Kanonaite-rich viridines from the Venn-Stavelot Massif,Belgian Ardennes

In highly oxidized Mn- and Al-rich layers of the low-grade metamorphic metasediments north of Salmchâteau, Belgium, three types of viridine porphyroblasts can be distinguished: (a) single-phase porphyroblasts, (b) composite porphyroblasts with a core consisting of very fine-grained kaolinite, which is interpreted as a pseudomorph after andalusite, and a rim of viridine and, (c) composite porphyroblasts with a viridine core, a first mantle of kaolinite (as a pseudomorph after andalusite) and a second rim of viridine. These grains form an assemblage with muscovite, paragonite, Mg-chlorite, piemontite, braunite, hematite, quartz, rutile, and apatite. In the single-phase porphyroblasts the chemical composition of the core, (Mn _0.51 ³⁺ Al_0.44Fe _0.04 ³⁺ )Al(O/SiO₄), grades continuously into (Mn _0.79 ³⁺ Al_0.15Fe _0.06 ³⁺ )Al(O/SiO₄) of the rim. The outer zones of the analyzed grains are thus richer in kanonaite-component, MnAl(O/SiO₄), than the original kanonaites described recently by Vrána et al. (1978) from the type locality in Zambia. Taking into consideration also other chemical data of viridines of the Salmchâteau locality there exists a complete solid solution series in the system Al₂SiO₅ (And)-MnAlSiO₅ (Kan)-FeAlSiO₅ at least from And₆₁Kan₃₇(FeAlSiO₅)₂ to And₁₅Kan₇₉(FeAl-SiO₅)₆ for the P-T conditions verified here ( 400 ° C/1–2 kb). The oxygen fugacity of the rocks is estimated to correspond to f _O2 of the MnO₂/ Mn₂O₃-buffer or to exceed it. Single-phase and composite porphyroblasts of the andalusite-type minerals contain S-shaped internal structures, which, in the marginal parts of the grains, are always oriented perpendicular to the schistosity planes. Together with the zoning of the composite grains these structures give clear evidence, that the andalusite formation is preceded by viridine growth in suitable rocks high in manganese. Thus, the viridines of the cores are formed under P-T conditions of less than 360–380 ° C/1–2 kb. The recurrent availability of Mn to form the kanonaite-rich rims in all three types of porphyroblasts may be explained by Mn release out of the recrystallizing hematites along the schistosity planes during decreasing temperatures.Although the Commision on New Minerals and Mineral Names, IMA, has deleted the classical name viridine it is preferred here and used as a varietal name for all intermediate members of the solid solution series between andalusite, Al₂(O/SiO₄), and the recently described kanonaite, MnAl(O/SiO₄) (Vrána et al., 1978). This is done with special regard to the petrographie purpose of this investigation, since it is impracticable and by simple optical methods in strongly zoned crystals even impossible to distinguish between manganian andalusite [Al(6)>Mn] and aluminous kanonaite [Al(6)相似文献   

The coticule rocks (spessartine quartzites) of the Venn-Stavelot Massif,Ardennes, a volcanoclastic Metasediment?

Thin spessartine-quartzite layers (coticules) are interstratified with Ordovician (Salmian) shales of the Venn-Stavelot Massif, Ardennes. These coticules indicate sudden interruptions in the sedimentation process of the shales. The lower contact of the coticules represents an abrupt change in the chemical composition from the underlying shales. In contrast, the upper limit of the coticules is chemically more diffuse. Phase relations of the phengitebearing spessartine-quartzites including paragonite, chlorite, and chloritoid or kaolinite as subordinate phases give evidence for a manganese-montmorillonitic source material of the coticules. This is in good agreement with the internal structures observed in the coticule layers (swelling and sliding effects, Liesegang structures). Since there is a positive correlation between the oxidation ratio of the enclosing shales and the chemical composition of the coticules, it is proposed that the source material of the coticules developed in situ by halmyrolysis out of tuffs. High oxidation ratios of the shales with iron fixed in the trivalent state but with divalent and thus mobile manganese led to the formation of coticule starting material, dominantly a manganese-montmorillonite. Halmyrolysis products formed under low oxidation ratio conditions of the enclosing shales are characterized by hydrosilicates rich in iron. The Obrochishte manganese deposit, Bulgaria, is discussed as a nonmetamorphic equivalent of the coticules of the Ardennes.     

Beiträge zur Limnologie des Walen-, Zürich-Ober-und Zürichsees

Chemical and physical conditions of the Walensee are discussed on the basis of values analyzed during an investigation period in the years 1972–1975. Thereby values of selected parameters have been compared with data of other Swiss lakes to estimate the trophic condition of the lake.      

Isotopenuntersuchungen zum Kalkumsatz im Löß

The ratio of oxygen and carbon isotopes is a criterion for the carbonate-genesis with distinct differences between marine, freshwater, soil and loess carbonates. In addition it becomes clear that there is a number of carbonate soils within the loess of the mediterranean region with distinguishable isotopic compositions. The components affecting the relation of the isotopes in these loesses and the intercalated soils are discussed. The carbonate-turnover during the sedimentation of the loess with carbonate-precipitation from remnant solutions seems to be of some importance. The relatively small participation of biogenic CO₂ in the carbonate-turnover ist evident.     

Random-Lag Singular Cross-Spectrum Analysis

We compare the distribution of stars of different spectral types, and hence mean age, within the central SMC and find that the asymmetric structures are almost exclusively composed of young main-sequence stars. Because of the relative lack of older stars in these features and the extremely regular distribution of red giant and clump stars in the SMC central body, we conclude that tides alone are not responsible for the irregular appearance of the central SMC. The dominant physical mechanism in determining the current-day appearance of the SMC must be star formation triggered by a hydrodynamic interaction between gaseous components. These results extend the results of population studies (see Gardiner & Hatzidimitriou) inward in radius and also confirm the suggestion of the spheroidal nature of the central SMC based on kinematic arguments (Dopita et al.; Hardy, Suntzeff, & Azzopardi). Finally, we find no evidence in the underlying older stellar population for a "bar" or "outer arm," again supporting our classification of the central SMC as a spheroidal body with highly irregular recent star formation.     

Lateglacial to early Holocene multiproxy record from Loch Assynt,NW Scotland

A core, recovered from a water depth of 53 m in Loch Assynt, North-West Scotland, has yielded a 9 m sequence comprising two distinct units, an upper, organic-rich unit (Unit I, ca. 6 m) overlying a sequence of laminated clays, silts and sands (Unit II, ca. 3 m). The upper unit is essentially Holocene in age based upon three bulk AMS radiocarbon dates while a fourth radiocarbon date from Unit II confirms a late-glacial age for that interval and supports a broadly linear age–depth relationship. Distinct variations in the magnetic susceptibility record of the lower unit can be visually correlated to major changes in the Greenland ice core (GISP2), this together with pollen evidence supports the radiocarbon dating suggesting an age of approximately 11,000 to around 17,000 cal. BP for Unit II, with evidence for the Younger Dryas (Loch Lomond) stadial and the Bolling–Allerød climatic phases. Variations in the magnetic susceptibility record of the late-glacial sediments are thought to relate to climatically driven changes in soil cover and erosion rates. The multiproxy record from Loch Assynt indicates relatively continuous, sub-aqueous sedimentation during the last ～17,000 years, providing an approximate age for the initiation of modern Loch Assynt and supporting recent dates of moraine retreat lines in the Loanan Valley from about 14–15 ka BP. Pollen and chironomid sampling provides further insights to the history of this relatively deep water body and compliment existing high-resolution palaeo-precipitation records for the mid to late Holocene interval from speleothem archives within the loch catchment.     

Refining the noble gas record of the Réunion mantle plume source: Implications on mantle geochemistry

We report isotope analyses of helium, neon, argon, and xenon using different extraction techniques such as stepwise dynamic and static crushing, and high-resolution stepwise heating of three mantle xenoliths from Réunion Island. He and Ne isotopic compositions were similar to previously reported Réunion data, yielding a more radiogenic composition when compared to the Hawaiian or Icelandic mantle plume sources. We furthermore observed correlated ¹²⁹Xe/¹³⁰Xe and ¹³⁶Xe/¹³⁰Xe ratios following the mantle trend with maximum values of 6.93 ± 0.14 and 2.36 ± 0.06, respectively. High-resolution argon analyses resulted in maximum ⁴⁰Ar/³⁶Ar ratios of 9000–11,000, in agreement with maximum values obtained in previous studies. We observed a well-defined hyperbolic mixing curve between an atmospheric and a mantle component in a diagram of ⁴⁰Ar/³⁶Ar vs. ²⁰Ne/²²Ne. Using a mantle ²⁰Ne/²²Ne of 12.5 (Ne–B) a consistent ⁴⁰Ar/³⁶Ar value of 11,053 ± 220 in sample ILR 84-4 was obtained, whereas extrapolations to a higher mantle ²⁰Ne/²²Ne ratio of 13.8 (solar wind) would lead to a much higher ⁴⁰Ar/³⁶Ar ratio of 75,000, far above observed maximum values. This favours a mantle ²⁰Ne/²²Ne of about 12.5 considered to be equivalent to Ne–B. Extrapolated and estimated ⁴⁰Ar/³⁶Ar ratios of the Réunion, Iceland, Loihi, and MORB mantle sources, respectively, tend to be linearly correlated with air corrected ²¹Ne/²²Ne and show the same systematic sequence of increasing relative contributions in radiogenic isotopes (Iceland–Loihi–Réunion–MORB) as observed for ⁴He/³He. In general, He–Ne–Ar isotope systematics of the oceanic mantle can be explained by following processes: (i) different degree of mixing between pure radiogenic and pure primordial isotopes generating the MORB and primitive plume (Loihi-type) endmembers; (ii) relatively recent fractionation of He relative to Ne and Ar, in one or both endmembers; (iii) after the primary fractionation event, different degrees of mixing between melts or fluids of MORB and primitive plume affinity generate the variety of observed OIB data, also on a local scale; (iv) very late-stage secondary fractionation during magma ascent and magma degassing leads to further strong variation in He/Ne and He/Ar ratios.     

Colloidally separated samples from Allende residues: Noble gases,carbon and an ESCA-study

A non-colloidal fraction separated by physical means from an $\text{HF}\text{HCl}$-resistant residue of the Allende carbonaceous meteorite exhibits a ratio of isotopically “normal” (Q-type) xenon to “anomalous” xenon (X-type) that is ~4 times larger than usually observed. Coincidentally this fraction contains carbon that is isotopically heavier by ~10%. than bulk Allende residue samples. ESCA analyses of companion colloidal separates show that the major portion of the S contained in our $\text{HF}\text{HCl}$-residues is elemental rather than a sulfide. They also confirm earlier observations that no elementally distinct surface coating is present, in accord with the absence of a surface-sited sulfur-bearing gas carrier, and that the oxygen content is increased after etching with nitric acid. For these separates noble gas data coupled with the ESCA data for nitrogen and the isotopic data for carbon point to the existence of heterogeneities among the noble gas-, carbon- and nitrogen-bearing phases and, thus, preservation of discrete components from the variety of source regions (or production mechanisms, or both) sampled by the Allende parent body. In sharp contrast to the success of physical and chemical methods in yielding samples in which one of the major noble gas components predominates to an extraordinary degree over the other, carbon isotopic compositions that have been inferred for the respective carrier phases in these same samples are strongly contradictory. Mass and isotope balance considerations lead us to conclude that a major fraction of the carbonaceous matter in Allende is noble-gas-poor, a result that could be confirmed by direct isolation of a sample, the carbon in which is dominated by this variety; and for that reason no simple relationship is discernable yet between observed isotopic compositions and either major noble gas component. Similar ambiguities may exist for nitrogen. The possible relationship between carbon-rich phases in ureilites and carbonaceous chondrites is considered.     

Multi-proxy reconstruction of the Holocene vegetation and land use dynamics in the Julian Alps,north-west Slovenia

Small mountain lakes are natural archives for understanding long-term natural and anthropogenic impact on the environment. This study focused on long-term (last ca. 13 000 years) vegetation changes and sedimentary processes in the catchment area of Lake Planina pri jezeru (1430 m a.s.l.) by using mineralogical, geochemical and palynological methods. Palynological results suggest that regional vegetation between 12 900 and 11 700 cal a bp was a herbaceous–forest tundra (Pinus, Artemisia, Poaceae ). Climate warming at the beginning of the Holocene (ca. 11 700 cal a bp ) caused the transition from a wetland (Cyperaceae) to an eutrophic lake with alternating anoxic (pyrite) and oxic conditions (gypsum). In addition, the surrounding area became forested (Picea, Larix, Ulmus). Fagus expanded at 10 200 cal a bp and Abies at 8200 cal a bp. Between 7500 and 4300 cal a bp , human impact on the environment was barely noticeable and mostly limited to grazing. During 4300–430 cal a bp human impact became more evident and gradually increased. The greatest influence was observed from 430 cal a bp onwards, when excessive exploitation of the surrounding area (logging and grazing) severely eutrophicated the lake.     

What Can Groundwater Monitoring Tell Us About Gas Migration? A Numerical Modeling Study

Groundwater monitoring to measure a variety of indicator parameters including dissolved gas concentrations, total dissolved gas pressure (TDGP), and redox indicators is commonly used to evaluate the impacts of gas migration (GM) from energy development in shallow aquifer systems. However, these parameters can be challenging to interpret due to complex free-phase gas source architecture, multicomponent partitioning, and biogeochemical reactions. A series of numerical simulations using a gas flow model and a reactive transport model were conducted to delineate the anticipated evolution of indicator parameters following GM in an aquifer under a variety of physical and biogeochemical conditions. The simulations illustrate how multicomponent mass transfer processes and biogeochemical reactions create unexpected spatial and temporal variations in several analytes. The results indicate that care must be taken when interpreting measured indicator parameters including dissolved hydrocarbon concentrations and TDGP, as the presence of dissolved gases in background groundwater and biogeochemical processes can cause potentially misleading conclusions about the impact of GM. Based on the consideration of multicomponent gas partitioning in this study, it is suggested that dissolved background gases such as N₂ and Ar can provide valuable insights on the presence, longevity and fate of free-phase natural gas in aquifer systems. Overall, these results contribute to developing a better understanding of indicators for GM in groundwater, which will aid the planning of future monitoring networks and subsequent data interpretation.