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  • 1
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract A wide variety of rock types are present in the O'Leary Peak and Strawberry Crater volcanics of the Pliocene to Recent San Francisco Volcanic Field (SFVF), AZ. The O'Leary Peak flows range from andesite to rhyolite (56–72 wt % SiO2) and the Strawberry Crater flows range from basalt to dacite (49–64 wt % SiO2). Our interpretation of the chemical data is that both magma mixing and crustal melting are important in the genesis of the intermediate composition lavas of both suites. Observed chemical variations in major and trace elements can be modeled as binary mixtures between a crustal melt similar to the O'Leary dome rhyolite and two different mafic end-members. The mafic end-member of the Strawberry suite may be a primary mantle-derived melt. Similar basalts have also been erupted from many other vents in the SFVF. In the O'Leary Peak suite, the mafic end-member is an evolved (low Mg/(Mg+ Fe)) basalt that is chemically distinct from the Strawberry Crater and other vent basalts as it is richer in total Fe, TiO2, Al2O3, MnO, Na2O, K2O, and Zr and poorer in MgO, CaO, P2O5, Ni, Sc, Cr, and V. The derivative basalt probably results from fractional crystallization of the more primitive, vent basalt type of magma. This evolved basalt occurs as xenolithic (but originally magmatic) inclusions in the O'Leary domes and andesite porphyry flow. The most mafic xenolith may represent melt that mixed with the O'Leary dome rhyolite resulting in andesite preserved as other xenoliths, a pyroclastic unit (Qoap), porphyry flow (Qoaf) and dacite (Darton Dome) magmas. Thermal constraints on the capacity of a melt to assimilate (and melt) a volume of solid material require that melt mixing and not assimilation has produced the observed intermediate lavas at both Strawberry Crater and O'Leary Peak. Textures, petrography, and mineral chemistry support the magma mixing model. Some of the inclusions have quenched rims where in contact with the host. The intermediate rocks, including the andesite xenoliths, contain xenocrysts of quartz, olivine and oligoclase, together with reversely zoned plagioclase and pyroxene phenocrysts. The abundance of intermediate volcanic rocks in the SFVF, as observed in detail at O'Leary Peak and Strawberry Crater, is due in part to crustal recycling, the result of basalt-driven crustal melting and the subsequent mixing of the silicic melts with basalts and derivative magmas.
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  • 2
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Contents of major and most trace elements of granitoids in three intrusions associated with the Cretaceous Independence volcanic complex, Montana, correlate well with SiO2. Major-element contents in granitoids in each intrusion are accurately modeled as mixtures of minimum melts and phenocryst assemblages (presumably restite). Restite assemblages are hypersthene+augite+plagioclase, hornblende+plagioclase, and biotite+plagioclase+quartz. Residues of melting are granulite or amphibolite. Melts in two of the bodies were LREE-enriched but unfractionated in MREE and HREE. REE patterns are consistent with residues dominated by pyroxene or amphibole and feldspar. Initial 207Pb/204Pb and 206Pb/204Pb of granitoids define a line interpreted as a secondary isochron established during crustal homogenization 3.3 Ga ago. The relatively low μ of source rocks (8.25) suggests that they did not spend long in U-rich environments. Source regions had variable trace element patterns; Th/Pb and U/Pb were correlated, Rb/Sr and Sm/Nd moderately well correlated, but Rb/Sr and U/Pb were decoupled. This is consistent with poor correlation of Rb, Sr and Ba with SiO2 in some granitoids and may suggest that minor phases that concentrate these elements were inhomogeneously distributed in source regions. The source probably consisted of LREE-rich, Rb-poor metamorphic rocks. Archean amphibolites, exposed in the Beartooth Mountains, are similar to the postulated source materials. They contain plagioclase, hornblende, minor quartz, biotite, and muscovite, and have low Rb/Sr and high LREE/HREE. Certain trace-element characteristics of the granitoids indicate that the deep crust in this part of Montana may be dominated by metamorphosed mafic-intermediate lavas that formed on the sea-floor. Metapelites, intercalated with amphibolites at the surface, were rare in granitoid source regions. This buried supracrustal pile was isotopically homogenized ≈3.3 Ga ago. Although some material melted ≈2.7 Ga ago to form granites that dominate the exposed basement, enough remained fertile that heating by mantle-derived magmas 85–90 Ma ago produced the granitic rocks at Independence.
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  • 3
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The phase relations of Al- and Fe-bearing silicates in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-CO2, in the presence of quartz and magnetite, are discussed on the basis of mineralogic and petrologic data from Precambrian iron-formations and blueschist facies meta-ironstone from the Franciscan Formation, California. These relations allow an estimation of the physiochemical conditions during low-grade metamorphism of iron-formations. Petrologic data together with available experimental and predicted thermodynamic data on the associated minerals place the upper stability limit of stilpnomelane in iron-formations at about 430–470° C and 5–6 kilobars. Fe-end member stilpnomelane can persist to a maximum temperature of 500° C and pressures up to 6–7 kilobars, although it is unlikely to occur in metamorphosed iron-formations. In iron-formation occurrences the stilpnomelane stability field is bordered by four equilibrium reactions with the assemblages stilpnomelane-zussmanite-chlorite-minnesotaite, stilpnomelane-zussmanite-chlorite-grunerite, stilpnomelane-biotite-chlorite-grunerite, and stilpnomelane-biotite-almandine-grunerite. The stability field is reduced by increasing X(CO2) and X Mg Stil , and is also a function of a(K +)/ a(H +) in the metamorphic fluid. If the value of a(K +)/ a(H +) is smaller than that defined by the above assemblages, stilpnomelane decomposes to chlorite, but if larger, it is replaced by biotite. At pressures less than 4 kilobars, the zussmanite field is restricted to a very high value of a(K +)/a(H +) (〉 5.0 in log units at 1.0 kilobar) where iron-formation assemblages are not stable.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Crystal dissolution may include three component processes: interface reaction, diffusion and complications due to convection. We report here a theoretical and experimental study of crystal dissolution in silicate melt without convection. A reaction-diffusion equation is developed and numerically solved. The results show that during non-convective crystal dissolution in silicate melt, the interface melt composition reaches a constant or stationary “saturation” composition in less than a second, hence interface reaction is not the rate-determining step and crystal dissolution in silicate melt is usually diffusion-controlled. Crystal dissolution experiments (designed to suppress convection) show that the concentration profiles of all components propagate into the melt according to the square root of run duration, and that the dissolution distance is also proportional to the square root of run duration. Thus our experiments confirm that the dissolution is diffusion controlled, which is consistent with our numerical calculations. For some principal equilibrium-determining components, concentration profiles conform approximately to the analytical solution of the diffusion equation with a constant effective binary diffusion coefficient. Diffusive dissolution rates (which are inversely proportional to square root of time) can thus be predicted from the phase equilibria and the effective binary diffusion coefficients. To predict steady-state convective dissolution rates, the thickness of the boundary layer must be known. If the convective compositional boundary layer thickness around a dissolving crystal aggregate or near the wall of a magma chamber during convection is about 2 cm or larger, then convective dissolution would rarely result in any significant alteration of original melt. Our dissolution experiments also illustrate the complexity of the diffusion process. Uphill diffusion is common, especially during olivine dissolution into andesitic melt where a majority of the components show the effect of diffusion up their own concentration gradients. Uphill diffusion has implications to the understanding of crystal zoning, and suggests caution is required in applying least squares mass balance analysis to magmatic rocks affected by processes involving diffusion.
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  • 5
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The spatial disposition, compositional zoning profiles, and size distributions of garnet crystals in 11 specimens of pelitic schist from the Picuris Range of New Mexico (USA) demonstrate that the kinetics of intergranular diffusion controlled the nucleation and growth mechanisms of porphyroblasts in these rocks. An ordered disposition of garnet centers and a significant correlation between crystal radius and near-neighbor distances manifest suppressed nucleation of new crystals in diffusionally depleted zones surrounding pre-existing crystals. Compositional zoning profiles require diffusionally controlled growth, the rate of which increases exponentially as temperature increases with time; an acceleration factor for growth rate can be estimated from a comparison of compositional profiles for crystals of different sizes in each specimen. Crystal size distributions are interpreted as the result of nucleation rates that accelerate exponentially with increasing temperature early in the crystallization process, but decline in the later stages because of suppression effects in the vicinity of earlier-formed nuclei. Simulations of porphyroblast crystallization, based upon thermally accelerated diffusionally influenced nucleation kinetics and diffusionally controlled growth kinetics, quantitatively replicate textural relations in the rocks. The simulations employ only two variable parameters, which are evaluated by fitting of crystal size distributions. Both have physical significance. The first is an acceleration factor for nucleation, with a magnitude reflecting the prograde increase during the nucleation interval of the chemical affinity for the reaction in undepleted regions of the rock. The second is a measure of the relative sizes of the porphyroblast and the diffusionally depleted zone surrounding it. Crystal size distributions for the Picuris Range garnets correspond very closely to those in the literature from a variety of other localities for garnet and other minerals. The same kinetic model accounts quantitatively for crystal size distributions of porphyroblastic garnet, phlogopite, sphene, and pyroxene in rocks from both regional and contact metamorphic occurrences. These commonalities indicate that intergranular diffusion may be the dominant kinetic factor in the crystallization of porphyroblasts in a wide variety of metamorphic environments.
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  • 6
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Zusammenfassung An völlig pseudomorphosierten Kristallen von Wolframit von Zinnwald wurde eine. ausgezeichnete orientierte Verwachsung von Scheelit mit dem präxistierenden Wolframit festgestellt. Es fallen zusammen (010)w und (001)s bzw. (100)w und (110),. Ebenso sind die aus Wolframit neuentstehenden Eisenglanzschüppchen orientiert mit diesem verwachsen; (0001) fällt mit (100), (1011) mit (223) usw. zusammen. Die strukturelle Erklärung beider Verwachsungen ist, jedenfalls nach den Dimensionen, so einfach, daß man über die Seltenheit der Verwachsungen erstaunt sein muß. An einem feinkristallinen, aus Scheelit entstandenen Wolframitaggregat von „S. Ajura in Spanien” wurde auch die umgekehrte Verwachsung, Wolframit orientiert zum präexistierenden Scheelit entdeckt. Sie ist außerordentlich auf fallend dadurch, daß in dem feinen Wolframitnetzwerk die tafeligen Kristalle sich unter rechten Winkeln schneiden und sämtlich gemeinsam nach der Spaltbarkeit einspiegeln.
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  • 9
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
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  • 10
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Zusammenfassung Das Periklingesetz verlangt Spiegelsymmetrie in der Zone senkrecht [010]; die kristallographische Symmetrie wiederholt sich in der Lage der Indikatrizen. Die Einmessung zeigt aber, daß (trotz normaler Verwachsung nach dem rhombischen Schnitt) die Symmetrie der verzwillingten Indikatrizen verringert ist: die (theoretische)ZE fungiert nur als Halbierungsebene der korrespondierten Mittellinien, die volle Symmetrie wird nicht verwirklicht. In unserem Falle ist die eingelagerte Zwillingspartie in bezug auf dieZE „korrekter” orientiert als das Hauptindividuum. - Die Konstruktion derZE nachBEREK führt auch bei der verminderten Symmetrie der verzwillingten Indikatrizen zur Auffindung der morphologisch definierten Zwillingsachse. - Man wird eben nicht vergessen dürfen, daß die Zwillingsgesetze Idealformulierungen sind, denen die Wirklichkeit nicht immer „gerecht” wird.
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