Intergranular coesite and coesite inclusions in dolomite from the Dabie Shan: Constraints on the preservation of coesite in UHP rocks

Intergranular coesite is extremely rare in, and bears crucial information on the formation and preservation of, ultrahigh‐pressure (UHP) rocks. Here, we report the first occurrence of intergranular coesite in a metasedimentary rock, which occurs in the Ganjialing area in the Dabie Shan, east‐central China, and contains abundant coesite inclusions in both garnet and dolomite. We investigated the content of structural water in these minerals with Fourier transform infrared spectroscopy. Our new results undermine the ubiquity of the “pressure‐vessel” model and highlight the role of reaction kinetics in preserving coesite due to the availability of water in UHP rocks.     

Holocene peatland development and hydrological variability inferred from bog‐pine dendrochronology and peat stratigraphy – a case study from southern Sweden

Dendrochronological analysis was applied to subfossil remains of Scots pine (Pinus sylvestris L.) buried in a South Swedish peat deposit. In combination with peat stratigraphy, this approach was explored for its potential to provide information on the local hydrological and depositional history at the site, forming the basis for a regional palaeohydrological analysis. A 726‐year ring‐width chronology was developed and assigned an absolute age of 7233–6508 cal a BP (5284–4559 BC) through cross‐dating with German bog‐pine chronologies, whereas two short additional records of older ages were radiocarbon dated. Registration of growth positions of individual trees allowed assessment of the spatial dynamics of the pine population in response to hydrological changes and peatland ontogeny. Annually resolved growth variability patterns in the pine population reveal several establishment and degeneration phases, probably reflecting fluctuations in bog‐surface wetness. A major establishment phase at 7200–6900 cal a BP reflects the onset of a period of lowered groundwater level, also indicated by increased peat humification, and a development consistent with regional temperature and lake level reconstructions revealed from other proxies. This study demonstrates that subfossil bog‐pine populations may provide annually to decadally resolved reconstructions of local groundwater variability, which are highly relevant in a long‐term palaeoclimatic context. Copyright © 2012 John Wiley & Sons, Ltd.     

The influence of the fluid phase composition on the solidus temperatures in the haplogranite system NaAlSi3O8-KAlSi3O8-SiO2-H2O-CO2

The solidus temperatures in the haplogranite-system NaAlSi₃O₈-KAlSi₃O₈-SiO₂-H₂O-CO₂ have been determined up to 15 kbar for a constant molar ratio of sodium to potassium of 11 and for fluid compositions ranging from pure water to pure carbon dioxide. The data for the water-saturated solidus are virtually identical with those of previous studies. At constant pressure, the solidus curve as a function of the fluid phase composition exhibits a point of inflection in the range of the water-rich compositions. This phenomenon is attributed to chemical interactions between the CO₂ and the H₂O in the silicate melt. The point of inflection disappears if the CO₂ in the gas phase is replaced by molecular nitrogen. The CO₂-saturated solidi have been measured at 2 and 5 kbars. The data at 5 kbar indicate a melting point depression in the order of 40° C compared to the dry solidus of Huang and Wyllie (1975). The experimental data can be used to estimate the melting temperatures of common quartz and feldspar bearing crustal rocks under the conditions of granulite facies metamorphism. Since for most fluid phase compositions, the solidus curves are very steep in the P, T-diagram, the beginning of melting is nearly exclusively determined by the fluid composition and almost independent of pressure between about 2 and more than 10 kbar. Therefore, the onset of partial melting in quartz and feldspar containing rocks under granulite facies conditions can be used to estimate the composition of a coexisting H₂O-CO₂ fluid phase if geothermometric data are available. The temperature range between the beginning of granulite facies metamorphism and the initiation of melting expands with increasing carbon dioxide content in the H₂O-CO₂ fluid phase. At a CO₂ molar fraction of 0.9, this range extends from about 600° C to 900° C and is almost independent of pressure.     

Cyclic organochlorines in North Sea sediments,relation with size and organic matter

Summary The geographical distribution of organic matter and particles <20 m in North Sea sediments as well as the correlation between organic content, <20 m fraction and PCB (as the sum of content of 24 individual components), p, p-DDE, HCB, -HCH and Lindane (-HCH) is reported. High amounts of organic matter and fine particles have been found in Fladen Ground, Kattegat/Skagerrak/Norwegian Channel, and the inner German Bight. Organic matter settling in the Kattegat/Skagerrak/Norwegian Channel is probably degraded by benthic organisms, including mineralization by bacteria. The smelter in south Norway and the rivers Ems and Elbe are point sources for HCB, the mud-area south-east of Helgoland is a point source for HCB and PCBs. No point source, however, was found for p,p-DDE. Sediment content of PCB, p,p-DDE and HCB show a good correlation to the amount of organic matter and to the <20 m fraction. -HCH and Lindane are only poorly adsorbed onto sediments.

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Zyklische organische Chlorverbindungen in Nordseesedimenten: Verteilung von organischer Substanz und feinpartikulärem Material

Zusammenfassung Es wird über die Verteilung von organischer Substanz und feinpartikulärem Material <20 m in Nordseesedimenten sowie über die Korrelation der Sedimentparameter mit den Konzentrationen von polychlorierten Biphenylen (24 Einzelisomere), p,p-DDE, HCB, -HCH und Lindan (-HCH) berichtet. Hohe Gehalte an organischer Substanz und der <20 m Fraktion wurden im Fladengrund, Kattegat/ Skagerrak/Norwegische Rinne sowie in der inneren Deutschen Bucht gefunden. Die Ergebnisse legen den Schluß nahe, daß ein Großteil des sedimentierten organischen Materials vor der norwegischen Küste von Benthosorganismen und Bakterien verstoffwechselt wird. Durch die mit Søderbergelektroden arbeitenden Metallschmelzen Südnorwegens sowie durch die Flüsse Elbe und Ems werden die Sedimente punktuell mit HCB belastet. Das Schlammgebiet südöstlich von Helgoland tritt als Emittent für PCBs in Erscheinung. Für p,p-DDE konnte keine Punktquelle nachgewiesen werden. Im Gegensatz zu -HCH und Lindan korrelieren die Sedimentkonzentrationen an PCBs, HCB und p,p-DDE gut mit den Gehalten an organischer Substanz bzw. feinpartikulärem Material.

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Les Organochlorines cycliques dans les Sédiments de la Mer du Nord, relation avec la taille et les matières organiques

Résumé La distribution géographique de matière organique et des particules de taille inférieure à 20 m dans les sédiments de la Mer du Nord ainsi que la corrélation entre le contenu organique, (la fraction <20 m) et PCB (somme des contenus de 24 composants individuels), p, p-DDE, HCB, -HCH et Lindane (-HCH) est décrite. D'importantes quantités de matières organiques et de particules fines ont été trouvées à Fladen Ground, Kattegat/Skagerrak/Norwegian Channel et dans le baie intérieur allemende. Les composants organiques qui se trouvent dans le Kattegat/ Skagerrak/Norwegian Channel sont probablement soumis à la dégradation par des organismes (benthic), et aussi à une minéralisation par bactéries. Les usines de métallurgie du Sud de la Norvège et la rivière l'Ems et l'Elbe sont des points sources pour HCB, les zones de boue au Sud Est de Helgoland sont des points sources pour HCB et PCB. Aucun point source ne fût trouvé cependant pour p, p-DDE. Le contenu en sédiment de PCB, p, p-DDE et HCB montre une bonne corrélation avec la quantité de matière organique et la fraction <20 m. -HCH et Lindane ne sont que peu absorbés dans les sédiments.

     

Eocene high pressure metamorphism in the Penninic units of the Tauern Window (Eastern Alps): evidence from40Ar−39Ar dating and petrological investigations

The⁴⁰Ar-³⁹Ar degassing spectra of white micas and amphiboles from three tectonic units of the central Tauern Window (Pennine basement and cover in the Eastern Alps) have been measured. White micas are classified as (1) pre-Alpine low-Si relic micas with an age value of 292 Ma, variously disturbed by the Alpine metamorphism; (2) Alpine phengitic micas of variable composition with an age between 32 and 36 Ma; (3) Alpine low-Si micas with a maximum age of 27 Ma. We attribute the higher Alpine ages to a blueschist facies event, whereas the lower age reflects the late cooling of the nappe pile. Blueschist facies phengites from the basement (Lower Schieferhülle) and the tectonic cover (Upper Schieferhülle) crystallized at a temperature below the closure temperature (T _c) for argon diffusion in white mica and record ages of 32 to 36 Ma. At the same time a thin, eclogite facies unit (Eclogite Zone) was thrust between the Lower and the Upper Schieferhülle and cooled from eclogite facies conditions at about 600°C at 20 kbar to blueschist facies conditions at 450°C or even 300°C at >10 kbar. Eclogite facies phengites closed for argon diffusion and record cooling ages, coinciding with the crystallization ages in the hanging and the footwall unit. Amphibole age spectra (actinolite, glaucophane, barroisite) are not interpretable in terms of geologically meaningful ages because of excess argon.