Variation of viscosity with temperature and composition in the plagioclase system

The viscosities of supercooled melts of 13 different plagioclase compositions have been measured in the range 10¹³–10⁸ dPas with a micro-penetration viscometer. Application of the most common empirical and theoretical viscosity-temperature models to the present data and plagioclase viscosity data available from the literature showed that the configurational entropy theory represents the best approach. Using this theory together with an entropy of mixing term evaluated from a modified two-lattice mixing model, the viscosities of intermediate plagioclase compositions have been calculated as a function of temperature and anorthite content solely from the appropriate data of the end members albite and anorthite. The agreement between experimental and calculated viscosity data is excellent.Dedicated to Professor Wolf von Engelhardt on the occasion of his 75th birthday     

Spinels as petrogenetic indicators: Activity-composition relations at low pressures

Provisional activity-composition relations have been developed for spinels in the system FeO-MgO-Fe₂O₃-Al₂O₃-Cr₂O₃-TiO₂. These relations are based on a Temkin type model for the configurational entropy combined with a third-degree Taylor's series expansion to describe the excess Gibbs energy of mixing that is the result of describing spinels in terms of a set of fictive ordered and inverse spinel components. The coefficients of this expansion have been identified as (1) reciprocal Mg-Fe exchange terms, (2) binary asymmetric Margules parameters, and (3) Margules and Bragg-Williams type terms which make explicit provision for mixing of Fe and Mg on two sites in ulvospinel. Provisional values have been assigned to these terms for the temperature range 900–1,250°C based on inferred and experimentally determined spinel miscibility gaps, Fe-Mg olivine-spinel exchange data reported in the literature, and 40 experimental determinations of Fe-Mg partitioning between olivines and spinels in basanitic to uganditic silicate liquids between 1,092 and 1,300°C ( QFM).     

Thermodynamic data from redox reactions at high temperatures. III. Activity-composition relations in Ni-Pd alloys from EMF measurements at 850–1250 K,and calibration of the NiO+Ni-Pd assemblage as a redox sensor

The assemblage NiO+Ni-Pd alloy has been calibrated as a precise oxygen fugacity sensor in the temperature range 850–1250 K at 1 bar, using an electrochemical technique with oxygen-specific CSZ electrolytes, and Ni+NiO and Cu+Cu₂O as the reference electrodes. Nine compositions were studied, ranging from 0.12 to 0.83 X _Ni ^alloy . Steady EMFs, implying equilibrium, were rapidly achieved in all cells, and were found to be reversible on increasing and decreasing temperature with a precision approaching 0.1 mV. The estimated accuracy of the measurements on each cell is ±0.2 mV (1, corresponding to ±0.003 log-bar units in fo₂ at 1273 K). Compositions of the Ni-Pd alloys were measured after each run by electron microprobe, and these compositions were then checked for internal consistency by measuring the lattice parameter by X-ray diffraction. Nickel-rich alloys show positive deviations from ideality and endothermic enthalpies of mixing, but palladium-rich compositions have exothermic enthalpies of mixing and strong negative deviations from ideality. The excess entropies of mixing are positive for all compositions, and correlate approximately with the excess volumes of mixing. The highly asymmetrical deviations from ideality are well described by a polynomial expression of the Redlich-Kister form, with three terms for the enthalpies, and two for the excess entropies and volumes of mixing. The experimental data from this study have been used to re-formulate the Ni-Pd oxygen fugacity sensor to give an expression; _O2 ^ss = _O2 ^NNO – 2RT ln X _Ni ^alloy – [2 · (1 – X _Ni ^alloy )² · [(–2165–7.958 · T) + (9409 – 0.888 · T) · (4 X _Ni ^alloy – 1) + 2089 · (6 X _Ni ^alloy – 1) · (2 X _Ni ^alloy – 1)]](850<T<1300) where _O2 ^ss is in J mol^-1, T is in kelvins, and the expression for _O2 ^NNO is that given by O'Neill and Pownceby (1993). Values in terms of log fo₂ may be obtained from the above by dividing by RT ln 10. The estimated standard error in _O2 ^ss is on the order of ±200 J mol^-1, which is approximately ±0.01 log-bar units in fo₂ at 1273 K.     

Effect of water on the heat capacity of polymerized aluminosilicate glasses and melts

The effect of water on heat capacity has been determined for four series of hydrated synthetic aluminosilicate glasses and supercooled liquids close to albite, phonolite, trachyte, and leucogranite compositions. Heat capacities were measured at atmospheric pressure by differential scanning calorimetry for water contents between 0 and 4.9 wt % from 300 K to about 100 K above the glass transition temperature (T_g). The partial molar heat capacity of water in polymerized aluminosilicate glasses, which can be considered as independent of composition, is (J/mol K). In liquids containing at least 1 wt % H₂O, the partial molar heat capacity of water is about 85 J/mol K. From speciation data, the effects of water as hydroxyl groups and as molecular water have tentatively been estimated, with partial molar heat capacities of 153 ± 18 and 41 ± 14 J/mol K, respectively. In all cases, water strongly increases the configurational heat capacity at T_g and exerts a marked depressing effect on T_g, in close agreement with the results of viscosity experiments on the same series of glasses. Consistent with the Adam and Gibbs theory of relaxation processes, the departure of the viscosity of hydrous melts from Arrhenian variations correlates with the magnitude of configurational heat capacities.     

Aluminosilicate melts: structure, composition and temperature

The anionic structure of aluminosilicate melts of intermediate degree of polymerization (NBO/T = 0.5) and with along the composition join (LS4-LA4) has been examined in-situ to ˜1480 °C, and compared with recent data for melts along the analog composition join and with less polymerized melts along the join and O_{_5}. With , the anionic equilibrium, (1) , adequately describes the structure. With , a second expression, (2) , is required because an additional structural unit, Q¹, is stabilized in the melts. The enthalpy, , of reaction (1) increases from − 36 ±4 kJ/mol in the absence of aluminum to 34± 5 kJ/mol at and 64 ± 4 kJ/mol at Al/(Al + Si) = 0.45. Similar trends are reported for other alkali aluminosilicate melts. Least-squares fitting of abundance of structural units as a function of temperature and bulk composition has been conducted. The unit abundance is dominantly a function of temperature, Al/(Al +Si), and bulk melt polymerization. Configurational entropy and heat capacity of mixing of melts above their glass transition temperatures have been calculated with the aid of the least-squares fitted equations. The values of these parameters indicate that as the ionization potential of the metal cations increases, configurational heat capacity of alkali aluminosilicate melts becomes temperature dependent. As a result, transport properties (viscosity, diffusivity, and conductivity) of such melts will not show Arrhenian behavior even in the high-temperature range. Further, discontinuous changes in entropy and heat capacity of mixing results from temperature-induced changes in types of structural units in the melts. Such discontinuous changes would also be reflected in discontinuous changes of temperature-dependent transport properties. Received: 26 September 1996 / Accepted: 18 October 1996     

High-temperature enthalpy at the orientational order-disorder transition in calcite: implications for the calcite/aragonite phase equilibrium

The enthalpy of calcite has been measured directly between 973 K and 1325 K by transposed-temperature- drop calorimetry. The excess enthalpy has been analysed in terms of Landau theory for this tricritical phase transition. The zero-point enthalpy and entropy allow estimates of the parameters a and C in the Landau expansion for free energy which expresses excess free energy G as a function of the order parameter Q and temperature T: G 1/2a(T _2c–T)Q ²+1/6CQ ⁶ with a=24 J·K·mol^-1, C = 30 kJ·mol^– T _c = 1260 ±5 K. The entropy of disorder below the transition has been formulated as a function of temperature allowing the calculation of the calcite/aragonite phase boundary when taking this extra entropy into account. There is remarkable agreement between the calculated equilibrium curve and previous experimental observations. The Landau theory predicts behaviour which fully accounts for the change in slope of the calcite/aragonite phase boundary, which is thus wholly due to the R¯3c –R¯3m transition in calcite.     

Effect of grossular-content in garnet on the partitioning of Fe and Mg between garnet and biotite

In contrast to Ferry (1980) (X _Ca)-values in garnet even lower than 0.1 have a significant effect on the calculated equilibrium temperature using the experimental calibration of the Fe and Mg paritioning between garnet and biotite. Garnet compositions and Mg/Fe — distribution coefficients from samples of the Eoalpine staurolite — in zone in the southern Ötztal are related by the quadratic regression equation: InK _D= -1.7500 (±0.0226) + 2.978 (±0.5317)X _Ca ^Gt -5.906(±2.359)(X _Ca ^Gt )² Temperatures derived by the Ferry and Spear (1978) calibration using chemistry — correctedK _D values are petrologically realistic.Analysis of our data supports non ideal mixing of grossular with almandine — pyrope solid solution. The derived excess mixing energies are quite small for the almandine — pyrope solution (W _FeMg= –133 cal/mole) and about +2775 cal/mole for the difference between pyrope-grossular and almandine-grossular solutions (W _MgCa —W _FeCa) at metamorphic conditions of 570° C and 5,000 bar. The mixing parameters proposed by Ganguly and Saxena (1984) are not confirmed by our data as they would result in significantly lower temperatures.     

Heat capacities and entropies of silicate liquids and glasses

The heat capacities of several dozen silicate glasses and liquids composed of SiO₂, TiO₂, Al₂O₃, Fe₂O₃, FeO, MgO, CaO, BaO, Li₂O, Na₂O, K₂O, and Rb₂O have been measured by differential scanning and drop calorimetry. These results have been combined with data from the literature to fit C _pas a function of composition. A model assuming ideal mixing (linear combination) of partial molar heat capacities of oxide components (each of which is independent of composition), reproduces the glass data within error. The assumption of constancy of ¯C _p,iis less accurate for the liquids, but data are not sufficient to adequately constrain a more complex model. For liquids containing alkali metal and alkaline earth oxides, heat capacities are systematically greater in liquids with high field strength network modifying cations. Entropies of fusion (per g-atom) and changes of configurational entropy with temperature, are similarly affected by composition. Both smaller cation size and greater charge are therefore inferred to lead to greater development of new structural configurations with increasing temperature in silicate liquids.     

Pressure dependence of melt viscosities on the join diopside-albite

The pressure dependence of melt viscosities on the join diopside-albite has been studied using falling-sphere viscometry. The five melt compositions investigated are: diopside, Ab₂₅Di₇₅, Ab₅₀Di₅₀, Ab₇₅Di₂₅ and albite. Experiments were performed at 1500° and 1600°C and at pressures of 5, 10, 15, 20 and 25 kbar. The positive and negative pressure dependence of the viscosity of diopside and albite, respectively, were confirmed. All intermediate compositions show an initial decrease in viscosity with increasing pressure; however, melt of Ab₂₅Di₇₅ composition passes through a minimum viscosity at approximately 12 kbar and 1600°C. This behavior is analogous to the variation in the viscosity of water with pressure at low temperature.

It is suggested that the three-dimensional, fully polymerized, albite structure dominates flow at low pressures. With increasing pressure, disruption of this structure and decrease in the average size of the flow units leads to domination by the diopside structure. The variation in viscosity with composition along the join at one atmosphere can be adequately modelled using the and (1965) configurational entropy model with an additional two-lattice configurational entropy of mixing term. The pressure dependence of viscosity in the diopside-albite system, however, cannot be predicted by the model, because there is an absence of information on the pressure dependence of the model parameters.

It is probable that relatively polymerized magmas (e.g. rhyolites to SiO₂-saturated basalts) show a negative pressure dependence of viscosity to depths where they originate in the lower crust or upper mantle. In contrast, the most depolymerized, naturally-occurring melts, such as strongly SiO₂-undersaturated basalts and picrites, may exhibit a viscosity minimum. The viscosity of these melts may be sufficiently high at depths within the upper mantle to inhibit their segregation, rise and eventual eruption at the surface.     

Hydrothermal alteration and oxygen and hydrogen isotope geochemistry of the D-68 zone Cu-Zn Massive Sulfide Deposit,Noranda District,Quebec, Canada

Summary Pervasive hydrothermal alteration zones in quartz-feldspar porphyry domes underly all massive sulfide lenses in the D-68 Zone Cu-Zn deposit, Noranda. Alteration pipes are mineralogically zoned and contain chloritic cores consisting of stringer sulfides, enveloped by sericitic haloes. Silicified rocks are found locally.Alteration took place at nearly constant volume. Na depletion, and K enrichment relative to the least altered rocks, are found in all alteration zones. Fe and Mg have been added to the chloritic zone and subtracted in the sericitic and silicic zones. Ca and Si are enriched mainly in the silicic zone. Al, Ti and Zr were the least mobile of the elements studied.Whole-rock ¹⁸O values vary from +5.6 to +6.2 per mil in chloritized rocks, +5.8 to + 7.3 per mil in sericitized rocks and + 7.2 to + 8.3 per mil in silicified rocks. D values for two chloritized samples are – 63 and – 70 per mil whereas in two sericitized samples they are close to –62 per mil. Quartz from the chlorite alteration zone is isotopically heavier (¹⁸O = 8.6 per mil) than that from the sericite alteration zone (¹⁸O = 6.4 per mil), suggesting equilibration with different hydrothermal fluid or different temperature of alteration. Assuming an alteration temperature of 300° + 50°C the fluid in equilibrium with quartz and chlorite had ¹⁸O and D values of about 1.5 ± 2.0 per mil and –23 ± 5 per mil, respectively. The fluid in equilibrium with quartz and sericite had ¹⁸O and D values of about –0.5 ± 2 per mil and –30 ± 5 per mil, respectively. On the basis of isotopic data, seawater was probably the major constituent of the hydrothermal fluids.

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Hydrothermale Umwandlung und Sauerstoff-Wasserstoff-Isotopengeochemie der Zone D-68 Cu-Zn Derberz Sulfidlagerstätte, Noranda District, Quebec, Canada

Zusammenfassung Hydrothermale Umwandlungszonen in porphyrischen Quarz-Feldspat Gesteinskörpern liegen unterhalb von Derberz Sulfidlinsen in der D-68 Zone Cu-Zn Lagerstätte, Noranda. Umgewandelte pipes sind mineralogisch zoniert; sie enthalten aus Sulfiden bestehende chloritische Kerne, die von sericitischen Höfen umhüllt werden. Lokal treten silicifizierte Gesteine auf.Die Umwandlung ging bei annähernd konstantem Volumen vor sich. Na-Verarmung und K-Anreicherung, bezogen auf die am wenigsten umgewandelten Gesteine, liegen in allen Umwandlungszonen vor. Fe und Mg wurden der Chloritzone zugeführt, in den Sericit- und Si-Zonen abgeführt. Ca und Si sind vor allem in der Si-Zone angereichert. Al, Ti und Zr waren von den untersuchten Elementen am wenigsten mobil.Gesamtgesteins-¹⁸O Werte variieren von +5,6 bis +6,2 in den chloritisierten Gesteinen, von +5,8 bis 7,3 in sericitisierten Gesteinen und von +7,2 bis +8,3 in den silicifizierten Gesteinen. Die D Werte für zwei chloritisierte Proben betragen –63 und –70, in zwei sericitisierten Proben liegen sie hingegen nahe bei –62. Quarz von der Chlorit-Umwandlungszone ist isotopisch schwerer (¹⁸O = 8,6) als von der Sericit-Umwandlungszone (¹⁸O = 6.4), was eine Gleichgewichtseinstellung mit verschiedenen hydrothermalen Lösungen oder eine verschiedene Umwandlungstemperatur nahelegt. Bei einer angenommenen Umwandlungstemperatur von 300 ± 50°C, hatte die im Gleichgewicht mit Quarz und Chlorit stehende Lösung ¹⁸O und D Werte von etwa 1,5 ± 2 bzw. –23 + 5. Die im Gleichgewicht mit Quarz und Sericit befindliche Lösung hatte ¹⁸O und D Werte von etwa –0,5 ± 2%o bzw. –30 ± 5. Aufgrund der Isotopendaten war wahrscheinlich Meerwasser der Hauptbestandteil der hydrothermalen Lösungen.

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With 7 Figures     

Viscosity and configurational entropy of silicate melts

With the configurational entropy theory of relaxation processes of Adam and Gibbs (1965), one predicts that the viscosity depends on temperature according to log $\text{η = A}{}_{\mathrm{e}}\text{+}\text{B}{}_{\mathrm{e}}\text{TS}{}_{\mathrm{<mtext>conf</mtext>}}$, where S_conf is the configurational entropy of the liquid. Thermochemical calculations of S_conf performed for some mineral compositions show the importance of non-configurational contributions to the entropy differences between amorphous and crystalline phases. Except for the case of SiO₂, the available thermodynamic data indicate that the above equation for viscosity accounts quantitatively for the experimentally determined temperature dependence of the viscosity of silicate melts. The Adam and Gibbs theory also provides a simple rationale for the non linear variation of the logarithmic viscosity with composition in mixed alkali silicate liquids at low temperatures, the minimum of viscosity resulting from the contribution of the entropy of mixing to S_conf.     

Synthesis and stability relations of wairakite,CaAl2 Si4 O12·2H2O

Hydrothermal investigation of the bulk composition CaO·Al₂O₃·4SiO₂ + excess H₂O has been conducted using conventional techniques over the temperature range 200–500° C and 500–5,000 bars P _fluid. The fully ordered wairakite was synthesized unequivocally in the laboratory, probably for the first time.The gradual, sluggish and continuous transformation from disordered to ordered wairakite evidently accounts for failure by previous investigators to synthesize ordered wairakite in runs of week-long duration. The dehydration of metastable disordered wairakite to metastable hexagonal anorthite, quartz and H₂O has been determined; this reaction takes place at temperatures exceeding 400° C, even at fluid pressures of 500 bars or less. The upper P _fluid-T boundary of the disordered phase is equivalent to the maximum temperature curve of synthetic wairakite presented by previous investigators. The hydrothermal breakdown of natural wairakite above its stability limit appears to be a very slow process.The equilibrium dehydration of wairakite to anorthite, quartz and H₂O occurs at 330±5° C at 500 bars, 348±5° C at 1,000 bars, 372±5° C at 2,000 bars and 385±5° C at 3,000 bars. Where fluid pressure equals total pressure, the thermal stability range of wairakite is about 100° C wide. At lower temperatures wairakite reacts with H₂O to form laumontite. Reconnaissance experiments dealing with the effect of CO₂ on stabilities of calcium zeolites suggest that wairakite or laumontite may be replaced by the assemblage calcite + montmorillonite in the presence of a CO₂-bearing fluid phase.The determined P _fluid -T field of wairakite is compatible with field observations in some metamorphic terrains where it is related to the shallow emplacement of granitic magma and with direct pressure-temperature measurements in certain active geothermal areas. Under inferred conditions of higher _CO2/_H2O ratios, essentially unmetamorphosed rocks grade directly into those characteristic of the greenschist facies; moderately high values of _CO2 in carbonate-bearing rocks result in the downgrade extension of the greenschist facies at the expense of zeolite-bearing assemblages.     

Koritnigit,Zn[H2O|HOAsO3], ein neues Mineral aus Tsumeb,Südwestafrika

Zusammenfassung Das neue Mineral Koritnigit ist ein wasserhaltiges Zinkhydrogenarsenat der Formel Zn[H₂O|HOAsO₃]. Die chemische Analyse (Elektronenmikrosonde und T.G.A.) ergab: As₂O₅ 51,75%, ZnO 35,97% und H₂O 12,3%, Summe 100,0%. Die HOAsO₃-Ionen wurden IR-spektroskopisch nachgewiesen. Koritnigit ist löslich in kalter, verdünnter HCl und HNO₃.Die Gitterkonstanten sind:a ₀=7,948(2),b ₀=15,829(5),c ₀=6,668(2) Å, =90,86(2), =96,56(2), =90,05(2)^o,V=833,2(4)ų,V=8. Die Raumgruppe ist . Die stärksten Linien des Pulverdiagramms sind: 7,90(10) (020,100), 3,83(7) ( ), 3,16(9) ( ) 2,926(4) (150), 2,679(4) ( ), 2,461(6) ( ), 2,186(5) ( ), 1,969(4) (400), 1,649(3) (004).Koritnigit ist wasserklar bis durchscheinend weiß. Idiomorphe Kristalle sind nicht bekannt. Die Spaltbarkeit nach {010} ist ausgezeichnet und auf {010} sind Spaltspuren nach [001] und nach [100] erkennbar. Härte 2.G=3,54 g·cm^–3,D _x =3,56 g·cm^–3. Koritnigit ist optisch zweiachsig positiv, 2V70(5)^o. Die Werte der Lichtbrechung sind:n =1,632(5),n =1,652(3) undn =1,693(3).Koritnigit wurde auf der 31. Sohle der Tsumeb-Mine, Südwestafrika gefunden. Er kommt als Sekundärmineral in Paragenese mit Cu-Adamin, Stranskiit und drei weiteren, vorerst nicht identifizierten mineralen in Zersetzungshohlräumen von Tennantit vor.

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Koritnigite, Zn[H₂O|HOAsO₃], a new mineral from Tsumeb, South West Africa

Summary The new mineral koritnigite is a hydrated zinc hydrogen arsenate with the formula Zn[H₂O|HOAsO₃]. Chemical analysis (electron microprobe and t.g.a.) gave: As₂O₅ 51.75%, ZnO 35.97%, and H₂O 12.3%, total 100.0%. The HOAsO₃ ions were determined by IR spectroscopy. Koritnigite is soluble in cold diluted HCl and HNO₃. The unit cell dimensions are:a ₀=7.948(2),b ₀=15.829(5),c ₀=6.668(2)Å, =90.86(2), =96.56(2), =90.05(2)^o,V=833.2(4) ų,Z=8. The space group is . The strongest lines of the powder pattern are: 7.90(10) (020, 100), 3.83(7) ( ), 3.16(9) ( ), 2.926(4) (150), 2.679(4) ( ), 2.461(6) ( ), 2.186(5) ( ), 1.969(4)(400), 1.649(3) (004).

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Herrn Univ. Prof. Dr.H. Meixner zum 70. Geburtstag gewidmet.     

Sulfur isotope study of source and deposits of stibnite in the Turhal Area,Turkey

The Turhal antimony sulfide ore deposits are hosted by a Permian-Jurassic sequence which consists of black phyllites at the base followed by interbedded phyllites and calcareous quartzites with metabasite interlayers and then by brown-gray phyllites with marble blocks. Four different styles and three distinct episodes of mineralization were distinguished according to deposition features of the ores and kinkbands in the stibnite crystals. Stibnite from stratiform, disseminated and vein occurrences as well as pyrite from black phyllites showed the following sulfur isotope composition (³⁴S): +2.8 and +3.0 for stratiform stibnite (n = 2), +3.6 and +5.5 for disseminated stibnite (n = 2), +2.5 to +7.8 for vein stibnite (n = 11) and -6.1 to +0.1 for pyrite (n = 3). The ³⁴S compositions of stibnite are interpreted as suggesting an ultimately single source for sulfur in the various styles of mineralization, i.e. synsedimentary volcanic exhalations for the stratiform and disseminated together with ores and hydrothermal mobilisation of these as well as leaching of volcanic rocks to form the vein ores. Deep basinal fluids probably under normal geothermal gradient conditions caused the leaching of the primary sulfides as suggested by the oxygen isotope composition of vein quartz associated with the ores. By contrast sulfur in pyrite is essentially a derivation of seawater sulfate through bacterial and/or chemical reduction.     

Die Strukturen des „Rogenpyrits“ (Kössener Schichten,Rät) als Beitrag zum Problem der „Vererzten Bakterien“

Zusammenfassung Der Rogenpyrit ist die häufigste Form des Pyrits in den Schlämmrückständen der Kössener Mergel des alpinen Rät. Anschliffuntersuchungen ergaben Strukturen, die mit den Vererzten Bakterien eine nahe Verwandtschaft zeigen und mit einem noch höheren Wahrscheinlichkeitsgrad auf organische Abkunft schließen lassen.Der Rogenpyrit tritt in zwei Typen auf, je nach der Größe der Primärkristalle, den kleinsten Bausteinen. Er wird wie die Vererzten Bakterien als pyritisierte farblose Schwefelbakterien bzw. deren Kolonien gedeutet.Der Rogenpyrit ist ein Faziesindikator: Seine Anwesenheit zusammen mit einer benthonischen Mikrofauna in den Kössener Schichten kennzeichnet diese Mergel als Halbfaulschlamm-Fazies.     

Developmental characteristics of aeolian dunes and environmental changes in the adjoining region of Puruogangri ice sheet, North Tibetan Plateau

A large area of moraine sediments and cryogenic weathering products, formed by glacial action and a cold environment, are the main source of aeolian sand in the high and cold region of the Qinghai–Tibetan Plateau in China. The evolution of aeolian dunes is closely related to the periglacial environment. Owing to the freezing of dune-land surfaces, the evolution of sand dunes is dominated by expanding dune bases and vertical accretion, thereby forming large barchan dunes. The migration rates of these large barchan dunes are very slow at an average rate of 1.7–0.7 cm·a^–1. The temperature mainly controls the environmental changes in the adjoining region of Puruogangri ice sheet. The ¹⁴C dating of humus layers in the studied area of the sand dune are 10,780±130, 9,549±130, 8,320±110, 7,450±100, 5,970±95, 5,330±90, 4,420±80, 3,460±80, 2,280±70, 980±70 aBP, respectively. The regions high temperature rising up during summer from the southwest monsoon intensity might be an important factor. As long as both water and temperature conditions are suitable, the plants will grow well, sand dunes will be stabilized, forming humus layers. Otherwise, sand dunes are bare and re-activate.     

Molybdenum mineralization at Alpeiner Scharte, Tyrol (Austria): results of in-situ U–Pb zircon and Re–Os molybdenite dating

Summary Vein-type Mo mineralization at Alpeiner Scharte occurs in the Penninic units of the western Tauern Window in the Eastern Alps. Three types of previously undated metagranitoids (central gneisses) are distinguished and preserve intrusive contacts with pre-Alpine metamorphosed supracrustal rocks. The granitic protoliths represent fractionated late to post-orogenic, calc-alkaline, I-type magmas with minor S-type components. The Mo veins are restricted to a biotite and alkali feldspar-rich gneiss variety and occur in E–W trending normally sub-vertical quartz veins with adjacent thin discontinuous garnet- and biotite-rich zones; the latter are interpreted as metamorphosed vein selvages. Prior to this work the age of the intrusive host rocks as well as the age of Mo mineralization were unknown.The pre-Alpine Mo deposit and its host rocks were affected by four Alpine deformation events (D₁–D₄) and Young-Alpine regional metamorphism. The P-T conditions of this metamorphic event were 550°C and 8kbar and are in agreement with results of previous regional studies.Zircon grains from two orthogneiss samples were dated with the U–Pb method using ion probe techniques. Zircons from the metagranitic host rock of the Mo-veins yielded an emplacement age of 306.8±3.8Ma (2). A second sample from a more leucocratic gneiss lacking Mo-veins gave 305.0±6.6Ma (2). Re–Os dating of molybdenite from the veins yielded an age of 306.8±3.1Ma, in good agreement with the U–Pb zircon ages.This study confirms one of two alternative hypotheses discussed in the literature. It supports the idea that vein-type Mo-mineralization in the western Tauern Window is genetically related to Late Carboniferous (Westphalian) granitoids that were emplaced during the late to post-orogenic stage of the Variscan orogeny. They do not constitute an Alpine metamorphic-hydrothermal deposit. This study further confirms the strength of the Re–Os molybdenite chronometer, in that it was unaffected by subsequent Alpine medium grade regional metamorphism.Present address: Kremstalstraße 32, A-4501 Neuhofen an der Krems, Austria     

Latest precambrian (Cadomian) zircon ages,Nd isotopic systematics and P-T evolution of granitoid orthogneisses of the Erzgebirge,Saxony and Czech Republic

Single zircons from two orthogneiss complexes, the Grey Gneiss and Red Gneiss, the lowermost tectonic units in the Erzgebirge, were dated. The grey Freiberg Gneiss is of igneous origin and has a ²⁰⁷Pb/²⁰⁶Pb emplacement age of 550±7 Ma. A quartz monzonite from Lauenstein contains idiomorphic zircons with a mean ²⁰⁷Pb/²⁰⁶Pb age of 555±7 Ma as well as xenocrysts ranging in age between 850 and 1910 Ma. Red gneisses from the central Erzgebirge contain complex zircon populations, including numerous xenocrysts up to 2464 Ma in age. The youngest, idiomorphic, zircons in all samples yielded uniform ²⁰⁷Pb/²⁰⁶Pb ages between 550±9 and 554±10 Ma. Nd isotopic data support the interpretation of crustal anatexis for the origin of both units. _Nd(t) values for the grey gneisses are –7.5 and –6.0 respectively, (mean crustal residence ages of 1.7–1.8 Ga). The red gneisses have a wider range in _Nd(t) values from –7.7 to –2.8 (T _DM ages of 1.4–1.8 Ga). The zircon ages document a distinct late Proterozoic phase of granitoid magmatism, similar in age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism ( 8 kbar/600–650° C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and red gneisses were overthrust by units with abundant high-pressure relicts and a contrasting P-T evolution. Zircon xenocryst and Nd model ages in the range 1000–1700 Ma are similar to those in granitoid rocks of Lusatia and the West-Sudetes, and document a pre-Cadomian basement in parts of east-central Europe that, chronologically, has similarities with the Sveconorwegian domain in the Baltic Shield.     

Cooling and uplift patterns in the Lepontine Alps South Central Switzerland and an age of vertical movement on the Insubric fault line

96 new fission track (FT) apatite and zircon, K/Ar and Rb/Sr biotite and muscovite ages are presented for 19 samples (mainly acid gneisses) from a 40 km traverse through the Lepontine Alps in the Maggia Valley, South Central Switzerland. Plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample. The entire profile shows a fairly uniform Late Neogene-Recent mean uplift rate of 0.5 mm/a, confirmed by a gradient of FT apatite age with elevation. Cooling from higher temperatures occurred earlier in the south, where uplift rates of 2.2 mm/a in the Steep Belt (root zone) indicate >9 km Early Miocene uplift of the northern Pennine block. This uplift started before 23 Ma and is interpreted as resulting from a major phase of backthrusting along the Insubric Line, and as dating the formation of the mylonite belt. Estimated cooling rates constrain the timing of Lepontine Mid-Tertiary metamorphism: 3 schematic models are proposed which also consider published Rb/Sr, K/Ar mica and hornblende and U/Pb monazite ages. Slow cooling, differential initial heating and subsequent cooling of different parts of the Central Alps and post-38 Ma cooling with syntectonic metamorphism at 27 Ma are postulated as alternative interpretations of isotopic data and geologic evidence. From extrapolation between K/Ar and Rb/Sr mica ages and apatite FT ages, 240±50° C is proposed as the closure temperature for the retention of fission tracks in zircon.     

An experimental investigation of heulandite-laumontite equilibrium at 1000 to 2000 bar P fluid

The univariant reaction governing the upper stability of heulandite (CaAl₂Si₇O₁₈·6H₂O), heulandite=laumontite+3 quartz+2H₂O (1), has been bracketed through reversal experiments at: 155±6° C, 1000 bar; 175±6° C, 1500 bar; and 180±8° C, 2000 bar. Reversals were established by determining the growth of one assemblage at the expense of the other, using both XRD and SEM studies. The standard molal entropy of heulandite is estimated to be 783.7±16 J mol^–1 K^–1 from the experimental brackets. Predicted standard molal Gibbs free energy and enthalpy of formation of heulandite are –9722.3±6.3 kJ mol^–1 and –10524.3±9.6 kJ mol^–1, respectively. The reaction (1), together with the reaction, stilbite=laumontite+3 quartz+3 H₂O, defines an invariant point at which a third reaction, stilbite=heulandite+ H₂O, meets. By combining the present experimental data with past work, this invariant point is located at approximately 600 bar and 140° C. Heulandite, which is stable between the stability fields of stilbite and laumontite, can occur only at pressures higher than that of the invariant point, for = P _total.These results are consistent with natural parageneses in low-grade metamorphic rocks recrystallized in equilibrium with an aqueous phase in which is very close to unity.