Marc Javoy

Isotope study on organic nitrogen of Westphalian anthracites from the Western Middle field of Pennsylvania (U.S.A.) and from the Bramsche Massif (Germany)

Abstract The objective of this study was to examine an aspect of the thermal cycling of organic nitrogen in sediments and metasediments. The cycling of organic nitrogen is important because sedimentary organic matter is a shuttle of nitrogen from the atmosphere to the lower crust and thermal decomposition of organic matter is a critical step in the recycling of nitrogen between the different nitrogen pools. Abundance and isotopic composition of organic nitrogen were determined in the particular case of two low sulfur Westphalian anthracites series from Pennsylvania and Bramsche Massif. They represent good examples of Euramerica coals spanning the whole range of anthractization in single fields. Gold cell experimental simulation of the denitrogenation process was conducted at moderate pressure to show that both suites make ideal metamorphic profiles without any shift due to change of facies or to hydrothermal disturbance. During anthracitization, organic nitrogen content decreases rapidly while organic nitrogen isotopic composition does not change with rank increase. The preservation of the isotopic signature implies that organic nitrogen isotopes could be used as indicators for the paleoecological and paleodepositional history reconstruction of the basins. The striking contrast between the rapid and sharp decrease of nitrogen organic content and the invariance of its isotopic composition during the whole anthracitization suggests that ammonia is an important product of the denitrogenation process.

Carbon and oxygen isotopes in eclogites, amphibolites, veins and marbles from the Western Gneiss region, Norway

The 13C12C and 18O16O ratios have been measured in carbonates and silicates from pods of eclogite, garnet-amphibolite and marbles and also some veins which outcrop in the Western Gneiss Region, Norway, in an attempt to clarify the origin of these rocks. n nThe eclogite carbonates (magnesite or dolomite) display carbon and oxygen isotope values (−9‰ < δ 13C < −2.4‰ PDB; +6.8‰ < δ 18O < +10.4‰ SMOW) which lie within the field of mantle-effluxed carbonate as represented by primary igneous carbonatites. Two garnet-amphibolites have heavier oxygen in the same δ 13C range. Two marbles have heavier carbon and differing δ 18O. Three types of carbonate-bearing vein show different isotopic relationships to the carbonate of their host rocks. n nOxygen isotope fractionations amongst minerals indicate preservation of a high-temperature equilibration (650–850°C) comparable to the temperature previously deduced from Feue5f8Mg partitioning amongst silicates (700–850°C). n nThe eclogite carbonate carbon and oxygen isotope compositions may have been produced either by metamorphism of crustal carbonates or by carbonation of carbonate-free rocks by mantle-effluxed CO2 before or during the eclogitisation processes. The whole-rock δ 18O-values (4–8‰ SMOW) are within the range of crustal basalts and gabbros thus supporting a crustal origin for these eclogites. Several other lines of petrological and geochemical evidence also support the model of crustal protoliths of these rocks. n nThese data may contribute to the geodynamic modelling of the carbon cycle in indicating that subducted carbon reinjected into the mantle could have essentially the same isotope composition as that of carbon effluxed from the mantle and that a crustal protolith origin for some erupted xenoliths gains in credibility.

New carbonatite complexes in the Archaean In'Ouzzal nucleus (Ahaggar, Algeria): mineralogical and geochemical data

Several massifs of very old carbonatites have been discovered in the Archaean granulitic block of InOuzzal (Western Ahaggar, Algeria). These carbonatites are original since they are associated with Silica — saturated syenitic magmatism and present, in the late stages of evolution, a very uncommon mineralogy, with silicate minerals, especially wollastonite, allanite, and quartz. The mineralogy, C and O isotopes and R.E.E. distributions indicate that the late stages of crystallization occurred under high SiO2 activities, and produced the uncommon mineralogy and extremely high R.E.E. concentrations in phosphate minerals apatite and britholite. Interaction with continental crust is a possible mechanism to explain the original features of these carbonatite complexes.