic and metamorphic evolution of metagabbros in the Münchberg Massif,N.E. Bavaria

The central gneiss complex of the Münchberg Massif consists of the Liegendserie at the base and the Hangendserie at the top. Metagabbros are found in the Liegendserie; eclogites occur in the Hangendserie. New isotope data revive the discussion whether a genetic relationship exists between metagabbros and eclogites in the Münchberg Massif. It is possible to relate the two types of rocks to the same protoliths by variable degrees of crustal contamination and magmatic accumulation. Therefore, a common magmatic origin may be assumed. Both the metagabbros and the eclogites were affected by amphibolite-facies metamorphism. The amphibolitization of the metagabbros was a prograde metamorphic event. Increasing temperature is indicated by inverse zonation of recrystallized plagioclase and increasing Mg/Fe ratios in garnet from core to rim. Geothermobarometry yields a temperature of 600°C and a pressure of about 11 kbar for the peak of metamorphism. In contrast, the eclogites underwent a first high-pressure stage at a minimum pressure of 14 kbar and a temperature estimated at 600°C and were subsequently overprinted under amphibolite-facies conditions at 10 kbar/700°C. A common magmatic origin of metagabbros (Liegendserie) and eclogites (Hangendserie) of the Münchberg Massif can no longer be discarded. However, the converging P-T-t paths reflect a different geodynamic evolution of the Liegendserie and the Hangendserie after magmatism and before amphibolite-facies metamorphism.     

Observations in the Sudan by Fouad N. Ibrahim

Reports

Observations in the Sudan by Fouad N. Ibrahim     

Biomarkers in sedimentary sulfides of precambrian age

Samples of the Mt Isa formation (Australia, c, 1.5 Ga), and the Shungit formation (U.S.S.R., c. 2.3 Ga) were studied by organic geochemical means. All samples were freed of any low-molecular-weight solvent soluble organic material in order to insure the authenticity of the analysed material. The isolation of biochemical compounds entrapped in sulfides was the major goal of this work.Organic compounds that were entrapped during the early stages of sulfide formation may obviously survive extended periods of time and can be released by a mild hydrogenation of the sulfides. Preliminary investigations of the hydrocarbon fraction indicate n-alkanes, monomethylalkanes, cycloalkanes, and an unknown series of branched alkanes as major constituents. Their distribution patterns show great selectivity with respect to structures and chain lengths of individual compounds. Differences between the hydrogenation reaction product and the sample extract may arise from the release of a different kind of lipid material through dissolution of the sulfides.     

Calculation of sulfur isotope fractionation in sulfides

The increment method has been successfully applied to calculate thermodynamic isotope fractionation factors of oxygen in silicates, oxides, carbonates, and sulfates. In this paper, we modified the increment method to calculate thermodynamic isotope fractionation factors of sulfur in sulfides, based on chemical features of sulfur-metal bonds and crystal features of sulfide minerals. To approximate the bond strength of sulfides, a new constant, known as the Madelung constant, was introduced. The increment method was then extended to calculate the reduced partition function ratios of sphalerite, chalcopyrite, galena, pyrrhotite, greenockite, bornite, cubanite, sulvanite, and violarite, as well as the isotope fractionation factors between them over the temperature range from 0 to 1000 °C. The order of ³⁴S enrichment in these nine minerals is pyrrhotite > greenockite > sphalerite > chalcopyrite > cubanite > sulvanite > bornite > violarite > galena. Our improved method constitutes another model for calculating the thermodynamic isotope fractionation factors of sulfur in sulfides of geochemical interest.     

Occurrences of iron sulfides in Ohio coals

The iron sulfides in seven different Ohio coals have been studied by polished-section ore microscopy, scanning electron microscopy augmented with EDS analyses, and secondary ion mass spectrometry. The iron sulfides in the coals exhibit a large array of textures and interrelationships which reflect site-specific environmental changes that occurred during the deposition of the sulfides. Sulfide deposition occurred principally during the depositional and diagenetic phases of the formation of the precursor peats. Pyrite and marcasite are present in most of the samples examined. Pyrite occurs as isolated and clustered euhedra, isolated and clustered framboids and spheres, cell-fillings, cleat- and fracture-fillings, replacements of plant debris, and as a porous or spongy-textured variety deposited within and around sulfide masses and grains. Marcasite occurs as polycrystalline spheres, polycrystalline rims and bands within and around clusters of pyrite spheres and framboids, as cell-fillings, and as replacements of plant debris. A typical sequence of iron sulfide deposition in texturally complex sulfide grains and masses is: (1) pyrite framboids or spheres; (2) deposition of marcasite around the relict framboids and clusters of framboids; and (3) spongy pyrite deposited as an outer fringe around sulfide masses and as infillings within the masses. The sulfides exhibit a persistent, although not universal, association with clays, and it is likely that much of the iron now present as sulfides was delivered to the depositional environment adsorbed on clay minerals. The iron sulfides tend to be localized in zones parallel to the banding in the coals. Such localization is most pronounced with respect to specific varieties of iron sulfides such as marcasite spheres, pyrite framboids, zones of pyrite euhedra, and occurrences of texturally complex grains and masses. Such zones are believed to represent depositional environments favorable for the precipitation of specific types of iron sulfides. These stratigraphic microenvironments changed during the times of deposition and diagenesis of the precursor peats and resulted in sequential deposition of the different forms of iron sulfides particularly evident in texturally complex sulfide grains. Chemically significant variables most likely were pH and availability of certain trace elements. The factors favoring precipitation of marcasite rather than pyrite are not clearly understood.The textures of the iron sulfides will prove to be important in understanding the reactivity of pyrite and marcasite in causing acid mine drainage and possibly spontaneous combustion of coal and mine waste, and will be important in the continuing development of effective methods of coal cleaning.     

Alteration and regeneration of sulfides by reaction with oxidizing hydrothermal solutions

Reactions between sulfides of heavy metals and solutions of hexavalent uranium sulfate at pH about 2, in the range of 200-360°C, led to replacement of pyrrhotite by marcasite-pyrite aggregate, development of bornite and pyrite after chalcopyrite, growth of regenerated chalcopyrite on the periphery of uranium oxides, replacement of chalcopyrite by bornite (with the accompanying complications, fig. 3), redepositions of chalcocite and native copper, and, in experiments with pyrite-galena and galena-marmatite pairs, redepositions of the minerals with the corresponding growth of pyrite. —V.P. Sokoloff.     

用微机实现地震测网上回声点的归位

由李文杉提出的T₀梯度法,在二层楔形模型下,给出了一种由T₀时间平面图直接构制等深线构造图的精确而简洁的步骤。但该文因局限于当时的背景,在方法的实施上采用了图解的方式,并以T₀图为基础。本文将在其模型和原理的基础上,给出利用计算机直接从地震测网上实施T₀梯度法空间归位的方法。