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1.
Crystal-Mush Compaction and the Origin of Pegmatitic Segregation Sheets in a Thick Flood-Basalt Flow in the Mesozoic Hartford Basin, Connecticut 总被引：9，自引：4，他引：5
Where the Holyoke flood-basalt flow in the Mesozoic HartfordBasin in Connecticut is thick and contains coarse-grained, horizontalsegregation sheets in its central part, the lower part of theflow is strongly depleted in incompatible elements; where theflow is thin and contains no segregation sheets it is homogeneousthroughout. This chemical variation can be explained only throughcompaction of the partly crystallized basalt. The compositionof the segregation sheets shows that they separated from thebasalt following only 33% crystallization. The segregation sheets,however, are clearly intrusive into the basalt, which must thereforehave already formed a crystal mush with finite strength at thislow degree of crystallinity. The incompatible element concentrationsindicate that the partly crystallized basalt underwent as muchas 28% compaction in the lowest 60 m of the flow. Between 60and 130 m above the base of the flow, the crystal mush becamedilated, and eventually ruptured with formation of the segregationsheets. No segregation sheet has a composition indicating separationafter more than 33% crystallization of the basalt. This is interpretedto indicate that compaction ceased at this stage because ofthe increasing strength of the mush and the increasing densityof the fractionating interstitial liquid KEY WORDS: crystal-mush compaction; segregation shtets; flood basalt; tholeiitie; Connecticut
*e-mail: philpotts{at}geol.uconn.edu 相似文献
2.
A depth migration method is presented that uses Radon-transformed common-source seismograms as input. It is shown that the Radon depth migration method can be extended to spatially varying velocity depth models by using asymptotic ray theory (ART) to construct wavefield continuation operators. These operators downward continue an incident receiver-array plane wave and an assumed point-source wavefield into the subsurface. The migration velocity model is constrained to have longer characteristic wavelengths than the dominant source wavelength such that the ART approximations for the continuation operators are valid. This method is used successfully to migrate two synthetic data examples:
- 1 a point diffractor, and
- 2 a dipping layer and syncline interface model.
3.
Differentiation of Ferro-Basaltic Magmas under Conditions Open and Closed to Oxygen: Implications for the Skaergaard Intrusion and Other Natural Systems 总被引：14，自引：6，他引：8
Because processes such as fractional crystallization and crystallizationunder conditions closed to oxygen are difficult to simulatein the laboratory there is a need for quantitative models ofmagma crystallization behaviour. Comparison of experimentaldata on an iron-rich basaltic composition with predictions ofthe MELTS free energy minimization algorithm shows that althoughliquidus temperatures and silicate mineral equilibria are predictedrelatively well, the saturation of Fe–Ti oxides is notWe have used the same experimental data to construct an alternativecrystallization model based on known equilibrium phase relations,mineral–melt partitioning of major elements, and massbalance constraints. The model is used to explore the consequencesof equilibrium and fractional crystallization in systems openand closed to oxygen. Liquid lines of descent for perfect equilibriumand perfect fractional crystallization are predicted to be verysimilar. In a system open to oxygen the model predicts thatmagnetite saturation leads to strongly decreasing iron and increasingsilica contents of residual liquids, whereas systems closedto oxygen crystallize less abundant magnetite, leading to aless pronounced iron depletion in the liquid. Predicted bulksolid compositions and variations of fo2, with falling temperatureagree well with those observed or inferred from the cumulatesof the Skaergaard intrusion, but none of the predicted liquidlines of descent are consistent with the extreme iron enrichmentproposed for this intrusion based on mass balance calculations.Compositional factors such as water and phosphorus are not thoughtto be the source of the discrepancy as the cumulates of theBasistoppen sill (which closely resemble those of Skaergaard)may be used to calculate a liquid line of descent in agreementwith that predicted by the model for fractional crystallizationclosed to oxygen. A comparison of the predicted T-fo2, pathsand liquid lines of descent with those inferred from naturalsystems suggests that volcanic centres such as Iceland and Hawaiievolve under conditions open to oxygen, whereas evidence fromplutonic environments (e.g. Skaergaard and Kiglapait layeredintrusions) suggests that they evolved under conditions moreclosed to oxygen. The compositional evolution of the melt phasein volcanic and plutonic systems may therefore be different,although the results of this study suggest that magnetite saturationwill limit Fe enrichment in all environments to <20wt% FeO*,consistent with enrichments reported for volcanic glasses. KEY WORDS: Skaergaard; ferro-basalt; iron enrichment; oxygen fugacity
*Bayerisches Geoinstitut, Universitt Bayreuth, D-95440 Bayreuth, Germany. Telephone: $ 49-921-553718. Fax $ 49-921-553769. e-mail: mike.toplis{at}uni-bayreuth.de 相似文献
4.
A. R. HILDEBRAND K. A. CARROLL E. F. TEDESCO D. R. FABER R. D. CARDINAL J. M. MATTHEWS R. KUSCHNIG G. A. H. WALKER B. GLADMAN J. PAZDER P. G. BROWN S. M. LARSON S. P. WORDEN B. J. WALLACE P. W. CHODAS K. MUINONEN A. CHENG 《Earth, Moon, and Planets》2004,95(1-4):33-40
Space-based observatories have several advantages over ground-based observatories in searching for asteroids and comets. In
particular, the Aten and Interior to Earth’s Orbit (IEO) asteroid classes may be efficiently sought at low solar elongations
along the ecliptic plane. A telescope in low Earth orbit has a sufficiently long orbital baseline to determine the parallax
for all Aten and IEO class asteroids discovered with this observing strategy. The Near Earth Object Space Surveillance Satellite
(NEOSSat) mission will launch a microsatellite to exploit this observing strategy complementing ground-based search programmes. 相似文献
5.
6.
Remobilization of Andesite Magma by Intrusion of Mafic Magma at the Soufriere Hills Volcano, Montserrat, West Indies 总被引：17，自引：3，他引：14
MURPHY M. D.; SPARKS R. S. J.; BARCLAY J.; CARROLL M. R.; BREWER T. S. 《Journal of Petrology》2000,41(1):21-42
The 1995–1999 eruption of the Soufriere Hills volcano,Montserrat, has produced a crystal-rich andesite containingquench-textured mafic inclusions, which show evidence of havingbeen molten when incorporated into the host magma. Individualcrystals in the andesite record diverse histories. Amphibolephenocrysts vary from pristine and unaltered to strongly oxidizedand pseudomorphed by anhydrous reaction products. Plagioclasephenocrysts are commonly reverse zoned, often with dusty sievetextures. Reverse zoned rims are also common on orthopyroxenephenocrysts. Pyroxene geothermometry gives an average temperatureof 858 ± 20°C for orthopyroxene phenocryst cores,whereas reverse zoned rims record temperatures from about 880to 1050°C. The heterogeneity in mineral rim compositions,zoning patterns and textures is interpreted as reflecting non-uniformreheating and remobilization of the resident magma body by intrusionof hotter mafic magma. Convective remobilization results inmixing together of phenocrysts that have experienced differentthermal histories, depending on proximity to the intruding maficmagma. The low temperature and high crystallinity are interpretedas reflecting the presence of a cool, highly crystalline magmabody beneath the Soufriere Hills volcano. The petrological observations,in combination with data on seismicity, extrusion rate and SO2fluxes, indicate that the current eruption was triggered byrecent influx of hot mafic magma. KEY WORDS: Montserrat; eruption; magma mixing; mafic inclusion; sieve texture 相似文献
7.
Experimental Constraints on the Conditions of Formation of Highly Calcic Plagioclase Microlites at the Soufrire Hills Volcano, Montserrat 总被引：3，自引：0，他引：3
High-pressure and -temperature experiments on a bulk-rock compositionrepresentative of the groundmass of the Soufrière HillsVolcano andesite have allowed the phase equilibria of the systemto be determined; these are then compared with the natural samples.Experimental conditions varied from 825 to 1100°C and from5 to 225 MPa; the main phases observed were clinopyroxene, crystallinesilica, amphibole and plagioclase. A relationship between plagioclasemicrolite size and anorthite content is identified in samplesof the natural andesite. Large crystals (>60 µm2 inarea) have cores of An60–75, whereas small crystals (<60µm2 in area) have cores of An40–60. Experimentalresults show that if the magma is heated to >950°C thehigh-anorthite microlite crystals can form at magma chamberpressures without any need for a change in bulk composition.It is proposed that convective self-mixing occurs within themagma chamber. Geothermometry of coexisting plagioclase–amphibolepairs confirms the complex crystallization history of the naturalsamples. Analysis of natural glass samples has identified compositionalvariations that can be related to the crystallinity of the sampleand also the groundmass plagioclase composition. Rapidly eruptedpumice samples have high glass contents, lower SiO2 glass compositionsand plagioclase microlites that are large in size (>60 µm2)and have a high anorthite content (>An60). Slowly erupteddome samples are highly crystalline and contain numerous plagioclasemicrolites of variable size and composition. KEY WORDS: glass evolution; experiment; Montserrat; plagioclase; self-mixing 相似文献
8.
The Stability of Igneous Anhydrite: Experimental Results and Implications for Sulfur Behavior in the 1982 El Chichon Trachyandesite and Other Evolved Magmas 总被引：11，自引：1，他引：11
Hydrothermal experiments on natural samples of trachyandesiteand dacite bulk composition show that anhydrite (CaSO4) mayoccur as a stable phenocryst phase at oxygen fugacities greaterthan or equal to 1.0 to 1.5 log fO2 units above the Ni–NiOequilibrium. The dissolved sulfur concentration in anhydritesaturated melts from MnO–Mn3O4 buffered experiments decreaseswith decreasing temperature, from approximately 2300 p.p.m.Sat 1025?C to 250 p.p.m.S at 850?C (all at 2 kb Pfluid = Ptotal).In FeS-saturated melts equilibrated at the Ni–NiO bufferand 2 kb pressure, the concentration of dissolved sulfur alsodecreases with decreasing temperature, varying from approximately400 p.p.m. S at 1025?C to less than 100 p.p.m. S at 850?C. AtNNO or lowerfO2s, decreasing melt FeO content due to crystalfractionation may explain the observed decrease in sulfur solubilitywith decreasing temperature. Sulfur solubility values equivalent to the approximately 0.6wt. per cent S present in fresh bulk pumice samples from the1982 eruptions of El Chichon volcano are not readily achievedunder any reasonable combinations of pressure, temperature,and oxidation state. Dissolved sulfur contents approaching 0.6wt. per cent might occur if the source regions of melts parentalto the El Chichon trachyandesite were at an fO2 several logunits above the Ni–NiO equilibrium. Because such elevatedoxidation states are far greater than the generally acceptedvalues for mantle-derived partial melts we believe the highsulfur content of the El Chichon pumices is not a primary feature;it reflects reaction with sulfur enriched material at some unknowndepth beneath the volcano. Published sulfur isotopic and petrologicdata suggest that hydrothermally altered rocks similar to thepyrite- and anydritebearing lithic fragments found in the 1982pumices could have provided a source of sulfur for crystallizationof magmatic anhydrite. The anhydrite was an important sourceof sulfur for evolution of a sulfur-rich vapor phase duringeruption of the magma. Although many calc-alkaline dacites and rhyolites appear toattain oxidation states high enough to stabilize anhydrite,the characteristically low bulk sulfur contents of these rockswill limit anhydrite abundances to less than approximately 0.1wt. per cent, assuming sufficient sulfur is present to achievesaturation. Such small amounts of a water soluble mineral couldbe easily removed by subaerial weathering processes, dissolvedduring vapor exsolution from a magma, or simply overlooked duringroutine petrographic examination. 相似文献
9.
Experiments at 15 kb in the tonalite-peridotite-H2O system provideinformation on some of the phase equilibrium factors that mayinfluence reaction and assimilation processes between quartznormativemagmas and ultramafic rocks in the deep crust and upper mantle.Experiments were done with 5 or 10 wt.% H2O added to powderednatural samples of tonalite, and mixtures of tonalite with 5or 10 wt.% peridotite added (TP5 and TP10, respectively). Theliquidus phase relations of these starting compositions wereinvestigated between 850 and 1100?C at 15 kb, using gold capsulesso that iron loss to the sample containers was not a problemand meaningful glass and mineral analyses could be obtained.Experiments on the tonalite alone show either liquidus garnet,for samples with 5% H2O added, or liquidus hornblende, for sampleswith 10% H2O. In contrast, orthopyroxene is the sole liquidusphase, irrespective of water content, in experiments using startingmixtures of 5 or 10 wt.% peridotite added to tonalite. Glassanalyses of partially crystallized tonalite define a crystallizationpath diverging significantly from the calc-alkaline trend towardshigher Ca/(Mg + Fe) in the CaO–(MgO + FeO)–?SiO2triangle. In contrast, glasses from partially crystallized mixturesof tonalite with 5 or 10 wt.% peridotite added define a liquidtrend close to natural calc-alkaline compositions in terms ofCa/(Mg + Fe). Of more general significance, the proximity ofa field ofliquidus orthopyroxene on the high (Mg + Fe) sideof compositions along the calc-alkaline trend serves to limitthe Mgenrichment of such melts by interaction with ultramaficrocks. Unless heat is added to the system, reaction of tonaliticcomposition melts with ultramafic rocks will produce only slightlyMg-enriched melts: increasing degree of reaction simply resultsin further precipitation of orthopyroxene + garnet ? clinopyroxeneonce melt compositions reach the orthopyroxene field boundary. 相似文献
10.
DOROTHY CARROLL 《Sedimentology》1963,2(2):149-155
Sediments cored to a depth of about 1 m in Bay St. George, Newfoundland, were examined for grain-size distribution and minerals. The sediments are light brown silty clays, the principal minerals of which are chlorite and muscovite mica. The scarce sand consists of fresh detrital grains of blue-green amphibole, biotite, epidote, zoisite, magnetite, garnet, hypersthene, apatite, chlorite, and scarce zircon. Quartz, plagioclase feldspar, and muscovite are abundant. These minerals are all present in the rocks of the adjacent land areas and have been deposited in the sediments with little alteration or change due to weathering. The clay minerals of the sediments are predominantly muscovite and chlorite with a slight admixture of vermiculite and montmorillonite. There is very little mixed-layering of these minerals. Montmorillonite may be due to diagenetic changes after deposition of these clays in the marine environment of the bay. 相似文献