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991.
Summary Supercooling of repeated influxes of high-MgO ( 13.5 wt% MgO) tholeiitic magma emplaced along the floor of the Kap Edvard Holm magma chamber led to the formation of fine grained, intraplutonic quench zones that define the bases of 12 macrorhythmic units. The quench zones grade upwards into poikilitic cumulates containing olivine dendrites, suggesting supercooling by 40°C. Supercooling led to early, precumulus crystallization of metastable, non-equilibrium plagioclase, followed by co-precipitation of cumulus plagioclase and olivine. Later nucleation of clinopyroxene followed by rapid growth led to crystallization of zoned oikocrysts during the cumulus stage, followed by postcumulus overgrowth and crystallization of discrete intercumulus grains. At least four generations of plagioclase crystals appear to be preserved.
With 5 Figures 相似文献
Gefüge in einem poikilitischen Olivingabbro-Kumulat: Hinweis auf Unterkühlung
Zusammenfassung Unterkühlung von mehrmals nachströmendem MgO-reichen (ca. 13,5 Gew.% MgO), tholeiitischen Magma, das sich am Boden der Magmenkammer vom Kap Edvard Holm sammelte, führte zur Bildung von feinkbrnigen Zonen mit Abschreckungsgefüge, die jeweils die Basis von 12 makrorhythmischen Einheiten bilden. Diese Zonen gehen im Hangenden in poikilitische Kumulate über, die Olivin-Dendriten enthalten, was auf eine Unterkühlung um ca. 40°C hindeutet. Die Unterkühlung führte zu einer frühen präkumulativen Kristallisation von metastabilem, nicht im Gleichgewicht befindlichem Plagioklas, gefolgt von gleichzeitiger Präzipitation von Kumulus-Plagioklas und -Olivin. Eine spätere Keimbildung von Klinopyroxen, die von raschem Wachstum abgelöst wurde, führte während der Akkumulation zur Kristallisation von zonierten Oikokrysten. Danach folgte eine postkumulative Überwachsung und die Kristallisation von Einzelkörnern in den Zwischenräumen. Zumindest vier Generationen von Plagioklaskristallen scheinen erhalten zu sein.
With 5 Figures 相似文献
992.
P. Černý R. Chapman J. Staně M. Nová H. Baadsgaard M. Rieder M. Kavalová L. Ottolini 《Mineralogy and Petrology》1995,55(1-3):177-201
Summary The chemistry, structural parameters, polytypism, optical properties and Rb-Sr isotopes were examined in 11 to 60 samples of biotite, muscovite and lepidolite from the pegmatites at Roná (the type locality of lepidolite; 323 ± 4Ma) and Dobrá Voda (306 ± 9Ma) in western Moravia. At both localities, early endocontact biotite is followed inwards by muscovite and lepidolite, which is concentrated in and around the core. At Roná, a 1M lepidolite follows after 2M1 muscovite but all later generations of lepidolite are 2M2, close to Tri50 Ply50 and in part associated with muscovite 2M1. At Dobrá Voda, all lepidolite types are 1M and free of muscovite, and the late varieties approximate Tri30 Ply70. At both localities, a trend of increasing HF is indicated during the progress of mica crystallization, culminating in precipitation of topaz. Polytypism of lepidolite is not correlatable with any compositional or growth feature, or their combination. Throughout the mica crystallization, Rb/Cs decreases but K/Rb becomes reversed after an initial decrease. Boron is partitioned preferentially into muscovite (up to 1.10 wt.% B2O3) but Be, Zn, Mn and Sc are enhanced in lepidolite. A slight increase in Fe, Ba and Cl in the last generation of lepidolite might be possibly due to mixing of residual pegmatite fluids with metamorphic pore solutions.
With 9 Figures 相似文献
Geochemische und strukturelle Entwicklung der Glimmer in den Pegmatiten von Roná und Dobrá Voda, Tschechische Republik
Zusammenfassung In 11 bis 60 Proben von Biotit, Muskovit und Lepidolith aus den Pegmatiten von Roná (Typlokalität des Lepidoliths; 323 ± 9 Ma) in Westmähren wurden Chemie, Struktur-parameter, Polytypie, optische Eigenschaften und Rb-Sr-Isotopie untersucht. An beiden Lokalitäten wird früher Biotit an Endokontakten nach Innen von Muskovit und Lepidolith gefolgt, letzterer ist in und um den Kern konzentriert. In Roná folgt 1M-Lepidolith auf 2M1-Muskovit, aber alle späteren Lepidolithgenerationen sind 2M2, nahe Tri50Ply50 und zum Teil mit 2M1-Muskovit vergesellschaftet. In Dobrá Voda sind alle Lepidolithe vom Typ 1M und frei von Muskovit, die späten Varietäten kommen Tri50Ply50 nahe. An beiden Lokalitäten ist während des Fortschreitens der Glimmerkristallisation eine Tendenz von steigendem HF angezeigt, die in der Ausfällung von Topas ihren Höhepunkt findet. Die Polytypie des Lepidoliths kann nicht mit irgendeiner Eigenheit der Zusammensetzung oder des Wachstums korreliert worden, auch nicht mit einer Kombination von diesen. Während der ganzen Glimmerkristallisation nimmt Rb/Cs ab, aber die Tendenz von K/Rb ändert sich nach anfänglichem Abfall. Das Bor verteilt sich bevorzugt auf den Muskovit (bis zu 1.10 Gew. -% B2O3), aber die Be-, Zn-, Mn- und Sc-Gehalte sind im Lepidolith erhöht. Ein leichter Ansteig von Fe, Ba und Cl in der letzten Lepidolithgeneration könnte vielleicht durch eine Mischung von pegmatitischen Restlösungen mit metamorphen Porenlösungen verursacht sein.
With 9 Figures 相似文献
993.
Mafic magma batches at Vesuvius: a glass inclusion approach to the modalities of feeding stratovolcanoes 总被引:1,自引:1,他引:0
P. Marianelli N. Métrich R. Santacroce A. Sbrana 《Contributions to Mineralogy and Petrology》1995,120(2):159-169
Glass inclusions in olivine and diopside phenocrysts from pyroclasts of various eruptions of Vesuvius are representative of the magmas that supplied the volcano in the last 4–5000 years. During this interval the volcano alternated between open conduit activity (e.g. 1944 and 1906 eruptions) with long pauses interupted by Plinian and sub-Plinian eruptions (e.g. 3360 B.P. Avellino, A.D. 79 Pompei, A.D. 472 Pollena). The eruptive behaviour was conditioned in all cases by the presence of shallow reservoirs: two cases are distinguished: (1) small and very shallow, 1906-type; (2) large and deeper Plinian-sub-Plinian magma chamber. Lapilli of 1906 lava fountains contain olivine (Fo89.5–90.4) including Cr-spinel [Cr/(Cr+Al)] (Cr#>75) and volatile-K-rich tephritic glasses, which represent the first recognized Vesuvius primary magmas. Mg-poorer olivine (Fo83–89) also occurs in 1906 and 1944 products; it formed within the shallow reservoir, together with pyroxene and leucite, between 1200 and 1130°C, from K-tephritic melts (MgO=6–8 wt%). The Plinian and sub-Plinian pumices contain diopside, phlogopite and minor olivine (Fo85–87) representing adcumulates wrenched from the chamber walls. Glass inclusions in diopside (and some olivine) range from K-basalt to K-tephrite (MgO=6–8 wt%), with homogenization temperature of 1130–1170°C. They have been regarded as representative of the magmas supplying the Plinian-sub-Plinian chamber(s). The Avellino glass inclusions have K-basaltic compositions, contrasting with the mostly K-tephritic Pompei and Pollena inclusions. They display lower C1 and P contents with respect to the younger tephritic melts, and these variations should reflect primary features of the mantle-derived magmas. The primary and the near-primary Vesuvius magmas, as illustrated by melt inclusions, emphasize high K, P and volatile (H2O, Cl, F, S) contents, with high K2O/H2O (2–2.5), Cl/F (2.5) and Cl/S (2–3) ratios, consistent with a metasomatized mantle source, and distinguishing the Vesuvius potassic primary magmas from those of the northern part of the Roman Province. 相似文献
994.
The Late Cretaceous Impact of the Trindade Mantle Plume: Evidence from Large-volume, Mafic, Potassic Magmatism in SE Brazil 总被引:15,自引:6,他引:9
GIBSON S. A.; THOMPSON R. N.; LEONARDOS O. H.; DICKIN A. P.; MITCHELL J. G. 《Journal of Petrology》1995,36(1):189-229
When the subcontinental lithospheric mantle undergoes heatingand/or extension, some of the earliest mafic melts to be generatedare those rich in volatUes and potassium. In some cases, e.g.when a plume impinges on thick cratonic lithosphere or whenthe amount of extension is very small, K-rich mafic igneousrocks may be the only surface expression of mantle melting.The Alto Paranaiba Igneous Province, in SE Brazil, is one ofthe world's most voluminous mafic potassic provinces (>15000km3),which until recently was relatively unknown. The magmas wereemplaced into a narrow Proterozoic mobile belt close to thesurface margin of the Sao Francisco craton, and it is one ofseveral Cretaceous alkaline igneous provinces that are locatedaround the margin of the Parana sedimentary basin in Braziland Paraguay.Detailed geochemical analyses of samples from throughoutthe Alto Paranaiba Igneous Province show that it is composedof a relatively diverse suite of ultrapotassic-potassic, ultramaficmqfic,silica-undersaturated lavas and hypabyssal intrusions, i.e.kimberlites, madupitic olivine lamproites and kamafugitic rocks.These all have very high concentrations of incompatible traceelements and are all strongly enriched in light rare earth relativeto heavy rare earth elements (e.g. La/Yb=50-230). Wide variationsin major element ratios, which are unrelated to the effectsof crystal fractionation in these magmas (e.g. CaO/Al2O3), suggestthat the mafic potassic rocks were derived from a heterogeneousmantle source. They show relatively restricted ranges of initial87Sr/86Sr (070436-070588) and Nd25 values of -4 to -8, intermediatebetween Group I and II South African kimberlites. TDM Nd isotopemodel ages of 900 Ma suggest that the magmas were derived bythe remobilization of subcontinental lithospheric mantle thathad been enriched by small-volume K-rich melt fractions sincethe Late Proterozoic.New K/Ar ages for mica separates show thatthe kimberlites, madupitic olivine lamproites and kamafugiticrocks were emplaced together with large carbonatite-bearingplutonic complexes at 85 Ma. Reconstructions of plate motionsshow that, at this time, the location of the Alto ParanaibaIgneous Province coincided with the postulated position of thepresent-day Trindade(or Martin Vaz) plume. We propose that thewidespread Late Cretaceous alkaline magmatism in SE Brazil mayhave been caused by impingement of this plume on the base ofthe subcontinental lithosphere. Heat penetrating the lithosphere,both by conduction and advection by asthenospheric-source decompressionmelts, may have caused melting of the readily fusible partsof the lithospheric mantle and the genesis of mafic potassicand (after fractionation) carbonatite magmas. The Proterozoicmobile belt (the Brasilia Belt) appears to have acted as a Hhinspofrelative to the adjacent Sao Francisco craton, allowing greaterupwelling and melting of the asthenosphere. Subsequently, asthe craton passed over the plume, volcanism was switched off'until the Early Tertiary when the plume reemerged from beneaththe westward drifting South America continent and was the magmasource for oceanic-islands and seamounts of the Trindade-Vitriachain.
Corresponding author 相似文献
995.
Melt Extraction during Formation of K-Feldspar + Sillimanite Migmatites, West of Revelstoke, British Columbia 总被引:1,自引:1,他引:0
An exposure of sillimanite-rich, strongly deformed, stromatic,K-feldspar-bearing migmatites in the Monashee Terrane west ofRevelstoke, British Columbia, has been examined to determinethe process of migmatization and to evaluate whether the systemwas open or closed during leucosome formation. An anatecticorigin for the migmatites is supported by: (1) the minimum meltcomposition of the leucosomes; (2) textures suggesting a fluidbehavior of the leucosomes and local pegmatitic textures; and(3) PT estimates (720820C; 759 kbar)above vaporabsent melting conditions of muscovitt + quartz. To establish whether melt was extracted or added during migmatization,measured volume percents of leucosome were compared with estimatesof melt production modeled by muscovite + quartz dehydrationmelting. Quantitative estimates of volume percent of leucosomeat present in the outcrop are between 20 and 30%. The amountof melt produced from the model muscovite dehydration meltingreaction is constrained by measured modal percent of sillimanite(1525%) in the outcrop and is dependent on modal proportionof muscovite in the unmelted protolith and the melt water contentUsing a muscovite-rich protolith and a melt water content of4 wt%, complete dehydration melting of muscovite results ina production of 54 vol % melt and 25 vol % sillmanite, indicatinga melt loss of 29 vol %. A melt water content of 6 wt% resultsin production of 41 vol % melt and 23 vol % sillimanite, indicatinga melt loss of 16 vol %. Melt loss may have occurred by meltmovement along foliation planes during flattening, during formationof shear bands or locally along subvertical fractures. Spatialproximity of the outcrop to the Monashee dcollement suggeststhat thrusting was localized to zones of high melt production,which in turn facilitated melt migration. KEY WORDS: migmatites; British Columbia; Monashee Tarrane; anatexis; melt extraction
*Corresponding author. Present address: Department of Earth and Planetary Sciences, The Univenity of New Mexico, Albuquerque, NM 817131, USA 相似文献
996.
The concentrations of Ni, Cu, Zn, Ga, Ge, and Se in five, fine-grained chondrule rims in the highly unequilibrated CO3 chondrite ALH A77307 (3.0) have been determined for the first time by synchrotron X-ray fluorescence (SXRF) microprobe at Brookhaven National Laboratory. These elements are especially useful for tracing the role of condensation and evaporation processes which occurred at moderate temperatures in the solar nebula. Understanding the distribution of moderately volatile elements between matrix and chondrules is extremely important for evaluating the different models for the volatile depletions in chondritic meteorites. The data show that the trace element chemistry of rims on different chondrules is remarkably similar, consistent with data obtained for the major and minor elements by electron microprobe. These results support the idea that rims are not genetically related to individual chondrules, but all sampled the same reservoir of homogeneously mixed dust. Of the trace elements analyzed, Zn and Ga show depletions relative to CI chondrite values, but in comparison with bulk CO chondrites all the elements are enriched by approximately 1.5 to 3.5 x CO. The abundance patterns for moderately volatile elements in ALH A77307 chondrule rims closely mimic those observed in the bulk chondrite, indicating that matrix is the major reservoir for these elements. The close matching of the patterns for the volatile depleted bulk chondrite and enriched matrix is especially striking for Na, which is anomalously depleted in ALH A77307 in comparison with average CO chondrite abundances. The depletion in Na is probably attributable to the effects of leaching in Antarctica. With the exception of Na, the volatile elements show a relatively smooth decrease in abundance as a function of condensation temperature, indicating that their behavior is largely controlled by their volatility. 相似文献
997.
Improved precision of radiometric dating of ore deposits can provide information about the thermal history of hydrothermal circulation in cooling plutons. In Jales a Hercynian porphyritic two-mica granite and pre-Ordovician mica schists are cut and intensely altered by the Campo gold-quartz vein. The unaltered granite must be younger than 320 ± 6 Ma, and gives mica Rb-Sr ages of 308.5 ± 2.4 (1) Ma (muscovite) and 294.5 ± 1.1 Ma (biotite). Alteration muscovites from the granite give a weighted mean Rb-Sr age of 308.1 ± 1.5 Ma, and a mean 39Ar-40Ar age of 300.7 ± 2.8 Ma. Alteration muscovites from the mica schists give similar 39Ar-40Ar ages, averaging 303.0 ± 2.8 Ma. The results suggest that circulation of the Campo mineralising fluids took place no more than 2–4 Ma after the granite cooled through the muscovite Rb-Sr closure temperature, about 500 °C, and that subsequent cooling to biotite closure at about 300 °C took place at less than 14°C/Ma. The mean cooling rate following emplacement was 15 to 25 °C/Ma. The most detailed comparable published data, for the Cornubian ore field, imply much faster cooling rates. 相似文献
998.
The stable isotopic composition (13C and 18O) and elemental (Sr and Mg) of marine molluscs are presented for Carditacea and Solenacea shells collected off the Mediterranean coast of Egypt. Based on shell microstructures and mineralogy, the bivalve shells are preserved in their original mineralogy and chemistry.The Sr and Mg concentrations of the bivalve shells have mean values of 1960 ppm and 226 ppm respectively. The stable isotopic composition generally show high values of 18O and 13C. The 18O values range from +0.1 to –1.8 PDB and most shells are highly enriched in13C; averaging +2.5 PDB. These elemental and isotopic signatures are analogous to modern marine bivalves from other localities.The oxygen and carbon isotopes, together with the calculated temperatures, suggest that the aragonitic bivalve shells were precipitated in isotopic equilibrium from warm marine waters. 相似文献
999.
High resolution simulations of January and July climate over the western Alpine region with a nested Regional Modeling system 总被引:1,自引:0,他引:1
M. R. Marinucci F. Giorgi M. Beniston M. Wild P. Tschuck A. Ohmura A. Bernasconi 《Theoretical and Applied Climatology》1995,51(3):119-138
Summary High resolution January and July present day climatologies over the central-western Alpine region are simulated with a Regional Climate Model (RegCM) nested within a General Circulation Model (GCM). The RegCM was developed at the National Center for Atmospheric Research (NCAR) and is run at 20 km grid point spacing. The model is driven by output from a present day climate simulation performed with the GCM ECHAM3 of the Max Planck Institute for Meteorology (MPI) at T106 resolution (~ 120 km). Five January and July simulations are conducted with the nested RegCM and the results for surface air temperature and precipitation are compared with a gridded observed dataset and a dataset from 99 observing stations throughout the Swiss territory. The driving ECHAM3 simulation reproduces well the position of the northeastern Atlantic jet, but underestimates the jet intensity over the Mediterranean. Precipitation over the Alpine region in the ECHAM3 simulation is close to observed in January but lower than observed in July. Compared to the driving GCM, the nested RegCM produces more precipitation in both seasons, mostly as a result of the stronger model orographic forcing. Average RegCM temperature over the Swiss region is 2–3 degrees higher than observed, while average precipitation is within 30% of observed values. The spatial distribution of precipitation is in general agreement with available gridded observations and the model reproduces the observed elevation dependency of precipitation in the summer. In the winter the simulated elevation of maximum precipitation amounts is lower than observed. Precipitation frequencies are overestimated, while precipitation intensities show a reasonable agreement with observations, especially in the winter. Sensitivity experiments with different cumulus parameterizations, soil moisture initialization and model topography are discussed. Overall, the model performance at the high resolution used here did not deteriorate compared to previous lower resolution experiments.The National Center for Atmospheric Research is sponsored by the National Science Foundation.With 11 Figures 相似文献
1000.
U Cubasch G C Hegerl A Hellbach H Höck U Mikolajewicz B D Santer R Voss 《Climate Dynamics》1995,11(2):71-84
Due to restrictions in the available computing resources and a lack of suitable observational data, transient climate change experiments with global coupled ocean-atmosphere models have been started from an initial state at equilibrium with the present day forcing. The historical development of greenhouse gas forcing from the onset of industrialization until the present has therefore been neglected. Studies with simplified models have shown that this cold start error leads to a serious underestimation of the anthropogenic global warming. In the present study, a 150-year integration has been carried out with a global coupled ocean-atmosphere model starting from the greenhouse gas concentration observed in 1935, i.e., at an early time of industrialization. The model was forced with observed greenhouse gas concentrations up to 1985, and with the equivalent C02 concentrations stipulated in Scenario A (Business as Usual) of the Intergovernmental Panel on Climate Change from 1985 to 2085. The early starting date alleviates some of the cold start problems. The global mean near surface temperature change in 2085 is about 0.3 K (ca. 10%) higher in the early industrialization experiment than in an integration with the same model and identical Scenario A greenhouse gas forcing, but with a start date in 1985. Comparisons between the experiments with early and late start dates show considerable differences in the amplitude of the regional climate change patterns, particularly for sea level. The early industrialization experiment can be used to obtain a first estimate of the detection time for a greenhouse-gas-induced near-surface temperature signal. Detection time estimates are obtained using globally and zonally averaged data from the experiment and a long control run, as well as principal component time series describing the evolution of the dominant signal and noise modes. The latter approach yields the earliest detection time (in the decade 1990–2000) for the time-evolving near-surface temperature signal. For global-mean temperatures or for temperatures averaged between 45°N and 45°S, the signal detection times are in the decades 2015–2025 and 2005–2015, respectively. The reduction of the cold start error in the early industrialization experiment makes it possible to separate the near-surface temperature signal from the noise about one decade earlier than in the experiment starting in 1985. We stress that these detection times are only valid in the context of the coupled model's internally-generated natural variability, which possibly underestimates low frequency fluctuations and does not incorporate the variance associated with changes in external forcing factors, such as anthropogenic sulfate aerosols, solar variability or volcanic dust. 相似文献