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1.
Natural feldspathoidal syenites may be approximated by assemblagescontaining some or all of the phases sodalite, nepheline, oneor two alkali feldspars, and aqueous chloride fluid in the systemNaAISi3O8-KAISi3O8-NaAISiO4-KAISiO4-NaCI-KCI-H2O. The stabilityof sodalite in these assemblages was studied in the range 500–700°C and 600–2000 bars fluid pressure. Sodalite appears to be a stable phase on the vapor-saturatedliquidus in this system over a wide range of pressure. At or near the vapor-saturated liquidus minimum in this system,three distinct types of sodalite-bearing syenite can crystallize.Nepheline-sodalite-one alkali feldspar rocks, nepheline-sodalite-twoalkali feldspars rocks and sodalite-analcime-bearing rocks crystallizebelow 1600 bars, between 1600 and 2750 bars and above 2750 barsfluid pressure, respectively. The effects of closed-system cooling on the assemblage sodalite-nepheline-twoalkali feldspars-aqueous fluid are different and distinguishablefrom the effects of metasomatism. Closed-system cooling resultsin replacement of K-feldspar by albite, feldspathoids remainingnearly unchanged, while metasomatism generally results in sismultaneousenrichment or impoverishment in sodalite plus K-feldspar. 相似文献
2.
Nepheline-alkali feldspar equilibria with alkali chloride aqueoussolutions have been determined for the temperature range 400to 700 °C at 1000 bars pressure. Nepheline-alkali feldsparequilibria with alkali chloride melts have been determined forthe temperature range 800 to 1100 °C at approximately 6bars pressure. (1) NaAlSiO4 + KCl(aq) = NaCl(aq) + KAlSiO4 (2) NaAlSiO4 + KCl(melt) = NaCl(melt) + KAlSiO4 (3) NaAlSi3O8(high) + KCl(aq) = NaCl(aq) + KAlSi3O8(San) (4) NaAlSi3O8(low) + KCl(aq) = NaCl(aq) + KAlSi3O8(Mic) (5) NaAlSi3O8(high) + KCl(melt) = NaCl(melt) + KAlSi3O4(San) (6) NaAlSi3O8(low) + KCl(melt) = NaCl(melt) + KAlSi3O8(Mic) From these, two diagrams of phase relationships were derivedfor the following exchange equilibria: (7) NaAlSiO4 + KAlSi3O8(San) = NaAlSi3O8(high) + KAlSiO4; (8) NaAlSiO4 + KAlSi3O8(Mic) = NaAlSi3O8(low) + KAlSiO4. The effect of pressure on these equilibria has been determinedby comparing the experimental data for 1000 and 5000 bars (t= 500 °C) and thermodynamic calculations. It has also beenshown that the effect of excess silica in nepheline solid solutionon the K—Na distribution between nepheline and alkalifeldspar is substantial and opposite to that of temperature.In the high temperature region an increase in silica contentin nepheline of 2 wt. per cent eliminates the effect on theredistribution of a temperature increase of 100 °C. Thesecation exchange data and unit cell data for the crystal phasesare used to calculate thermodynamic mixing properties of nephelinesolid solution and alkali feldspar solid solution for a widerange of temperature and pressure. 相似文献
3.
The system MgO-CO2-H2O has been studied up to 1,400? C and 4,000bars pressure using the sealed-capsule quenching technique.No melting was observed. At 1,000 bars pressure magnesite dissociatesat 780? C, and brucite at 635? C, to periclase and vapor. Theunivariant reaction MgCO3?Mg(OH)2 MgO + V proceeds at 630?C, at 1,000 bars and at 700? C, at 4,000 bars. Solubility measurementsshow that, at 1,000 bars and temperatures up to 1,000? C, lessthan 1.5 weight per cent MgO is dissolved in the vapor phase.Brucite is unstable in the presence of vapors containing morethan a small amount of CO2. The maximum percentage of CO2 ina vapor that can coexist with brucite increases with decreasingpressure and with increasing temperature: 6 weight per centCO2 is the maximum at 630? C, 1,000 bars, and 4 weight per centat 700? C, 4,000 bars. The phase relations in the isobaric TXprism for 1,000 bars pressure are described. The results illustratetwo dissociation reactions, decarbonation and dehydration, occurringin the presence of a vapor phase containing two volatile components,H2O and CO2. Applications to metamorphism are briefly discussed. 相似文献
4.
The variation of the birefringence with temperature, from roomtemperature up to c. 900? C, was studied of 34 nephelines fromdifferent geological environments. The measurements were madewith a Berek compensator using a microscope heating stage. Two optical types of nepheline were distinguished on the basisof the absolute value for birefringence at room temperatureand on the basis of the optical -value defined as >=(w-E)900?-(w-E)20?.The optical type I is characterized by a high birefringenceat room temperature and by a highly negative optical -value,whereas the optical type II shows a low birefringence at roomtemperature and a highly positive optical -value. The nephelinespecimens studied represent optically a continuous gradationfrom type I to type IL Both optical types are represented amongmediopotassic and perpotassic nephelines. A nepheline of the optical type I is considered to representthe ordered form of the mineral and a nepheline of the opticaltype II the disordered form. All plutonic nephelines studiedbelong to the optical type I whereas the volcanic nephelinesrange from the type I to the type II. Using a high temperature attachment for the Norelco diffractometer,the thermal expansion was measured of a few nepheline specimens.A nepheline belonging to the optical type I differs slightlyin thermal expansion from that of the optical type 11. Thisdifference in thermal expansion may qualitatively explain thedifference between the two optical types. The crystal structural nature of the supposed order-disordertransition in nepheline is not known. The analogy with the feldsparswould suggest a varying degree of ordering in the distributionof the silicon and aluminium cations in the oxygen tetrahedra. 相似文献
5.
Genesis of Peraluminous Granites I. Experimental Investigation of Melt Compositions at 3 and 5 kb and Various H2O Activities 总被引：1，自引：0，他引：1
Melting experiments have been performed on a peraluminous quartzo-feldspathicgneiss from Northern Portugal. This gneiss occurs as xenolithsin the Tourem anatectic complex and is the most probable sourcerock for the surrounding anatectic granites. On the basis offield, petrological, and geochemical data, it can be shown thatanatexis took place in the stability field of cordierite + biotiteand that the evolution of magmas is the result of processesinvolving segregation of partial melt and separation of restiteminerals. Experiments were performed at 700, 750, and 800 ?C, 3 and 5kb, and various H2O activities (aH2O) to clarify the influenceof aH2O and melt fraction on the composition of the generatedmelts. Biotite and cordierite are the two main ferromagnesianphases observed in the run products. Cordierite is formed byincongment melting of biotite. For relatively low melt fractions (< 30–35 wt. %),the partial melts coexisting with quartz, alkali feldspar, andplagioclase have a minimum or near-minimum melt composition.The melts become richer in potassium with decreasing aH2O. Thisresult using a natural rock as starting material is in goodagreement with results achieved in the synthetic Qz-Ab-Or system.In the stability field of biotite and cordierite, the influenceof aH2O on melt composition is at least as important as theeffect of changing P and/or T. For higher melt fractions the composition of the melt is stronglycontrolled by the disappearance of alkali feldspar and the meltsbecome richer in An and poorer in Or with increasing degreeof melting. The wide range of compositions (especially for K2O, which variesfrom 3.5 to 5.4%) observed in the experimental peraluminousmelts demonstrates that a wide variety of granitoid magmas maybe produced from the same source rocks. The application of theexperimental results to the Tourem anatectic complex shows thatmelting occurred at H2O-undersaturated conditions (4–5wt. % H2O in the melts, corresponding to aH2O of {small tilde}0.5at 5 kb). Experimental melts similar in composition to the mostleucocratic granite of the Tourem anatectic complex (consideredto approximate the composition of the generated melt) were obtainedaround 800 ?C, suggesting that this temperature was attainedduring the peak of anatexis. 相似文献
6.
Single-phase 2M1 muscovite-paragonite crystalline solutionsin the range 0?00–0?10 and 0?70–1?00 Xms have beensynthesized by hydrothermal treatment of gels of appropriatecompositions at 600–700?C, and 7 to 18 kb PH2O. The molarvolumes of these micas may be expressed as V(J/b?mol) = 13?1845+1?463Xms+0?0160 Xms2–0?1679 Xms3 (?0?005), which translateto a substantial positive excess molar volume of mixing. Na-K ion exchange experiments between presynthesized 2M1 micacrystalline solutions and 2 molal aqueous (Na,K)Cl fluids failedto proceed to completion despite 98 day runs at 500–600?C,6 kb Ptotal. Results of analogous exchange experiments provedencouraging however, when a much finer-grained 1M mica was usedas starting material. Applying the tie line rotation technique,reversal of ion exchange experiments could be achieved in the2-phase fields, not, however, in the 3-phase field of the ms-pg-NaCl-KClreciprocal ternary. Using gels as starting material, reversalexperiments were eventually successful both in the 2-phase andthe 3-phase fields; the results of reversal experiments withinthe two-phase fields being identical to those obtained earlierusing 1M micas. Four isobaric-isothermal sections through the ms-pg-NaCl-KClternary were reversibly determined at 450?C/5 kb, 550?C/6 kb,550?C/15 kb, and 620?C/7 kb. At 450?C, the coexisting mica compositionsin the 3-phase field (2 micas plus 1 fluid) are 0?10 and 0?77Xms, at 550?C they are 0?10 and 0?60 Xms, and finally, at 620?Cthese are 0?12 and 0?51 Xms. To the extent that internal equilibriumwas accomplished between the coexisting micas, these data wouldindicate a wide solvus at 450?C, narrowing gradually with increasingtemperature to 620?C. The critical temperature will be wellin excess of 620?C, although the mica at the critical conditionwill prove to be metastable with respect to the assemblage alkalifeldspars+corundum+H2O. The companion paper by Chatterjee & Flux (1986) presentsa thermodynamic analysis of the above experimental data. 相似文献
7.
The results of synthesis experiments in the system KAlSiO4—Mg2SiO4—SiO2—SiO2H2Ohave been used to outline the melting and sub-solidus phaserelations at temperatures from 700 to 1200 C and pressuresto 3 kilobars. Studies in this system provide a framework withinwhich petrologic features of the near-surface potassic rocks,some lamprophyres, charnockitic granites, kimberlites, and alliedmica peridotites may be discussed. On the basis of the experimentalstudies the pressure-temperature stability limits of coexistingphases are considered. The bivariant phase relations providea means by which the olivine biotite and pyroxene biotitereaction relations observed in potassic rocks may be accountedfor. The phase relations provide a mechanism for crossing the‘equilibrium thermal divides’ forsterite-potashfeldspar and enstatite-potash feldspar, from the silica-undersaturatedto the silica-oversaturated region. The petrologic importanceof water-undersaturatsed magmas is stressed throughout the discussion. 相似文献
8.
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
9.
The early augite syenite unit in the 1·13-Ga-old Ilímaussaqintrusive complex, South Greenland, consists of a magmatic assemblageof ternary alkali feldspar + fayalitic olivine + augite + titanomagnetite+ apatite + baddeleyite ± nepheline ± quartz ±ilmenite ± zircon. Feldspar, nepheline and QUILF thermometryyield T = 1000–700°C, at P = 1 kbar, which is derivedfrom fluid inclusion data from other parts of the complex. Ternaryfeldspar was the first major liquidus phase. It crystallizedat temperatures between 950 and 1000°C from a homogeneousmagma with aSiO2 = 0·8 and fO2 about 1·5–2log units below the fayalite–magnetite–quartz (FMQ)buffer. Later, closed system fractionation produced nepheline-bearingassemblages with aSiO2 = 0·4 and log fO2 = FMQ –3 to FMQ – 5. Assimilation of wall rocks produced localvariations of melt composition. Four traverses through the unitwere sampled parallel to the assumed direction of crystallization.They exhibit significant differences in their mineral assemblagesand compositions. The chemical zoning and calculated intensiveparameters of four sample suites reflect both closed systemfractional crystallization and local assimilation of wall rocks. KEY WORDS: alkaline magmatism; assimilation; fractionation; redox equilibria; QUILF 相似文献
10.
Enthalpies and Volumes Related to K--Na Mixing and Al-Si Order/Disorder in Alkali Feldspars 总被引：1，自引：0，他引：1
In order to investigate the thermodynamic properties of alkalifeldspars, three new feldspar ion-exchange series have beensynthesized, two based on monoclinic parent materials havingintermediate degrees of Al—Si order, the other on Amelialow albite. Acid solution calorimetric measurements have beencarried out in 20?1% HF at 50?C under isoperibolic conditionson 30 members of the three series, and compared with revisedvalues for a previously reported sanidine—analbite series.Molar volumes have been determined for all feldspars, and foran additional series based on Eifel sanidine. Enthalpies of K—Na mixing (Aex) calculated from the 50?Cheats of solution are dependent on Al—Si distributionfor both topochemically monoclinic and triclinic alkali feldspars,and in general can be expressed as
where NOr and NAb are mole fractions of KAlSi3O8 and NaAlSi3O8,respectively, and Z is an ordering parameter defined as twicethe difference in the mole fraction of Al in the T1 vs the T2tetrahedral sites. Aex values for all but the most disorderedseries are maximized toward sodic compositions, and increaseboth in magnitude and asymmetry as ordering increases. For topochemically monoclinic alkali feldspar series, volumesof K—Na mixing(Vex) are asymmetric with NOr, but withinthe precision of present data do not depend on Al—Si distribution:
Microcline-low albite feldspars appear to have volumes of mixingwith the opposite asymmetry, but expressions of for these differ somewhat among various investigators. Since no single thermodynamic mixing property is markedly asymmetricwith respect to composition, the excess Gibbs energies impliedfrom solvus data for alkali feldspars, and maximized at sodiccompositions, are apparently the result of additive effectsof subtle asymmetries in the volumes, enthalpies, and entropiesof K—Na mixing in these minerals. The thermodynamic properties of an alkali feldspar at any compositionare significantly affected by the distribution of Al and Sibetween T1 and T2 tetrahedral sites. The enthalpy of formationat 50?C of a monoclinic potassium feldspar with perfect order(Z=1) differs by 2?19 kcal/mol from one with a completely randomAl—Si distribution (Z=0), while a value of 2?86 kcal/molapplies to analagous sodium end members. ConverselyY-ordering(between T1O andT1m sites) seems to have little or no effecton the enthalpy of formation of either end member, evidencedby the fact that most of the enthalpy differences for the lowmicrocline to sanidine and corresponding low albite to analbitetransitions (1?73 and 2?79 kcal/mol, respectively) can be attributedto Al—Si exchanges between T1 and T2 sites. Observed enthalpydifferences in alkali feldspars are probably related to strainat domain boundaries, whether the domains are extremely small,or somewhat larger as in modulated structures. Neither Z-nor Y-ordering has a substantial effect on the molarvolumes of alkali feldspars. 相似文献
11.
GARCIA-CASCO ANTONIO; HAISSEN FAOUZIYIA; CASTRO ANTONIO; EL-HMIDI HASSAN; TORRES-ROLDAN RAFAEL LUIS; MILLAN GUILLERMO 《Journal of Petrology》2003,44(10):1727-1757
We document experiments on a natural metapelite in the range650–775°C, 6–14 kbar, 10 wt % of added water,and 700–850°C, 4–10 kbar, no added water. Staurolitesystematically formed in the fluid-present melting experimentsabove 675°C, but formed only sporadically in the fluid-absentmelting experiments. The analysis of textures, phase assemblages,and variation of phase composition and Fe–Mg partitioningwith P and T suggests that supersolidus staurolite formed at(near-) equilibrium during fluid-present melting reactions.The experimental results are used to work out the phase relationsin the system K2O–Na2O–FeO–MgO–Al2O3–SiO2–H2Oappropriate for initial melting of metapelites at the upperamphibolite facies. The P–T grid developed predicts theexistence of a stable P–T field for supersolidus staurolitethat should be encountered by aluminous Fe-rich metapelitesduring fluid-present melting at relatively low temperature andintermediate pressures (675–700°C, 6–10 kbarfor XH2O = 1, in the KNFMASH system), but not during fluid-absentmelting. The implications of these findings for the scarcityof staurolite in migmatites are discussed. KEY WORDS: metapelites; migmatites; partial melting; P–T grid; staurolite 相似文献
12.
Sub-potassic nephelines in the system NaAlSiO4(Ne)-KAlSiO4(Ks) were synthesized under a variety of conditions and studied at room temperature and up to 1000 °C using an X-ray powder diffractometer. At low temperatures they do not have the hexagonal structure determined by Hahn and Buerger (1955) for natural nepheline. Samples with 0.7 to 2.5 mole % Ks have an orthorhombic supercell with parameters equivalent to a, 3a, 3c where a and c are Hahn and Buerger structure cell parameters. Nephelines with 0 to 0.7% Ks consist of two phases with different c axes; one of these phases has the orthorhombic supercell.Pure-Na nephelines (NaAlSiO4) invert to a hexagonal phase with the Hahn and Buerger structure at 190 °±10 °C; this inversion temperature decreases with increasing Ks and a sample with 0.5% Ks inverts at 170 °±5 °C. The inversion is reversible and is displacive. Another reversible inversion begins at 875 °±10 °C in pure-Na nepheline; this inversion increases in temperature with increasing Ks and a sample with 1.8% Ks begins to invert at 960 °±10 °C.Superstructures with anomalous low-temperature cell parameters in sub-potassic nephelines are attributed to reversible collapse of the framework about the larger cation sites which must be occupied by small Na in subpotassic nephelines. Superstructures in natural nephelines are also related to framework collapse at a displacive inversion. 相似文献
13.
Phase relations in the system NaAlSiO4-NaGaSiO4 to 945° C at 1 kbar P(H2O) are dominated by stability of Na(Al,Ga)SiO4 with the beryllonite-type structure. The nepheline structure is restricted to NaAlSiO4-rich compositions at moderate and high temperature. Structure-composition relationships are controlled by space-fitting requirements of both framework and cavity cations, as in related systems. The two-phase (nepheline-type+beryllonite-type) field has been delineated from the end-member NaAlSiO4 composition up to the peritectic point at about 945° C (and 60 mol% NaGaSiO4), using a volume-composition relationship for the beryllonite-type phase, phase appearance, and electron microprobe analysis. At end-member NaGaSiO4 composition, the beryllonite-type phase is stable to the melting point (902±5° C). At end-member NaAlSiO4 composition, the beryllonite-type?nepheline-type transformation occurs at 348±2° C, and is associated with an increase in molar volume of 2.4% and enthalpy of 5170±40J·mol?1. Thus, end-member NaAlSiO4 nepheline, and probably all sub-potassic nephelines as well, are metastable at very-low geological temperatures. 相似文献
14.
Crystallization Pressure and Cooling History of the Giles Layered Igneous Complex, Central Australia 总被引：1，自引：0，他引：1
The Giles Complex, central Australia, consists of a series oflarge layered gabbroic/ultramafic intrusions emplaced in acidicand intermediate granulites of the Middle Proterozoic Musgraveblock. Lithologies range from well-layered dunite, wehrlite,and pyroxenite in the lower primitive series, to massive olivinegabbro, gabbronorite, and anorthosite in the main units, andferrodiorites, vanadife-rous magnetite layers, and granophyresin the upper, most fractionated parts. Unlike many layered intrusions,the Giles Complex is tectonically dismembered to an extent thata reconstruction of the original morphology is difficult. The Complex is believed to be a type example for medium- tohigh-pressure differentiation. (1) Chilled margin samples (wherepreserved) are orthopyroxene-phyric, and liquidus olivine isreplaced by liquidus orthopyroxene at an mg-number of 0.77,suggesting a pressure-related expansion of the orthopyroxenestability field (Goode & Moore, 1975). (2) Tschermaks substitutioninto pyroxene and plagioclase-orthoclase solid solution areextensive, indicating unusually high crystallization temperaturerelated to high pressure; antiperthites in the Giles Complexare amongst the most calcic reported for terrestrial rocks.(3) The lower primitive cumulate units of the Complex are coroniticand feature a variety of subsolidus high-pressure reaction textures;olivine and cumulus chromite have reacted with calcic plagioclaseto orthopyroxene-clinopyroxene-spinel, olivine-spinel, and clinopyroxene-spinelsymplectites. The principal reaction mechanism for the symplectites was continuousmass transfer of alumina from plagioclase toward spinel, asthe Complex passed from the olivine-plagioclase stability fieldinto the pyroxene-spinel field during cooling. Geothermometersapplicable to the cumulates record a wide range of equilibrationtemperatures from late-magmatic to granulite-metamorphic conditions.FeMg1 exchange gives closure temperatures around 600–700?C,whereas Al2Mg1Si1 net-transfer equilibria have preserved highertemperatures around 750–900 ?C. Defocused beam bulk analysesof exsolved cumulus clinopyroxenes and intercumulus plagioclasesrecover magmatic compositions; i. e., two-pyroxene solvus CaMg-1temperatures plot around 1120?50?C, whereas two-feldspar thermometersgive 1200?C. Pressures are calculated from thermochemical data with the heterogeneousequilibria 2 fo + an = en + di + sp, fo + an = di + Mg-Ts, andfo + an = en + Ca-Ts, after correcting spinel activities forselective retrograde FeMg-1 exchange during cooling. These equilibria,combined with orthopyroxene-spinel Al2Mg-1Si-1 temperaturesfor metamorphic assemblages and two-pyroxene temperatures forcumulus phases define a medium-pressure cooling path extendingfrom 1150 ?C (at 6?5 kb) to 750 ?C (at 6?2 kb). The resultssuggest an isobaric cooling path for the Giles Complex, withno evidence for a post-intrusive metamorphic overprint. Themagmas intruded at lower to middle crustal levels after thepervasive deformation in the Musgrave block, and probably afterthe peak metamorphic event. 相似文献
15.
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
16.
Effect of Carbon Dioxide on Dehydration Melting Reactions and Melt Compositions in the Lower Crust and the Origin of Alkaline Rocks 总被引：9，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
17.
Ultramafic xenoliths were recovered in four alkalic lava flowsfrom Loihi Seamount at depths between 2200 and 1400m. No xenolithbearing flows were sampled near the summit despite a concentrateddredge program. The flows, three of alkalic basalt and one ofbasanite, contain common olivine megacrysts and small xenolithsof dunite, rarer harzburgite, and a single wehrlite. Olivinemegacrysts as large as 8 mm are Fo84–88 6 and containmagnesiochromite inclusions with 1?1–3?5 wt.% TiO2 Dunitecontains Fo83 5–88?5 olivine, magnesiochromite with l?5–6?9wt.% TiO2 (avg. 3?2 wt.%), and extremely rare chrome-rich diopside.The wehrlite contains euhedral Fo85 9 olivine and magnesiochromitewith 1?9–4?7 wt.% TiO2 poikilitically enclosed in chrome-richdiopside (Wo45 4En48 0Fs6?6).Most of the olivine megacrysts,dunite, and the wehrlite are cumulates of Loihi alkalic lavasthat accumulated in a magma storage zone located at least 16kmbelow sea level. The rarity of dunite related to tholeiiticmagmas supports the interpretation that the alkalic lavas atLoihi generally predate the tholeiitic lavas. The harzburgitexenoliths have cataclastic textures and contain Fo89 5–926 olivine, enstatite (Wo2 0–2?7En90?0–88 7Fe8?0–8?6),Cr-rich endiopside (Wo43 4–44 5En52 0–50 0Fs4 6–45), and translucent red-brown magnesiochromite. The harzburgitexenoliths, which have 2-pyroxene temperatures of 1066 ? 35?C,originated in the uppermost mantle in a region of high strainrate, probably near the boundary between the mantle and theoverlying ocean crust. The presence of upper mantle xenolithsindicates that the magma storage zone is located below the baseof the ocean crust within the uppermost mantle. 相似文献
18.
13C/12C ratios have been analysed in graphite adcumulate crystalsassociated with the olivine and orthopyroxene of a suite ofultramafic xenoliths in the alkali basalts of Tissemt (Egg?r?,Algerian Sahara). The 13C of these graphites varies from –24?6to – 14?4 per milk with internal variations of up to 1?6per mille within a given sample. Carbonates, linked to the volcanicphase and precipitated at medium temperatures have 13C valuesfrom –0?8 to + 3?5 and 18O values from 21?6 to 31?5 Thehost lava has a high carbon content of 675 p.p.m. with a global13C of –4?2. It can be separated into three parts: a majorpeak is obtained between 670 and 985 ?C and also liberated byslow acid attack with a 13C of up to +7?3 and representing CO2trapped in the natrolite; a high temperature peak (above 1160?C with 47 p.p.m at –25?8) will represent carbon trappedin silicates and there is an additional carbon not well determinedextracted from 300 ?C to high temperature which represent 215p.p.m. at –23?0 with a possible fraction as low as –68?0.The graphite is interpreted as the result of the precipitationof carbon from an interstitial liquid previously outgassed thensupersaturated by the ongoing crystallization process. The volcanicevent can be considered to a first approximation as remobilizingthe carbon from the xenoliths by oxidation to CO2, subsequentpartial reduction upon cooling and precipitation of carbonateand trapping of residual CO2 within zeolite. Only volatilereducedspecies can be significantly outgassed so that the total 13Cof –4?2 is probably close to (or a little higher than)the original 13C. 相似文献
19.
Experimental phase equilibrium data on compositions of coexistingpyroxenes in the quadrilateral enstatite-diopside-ferrosilite-hedenbergitehave been used to model pyroxene solid solutions and to formulatepyroxene geothermometers. Each pyroxene is treated as a solidsolution of four quad-components using the Kohler formulation
where Gij* is the excess free energy of mixing in a binary solutioncalculated with binary mole fractions (e.g. Xio = Xi/(Xi+Xj))and Xi is the mole fraction in a multicomponent solution. Thefit to the experimental data is achieved by minimizing the totalGibbs free energy of the assemblage. The following set of thermochemicaldata and simple mixture parameters (Wij) are found to be bestsuited. Standard (T = 298?15 K) enthalpy and entropy of formationfrom elements for fictive orthohedenbergite are –1416?8kJ and 84?88 J K–1 mol –1 respectively. The heatcapacity is given by 114?67+17?09E-3T–31?40E5T–2.The Wij data are: Opx: W12 = W21 = 25 W13 = (13?1–0-015T),W31 = (3?37–0?005T), W23 = 20, W32 = 16, W24 = 5, W42= 7, W34 = 15, W43 = 15; Cpx: W12 = (25?484+0?0812P), W21 =(31?216–0?0061P),W31 = W13 = 0W14 = (93?3–0?045T), W41 = (–20?0+0?028T),W23 = 24, W32 = 15, W24 = 12, W42 = 12, W34 = (16?941+0?00592P),W43 = (20?697–0?00235P). Coexisting pyroxene compositionshave been computed in the temperature range of 700 to 1400?C. Two geothermometers have been constructed, one based on atomicfraction of iron (Fe/(Fe + Mg)) in orthopyroxene and the Fe-Mgdistribution coefficient and the other, based on wollastonitecontent of clinopyroxene. The two scales yield different temperatureswhen applied to the same rock. In igneous pyroxenes, the Catransfer ceased at 150 to 200?C above the closure temperatureof the Fe-Mg ion-exchange. In metamorphic rocks an oppositeeffect seems to have prevailed. 相似文献
20.
Experimental Crystallization of Leucogranite Magmas 总被引：25，自引：8，他引：17
Both crystallization and melting experiments have been carriedout on two natural, biotite-muscovite (DK) and tourmaline-muscovite(GB) High Himalayan leucogranites (HHL) at 4 kbar, logfO2 =FMQ–05, aH2O = 1–0•03, and at five temperaturesbetween 803 and 663C H2O contents of the quenched glasses wereanalysed by ion microprobe. Plagioclase and biotite are theliquidus phases for reduced melt H2O contents and H2O-rich conditions,respectively. H2O saturation limits range from 8 to 10 wt%.DK has a wider crystallization interval than GB (150 vs 80Cfor conditions close to H2O saturation), and a slightly higherH2O-saturated solidus (645 compared with 630C for GB). Tourmalinenever crystallized spontaneously from the melt. Tourmaline seedsalways reacted out to biotite in the biotite-muscovite sample,whereas they remained stable in the tourmaline-muscovite sample.Biotite is replaced by hercynite as the main ferromagnesianphase at high temperature and reduced aH2O. Muscovite crystallizationis restricted to near-solidus conditions. The compositions ofplagioclase, alkali feldspar, biotite and muscovite are givenas a function of bulk composition, temperature and aH2O. Glasscompositions are richer in normative quartz than the 4 kbarH2O-saturated Qz–Ab–Or eutectic, and become moreperaluminous and less mafic with increasing fractionation. Biotitecrystallization in peraluminous liquids is favoured by elevatedFe, Mg and Ti contents. Muscovite crystallization is not promotedunder H2O-saturated conditions. Tourmaline stability is stronglydependent on aH2O. For GB, tourmaline is present at elevatedtemperatures for intermediate values of aH2O (803 C, 0–7),but not above 650C for H2O-saturated conditions. Comparisonof the natural crystallization sequence with experiments suggestsinitial water contents between 5 and 75 wt % for the DK magma,and > 7 wt% for the GB magma. Plagioclase core compositionsgive minimum temperatures of 700C for GB and 750C for DK,consistent with an emplacement of these HHL as almost entirelyliquid bodies. The restricted occurrence of biotite in the GBgranite suggests that it reacted out during the magmatic evolution,owing to a marked change in fO2 toward more oxidizing conditions.Tourmaline leucogranites can be generated from biotite leucogranitesby fractional crystallization under conditions of increasingdegree of oxidation. KEY WORDS: leucogranite; melting experiments; crystlization experiments; Himalayas; phase relations
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