Alkali in phlogopite and amphibole and their effects on phase relations in metasomatized peridotites: a high-pressure study   总被引：2，自引：2，他引：0  

P. Fumagalli  S. Zanchetta  S. Poli 《Contributions to Mineralogy and Petrology》2009,158(6):723-737

Subsolidus phase relations for a K-doped lherzolite are investigated in the model system K₂O–Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O at 1.5–6.0 GPa and 680–1,000°C. Phlogopite is ubiquitous and coexists with Ca-amphibole up to 3.2 GPa and 900°C. High-pressure phlogopites show a peculiar mineral chemistry dependent on pressure: e.g., at 5.5 GPa and 680°C, excess of Si (up to 3.4 apfu) coupled with deficiency in Al (as low as 0.58 apfu) and K + Na (as low as 0.97 apfu), suggest a significant amount of a talc/10 Å phase component ([v]^XIISi₁K_?1Al _?1 ^IV , where [v]^XII is interlayer vacancy). Mixed layering or solid solution relations between high-pressure phlogopites and the 10 Å phase, Mg₃Si₄O₁₀(OH)₂ nH₂O, are envisaged. Phlogopite modal abundance, derived by weighted least squares, is maximum at high-pressure and relative low-temperature conditions and therefore along the slab–mantle interface (10.3 ± 0.7 wt.%, at 4.8 GPa, 680°C). In phlogopite-bearing systems, Ca-amphibole breaks down between 2.5 and 3.0 GPa, and 1,000°C, through the water conservative reaction 5(pa + 0.2 KNa_?1) + 17en + 15phl = (10di + 4jd) + 5py + 12fo + 20(phl + 0.2 talc), governed by bulk composition and pressure-dependent variations of K/OH in K-bearing phases and as a result, it does not necessarily imply a release of fluid.  相似文献   

Strange attractors in magmas: evidence from lava flows     

G. Poli  D. Perugini 《Lithos》2002,65(3-4):287-297

Magma mixing structures from three different lava flows (Salina, Vulcano and Lesbos) are studied in order to assess the possible chaotic origin of magma mixing processes. Structures are analysed using a new technique based on image analysis procedures that extract time series that are representative of the relative change in composition through the structures. These time series are then used to reconstruct the attractors underlying the magma mixing process and to calculate the fractal dimension of the attractors. Results show that attractors exist and possess fractional dimensions. This evidence suggests that the mixing of magmas is a chaotic process governed by a low number of degrees of freedom. In addition, fractal dimension analyses allows us to discriminate between different regimes of mixing in the three lava flows. In particular our analyses suggest that the lava flow of Salina underwent more turbulent mixing than the lava flows of Lesbos and Vulcano.  相似文献   

Volcanology and petrology of volcanic products from the island of Milos and neighbouring islets     

M. Fytikas  F. Innocenti  N. Kolios  P. Manetti  R. Mazzuoli  G. Poli  F. Rita  L. Villari 《Journal of Volcanology and Geothermal Research》1986,28(3-4)

New geochronological and volcanological data from volcanics of the island group of Milos (South Aegean active volcanic arc), allow four main cycles of volcanic activity to be distinguished, ranging in age between 3.5 and 0.1 m.y. B.P. The oldest volcanic activity consists almost totally of pyroclastics and submarine products, followed by subaerial ones. The eruptive centers migrated in time and concentrated, during the most recent volcanic phase, in the central part of Milos and on the island of Antimilos. The oldest products are dominated by andesites and dacites, while the most recent ones mainly consist of rhyolites. The volcanic evolution and the geochemical characters of the erupted products suggest that the feeding system of the oldest volcanism was located in the deep continental crust, where contamination and fractional crystallization jointly occurred. The younger cycle of volcanic activity was fed from small and relatively shallow magma chambers, where contamination processes played a minor role. These magma bodies are considered to responsible for the shallow thermal anomaly giving rise to the high enthalpy field on Milos.  相似文献   

Petrology of the late-Carboniferous Punta Falcone gabbroic complex,nothern Sardinia,Italy     

Simone Tommasini  Giampiero Poli 《Contributions to Mineralogy and Petrology》1992,110(1):16-32

The Punta Falcone gabbroic complex represents an evolved high-alumina basalt which rose from the mantle through the lower crust, and subsequently intruded a granite magma in middle crustal levels, during the calc-alkaline magmatic activity which took place in the Sardinian and Corsican islands in the Carboniferous. The gabbroic complex has a stratified sub-vertical structure, and consists of three zones developing from the bottom to the top of the magma chamber. An interaction zone can be recognized along contacts with the surrounding granite stock, and it is characterized by finer-grained and more evolved rocks than the interior of the gabbroic complex. Processes occurring in its interior zone have been substantially different from those occurring in its marginal interaction zone. Petrographical and geochemical features indicate that the differentiation of the interior of the gabbroic complex can be accounted for by low pressure, closed-system in-situ crystallization. The different gabbroic units represent mixtures between cumulus phases and trapped liquid. Plagioclase + pyroxenes, and successively plagioclase + calcic amphibole + oxides nucleated and grew in-situ on the floor and walls of the chamber. Floating of plagioclase towards the top of the magma chamber resulted in the accumulation of the denser liquid at the bottom. Compaction phenomena and convective fractionation processes permitted the development of the pile of cumulus crystals with their trapped liquid, and the migration of part of this evolved liquid towards the top of the magma chamber. On the basis of major and trace element modelling a mathematical artifice has been developed to evaluate cumulus-intercumulus processes that occurred in the interior of the gabbroic complex. Accordingly, the formation of the different units can be modelled by mixtures between the parental magma and different percentages of minerals formed during the first stages of crystallization. Contemporaneously with the differentiation of the interior zone, the envelope of fine-grained rocks enclosing and grading into the coarser inner part of the gabbroic complex experienced both chemical and physical processes. Chemical processes resulted in the evolution of the marginal interaction zone by crystal fractionation plus contamination by the acid magma. Physical processes were closely related to the thermodynamic instability of this marginal zone, and consisted of mingling and back veining phenomena which developed interdigitations of granite veins along contacts. In addition, an increase of the melt fraction of the granite magma, superheated by the latent heat of crystallization of the mafic magma, caused the occurrence of tilting of the mafic magma chamber, and resulted in the development of the sub-vertical structure of the gabbroic complex.  相似文献   

Major,trace element and Sr isotopic composition of lavas from Vico volcano (Central Italy) and their evolution in an open system     

M. Barbieri  A. Peccerillo  G. Poli  L. Tolomeo 《Contributions to Mineralogy and Petrology》1988,99(4):485-497

Major, trace element and Sr isotopic compositions have been determined on 21 lava samples from Vico volcano, Roman Province, Central Italy. The rocks investigated range from leucite tephritic phonolites to leucite phonolites and trachytes. Trace element compositions are characterized by high enrichments of incompatible elements which display strong variations in rocks with a similar degree of evolution. Well-defined linear trends are observed between pairs of incompatible trace elements such as Th-Ta, Th-La, Th-Hf. A decrease of Large Ion Lithophile (LIL) elements abundance contemporaneously with the formation of a large central caldera is one of the most prominent characteristics of trace element distribution. Sr isotope ratios range from 0.71147 to 0.71037 in the pre-caldera lavas and decreases to values of 0.70974–0.70910 in the lavas erupted after the caldera collapse. Theoretical modelling of geochemical and Sr isotopic variations indicates that, while fractional crystallization was an important evolutionary process, AFC and mixing also played key roles during the evolution of Vico volcano. AFC appears to have dominated during the early stages of the volcanic history when evolved trachytes with the highest Sr isotope ratios were erupted. Mixing processes are particularly evident in volcanites emplaced during the late stages of Vico evolution. According to the model proposed, the evolution of potassic magmas emplaced in a shallow-level reservoir was dominated by crystal fractionation plus wall rock assimilation and mixing with ascending fresh mafic magma. This process generated a range of geochemical and isotopic compositions in the mafic magmas which evolved by both AFC and simple crystal liquid fractionation, producing evolved trachytes and phonolites with variable trace element and Sr isotopic compositions.  相似文献   

Laboratory methods for evaluating migrated high molecular weight hydrocarbons in marine sediments at naturally occurring oil seeps     

Graham A. Logan  Michael A. Abrams  Nicola F. Dahdah  Emmanuelle Grosjean 《Organic Geochemistry》2009,40(3):365-375

A laboratory study has been conducted to determine the best methods for the detection of C₁₀–C₄₀ hydrocarbons at naturally occurring oil seeps in marine sediments. The results indicate that a commercially available method using n-C₆ to extract sediments and gas chromatography–flame ionization detection (GC–FID) to screen the resulting extract is effective at recognizing the presence of migrated hydrocarbons at concentrations from 50 to 5000 ppm. When non-biodegraded, the amount of oil charge is effectively tracked by the sum of n-alkanes in the gas chromatogram. However, once the charge oil becomes biodegraded, with the loss of n-alkanes and isoprenoids, the amount of oil is tracked by the quantification of the unresolved complex mixture (UCM). Gas chromatography–mass spectrometry (GC–MS) was also found to be very effective for the recognition of petroleum related hydrocarbons and results indicate that GC–MS would be a very effective tool for screening samples at concentrations below 50 ppm oil charge.  相似文献   

Contour current imprints and contourite drifts in the Bahamian archipelago     

Thierry Mulder  Emmanuelle Ducassou  Vincent Hanquiez  Mlanie Principaud  Kelly Fauquembergue  Elsa Tournadour  Ludivine Chabaud  John Reijmer  Audrey Recouvreur  Herv Gillet  Jean Borgomano  Anais Schmitt  Paul Moal 《Sedimentology》2019,66(4):1192-1221

New data collected along the slopes of Little and Great Bahama Bank and the abyssal plain of the Bahama Escarpment provides new insights about contour current‐related erosive structures and associated deposits. The Bahamian slope shows abundant evidence of bottom current activity such as furrows, comet‐like structures, sediment waves and drifts. At a seismic scale, large erosion surfaces and main periods of drift growth resulted from current acceleration related to plate tectonic processes and progressive opening and closure of gateways and long‐term palaeoclimate evolution. At present‐day, erosion features and contourite drifts are either related to relatively shallow currents (<1000 m water depth) or to deep currents (>2500 m water depth). It appears that the carbonate nature of the drifts does not impact the drift morphology at the resolution addressed in the present study. Classical drift morphologies defined in siliciclastic environments are found, such as mounded, plastered and separated drifts. In core, contourite sequences show a bi‐gradational trend that resembles classical contourite sequences in siliciclastic deposits showing a direct relationship with a change in current velocity at the sea floor. However, in a carbonate system the peak in grain size is associated with increased winnowing rather than increased sediment supply as in siliciclastic environments. In addition, the carbonate contourite sequence is usually thinner than in siliciclastics because of lower sediment supply rates. Little Bahama Bank and Great Bahama Bank contourites contain open‐ocean input and slope‐derived debris from glacial episodes. Inner platform, platform edge and open ocean pelagic input characterize the classical periplatform ooze during interglacials. In all studied examples, the drift composition depends on the sea floor topography surrounding the drift location and the type of sediment supply. Carbonate particles are derived from either the slope or the platform in slope and toe of slope drifts, very deep contourites have distant siliciclastic sources of sediment supply. The recent discovery of the importance of a large downslope gravitary system along Bahamian slopes suggests frequent interactions between downslope and along‐slope (contour currents) processes. The interlayering of mass flow deposits and contourites at a seismic scale or the presence of surface structures associated with both contour currents and mass flow processes shows that both processes act at the same location. Finally, contour currents have an important impact on the repartition of deep‐water coral mounds. Currents can actively interact with mounds as a nutrient and oxygen supplier or have a passive interaction, with mounds solely being obstacles orienting erosion and deposition.  相似文献   

Formation of magnesium‐smectite during lacustrine carbonates early diagenesis: Study case of the volcanic crater lake Dziani Dzaha (Mayotte – Indian Ocean)     

Vincent P. Milesi  Didier Jzquel  Mathieu Debure  Pierre Cadeau  Franois Guyot  Grard Sarazin  Francis Claret  Emmanuelle Vennin  Carine Chaduteau  Aurlien Virgone  Eric C. Gaucher  Magali Ader 《Sedimentology》2019,66(3):983-1001

The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l^?1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the <2 μm clay fraction indicate the presence of a saponite‐like mineral, a Mg‐rich smectite. The saponite‐like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite‐like mineral instead of Al‐free Mg‐silicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic link may exist between saponite and the development of organic‐rich carbonate rocks, which may be fuelled by the input of CO₂‐rich volcanic gases. This provides novel insights into the composition and formation of saponite‐rich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs.  相似文献   

Eclogite-facies vein systems in the Marun-Keu complex (Polar Urals,Russia): textural,chemical and thermal constraints for patterns of fluid flow in the lower crust   总被引：1，自引：0，他引：1  

José?F.?MolinaEmail author  Stefano?Poli  H?kon?Austrheim  Johannes?Glodny  Anatolij?Rusin 《Contributions to Mineralogy and Petrology》2004,147(4):484-504

Metasomatic amphibole-eclogite sequences grew in selvages of quartz veins from the Marun-Keu complex (Polar Urals, Russia) during high-pressure metamorphism. Relicts of a pre-metasomatic eclogite-facies assemblage are present in the wallrock layers as irregular patches. Wallrock interstitial quartz trails lying at a high angle to reaction fronts provide evidence for grain-scale pore channelisation which may be produced by intergranular-fluid compositional gradients parallel to the quartz trails. Disequilibrium at vein-wallrock scale is inferred from wallrock mineral heterogeneity and from variable initial Sr isotope ratios in mineral separates. Mass-balance calculations between relicts and wallrock assemblages reveal chemical imbalances caused by open system-behaviour with two way mass-transfer. The vein-wallrock system registers a prograde history from 408–434 °C (relicts) to 526–668 °C (vein precipitates). Vein and metasomatic assemblages formed during a single fluid-rock interaction process, implying high heating rates (100 °C/Ma), which could result from heat advection by large-scale fluid circulation.Editorial responsibility: W. Schreyer  相似文献   

The ‘small-world’ nature of fracture/conduit networks: Possible implications for disequilibrium transport of magmas beneath mid-ocean ridges     

L. Valentini  D. Perugini  G. Poli 《Journal of Volcanology and Geothermal Research》2007

In this contribution we show that natural fracture/conduit networks can be studied by using a new method based on Graph Theory. A number of natural networks at different length scales (from the meter to the millimeter) are analysed and results show that they have typical attributes of ‘small-world’ networks, a special class of networks characterized by high global and local transport efficiency. To our knowledge, this topological feature of natural fracture networks is recognized here for the first time. By starting from results on natural fracture/conduit networks, the possible implications are discussed by focusing on disequilibrium transport of magmas in the upper mantle beneath mid-ocean ridges. Results indicate that the ‘small-world’ topology of natural fracture/conduit networks is an important characteristic to ensure disequilibrium delivery of melts through the upper mantle, thus offering a good explanation of geochemical features of magmas. The remarkable point here is that the modelling of melt migration has been constrained by using real fracture network systems. The results presented in this work may contribute to a better understanding of melt migration in fracture network systems and of the way geochemical features of magmas may be influenced by their transport history.  相似文献