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1.
The sulfur content in basaltic melts coexisting with eithersulfide or sulfate melts was determined experimentally. Theexperimental conditions were in the range of 1300–1355°Cand 1·0–1·6 GPa, conditions appropriatefor the melting of the upper mantle above subduction zones.Under these conditions, both sulfide and sulfate were presentas immiscible liquids, as inferred from the round geometriesof the quenched sulfide and sulfate phases. The measured S contentin basaltic melts saturated with sulfate liquids ([S] = 1·5± 0·2 wt %) was 10 times higher than the S contentin basaltic melts saturated with sulfide liquids ([S] = 0·14± 0·02 wt %). In our experiments, sulfate liquidswere stable at fO2 as low as FMQ = +1·85 [FMQ = log (fO2)sample– log (fO2)FMQ, where FMQ is the fayalite–magnetite–quartzoxygen buffer], and evidence from other sources indicates thatsulfates will be stable at lower fO2 in melts with lower activitiesof silica. Because chalcophile and highly siderophile elements,such as Cu, Ni, Au, and Pd, are partitioned preferentially intosulfide phases, melting of sufficiently oxidized sources, inwhich sulfides are not stable, would favor incorporation ofthese elements into the silicate melt produced. Such melts wouldhave a higher potential to generate ore deposits. This studyshows that the high sulfur contents of such oxidized basaltsalso means that relatively small amounts of such magmas canprovide significant amounts of sulfur to exsolving volatilephases and account for the bulk of the sulfur expelled in somevolcanic eruptions, such the 1991 eruption of Mount Pinatubo. KEY WORDS: basalt; mantle; oxidation state; sulfate; sulfur 相似文献
2.
Fe-Ni-Co-O-S Phase Relations in Peridotite-Seawater Interactions 总被引：4，自引：0，他引：4
Serpentinization of abyssal peridotites is known to produceextremely reducing conditions as a result of dihydrogen (H2,aq)release upon oxidation of ferrous iron in primary phases toferric iron in secondary minerals by H2O. We have compiled andevaluated thermodynamic data for Fe–Ni–Co–O–Sphases and computed phase relations in fO2,g–fS2,g andaH2,aq–aH2S,aq diagrams for temperatures between 150 and400°C at 50 MPa. We use the relations and compositions ofFe–Ni–Co–O–S phases to trace changesin oxygen and sulfur fugacities during progressive serpentinizationand steatitization of peridotites from the Mid-Atlantic Ridgein the 15°20'N Fracture Zone area (Ocean Drilling ProgramLeg 209). Petrographic observations suggest a systematic changefrom awaruite–magnetite–pentlandite and heazlewoodite–magnetite–pentlanditeassemblages forming in the early stages of serpentinizationto millerite–pyrite–polydymite-dominated assemblagesin steatized rocks. Awaruite is observed in all brucite-bearingpartly serpentinized rocks. Apparently, buffering of silicaactivities to low values by the presence of brucite facilitatesthe formation of large amounts of hydrogen, which leads to theformation of awaruite. Associated with the prominent desulfurizationof pentlandite, sulfide is removed from the rock during theinitial stage of serpentinization. In contrast, steatitizationindicates increased silica activities and that high-sulfur-fugacitysulfides, such as polydymite and pyrite–vaesite solidsolution, form as the reducing capacity of the peridotite isexhausted and H2 activities drop. Under these conditions, sulfideswill not desulfurize but precipitate and the sulfur contentof the rock increases. The co-evolution of fO2,g–fS2,gin the system follows an isopotential of H2S,aq, indicatingthat H2S in vent fluids is buffered. In contrast, H2 in ventfluids is not buffered by Fe–Ni–Co–O–Sphases, which merely monitor the evolution of H2 activitiesin the fluids in the course of progressive rock alteration.The co-occurrence of pentlandite–awaruite–magnetiteindicates H2,aq activities in the interacting fluids near thestability limit of water. The presence of a hydrogen gas phasewould add to the catalyzing capacity of awaruite and would facilitatethe abiotic formation of organic compounds. KEY WORDS: serpentinization; ODP Expedition 209; sulfide; oxygen fugacity; sulfur fugacity; hydrothermal system; metasomatism; Mid-Atlantic Ridge 相似文献
3.
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 176–717 bar and H2O—CO2vapor 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 cm–1band of hydroxyl groups and the 5200 and 1630 cm–1 bandsof molecular water are 0•67±0•03, 0•62±0•07,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, 667–709, 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, 479–492, 1959;Hamilton et al., Journal of Petrology 5, 21–39, 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 of0•11 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, 1587–1595,1991), the solubility of carbonate in the melt at 1 bar and1200°C is 0•5 p.p.m. Our revised determination of CO2solubility is 20% higher than that reported by Stolper &Holloway (Earth and Planetary Science Letters 87, 397–408,1988). KEY WORDS: mid-ocean ridge basalts; water and carbon dioxide solubility; experimental petrology 相似文献
4.
The Solubility of Sulphur in Hydrous Rhyolitic Melts 总被引：1，自引：1，他引：1
Experiments performed at 2 kbar, in the temperature range 800–1000°C,with fO2 between NNO–2·3 and NNO+2·9 (whereNNO is the nickel–nickel oxide buffer), and varying amountsof sulphur added to hydrous metaluminous rhyolite bulk compositions,were used to constrain the solubility of sulphur in rhyolitemelts. The results show that fS2 exerts a dominant control onthe sulphur solubility in hydrous silicate melts and that, dependingon fO2, a rhyolitic melt can reach sulphur contents close to1000 ppm at high fS2. At fO2 below NNO+1, the addition of ironto a sulphur-bearing rhyolite magma produces massive crystallizationof pyrrhotite and does not enhance the sulphur solubility ofthe melt. For a given fO2, the melt-sulphur-content increaseswith fS2. For fixed fO2 and fS2, temperature exerts a positivecontrol on sulphur solubilities, at least for fO2 below NNO+1.The mole fraction of dissolved sulphur exhibits essentiallylinear dependence on fH2S at low fO2 and, although the experimentalevidence is less clear, on fSO2 at high fO2. The minimum insulphur solubility corresponds to the redox range where bothfH2S and fSO2 are approximately equal. A thermodynamic modelof sulphur solubility in hydrous rhyolite melts is derived assumingthat total dissolved sulphur results from the additive effectsof H2S and SO2 dissolution reactions. The model reproduces wellthe minimum of sulphur solubility at around NNO+1, in additionto the variation of the sulphide to sulphate ratio with fO2.A simple empirical model of sulphur solubility in rhyoliticmelts is derived, and shows good correspondence between modeland observations for high-silica rhyolites. KEY WORDS: sulphur; solubility; rhyolite; thermodynamics; fO2; fS2 相似文献
5.
The equilibrium between chromite and melt has been determinedon four basalts at temperatures of 1200–1400?C over arange of oxygen fugacity (fo2) and pressures of 1 atm and 10kb. The Cr content of chromite-saturated melts at 1300?C and1 atm ranges from 0?05 wt.% Cr2O3 at a log fo2= –3 to1?4 wt.% at a log fo2=–12?8. The Cr2+/Cr3+ of melt increaseswith decreasing fo2 and is estimated by assuming a constantpartitioning of Cr3+ between chromite and melt at constant temperature.The estimated values of Cr2+/Cr3+ in the melt are at fo2 valuesof 4–5 orders of magnitude lower than the equivalent Fe2+/Fe3+values. The Cr/(Cr+Al) of chromite coexisting with melt at constanttemperature changes little with variation of fo2 below log fo2=–6.Five experiments at 10 kb indicate that Cr2O3 dissolved in themelt is slightly higher and the Cr/(Cr + Al) of coexisting chromiteis slightly lower than experiments at 1 atm pressure. Thus variationin total pressure cannot explain the large variations of Cr/(Cr+ Al) that are common to mid-ocean ridge basalt (MORB) chromite. Experiments on a MORB at 1 atm at fo2 values close to fayalite-magnetite-quartz(FMQ) buffer showed that the Al2O3 content of melt is highlysensitive to the crystallization or melting of plagioclase,and consequently coexisting chromite shows a large change inCr/(Cr + Al). It would appear, therefore, that mixing of a MORBmagma containing plagioclase with a hotter MORB magma undersaturatedin plagioclase may give rise to the large range of Cr/(Cr +Al) observed in some MORB chromite. 相似文献
6.
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 Ni–NiO 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. Riebeckite–arfvedsoniteappears 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 50–60°C,and that of amphibole by 30°C. Riebeckite–arfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40–100°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 fayalite–quartz–magnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H2O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria 相似文献
7.
The redox state of sillimanite zone (650–700°C, 5–6kbar) metasediments of the Barrovian type area, Scotland, wasinvestigated using estimates of metamorphic oxygen fugacity(fO2), sulfur fugacity (fS2), and fluid chemistry based on newdeterminations of mineral and rock compositions from 33 samples.A total of 94% of the samples lack graphite, contain both ilmenite–hematitesolid solutions (RHOMOX) and magnetite, and had metamorphicfO2 about 2 log10 units above the quartz–fayalite–magnetite(QFM) buffer. The regional variation in metamorphic fO2 forthese rocks was minimal, about ±0·3 log10 units,reflecting either a protolith that was homogeneous with respectto redox state, or an initially variable protolith whose redoxstate was homogenized by metamorphic fluid–rock interaction.RHOMOX inclusions in garnet porphyroblasts that become richerin ilmenite from the interior to the edge of the host porphyroblastsuggest that at least some syn-metamorphic reduction of rockoccurred. Significant variations in bulk-rock oxidation ratio(OR) that are probably inherited from sedimentary protolithsare found from one layer to the next; OR ranges mostly between 相似文献
8.
KADIK A.; PINEAU F.; LITVIN Y.; JENDRZEJEWSKI N.; MARTINEZ I.; JAVOY M. 《Journal of Petrology》2004,45(7):1297-1310
Studies of iron-bearing silicate melt (ferrobasalt) + iron metallicphase + graphite + hydrogen equilibria show that carbon andhydrogen solubilities in melts are important for the evolutionof the upper mantle. In a series of experiments conducted at3·7 GPa and 1520–1600°C, we have characterizedthe nature (oxidized vs reduced) and quantified the abundancesof C- and H-compounds dissolved in iron-bearing silicate melts.Experiments were carried out in an anvil-with-hole apparatuspermitting the achievement of equal chemical potentials of H2in the inner Pt capsule and outer furnace assembly. The fO2for silicate melt–iron equilibrium was 2·32 ±0·04 log units below iron–wüstite (IW). Theferrobasalt used as starting material experienced a reductionof its iron oxides and silicate network. The counterpart wasa liberation of oxygen reacting with the hydrogen entering thecapsule. The amount of H2O dissolved in the glasses was measuredby ion microprobe and by step-heating and was found to be between1 and 2 wt %. The dissolved carbon content was found to be 1600ppm C by step-heating. The speciation of C and H componentswas determined by IR and Raman spectroscopy. It was establishedthat the main part of the liberated oxygen was used to formOH– and to a much lesser extent H2O, and only traces ofH2, CO2 and 相似文献
9.
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 1150–950°C at oxygen fugacitiescorresponding to the quartz–fayalite–magnetite (QFM)and MnO–Mn3O4 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 (0·35–4·7wt % 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 (MnO–Mn3O4buffer) 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
10.
Experimental studies were carried out to evaluate phase relationsinvolving titanite–F–Al-titanite solid solutionin the system CaSiO3–Al2SiO5–TiO2–CaF2. Theexperiments were conducted at 900–1000°C and 1·1–4·0GPa. The average F/Al ratio in titanite solid solution in theexperimental run products is 1·01 ± 0·06,and XAl ranges from 0·33 ± 0·02 to 0·91± 0·05, consistent with the substitution [TiO2+]–1[AlF2+]1.Analysis of the phase relations indicates that titanite solidsolutions coexisting with rutile are always low in XAl, whereasthe maximum XAl of titanite solid solution occurs with fluoriteand either anorthite or Al2SiO5. Reaction displacement experimentswere performed by adding fluorite to the assemblage anorthite+ rutile = titanite + kyanite. The reaction shifts from 1·60GPa to 1·15 ± 0·05 GPa at 900°C, from1·79 GPa to 1·375 ± 0·025 GPa at1000°C, and from 1·98 GPa to 1·575 ±0·025 GPa at 1100°C. The data show that the activityof CaTiSiO4O is very close to the ideal molecular activity model(XTi) at 1100°C, but shows a negative deviation at 1000°Cand 900°C. The results constrain 相似文献
11.
Al2SiO5 reaction textures in aluminous schist and quartziteof the northern Picuris range, north-central New Mexico, recorda paragenetic sequence of kyanite to sillimanite to andalusite,consistent with a clockwise P–T loop, with minor decompressionnear the Al2SiO5 triple-point. Peak metamorphic temperaturesare estimated at 510–525°C, at 4·0–4·2kbar. Kyanite and fibrolite are strongly deformed; some prismaticsillimanite, and all andalusite are relatively undeformed. Monaziteoccurs as inclusions within kyanite, mats of sillimanite andcentimetre-scale porphyroblasts of andalusite, and is typicallyaligned subparallel to the dominant regional foliation (S0/S1or S2) and extension lineation (L1). Back-scatter electron imagesand X-ray maps of monazite reveal distinct core, intermediateand rim compositional domains. Monazite–xenotime thermometryfrom the intermediate and rim domains yields temperatures of405–470°C (±50°C) and 500–520°C(±50°C), respectively, consistent with the progradeto peak metamorphic growth of monazite. In situ, ion microprobeanalyses from five monazites yield an upper intercept age of1417 ± 9 Ma. Near-concordant to concordant analyses yield207Pb–206Pb ages from 1434 ± 12 Ma (core) to 1390± 20 Ma (rim). We find no evidence of older regionalmetamorphism related to the 1650 Ma Mazatzal Orogeny. KEY WORDS: Al2SiO5; metamorphism; monazite; thermochronometry; triple-point 相似文献
12.
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 (76–77 wt % SiO2) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (66–70 wt % SiO2), dense (40–60% vesicularity)and crystal rich (30–40 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770–790°C with melt water contents of 3·1–4·4wt %. The pre-eruptive Toconao magma was cooler (730–750°C)and its melt more water rich (6·3–6·8 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 0·7094–0·7131;143Nd/144Nd 0·51222–0·51230) 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
13.
Empirical Solution Model for Alkalic to Tholeiitic Basic Magmas 总被引：2，自引：0，他引：2
Currently available models to simulate naturally occurring mineral–meltequilibria use mineral components limited to tholeiitic basaltcompositions and thus they cannot be used for alkali-rick basaltsand basanites. To expand mineral–melt equilibria calculationsto alkali-rich composition space at low pressures, we have derivedequations that describe chemical equilibria between olivine–melt,pyroxene–melt, plagioclase–melt, nepheline–meltand leucite–melt components. Excess free energies of reactionsbetween the end-member mineral and melt components at equilibriumhave been expressed as a function of melt composition, temperatureand fo2. The database used to calculate the mineral–meltexpressions consists of a total of >350 anhydrous experimentsconducted under controlled oxygen fugacity defined by the quartz–fayalite–magnetite(QFM) oxygen buffer. Rocks used in these experiments range frombasanites, nephelinites and alkali olivine basalts, to tholeiiticbasalts and basaltic andesites. Using bulk compositions of startingmaterials both in this experimental database and in others thatwere not incorporated into the regression of modeled parameters,modeled equations successfully predict, at a given temperatureand fo2, compositions of multiply saturated melts as well asthe compositions of coexisting minerals. Standard deviationsof the calculated mole fractions of mineral components () areas follows: anorthite 002; forsterite 002; clinoenstatite002; enstatite 0003; nepheline 002; and leucite 001. Standarddeviations () of the calculated melt compositions in terms ofweight percent of oxides are: SiO2 0•96; Al2O3 132; Fe2O3023; FeO 121; MgO 084; CaO 079; Na2O 058; and K2O 069.All calculations were carried out using a non-linear Newton–Raphsonnumerical procedure. KEY WORDS: mineral–melt equilibria; alkalic–tholeiitic basalts; equilibrium thermodynamics
*Corresponding author 相似文献
14.
Fe–Mg exchange is the most important solid solution involvedin partial melting of spinel lherzolite, and the system CaO–MgO–Al2O3–SiO2–FeO(CMASF) is ideally suited to explore this type of exchange duringmantle melting. Also, if primary mid-ocean ridge basalts arelargely generated in the spinel lherzolite stability field bynear-fractional fusion, then Na and other highly incompatibleelements will early on become depleted in the source, and themelting behaviour of mantle lherzolite should resemble the meltingbehaviour of simplified lherzolite in the CMASF system. We havedetermined the isobarically univariant melting relations ofthe lherzolite phase assemblage in the CMASF system in the 0·7–2·8GPa pressure range. Isobarically, for every 1 wt % increasein the FeO content of the melt in equilibrium with the lherzolitephase assemblage, the equilibrium temperature is lower by about3–5°C. Relative to the solidus of model lherzolitein the CaO–MgO–Al2O3–SiO2 system, melt compositionsin the CMASF system are displaced slightly towards the alkalicside of the basalt tetrahedron. The transition on the solidusfrom spinel to plagioclase lherzolite has a positive Clapeyronslope with the spinel lherzolite assemblage on the high-temperatureside, and has an almost identical position in P–T spaceto the comparable transition in the CaO–MgO–Al2O3–SiO2–Na2O(CMASN) system. When the compositions of all phases are describedmathematically and used to model the generation of primary basalts,temperature and melt composition changes are small as percentmelting increases. More specifically, 10% melting takes placeover 1·5–2°C, melt compositions are relativelyinsensitive to the degree of melting and bulk composition, andequilibrium and near-fractional melting yield similar melt compositions.FeO and MgO are the oxides that exhibit the greatest changein the melt with degree of melting and bulk composition. Theamount of FeO decreases with increasing degree of melting, whereasthe amount of MgO increases. The coefficients for Fe–Mgexchange between the coexisting crystalline phases and melt,KdFe–Mgxl–liq, show a relatively simple and predictablebehaviour with pressure and temperature: the coefficients forolivine and spinel do not show significant dependence on temperature,whereas the coefficients for orthopyroxene and clinopyroxeneincrease with pressure and temperature. When melting of lherzoliteis modeled in the CMASF system, a strong linear correlationis observed between the mg-number of the lherzolite and themg-number of the near-solidus melts. Comparison with meltingin the CMASN system indicates that Na2O has a strong effecton lherzolite melting behaviour only at small degrees of melting. KEY WORDS: CMASF; lherzolite solidus; mantle melting 相似文献
15.
The magmatic processes leading to porphyry-Cu mineralizationat Santa Rita are reconstructed on the basis of petrographicstudies, thermobarometry, and laser-ablation inductively-coupled-plasmamass-spectrometry analyses of silicate melt and sulfide inclusionsfrom dikes ranging from basaltic andesite to rhyodacite. Combinedresults suggest that magma evolution at Santa Rita is similarto that of sulfur-rich volcanoes situated above subduction zones,being characterized by repeated injection of hot, mafic magmainto an anhydrite-bearing magma chamber of rhyodacitic composition.The most mafic end-member identified at Santa Rita is a shoshoniticbasaltic andesite that crystallized at 1000–1050°C,1–3 kbar and log fO2 = NNO + 0·7 to NNO + 1·0,whereas the rhyodacite crystallized at 730–760°C andlog fO2 = NNO + 1·3 to NNO + 1·9. Mixing betweenthe two magmas caused precipitation of 0·1–0·2wt % magmatic sulfides and an associated decrease in the Cucontent of the silicate melt from 300–500 ppm to lessthan 20 ppm. Quantitative modeling suggests that temporal storageof ore-metals in magmatic sulfides does not significantly enhancethe amount of copper ultimately available to ore-forming hydrothermalfluids. Magmatic sulfides are therefore not vital to the formationof porphyry-Cu deposits, unless a mechanism is required thatholds back ore-forming metals until late in the evolution ofthe volcanic–plutonic system. KEY WORDS: porphyry-Cu; sulfur; sulfides; magma mixing; LA-ICP-MS 相似文献
16.
Kinetic Controls on the Partitioning of Trace Elements Between Silicate and Sulfide Liquids 总被引：1，自引：0，他引：1
Mass transfer between sulfide and silicate liquids is oftendiscussed by reference to a mass ratio between hypotheticalliquid reservoirs at equilibrium, defined as R. In an open systemnot at equilibrium the mass of silicate melt from which a growingsulfide droplet appears to have scavenged chalcophile elementswill depend on diffusivities Di and DFeS of the element andof the FeS component in the silicate melt, respectively, onthe degree of supersaturation of the melt with FeS CFeSo, onthe radius a of the droplet and on the velocity v with whichthe droplet is advected through the melt, so that 相似文献
17.
Melting experiments were performed on a komatiitic basalt with17 wt% MgO from Munro Township, Ontario, at I-atm pressure andan oxygen fugacity controlled approximately to the fayalite-magnetite-quartzbuffer. The experiments showed that olivine appears at 1344±5°C,spinel at 1334±6°C plagioclase at 1185±5°C,augite at 1176±5°C and pigeonite at 1154±6°C.Compositionally, olivine varies from Fo90 to Fo74 and displaysan average KFe/MgD (ol/liq) of 0•32. The spinels are chromitesand chromian spinels with Mg/(Mg + Fe2+) ratios between 0•66and 0•;32, which show a marked correlation with meltingtemperature. The pyroxenes show an average KFe/MgD (px/liq)of 0•26, identical for augite and pigeonite. Plagiodaseranges compositionally between An82 and An72 Plotted in thepseudo-quaternary basalt phase diagram, the liquid line of descentis similar to that observed for quartz tholeiitic magmas. Therefore,the low-pressure, late-stage evolution products of komatiiteand basaltic komatiite parental magmas will chemically and mineralogicallybe ferrobasaltic quartz tholeiites. High-temperature and high-pressuremodeling suggests that the main observed compositional variationof Munro komatiites can be explained by low-pressure crystalfractionation and accumulation of olivine into komatiite liquidswith below 21•5–23•5 wt% MgO and eruptive temperaturesbelow 1435–1465°C for oxygen fugacities between thefayalite-magnetite quartz (FMQ) and iron-wiistite (IW) buffers.The maximum magnesium content of liquid komatiites, assumingequilibrium Fo94 olivine, is 27–29 wt% MgO and eruptivetemperatures are between 1515 and 1540°C. KEY WORDS: komatiites; experimental petrology; Munro Township; Ontario 相似文献
18.
Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions 总被引：1，自引：0，他引：1
Many basaltic flood provinces are characterized by the existenceof voluminous amounts of silicic magmas, yet the role of thesilicic component in sulphur emissions associated with trapactivity remains poorly known. We have performed experimentsand theoretical calculations to address this issue. The meltsulphur content and fluid/melt partitioning at saturation witheither sulphide or sulphate or both have been experimentallydetermined in three peralkaline rhyolites, which are a majorcomponent of some flood provinces. Experiments were performedat 150 MPa, 800–900°C, fO2 in the range NNO –2 to NNO + 3 and under water-rich conditions. The sulphur contentis strongly dependent on the peralkalinity of the melt, in additionto fO2, and reaches 1000 ppm at NNO + 1 in the most stronglyperalkaline composition at 800°C. At all values of fO2,peralkaline melts can carry 5–20 times more sulphur thantheir metaluminous equivalents. Mildly peralkaline compositionsshow little variation in fluid/melt sulphur partitioning withchanging fO2 (DS 270). In the most peralkaline melt, DS risessharply at fO2 > NNO + 1 to values of >500. The partitioncoefficient increases steadily for Sbulk between 1 and 6 wt% but remains about constant for Sbulk between 0·5 and1 wt %. At bulk sulphur contents lower than 4 wt %, a temperatureincrease from 800 to 900°C decreases DS by 10%. These results,along with (1) thermodynamic calculations on the behaviour ofsulphur during the crystallization of basalt and partial meltingof the crust and (2) recent experimental constraints on sulphursolubility in metaluminous rhyolites, show that basalt fractionationcan produce rhyolitic magmas having much more sulphur than rhyolitesderived from crustal anatexis. In particular, hot and dry metaluminoussilicic magmas produced by melting of dehydrated lower crustare virtually devoid of sulphur. In contrast, peralkaline rhyolitesformed by crystal fractionation of alkali basalt can concentrateup to 90% of the original sulphur content of the parental magmas,especially when the basalt is CO2-rich. On this basis, we estimatethe amounts of sulphur potentially released to the atmosphereby the silicic component of flood eruptive sequences. The peralkalineEthiopian and Deccan rhyolites could have produced 1017 and1018 g of S, respectively, which are comparable amounts to publishedestimates for the basaltic activity of each province. In contrast,despite similar erupted volumes, the metaluminous Paraná–Etendekasilicic eruptives could have injected only 4·6 x 1015g of S in the atmosphere. Peralkaline flood sequences may thushave greater environmental effects than those of metaluminousaffinity, in agreement with evidence available from mass extinctionsand oceanic anoxic events. KEY WORDS: silicic flood eruptions; sulphur; experiment; Ethiopia; Deccan 相似文献
19.
Boninites are an important ‘end-member’ supra-subductionzone magmatic suite as they have the highest H2O contents andrequire the most refractory of mantle wedge sources. The pressure–temperatureconditions of boninite origins in the mantle wedge are importantto understanding subduction zone initiation and subsequent evolution.Reaction experiments at 1·5 GPa (1350–1530°C),2 GPa (1400–1600°C) and 2·5 GPa (1450–1530°C)between a model primary high-Ca boninite magma composition anda refractory harzburgite under anhydrous and H2O-undersaturatedconditions (2–3 wt % H2O in the melt) have been completed.The boninite composition was modelled on melt inclusions occurringin the most magnesian olivine phenocrysts in high-Ca boninitesfrom the Northern Tongan forearc and the Upper Pillow Lavasof the Troodos ophiolite. Direct melting experiments on a modelrefractory lherzolite and a harzburgite composition at 1·5GPa under anhydrous conditions (1400–1600°C) havealso been completed. Experiments establish a P, T ‘meltinggrid’ for refractory harzburgite at 1·5, 2 and2·5 GPa and in the presence of 2–3 wt % H2O. Theeffect of 2–3 wt % dissolved H2O produces a liquidus depressionin primary boninite of 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 3–7 kbar and 800–1000°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 2·6–5·0at moderate to low cordierite H2O contents (0·6–1·1wt %). At very low aH2O, cordierite contains less than 0·2–0·3wt % 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