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1.
An Experimental Study of Water and Carbon Dioxide Solubilities in Mid-Ocean Ridge Basaltic Liquids. Part I: Calibration and Solubility Models 总被引:14,自引:7,他引:7
DIXON JACQUELINE EABY; STOLPER EDWARD M.; HOLLOWAY JOHN R. 《Journal of Petrology》1995,36(6):1607-1631
Experiments were conducted to determine the solubilities ofH2O and CO2 and the nature of their mixing behavior in basalticliquid at pressures and temperature relevant to seqfloor eruption.Mid-ocean ridge basaltic (MORB) liquid was equilibrated at 1200°Cwith pure H2O at pressures of 176717 bar and H2OCO2vapor at pressures up to 980 bar. Concentrations and speciationof H2O and CO2 dissolved in the quenched glasses were measuredusing IR spectroscopy. Molar absorptivities for the 4500 cm1band of hydroxyl groups and the 5200 and 1630 cm1 bandsof molecular water are 067±003, 062±007,and 25±3 l/mol-cm, respectively. These and previouslydetermined molar absorptivities for a range of silicate meltcompositions correlate positively and linearly with the concentrationof tetrahedral cations (Si+Al). The speciation of water in glass quenched from vapor-saturatedbasaltic melt is similar to that determined by Silver &Stolper (Journal of Petrology 30, 667709, 1989) in albiticglass and can be fitted by their regular ternary solution modelusing the coefficients for albitic glasses. Concentrations ofmolecular water measured in the quenched basaltic glasses areproportional to f H2O in all samples regardless of the compositionof the vapor, demonstrating that the activity of molecular waterin basaltic melts follows Henry's law at these pressures. Abest fit to our data and existing higher-pressure water solubilitydata (Khitarov et al., Geochemistry 5, 479492, 1959;Hamilton et al., Journal of Petrology 5, 2139, 1964),assuming Henrian behavior for molecular water and that the dependenceof molecular water content on total water content can be describedby the regular solution model, gives estimates for the Vo, mH2Oof 12±1 cm3/mol and for the 1-bar water solubility of011 wt%. Concentrations of CO2 dissolved as carbonate in the melt forpure CO2-saturated and mixed H2O-CO2-saturated experiments area simple function of fCO2 These results suggest Henrian behaviorfor the activity of carbonate in basaltic melt and do not supportthe widely held view that water significantly enhances the solutionof carbon dioxide in basaltic melts. Using a Vo, mr of 23 cm3/mol(Pan et al., Geochimica et Cosmochimica Acta 55, 15871595,1991), the solubility of carbonate in the melt at 1 bar and1200°C is 05 p.p.m. Our revised determination of CO2solubility is 20% higher than that reported by Stolper &Holloway (Earth and Planetary Science Letters 87, 397408,1988). KEY WORDS: mid-ocean ridge basalts; water and carbon dioxide solubility; experimental petrology 相似文献
2.
Magmatic Evolution of the La Pacana Caldera System, Central Andes, Chile: Compositional Variation of Two Cogenetic, Large-Volume Felsic Ignimbrites 总被引:10,自引:3,他引:10
LINDSAY J. M.; SCHMITT A. K.; TRUMBULL R. B.; DE SILVA S. L.; SIEBEL W.; EMMERMANN R. 《Journal of Petrology》2001,42(3):459-486
La Pacana is one of the largest known calderas on Earth, andis the source of at least two major ignimbrite eruptions witha combined volume of some 2700 km3. These ignimbrites have stronglycontrasting compositions, raising the question of whether theyare genetically related. The Toconao ignimbrite is crystal poor,and contains rhyolitic (7677 wt % SiO2) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (6670 wt % SiO2), dense (4060% vesicularity)and crystal rich (3040 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770790°C with melt water contents of 3·14·4wt %. The pre-eruptive Toconao magma was cooler (730750°C)and its melt more water rich (6·36·8 wt% H2O). A pressure of 200 MPa is inferred from mineral barometryfor the Atana magma chamber. Isotope compositions are variablebut overlapping for both units (87Sr/86Sri 0·70940·7131;143Nd/144Nd 0·512220·51230) and are consistentwith a dominantly crustal origin. Glass analyses from Atanapumices are similar in composition to those in Toconao tubepumices, demonstrating that the Toconao magma could representa differentiated melt of the Atana magma. Fractional crystallizationmodelling suggests that the Toconao magma can be produced by30% crystallization of the observed Atana mineral phases. Toconaomelt characteristics and intensive parameters are consistentwith a volatile oversaturation-driven eruption. However, thelow H2O content, high viscosity and high crystal content ofthe Atana magma imply an external eruption trigger. KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber 相似文献
3.
Calculation of Phase Relations Involving Haplogranitic Melts Using an Internally Consistent Thermodynamic Dataset 总被引:5,自引:4,他引:5
A simple thermodynamic model is developed for silicate meltsin the system CaONa2OK2OAl2O3SiO2H2O(CNKASH). The Holland & Powell (Journal of Metamorphic Geology,16, 289302, 1998) internally consistent thermodynamicdataset is extended via the incorporation of the experimentallydetermined melting relationships in unary and binary subsystemsof CNKASH. The predictive capability of the model is evaluatedvia the experimental data in ternary and quaternary subsystems.The resulting dataset, with the software THERMOCALC, is thenused to calculate melting relationships for haplogranitic compositions.Predictions of the PT stabilities of assemblages in water-saturatedand -undersaturated bulk compositions are illustrated. It isnow possible to make useful calculations of the melting behaviourof appropriate composition rocks under crustal conditions. KEY WORDS: thermodynamics; melts; granite; dataset 相似文献
4.
De Souza Zorano Sergio; Martin Herve; Peucat Jean-Jacques; Jardim De Sa Emanuel Ferraz; Macedo Maria Helena De Freitas 《Journal of Petrology》2007,48(11):2149-2185
The Paleoproterozoic metaplutonic rocks of the CaicóComplex Basement (Seridó region, NE Brazil) provide importantand crucial insights into the petrogenetic processes governingcrustal growth and may potentially be a proxy for understandingthe Archean–Proterozoic transition. These rocks consistof high-K calc-alkaline diorite to granite, with Rb–Sr,U–Pb, Pb–Pb and Sm–Nd ages of c. 2·25–2·15Ga. They are metaluminous, with high YbN, K2O/Na2O and Rb/Sr,low ISr ratios, and are large ion lithophile elements (LILE)enriched. Petrographic and geochemical data demonstrate thatthey belong to differentiated series that evolved by low-pressurefractionation, thus resulting in granodioritic liquids. We proposea model in which the petrogenesis of the Caicó Complexorthogneisses begins with partial melting of a metasomaticallyenriched spinel- to garnet-bearing lherzolite (with high-silicaadakite melt as the metasomatic agent), generating a basic magmathat subsequently evolved at depth through fractional crystallizationof olivine, followed by low-pressure intracrustal fractionation.A subduction zone setting is proposed for this magmatism, toaccount for both negative anomalies in high field strength elements(HFSE) and LILE enrichment. Mantle-derived juvenile magmatismwith the same age is also known in the São Franciscoand West Africa cratons, as well as in French Guyana, and thusthe Archean–Proterozoic transition marks a very importantcontinental accretion event. It also represents a transitionfrom slab-dominated (in the Archean) to wedge-dominated post-Archeanmagmatism. KEY WORDS: calc-alkaline; magmatism; NE Brazil; Paleoproterozoic; petrogenesis 相似文献
5.
The phase relationships of three peralkaline rhyolites fromthe Kenya Rift have been established at 150 and 50 MPa, at oxygenfugacities of NNO - 1·6 and NNO + 3·6 (log fO2relative to the NiNiO solid buffer), between 800 and660°C and for melt H2O contents ranging between saturationand nominally anhydrous. The stability fields of fayalite, sodicamphiboles, chevkinite and fluorite in natural hydrous silicicmagmas are established. Additional phases include quartz, alkalifeldspar, ferrohedenbergite, biotite, aegirine, titanite, montdoriteand oxides. Ferrohedenbergite crystallization is restrictedto the least peralkaline rock, together with fayalite; it isreplaced at low melt water contents by ferrorichterite. Riebeckitearfvedsoniteappears only in the more peralkaline rocks, at temperaturesbelow 750°C (dry) and below 670°C at H2O saturation.Under oxidizing conditions, it breaks down to aegirine. In themore peralkaline rocks, biotite is restricted to temperaturesbelow 700°C and conditions close to H2O saturation. At 50MPa, the tectosilicate liquidus temperatures are raised by 5060°C,and that of amphibole by 30°C. Riebeckitearfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40100°C compared with the solidusof the metaluminous granite system, as a result of the abundanceof F and Cl. Low fO2 lowers solidus temperatures by at least30°C. Comparison with studies of metaluminous and peraluminousfelsic magmas shows that plagioclase crystallization is suppressedas soon as the melt becomes peralkaline, whatever its CaO orvolatile contents. In contrast, at 100 MPa and H2O saturation,the liquidus temperatures of quartz and alkali feldspar arenot significantly affected by changes in rock peralkalinity,showing that the incorporation of water in peralkaline meltsdiminishes the depression of liquidus temperatures in dry peralkalinesilicic melts compared with dry metaluminous or peraluminousvarieties. At 150 MPa, pre-eruptive melt H2O contents rangefrom 4 wt % in the least peralkaline rock to nearly 6 wt % inthe two more peralkaline compositions, in broad agreement withprevious melt inclusion data. The experimental results implymagmatic fO2 at or below the fayalitequartzmagnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H2O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria 相似文献
6.
Sediment Melts at Sub-arc Depths: an Experimental Study 总被引:14,自引:0,他引:14
The phase and melting relations in subducted pelites have beeninvestigated experimentally at conditions relevant for slabsat sub-arc depths (T = 600–1050°C, P = 2·5–4·5GPa). The fluid-present experiments produced a dominant paragenesisconsisting of garnet–phengite–clinopyroxene–coesite–kyanitethat coexists with a fluid phase at run conditions. Garnet containsdetectable amounts of Na2O (up to 0·5 wt%), P2O5 (upto 0·56 wt%), and TiO2 (up to 0·9 wt%) in allexperiments. Phengite is stable up to 1000°C at 4·5GPa and is characterized by high TiO2 contents of up to 2 wt%.The solidus has been determined at 700°C, 2·5 GPaand is situated between 700 and 750°C at 3·5 GPa.At 800°C, 4·5 GPa glass was present in the experiments,indicating that at such conditions a hydrous melt is stable.In contrast, at 700°C, 3·5 and 4·5 GPa, asolute-rich, non-quenchable aqueous fluid was present. Thisindicates that the solidus is steeply sloping in P–T space.Fluid-present (vapour undersaturated) partial melting of thepelites occurs according to a generalized reaction phengite+ omphacite + coesite + fluid = melt + garnet. The H2O contentof the produced melt decreases with increasing temperature.The K2O content of the melt is buffered by phengite and increaseswith increasing temperature from 2·5 to 10 wt%, whereasNa2O decreases from 7 to 2·3 wt%. Hence, the melt compositionschange from trondhjemitic to granitic with increasing temperature.The K2O/H2O increases strongly as a function of temperatureand nature of the fluid phase. It is 0·0004–0·002in the aqueous fluid, and then increases gradually from about0·1 at 750–800°C to about 1 at 1000°C inthe hydrous melt. This provides evidence that hydrous meltsare needed for efficient extraction of K and other large ionlithophile elements from subducted sediments. Primitive subduction-relatedmagmas typically have K2O/H2O of 0·1–0·4,indicating that hydrous melts rather than aqueous fluids areresponsible for large ion lithophile element transfer in subductionzones and that top-slab temperatures at sub-arc depths are likelyto be 700–900°C. KEY WORDS: experimental petrology; pelite; subduction; UHP metamorphism; fluid; LILE 相似文献
7.
Effect of Carbon Dioxide on Dehydration Melting Reactions and Melt Compositions in the Lower Crust and the Origin of Alkaline Rocks 总被引:7,自引:2,他引:7
Dehydration melting experiments of alkali basalt associatedwith the Kenya Rift were performed at 0·7 and 1·0GPa, 850–1100°C, 3–5 wt % H2O, and fO2 nearnickel–nickel oxide. Carbon dioxide [XCO2 = molar CO2/(H2O+ CO2) = 0·2–0·9] was added to experimentsat 1025 and 1050°C. Dehydration melting in the system alkalibasalt–H2O produces quartz- and corundum-normative trachyandesite(6–7·5 wt % total alkalis) at 1000 and 1025°Cby the incongruent melting of amphibole (pargasite–magnesiohastingsite).Dehydration melting in the system alkali basalt–H2O–CO2produces nepheline-normative tephriphonolite, trachyandesite,and trachyte (10·5–12 wt % total alkalis). In thelatter case, the solidus is raised relative to the hydrous system,less melt is produced, and the incongruent melting reactioninvolves kaersutite. The role of carbon dioxide in alkalinemagma genesis is well documented for mantle systems. This studyshows that carbon dioxide is also important to the petrogenesisof alkaline magmas at the lower pressures of crustal systems.Select suites of continental alkaline rocks, including thosecontaining phonolite, may be derived by low-pressure dehydrationmelting of an alkali basalt–carbon dioxide crustal system. KEY WORDS: alkali basalt; alkaline rocks; carbon dioxide; dehydration melting; phonolite 相似文献
8.
Aizawa Yoshitaka; Barnhoorn Auke; Faul Ulrich H.; Fitz Gerald John D.; Jackson Ian; Kovacs IstvAn 《Journal of Petrology》2008,49(4):841-855
As a pilot study of the role of water in the attenuation ofseismic waves in the Earth's upper mantle, we have performeda series of seismic-frequency torsional forced-oscillation experimentson a natural (Anita Bay) dunite containing accessory hydrousphases, at high temperatures to 1300°C and confining pressure(Pc) of 200 MPa, within a gas-medium high-pressure apparatus.Both oven-dried and pre-fired specimens wrapped in Ni–Fefoil within the (poorly) vented assembly were recovered essentiallydry after 50–100 h of annealing at 1300°C followedby slow staged cooling. The results for those specimens indicatebroadly similar absorption-band viscoelastic behaviour, butwith systematic differences in the frequency dependence of strain-energydissipation Q–1, attributed to differences in the smallvolume fraction of silicate melt and its spatial distribution.In contrast, it has been demonstrated that a new assembly involvinga welded Pt capsule retains aqueous fluid during prolonged exposureto high temperatures—allowing the first high-temperaturetorsional forced-oscillation measurements under high aqueousfluid pore pressure Pf. At temperatures >1000°C, a markedreduction in shear modulus, without concomitant increase inQ–1, is attributed to the widespread wetting of grainboundaries resulting from grain-scale hydrofracturing and themaintenance of conditions of low differential pressure Pd =Pc – Pf . Staged cooling from 1000°C is accompaniedby decreasing Pf and progressive restoration of significantlypositive differential pressure resulting in a microstructuralregime in which the fluid on grain boundaries is increasinglyrestricted to arrays of pores. The more pronounced viscoelasticbehaviour observed within this regime for the Pt-encapsulatedspecimen compared with the essentially dry specimens may reflectboth water-enhanced solid-state relaxation and the direct influenceof the fluid phase. The scenario of overpressurized fluids andhydrofracturing in the Pt-encapsulated dunite specimen may havesome relevance to the high Q–1 and low-velocity zonesobserved in subduction-zone environments. The outcomes of thisexploratory study indicate that the presence of water can havea significant effect on the seismic wave attenuation in theupper mantle and provide the foundation for more detailed studieson the role of water. KEY WORDS: seismic wave attenuation; water; dunite; hydrous mineral; shear modulus; viscoelasticity; olivine; grain-scale hydrofracturing 相似文献
9.
Enriched End-member of Primitive MORB Melts: Petrology and Geochemistry of Glasses from Macquarie Island (SW Pacific) 总被引:13,自引:4,他引:9
KAMENETSKY VADIM S.; EVERARD JOHN L.; CRAWFORD ANTHONY J.; VARNE RICK; EGGINS STEPHEN M.; LANYON RUTH 《Journal of Petrology》2000,41(3):411-430
Macquarie Island is an exposure above sea-level of part of thecrest of the Macquarie Ridge. The ridge marks the Australia–Pacificplate boundary south of New Zealand, where the plate boundaryhas evolved progressively since Eocene times from an oceanicspreading system into a system of long transform faults linkedby short spreading segments, and currently into a right-lateralstrike-slip plate boundary. The rocks of Macquarie Island wereformed during spreading at this plate boundary in Miocene times,and include intrusive rocks (mantle and cumulate peridotites,gabbros, sheeted dolerite dyke complexes), volcanic rocks (N-to E-MORB pillow lavas, picrites, breccias, hyaloclastites),and associated sediments. A set of Macquarie Island basalticglasses has been analysed by electron microprobe for major elements,S, Cl and F; by Fourier transform infrared spectroscopy forH2O; by laser ablation–inductively coupled plasma massspectrometry for trace elements; and by secondary ion mass spectrometryfor Sr, Nd and Pb isotopes. An outstanding compositional featureof the data set (47·4–51·1 wt % SiO2, 5·65–8·75wt % MgO) is the broad range of K2O (0·1–1·8wt %) and the strong positive covariation of K2O with otherincompatible minor and trace elements (e.g. TiO2 0·97–2·1%;Na2O 2·4–4·3%; P2O5 0·08–0·7%;H2O 0·25–1·5%; La 4·3–46·6ppm). The extent of enrichment in incompatible elements in glassescorrelates positively with isotopic ratios of Sr (87Sr/86Sr= 0·70255–0·70275) and Pb (206Pb/204Pb =18·951–19·493; 207Pb/204Pb = 15·528–15·589;208Pb/204Pb = 38·523–38·979), and negativelywith Nd (143Nd/144Nd = 0·51310–0·51304).Macquarie Island basaltic glasses are divided into two compositionalgroups according to their mg-number–K2O relationships.Near-primitive basaltic glasses (Group I) have the highest mg-number(63–69), and high Al2O3 and CaO contents at a given K2Ocontent, and carry microphenocrysts of primitive olivine (Fo86–89·5).Their bulk compositions are used to calculate primary melt compositionsin equilibrium with the most magnesian Macquarie Island olivines(Fo90·5). Fractionated, Group II, basaltic glasses aresaturated with olivine + plagioclase ± clinopyroxene,and have lower mg-number (57–67), and relatively low Al2O3and CaO contents. Group I glasses define a seriate variationwithin the compositional spectrum of MORB, and extend the compositionalrange from N-MORB compositions to enriched compositions thatrepresent a new primitive enriched MORB end-member. Comparedwith N-MORB, this new end-member is characterized by relativelylow contents of MgO, FeO, SiO2 and CaO, coupled with high contentsof Al2O3, TiO2, Na2O, P2O5, K2O and incompatible trace elements,and has the most radiogenic Sr and Pb regional isotope composition.These unusual melt compositions could have been generated bylow-degree partial melting of an enriched mantle peridotitesource, and were erupted without significant mixing with commonN-MORB magmas. The mantle in the Macquarie Island region musthave been enriched and heterogeneous on a very fine scale. Wesuggest that the mantle enrichment implicated in this studyis more likely to be a regional signature that is shared bythe Balleny Islands magmatism than directly related to the hypotheticalBalleny plume itself. KEY WORDS: mid-ocean ridge basalts; Macquarie Island; glass; petrology; geochemistry 相似文献
10.
SATO HIROAKI; HOLTZ FRANCOIS; BEHRENS HARALD; BOTCHARNIKOV ROMAN; NAKADA SETSUYA 《Journal of Petrology》2005,46(2):339-354
High-temperaturepressure experiments were carried outto determine the chlorinehydroxyl exchange partitioncoefficient between hornblende and melt in the 1992 Unzen dacite.Cl in hornblende and melt was analyzed by electron microprobe,whereas OH in hornblende and melt was calculated assuming anionstoichiometry of hornblende and utilizing the dissociation reactionconstant for H2O + O = 2(OH) in water-saturated melt, respectively.The partition coefficient strongly depends on the Mg/(Mg + Fe)ratio of hornblende, and is expressed as ln K1 = (Cl/OH)hb/(Cl/OH)melt= 2·37 4·6[Mg/(Mg + Fe)]hb at 23kbar and 800850°C. The twofold variation in Cl contentin the oscillatory zoned cores of hornblende phenocrysts inthe 19911995 dacite cannot be explained by the dependenceof the Cl/OH partition coefficient on the Mg/(Mg + Fe)hb ratio,and requires c. 80% variation of the Cl/OH ratio of the coexistingmelt. Available experimental data at 200 MPa on Cl/OH fractionationbetween fluid and melt suggest that c. 1·21·8wt % degassing of water from the magma can explain the required80% variation in the Cl/OH ratio of the melt. The negative correlationbetween Al content and Mg/(Mg + Fe) ratio in the oscillatoryzoned cores of the hornblende phenocrysts is consistent withrepeated influx and convective degassing of the fluid phasein the magma chamber. KEY WORDS: chlorine; element partitioning; hornblende; oscillatory zoning; Unzen volcano 相似文献
11.
Primitive Magma From the Jericho Pipe, N.W.T., Canada: Constraints on Primary Kimberlite Melt Chemistry 总被引:12,自引:7,他引:5
We report the first estimates of primary kimberlite melt compositionfrom the Slave craton, based on samples of aphanitic kimberlitefrom the Jericho kimberlite pipe, N.W.T., Canada. Three samplesderive from the margins of dykes where kimberlite chilled againstwall rock (JD51, JD69 and JD82) and are shown to be texturallyconsistent with crystallization from a melt. Samples JD69 andJD82 have geochemical characteristics of primitive melts: theyhave high MgO (2025 wt %), high mg-numbers (8688),and high Cr (13001900 ppm) and Ni (8001400 ppm)contents. They also have high contents of CO2 (1017 wt%). Relative to bulk macrocrystal kimberlite, they have lowermg-numbers and lower MgO but are enriched in incompatible elements(e.g. Zr, Nb and Y), because the bulk kimberlite compositionsare strongly controlled by accumulation of mantle olivine andother macrocrysts. The compositions of aphanitic kimberlitefrom Jericho are similar to melts produced experimentally bypartial melting of a carbonate-bearing garnet lherzolite. Onthe basis of these experimental data, we show that the primarymagmas from the Jericho kimberlite could represent 0·70·9%melting of a carbonated lherzolitic mantle source at pressuresand temperatures found in the uppermost asthenosphere to theSlave craton. The measured CO2 contents for samples JD69 andJD82 are only slightly lower than the CO2 contents of the correspondingexperimental melts; this suggests that the earliest hypabyssalphase of the Jericho kimberlite retained most of its originalvolatile content. As such these samples provide a minimum CO2content for the primary kimberlite magmas from the Slave craton. KEY WORDS: kimberlite; melt; primitive; primary magma; Slave craton 相似文献
12.
Biotites as Indicators of Fluorine Fugacities in Late-Stage Magmatic Fluids: the Gardar Province of South Greenland 总被引:5,自引:0,他引:5
The major element chemistry and fluorine contents of biotitesfrom the Gardar alkaline province of South Greenland providean insight into the F contents of late-stage fluids associatedwith the magmatism. Biotites were taken from composite intrusionsranging from alkali gabbro to syenites, nepheline syenites andalkali granites. In each complex they show a large range ofFe/(Fe+Mg) (from 02 to 1) and exhibit strong Fe-F avoidance.There is considerable variation in F for any value of Fe/(Fe+Mg)but for each centre maximum F values plotted against Fe/(Fe+Mg)define a nearly straight line of characteristic slope towardslow or zero F at pure annite. Micas in the SiO2 undersaturatedcentres have higher F contents than those from oversaturatedcentres. Cl is low (069 wt%) except in the Kûngnâtintrusion, where it reaches 14 wt%. Phase equilibriumand textural considerations suggest that most or all biotitesgrew subsolidus in a pervasive deuteric fluid. 18O values suggestthat these fluids were largely magmatic in character and thatextensive reactions with envelope fluids did not occur. Metasomaticresetting of F in biotites in early intrusive units in the aureoleof later units can be demonstrated. Experimental data of Munoz (Mineralogical Society of America,Reviews in Mineralogy 13, 469494, 1984) were used tocalculate families of curves showing variation in F with Fe/(Fe+Mg)for biotites in equilibrium with fluids of fixed fugacity ratio,f(HF)/f(H2O), at fixed P and T. The resulting curves cut sharplyacross the maximum fluorine lines observed in the natural examples.As it seems highly unlikely that changes in fluid compositionand T, acting together, could produce the observed linear relationship,we conclude that the partitioning of F between fluid and micain the plutonic environment is not well modelled by the experiments.Possible explanations are short-range order (SRO) of Fe andMg on octahedral sites in biotite (Mason, Canadian Mineralogist30, 343354, 1992) or effects resulting from differentF speciation in alkaline fluids. If perfect Fe-F avoidance isassumed, Fe-Mg SRO can increase maximum F content. The F levelsseen in the silica-saturated centres are broadly consistentwith a cooling-rate related control. It is possible that thehigher F in biotites in the undersaturated centres reflectsdifferent speciation in the fluid rather than higher F contents,with strongly bound SiF4° complexes more common. KEY WORDS: biotites; Gardar; fluorine; alkaline rocks; fluids 相似文献
13.
An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200 MPa 总被引:10,自引:2,他引:10
Crystallization experiments were performed at 200 MPa in thetemperature range 1150950°C at oxygen fugacitiescorresponding to the quartzfayalitemagnetite (QFM)and MnOMn3O4 buffers to assess the role of water andfO2 on phase relations and differentiation trends in mid-oceanridge basalt (MORB) systems. Starting from a primitive (MgO9·8 wt %) and an evolved MORB (MgO 6·49 wt %),crystallization paths with four different water contents (0·354·7wt % H2O) have been investigated. In primitive MORB, olivineis the liquidus phase followed by plagioclase + clinopyroxene.Amphibole is present only at water-saturated conditions below1000°C, but not all fluid-saturated runs contain amphibole.Magnetite and orthopyroxene are not stable at low fO2 (QFM buffer).Residual liquids obtained at low fO2 show a tholeiitic differentiationtrend. The crystallization of magnetite at high fO2 (MnOMn3O4buffer) results in a decrease of melt FeO*/MgO ratio, causinga calc-alkaline differentiation trend. Because the magnetitecrystallization temperature is nearly independent of the H2Ocontent, in contrast to silicate minerals, the calc-alkalinedifferentiation trend is more pronounced at high water contents.Residual melts at 950°C in a primitive MORB system havecompositions approaching those of oceanic plagiogranites interms of SiO2 and K2O, but have Ca/Na ratios and FeO* contentsthat are too high compared with the natural rocks, implyingthat fractionation processes are necessary to reach typicalcompositions of natural oceanic plagiogranites. KEY WORDS: differentiation; MORB; oxygen fugacity; water activity; oceanic plagiogranite 相似文献
14.
Subduction Influence on Oxygen Fugacity and Trace and Volatile Elements in Basalts Across the Cascade Volcanic Arc 总被引:2,自引:0,他引:2
Fluids or melts derived from a subducting plate are often citedas a mechanism for the oxidation of arc magmas. What remainsunclear is the link between the fluid, oxygen fugacity, andother major and trace components, as well as the spatial distributionof the impact of those fluids. To test the potential effectsof addition of a subduction-derived fluid or melt to the sub-arcmantle, olivine-hosted melt inclusions from primitive basalticlavas sampled from across the central Oregon Cascades (43°–45°N)have been analyzed for major, trace and volatile elements andfO2. Oxygen fugacity was determined in melt inclusions fromsulfur speciation determined by electron microprobe and fromolivine–chromite oxygen geobarometry. The overall rangein fO2 based on sulfur speciation measurements is from <–0·25log units to + 1·9 log units (FMQ, where FMQ is fayalite–magnetite–quartzbuffer). Oxygen fugacity is positively correlated with fluid-mobiletrace element and light rare earth element contents in basaltsgenerated by relatively low-degree partial melting. Establishinga further correlation between fO2 and fluid-mobile trace elementabundances with position along the arc requires the basaltsto be subdivided into shoshonitic, calc-alkaline, low-K tholeiiteand enriched intraplate basalt groups. Melt inclusions fromenriched intraplate and shoshonitic lavas show increasing fO2and trace element abundances closer to the trench, whereas calc-alkalinemelt inclusions exhibit no significant across-arc variations.Low-K tholeiitic melt inclusions record an increase in incompatibletrace elements closer to the trench; however, there is no correlatedincrease in fO2. The correlation observed in enriched intraplateand shoshonitic melt inclusions is interpreted to reflect aprogressively greater proportion of a fluid-rich, oxidized subductioncomponent in magmas generated nearer the subduction zone. Significantly,calc-alkaline melt inclusions with high ratios of large ionlithophile elements to high field strength elements, characteristicof typical arc magmas, have oxidation states indistinguishablefrom low-K tholeiite and enriched intraplate basalt melt inclusions.The lack of across-arc geochemical variation in calc-alkalinemelt inclusions may suggest that these basalts are not necessarilythe most appropriate magmas for examining recent addition ofa subduction component to the sub-arc mantle. Flux and batchmelt model results produce a wide range of predicted amountsof melting and subduction component added to the mantle source;however, general trends characterized by increased melting andproportion of the subduction component from enriched intraplate,to low-K tholeiite, to calc-alkaline are robust. The model resultsdo not require enriched intraplate, low-K tholeiite and calc-alkalinemagmas to be produced from the same more fertile mantle source.However, enriched intraplate magmas, in contrast to calc-alkalineand low-K tholeiite magmas, cannot be generated from a depletedmantle source. Flux or batch melting of either the more fertileor depleted mantle sources used to generate the low-K tholeiite,calc-alkaline, and enriched intraplate magmas cannot reproduceshoshonitic compositions, which require a significantly depletedmantle source strongly metasomatized by a subduction component.The potential mantle source for shoshonitic basalts has a predictedfO2 (after oxidation) from + 0·3 to + 2·4 logunits (FMQ) whereas the mantle source for low-K tholeiite, calc-alkaline,and enriched intraplate magmas may range from –1·1to + 0·7 log units (FMQ). KEY WORDS: basalt; Cascades; melt inclusions; oxidation state; volatiles 相似文献
15.
Melting of Biotite + Plagioclase + Quartz Gneisses: the Role of H2O in the Stability of Amphibole 总被引:4,自引:0,他引:4
Biotite + plagioclase + quartz (BPQ) is a common assemblagein gneisses, metasediments and metamorphosed granitic to granodioriticintrusions. Melting experiments on an assemblage consistingof 24 vol. % quartz, 25 vol. % biotite (XMg = 0·380·40),42 vol. % plagioclase (An2629), 9 vol. % alkali feldsparand minor apatite, titanite and epidote were conducted at 10,15 and 20 kbar between 800 and 900°C under fluid-absentconditions and with small amounts (2 and 4 wt %) of water addedto the system. At 10 kbar when 4 wt % of water was added tothe system the biotite melting reaction occurred below 800°Cand produced garnet + amphibole + melt. At 15 kbar the meltingreaction produced garnet + amphibole + melt with 2 wt % addedwater. At 20 kbar the amphibole occurred only at high temperature(900°C) and with 4 wt % added water. In this last case themelting reaction produced amphibole + clinopyroxene ±garnet + melt. Under fluid-absent conditions the melting reactionproduced garnet + plagioclase II + melt and left behind a plagioclaseI ± quartz residuum, with an increase in the modal amountof garnet with increasing pressure. The results show that itis not possible to generate hornblende in such compositionswithout the addition of at least 24 wt % H2O. This reflectsthe fact that conditions of low aH2O may prevent hornblendefrom being produced with peraluminous granitic liquids fromthe melting of biotite gneiss. Thus growth of hornblende inanatectic BPQ gneisses is an indication of addition of externalH2O-rich fluids during the partial melting event. KEY WORDS: biotite; dehydration; gneisses; hornblende; melt 相似文献
16.
K-feldspar megacrysts (Kfm) are used to investigate the magmaticevolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba,Italy). Dissolution and regrowth of Kfm during magma mixingor mingling events produce indented resorption surfaces associatedwith high Ba contents. Diffusion calculations demonstrate thatKfm chemical zoning is primary. Core-to-rim variations in Ba,Rb, Sr, Li and P support magma mixing (i.e. high Ba and P andlow Rb/Sr at rims), but more complex variations require othermechanisms. In particular, we show that disequilibrium growth(related to variations in diffusion rates in the melt) may haveoccurred as a result of thermal disturbance following influxof mafic magma in the magma chamber. Initial 87Sr/86Sr ratios(ISr) (obtained by microdrilling) decrease from core to rim.Inner core analyses define a mixing trend extending towardsa high ISrRb/Sr melt component, whereas the outer coresand rims display a more restricted range of ISr, but a largerrange of Rb/Sr. Lower ISr at the rim of one megacryst suggestsmixing with high-K calc-alkaline mantle-derived volcanics ofsimilar age on Capraia. Trace element and isotopic profilessuggest (1) early megacryst growth in magmas contaminated bycrust and refreshed by high ISr silicic melts (as seen in theinner cores) and (2) later recharge with mafic magmas (as seenin the outer cores) followed by (3) crystal fractionation, withpossible interaction with hydrothermal fluids (as seen in therim). The model is compatible with the field occurrence of maficenclaves and xenoliths. KEY WORDS: Elba; monzogranite; K-feldspar megacrysts; zoning; magma mixing; trace element; Sr isotopes; petrogenesis 相似文献
17.
High-Mg chloritoid (XMg = 0·40–0·47) andrelatively high-Mg staurolite (XMg = 0·25–0·28)coexisting with kyanite and garnet were identified in a mica–garnet-richrock associated with very high-pressure eclogites in the BugheaComplex of the Leaota Massif (South Carpathians). Major andtrace element geochemical data for both fresh eclogites andassociated rocks which represent a metasomatic or retrogradealteration rind of the eclogites, indicate a pelitic precursor.Magnesian chloritoid was found as inclusions in garnet as partof a chloritoid–kyanite–garnet assemblage whichis indicative of high-pressure conditions. The host garnet showsa typically prograde chemical zoning pattern. The chloritoid-bearingassemblage is confined to the inner part of the garnet porphyroblasts,whereas the matrix assemblage in equilibrium with Mg-rich garnetrims has exceeded the thermal stability limit of chloritoid.Pressure–temperature pseudosections for simplified compositionsapproaching the rock bulk-chemistry show a high-pressure fieldfor the identified chloritoid-bearing assemblage in good agreementwith pressure–temperature estimates in the CFMASH andKCFMASH chemical subsystems using analysed mineral compositions.The derived pressure–temperature path is clockwise, indicatingoverprinting during exhumation from 1·8 GPa and 580°Cto 1·15 GPa and 620°C, at a water activity approachingaH2O = 1. These conditions were attained in a subduction mélangeindicating transient thermal perturbations of a subduction channel. KEY WORDS: high-pressure metapelite; Mg-rich chloritoid; P–T path; P–T pseudosection; very high-pressure eclogite 相似文献
18.
Formation of Diatexite Migmatite and Granite Magma during Anatexis of Semi-pelitic Metasedimentary Rocks: an Example from St. Malo, France 总被引:8,自引:2,他引:8
Petrological and geochemical variations are used to investigatethe formation of granite magma from diatexite migmatites derivedfrom metasedimentary rocks of pelitic to greywacke compositionat St. Malo, France. Anatexis occurred at relatively low temperaturesand pressures (<800°C, 47 kbar), principally throughmuscovite dehydration melting. Biotite remained stable and servesas a tracer for the solid fraction during melt segregation.The degree of partial melting, calculated from modal mineralogyand reaction stoichiometry, was <40 vol. %. There is a continuousvariation in texture, mineralogy and chemical composition inthe diatexite migmatites. Mesocratic diatexite formed when metasedimentaryrocks melted sufficiently to undergo bulk flow or magma flow,but did not experience significant meltresiduum separation.Mesocratic diatexite that underwent melt segregation duringflow generated (1) melanocratic diatexites at the places wherethe melt fraction was removed, leaving behind a biotite andplagioclase residuum (enriched in TiO2, FeOT, MgO, CaO, Sc,Ni, Cr, V, Zr, Hf, Th, U and REE), and (2) a complementary leucocraticdiatexite (enriched in SiO2, K2O and Rb) where the melt fractionaccumulated. Leucocratic diatexite still contained 515vol. % residual biotite (mg-number 4044) and 1020vol. % residual plagioclase (An22). Anatectic granite magmadeveloped from the leucodiatexite, first by further meltresiduumseparation, then through fractional crystallization. Most biotitein the anatectic granite is magmatic (mg-number 1822). KEY WORDS: anatexis; diatexite; granite magma; melt segregation; migmatite 相似文献
19.
Crystallization and Breakdown of Metasomatic Phases in Graphite-bearing Peridotite Xenoliths from Marsabit (Kenya) 总被引:1,自引:0,他引:1
Mantle-derived xenoliths from the Marsabit shield volcano (easternflank of the Kenya rift) include porphyroclastic spinel peridotitescharacterized by variable styles of metasomatism. The petrographyof the xenoliths indicates a transition from primary clinopyroxene-bearingcryptically metasomatized harzburgite (light rare earth element,U, and Th enrichment in clinopyroxene) to modally metasomatizedclinopyroxene-free harzburgite and dunite. The metasomatic phasesinclude amphibole (low-Ti Mg-katophorite), Na-rich phlogopite,apatite, graphite and metasomatic low-Al orthopyroxene. Transitionalsamples show that metasomatism led to replacement of clinopyroxeneby amphibole. In all modally metasomatized xenoliths melt pockets(silicate glass containing silicate and oxide micro-phenocrysts,carbonates and empty vugs) occur in close textural relationshipwith the earlier metasomatic phases. The petrography, majorand trace element data, together with constraints from thermobarometryand fO2 calculations, indicate that the cryptic and modal metasomatismare the result of a single event of interaction between peridotiteand an orthopyroxene-saturated volatile-rich silicate melt.The unusual style of metasomatism (composition of amphibole,presence of graphite, formation of orthopyroxene) reflects lowP –T conditions (850–1000°C at < 1·5GPa) in the wall-rocks during impregnation and locally low oxygenfugacities. The latter allowed the precipitation of graphitefrom CO2. The inferred melt was possibly derived from alkalinebasic melts by melt–rock reaction during the developmentof the Tertiary–Quaternary Kenya rift. Glass-bearing meltpockets formed at the expense of the early phases, mainly throughincongruent melting of amphibole and orthopyroxene, triggeredby infiltration of a CO2-rich fluid and heating related to themagmatic activity that ultimately sampled and transported thexenoliths to the surface. KEY WORDS: graphite; peridotite xenoliths; Kenya Rift; modal metasomatism; silicate glass 相似文献
20.
Experiments defining the distribution of H2O [Dw = wt % H2O(melt)/wt% H2O(crd)]) between granitic melt and coexisting cordieriteover a range of melt H2O contents from saturated (i.e. coexistingcordierite + melt + vapour) to highly undersaturated (cordierite+ melt) have been conducted at 37 kbar and 8001000°C.H2O contents in cordierites and granitic melts were determinedusing secondary ion mass spectrometry (SIMS). For H2O vapour-saturatedconditions Dw ranges from 4·3 to 7 and increases withrising temperature. When the system is volatile undersaturatedDw decreases to minimum values of 2·65·0at moderate to low cordierite H2O contents (0·61·1wt %). At very low aH2O, cordierite contains less than 0·20·3wt % H2O and Dw increases sharply. The Dw results are consistentwith melt H2O solubility models in which aH2O is proportionalto Xw2 (where Xw is the mole fraction of H2O in eight-oxygenunit melt) at Xw 相似文献