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1.
BERMAN  R. G. 《Journal of Petrology》1988,29(2):445-522
Internally consistent standard state thermodynamic data arepresented for 67 minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2.The method of mathematical programming was used to achieve consistencyof derived properties with phase equilibrium, calorimetric,and volumetric data, utilizing equations that account for thethermodynamic consequences of first and second order phase transitions,and temperature-dependent disorder. Tabulated properties arein good agreement with thermophysical data, as well as beingconsistent with the bulk of phase equilibrium data obtainedin solubility studies, weight change experiments, and reversalsinvolving both single and mixed volatile species. The reliabilityof the thermodynamic data set is documented by extensive comparisons(Figs. 4–45) between computed equilibria and phase equilibriumdata. The high degree of consistency obtained with these diverseexperimental data gives confidence that the refined thermodynamicproperties should allow accurate prediction of phase relationshipsamong stoichiometric minerals in complex chemical systems, andprovide a reasonable basis from which activity models for mineralsmay be derived.  相似文献   

2.
The reaction 2 zoisite + CO2 = 3 anorthite + calcite + H2O hasbeen reversed experimentally in cold-seal pressure vessels usingnatural phases and H2O–C02 fluids generated by water-silveroxalate mixtures. Equilibrium has been determined at 5000 50bars, 599 9 °C and 0–075 ± 0–010 XCO2.Extrapolation using the MRK equation of Kerrick & Jacobs(1981) gives an equilibrium curve of negative T–X slopeconsistent with bracketing runs at 500, 550 and 650 °C.The curve agrees only with a new bracket of Nitsch (in Hoschek,1980), and is at higher XCo2 than all other experimental determinationsand at lower XCO2 than those calculated from the thermodynamicdata of Helgeson et al. (1978). Discrepancies are attributedto differences in starting materials and small errors in thethermodynamic properties of the phases. Reaction direction and equilibrium have been determined by observingsurface textures of run products by SEM. Growth and solutiontextures are non-equivalent, permitting unequivocal determinationof reaction direction even where the extent of reaction is small,an advantage over conventional and insensitive XRD methods whichmeasure bulk changes in the charge. Dissolution features ofanorthite and zoisite are defect-related indicating controlby surface reaction, whereas calcite dissolves by both surfacereaction and diffusion controlled processes. Margarite forms in most runs below 585 °C. Textural features,its restriction to the margarite stability field and comparisonwith feldspar solubility data demonstrate it is an equilibriumphase formed by incongruent solubility of anorthite and zoisitein H2O-CO2 fluids. Quench phases formed from the solute areconsequently silica-rich, with implications for metasomaticprocesses in feldspar–epidote–bearing rock and fluidsystems. Absence of margarite from runs with anorthite, zoisiteand calcite in the zoisite stability field is apparently dueto the fast growth rate of zoisite. The full equilibrium assemblageis zoisite–anorthite–calcite–margarite atthese temperatures, and the degeneracy of the model system isunobtainable in experiments, and presumably, in nature.  相似文献   

3.
In the system CaO-MgO-Al2O3-SiO2-Na2O-H2O under 5 kb pressurethe invariant equilibrium forsterite-orthopyroxene-Ca-rich clinopyroxene-amphibole-plagioclase-liquid-vapourhas been identified at 960?12 ?C. A similar invariant assemblagewith spinel replacing Ca-rich clinopyroxene exists at 950?8?C. The liquid in the former equilibrium contains 16.5 per cent(wt.) normative quartz and 3 per cent Na2O; the plagioclaseis more calcic than An87; the pyroxenes contain about 3 percent Al2O3 and the amphibole is hypersthene-normative. Two anhydrousthermal maxima, the olivine-Ca-rich clinopyroxene-plagioclaseand the orthopyroxene-Ca-rich clinopyroxene-plagioclase dividezones are not encountered in this system, and nepheline-normativeliquids may crystallize amphibole?olivine?Ca-rich clinopyroxeneto produce quartz-normative residual liquids of andesite-typecomposition. A thermal maximum involving amphibole-olivine-Ca-richclinopyroxene-liquid-vapour exists for liquids containing approximately11 per cent normative nepheline and liquids more undersaturatedthan this will crystallize these phases to produce extremelynephelinitic liquids. Phase diagrams are presented which facilitate the predictionof crystallization sequences and liquid evolution paths forany basic or intermediate composition under the conditions employedhere.  相似文献   

4.
Phase equilibrium studies have been carried out on the compositionjoin NaAlSiO4-CaCO3-H2O with 25 wt per cent H2O at 1 kb pressurein the temperature range 600–960 °C. Liquid, in equilibriumwith crystalline phases and a sodic, aqueous vapor phase persistsacross the join down to temperatures of about 600 °C. Fractionalcrystallization of a carbonated nepheline-rich liquid in equilibriumwith vapor is capable of generating successively the crystallineassemblages (1) nepheline, (2) melilite+nepheline, (3) hydroxyhaüyne+melilite,(4) cancrinite+melilite, and (5) calcite+cancrinite+melilite.Late-stage liquid fractions are rich in CaCO3, whereas the vaporphase is enriched in Na. The experimental assemblages are strikinglysimilar to rocks in alkalic rock-carbonatite complexes in generaland in the Oka, Quebec, complex in particular. The successionof assemblages at Oka and at other melilite rock-bearing complexesmay be interpreted as the products of fractionation of a carbonatednephelinite magma by analogy with the experimental results.The sodium-bearing vapor phase of the experiments may be analogousto the fenitizing agent of some carbonatite complexes.  相似文献   

5.
We present, as a progress report, a revised and much enlarged version of the thermodynamic dataset given earlier (Holland & Powell, 1985). This new set includes data for 123 mineral and fluid end-members made consistent with over 200 P–T–XCO2–fO2 phase equilibrium experiments. Several improvements and advances have been made, in addition to the increased coverage of mineral phases: the data are now presented in three groups ranked according to reliability; a large number of iron-bearing phases has been included through experimental and, in some cases, natural Fe:Mg partitioning data; H2O and CO2 contents of cordierites are accounted for with the solution model of Kurepin (1985); simple Landau theory is used to model lambda anomalies in heat capacity and the Al/Si order–disorder behaviour in some silicates, and Tschermak-substituted end-members have been derived for iron and magnesium end-members of chlorite, talc, muscovite, biotite, pyroxene and amphibole. For the subset of data which overlap those of Berman (1988), it is encouraging to find both (1) very substantial agreement between the two sets of thermodynamic data and (2) that the two sets reproduce the phase equilibrium experimental brackets to a very similar degree of accuracy. The main differences in the two datasets involve size (123 as compared to 67 end-members), the methods used in data reduction (least squares as compared to linear programming), and the provision for estimation of uncertainties with this dataset. For calculations on mineral assemblages in rocks, we aim to maximize the information available from the dataset, by combining the equilibria from all the reactions which can be written between the end-members in the minerals. For phase diagram calculations, we calculate the compositions of complex solid solutions (together with P and T) involved in invariant, univariant and divariant assemblages. Moreover we strongly believe in attempting to assess the probable uncertainties in calculated equilibria and hence provide a framework for performing simple error propagation in all calculations in thermocalc, the computer program we offer for an effective use of the dataset and the calculation methods we advocate.  相似文献   

6.
The pressure-temperature-compositional (P-T-X) dependence ofthe solubility of Al2O3 in orthopyroxene coexisting with garnethas been experimentally determined in the P-T range 5–30kilobars and 800–1200 ?C in the system FeO—MgO—Al2O3—SiO2(FMAS). These results have been extended into the CaO—FeO—MgO—Al2O3—SiO2(CFMAS) system in a further set of experiments designed to determinethe effect of the calcium content of garnet on the Al2O3 contentsof coexisting orthopyroxene at near-constant Mg/(Mg + Fe). Startingmaterials were mainly glasses of differing Mg/(Mg + Fe) or Ca/(Ca+ Mg + Fe) values, seeded with garnet and orthopyroxene of knowncomposition, but mineral mixes were also used to demonstratereversible equilibrium. Experiments were performed in a piston-cylinderapparatus using a talc/pyrex medium. Measured orthopyroxene and corrected garnet compositions werefitted by multiple and stepwise regression techniques to anequilibrium relation in the FMAS system, yielding best-fit,model-dependent parameters Goy= –5436 + 2.45T cal mol–1,and WM1FeA1= –920 cal mol–1. The volume change ofreaction, Vo, the entropy change, So970 and the enthalpy changeHo1,970, were calculated from the MAS system data of Perkinset al. (1981) and available heat capacity data for the phases.Data from CFMAS experiments were fitted to an expanded equilibriumrelation to give an estimate of the term WgaCaMg = 1900 ? 400cal/mole cation, using the other parametric values already obtainedin FMAS. The experimental data allow the development of a arnet-orthopyroxenegeobarometer applicable in FMAS and CFMAS: where This geobarometer is applicable to both pelitic and metabasicgranulites containing garnet orthopyroxene, and to garnet peridoditeand garnet pyroxenite assemblages found as xenoliths in diatremesor in peridotite massifs. It is limited, however, by the necessityof an independent temperature estimate, by errors associatedwith analysis of low Al2O3 contents in orthopyroxenes in high-pressureor low-temperature parageneses, and by uncertainties in thecomposition of garnet in equilibrium with orthopyroxene. Ananalysis of errors associated with this formulation of the geobarometersuggests that it is subject to great uncertainty at low pressuresand for Fe-rich compositions. The results of application ofthis geobarometer to natural assemblages are presented in acompanion paper.  相似文献   

7.
We have used Kieffer's vibrational model to calculate heat capacities and entropies for Al2O3 corundum and MgSiO3 ilmenite, using available vibrational and elastic data for these phases. The calculated heat capacity for corundum is within 1–2 percent of the experimental values between 100 K and 1,800 K, while that for MgSiO3 ilmenite is within 1–2 percent of the experimental data between 350 K and 500 K. We have calculated the heat capacity for MgSiO3 ilmenite from 50 K to 1,800 K, which extends the range of available heat capacity data for this phase. The results of this calculation suggest that there may be differences in the vibrational properties of corundum and MgSiO3 ilmenite. Finally, we have used the results of our calculation to obtain a transition entropy of near -18.8 J/mol.K for the MgSiO3 pyroxene-ilmenite reaction.  相似文献   

8.
Two series of anhydrous experiments have been performed in anend-loaded piston cylinder apparatus on a primitive, mantle-derivedtholeiitic basalt at 1·0 GPa pressure and temperaturesin the range 1060–1330°C. The experimental data provideconstraints on phase equilibria, and solid and liquid compositionsalong the liquid line of descent of primary basaltic magmasdifferentiating in storage reservoirs located at the base ofthe continental crust. The first series are equilibrium crystallizationexperiments on a single basaltic bulk composition; the secondseries are fractionation experiments where near-perfect fractionalcrystallization was approached in a stepwise manner using 30°Ctemperature steps and starting compositions corresponding tothe liquid composition of the previous, higher-temperature glasscomposition. Liquids in the fractional crystallization experimentsevolve with progressive SiO2 increase from basalts to dacites,whereas the liquids in the equilibrium crystallization experimentsremain basaltic and display only a moderate SiO2 increase accompaniedby more pronounced Al2O3 enrichment. The principal phase equilibriacontrols responsible for these contrasting trends are suppressionof the peritectic olivine + liquid = opx reaction and earlierplagioclase saturation in the fractionation experiments comparedwith the equilibrium experiments. Both crystallization processeslead to the formation of large volumes of ultramafic cumulatesrelated to the suppression of plagioclase crystallization relativeto pyroxenes at high pressures. This is in contrast to low-pressurefractionation of tholeiitic liquids, where early plagioclasesaturation leads to the production of troctolites followed by(olivine-) gabbros at an early stage of differentiation. KEY WORDS: liquid line of descent; tholeiitic magmas; equilibrium crystallization; fractional crystallization  相似文献   

9.
The common upper mantle assemblage olivine-orthopyroxene-spinelmay be used to calculate the oxygen fugacity at which mantle-derivedperidotites have equilibrated. The equilibrium has been calibratedusing the large amount of existing data on the thermodynamicproperties of each phase in this assemblage. A by-product ofthis procedure is a new calibration of the olivine-spinel Mg-Fe2+exchange geothermometer. Application of the equilibrium to avariety of peridotite xenoliths indicates that the oxygen fugacityof the upper mantle lies between the quartz-fayalite-magnetite(QFM) and w?stite-magnetite (WM) oxygen buffers; the few apparentexceptions to this rule may be due to analytical error, particularlyin the Fe3+ content of the spinet phase. In fact, the determinationof Fe3+ in spinet is at present the limiting factor in the accurateapplication of the method: within this limitation, the presentlyavailable evidence suggests that the oxygen fugacity of themantle may be laterally homogeneous over wide regions, but mayalso show small differences between these regions. The fluidspecies in the system C-H-O at such oxygen fugacities are predominantlyCO2 and/or H2O, and not CH4/H2 The minimum possible oxygen fugacity of the mantle is givenby the nickel content of olivine in equilibrium with orthopyroxene;for typical mantle compositions this minimum curve is virtuallycoincident with the iron-w?stite (IW) oxygen buffer.  相似文献   

10.
Direct measurements of liquid heat capacity, using a Setaram HT1500 calorimeter in step-scanning mode, have been made in air on six compositions in the Na2O-FeO-Fe2O3-SiO2 system, two in the CaO-FeO-Fe2O3-SiO2 system and four of natural composition (basanite, andesite, dacite, and peralkaline rhyolite). The fitted standard deviations on our heat capacity measurements range from 0.6 to 3.6%. Step-scanning calorimetry is particularly useful when applied to iron-bearing silicate liquids because: (1) measurements are made over a small temperature interval (10K) through which the ferric-ferrous ratio of the liquid remains essentially constant during a single measurement; (2) the sample is held in equilibrium with an atmosphere that can be controlled; (3) heat capacity is measured directly and not derived from the slope of enthalpy measurements with temperature. Liquid compositions in the sodic and calcic systems were chosen because they contain large concentrations of Fe2O3 (up to 19 mol%), and their equilibrium ferric-ferrous ratios were known at every temperature of measurement. These measurement have been combined with heat capacity (Cp) data in the literature on iron-free silicate liquids to fit Cp as a function of composition. A model assuming no excess heat capacity (linear combination of partial molar heat capacities of oxide components) reproduces the liquid data within error (±2.2% on average). The derived partial molar heat capacity of the Fe2O3 component is 240.9 ±7.9 J/g.f.w.-K, with a standard error reduced by more than a factor of two from that in earlier studies. The model equation, based primarily on simple, synthetic compositions, predicts the heat capacity of the four magmatic liquids within 1.8% on average.  相似文献   

11.
The enthalpy of drop-solution in molten 2PbO·B2O3 of synthetic and natural lawsonite, CaAl2(Si2O7)(OH)2·H2O, was measured by high-temperature oxide melt calorimetry. The enthalpy of formation determined for the synthetic material is (fHOxides=-168.7Dž.4 kJ mol-1, or (fH0298=-4,872.5dž.0 kJ mol-1. These values are in reasonable agreement with previously published data, although previous calorimetric work yielded slightly more exothermic data and optimisation methods resulted in slightly less exothermic values. The equilibrium conditions for the dehydration of lawsonite to zoisite, kyanite and quartz/coesite at pressures and temperatures up to 5 GPa and 850 °C were determined by piston cylinder experiments. These results, other recent phase equilibrium data, and new calorimetric and thermophysical data for lawsonite and zoisite, Ca2Al3(SiO4)(Si2O7)O(OH), were used to constrain a mathematical programming analysis of the thermodynamic data for these two minerals in the chemical system CaO-Al2O3-SiO2-H2O (CASH). The following data for lawsonite and zoisite were obtained: (fH0298 (lawsonite)=-4,865.68 kJ mol-1 , S0298 (lawsonite)=229.27 J K-1 mol-1 , (fH0298 (zoisite)=-6,888.99 kJ mol-1 , S0298 (zoisite)=297.71 J K-1 mol-1 . Additionally, a recalculation of the bulk modulus of lawsonite yielded K=120.7 GPa, which is in good agreement with recent experimental work.  相似文献   

12.
The experimental work on biotites has primarily involved compositionsalong the annite-phlogopite join, but most natural biotitescontain significantly larger amounts of aluminum. At the sametime, the aluminum content of natural biotites varies considerably.The available evidence indicates that these variations in thealuminum content of biotite depend on the conditions of formationand the whole rock chemistry. Experiments on the phase relations of aluminous iron biotitesin the silica deficient system KAlSiO4-KAlSi3O8-Al2O3-Fe-O-H(pfluid = 2 kb) indicate that compositions up to Ann75 can besynthesized on the join annite [K2Fe6Al2Si6O20(OH)4]-aluminumbiotite [K2Al6Al2Al6O20(OH)4]. The aluminous biotites are stableto higher temperatures than annite. An isobaric divariant equilibrium,Bioss-Mtss-Sa-Lc-V, extends to higher oxygen fugacities fromthe Ann-Mt-Sa-Lc-V curve of Eugster & Wones (1962). Compositioncontours on this surface indicate that both the magnetite andbiotite become more aluminous with increasing temperature and/oroxygen fugacity. The Bioss-Mtss-Sa-Lc-V reaction surface isterminated by equilibria involving the additional phases muscovite,corundum, and hercynite respectively as the conditions becomemore reducing. At 2 kb fluid pressure; aluminum-rich iron biotiteis stable to 555 °C on the HM buffer, 763 °C on theMt-Hc-Cor buffer, 820 °C on NNO, and about 860 °C onQFM. The data obtained can be applied to a number of biotitesyenites and appears to explain why iron-rich aluminum biotitesoccur in these rocks.  相似文献   

13.
The stability of the high-pressure phase Mg-sursassite, previously MgMgAl-pumpellyite, in ultramafic compositions has been determined in experiments in the system MgO-Al2O3-SiO2-H2O (MASH). The breakdown of Mg-sursassite + forsterite + enstatite to pyrope + vapour with increasing temperature was bracketed at 6.0 and 7.0 GPa. Below 6.0 GPa, Mg-sursassite + forsterite + vapour reacts to chlorite + enstatite. This reaction provides a mechanism for transfer of water from chlorite- to Mg-sursassite-bearing assemblages. At pressures of 7.0 GPa and above, the assemblage Mg-sursassite + phase A + enstatite was found. Phase relations involving Mg-sursassite and phase A are considered. For bulk compositions with a low water content, the vapour-absent reaction Mg-sursassite + forsterite = pyrope + phase A + enstatite determines the upper-pressure stability of Mg-sursassite, and provides a mechanism for the complete transfer of water from Mg-sursassite to phase A-bearing assemblages. Mg-sursassite plays an important role in peridotite compositions in the subducting slab because, at temperatures below 700 °C, it can transfer water from hydrous phases such as antigorite and chlorite to high-pressure stable phases such as phase A.  相似文献   

14.
RUMBLE  DOUGLAS  III 《Journal of Petrology》1978,19(2):317-340
The rocks of the Clough Formation, Black Mountain, New Hampshire,were regionally metamorphosed at 5.5 (±0.5) kb and 495°± 10 °C during the Acadian orogeny. Mineral assemblagesattained chemical equilibrium during metamorphism on the scaleof single sedimentary beds up to 1 m thick. An aqueous, intergranular,metamorphic fluid was probably present; however, the concentrationsof the species H2O, H2, and O2 as well as the abundance of 18Oin the fluid varied from bed to bed. Neither isobaric nor polybaricosmotic equilibrium of H2 was attained between sedimentary beds.Fluid composition was controlled in each bed by the inherentbuffer capacity of the solid phases. Despite the effects ofprogressive dehydration during metamorphism, the buffer capacitiesof the mineral assemblages were sufficiently great that vestigesof premetamorphic heterogeneity of volatile components havebeen preserved.  相似文献   

15.
ERNST  W. G. 《Journal of Petrology》1977,18(3):371-398
Certain ultramafic-mafic lenses exposed in Ticino along thecontact zone between the underlying Simano and overlying Adulanappes display relatively high-pressure phase assemblages. AtAlpe Arami, metabasaltic layers associated with pyropic garnet-bearingIherzolite consist mainly of an early eclogitic assemblage characterizedby Alm39Py37Gross23Spess01+ Di49Hd08Jd43+rutile±kyanite.Iron-magnesium fractionation between garnet+omphacite pairsyields a KD, (Fe2+/Mg)garnet/(Fe2+/Mg)cllnopyroxence, of about6. This earlier assemblage has been replaced by a later, somewhatpargasitic hornblende+oligoclase+clinozoisite phase compatibility.Associated primary garnet peridotites contain Ca-rich clinopyroxeneand Al2O3-poor orthopyroxene. Both rock types have been affectedby a still later period of incipient chloritization. Available phase equilibrium and element partitioning data arecompatible with an inferred P-T condition of origin for theAlpe Arami mafic-ultramafic complex of 965–1000 °C,30–50 kilobars, indicating deep upper mantle generation.Amphibolites could have been produced during depressurizationaccompanying ascent of the mass through the upper mantle, butinasmuch as plagioclase accompanies the hornblende, the assemblageprobably crystallized after emplacement of the complex in theLepontine terrane prior to the termination of the Late Alpineregional metamorphism. Incipient production of high-rank greenschistphases certainly reflects a crustal event.  相似文献   

16.
LUTH  W. C. 《Journal of Petrology》1967,8(3):372-416
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   

17.
The Fish Canyon Tuff, Colorado, forms one of the largest (3000km3 known silicic eruptions in Earth history. The tuff is ahomogeneous quartz latite consisting of 40% phenocrysts (plagioclase,sanidine, biotite, hornblende, quartz, magnetite, apatite, sphene,and ilmenite) in equilibrium with a highly evolved rhyoliticmelt now represented by the matrix glass. Melt inclusions trappedin hornblende and quartz phenocrysts are identical to the newlyanalyzed matrix glass composition indicating that hornblendeand quartz crystallized from a highly evolved magma that subsequentlyexperienced little change. This study presents experimentalphase equilibrium data which are used to deduce the conditions(P, T, fO2, fH2O, etc.) in the Fish Canyon magma chamber priorto eruption. These new data indicate that sanidine and quartzare not liquidus phases until 780?C temperatures are achieved,consistent with Fe-Ti oxide geothermometry which implies thatthe magmatic temperature prior to eruption was 760?30?C. NaturalFe-Ti oxide pairs also suggest that log fO2 was -12.4 (intermediatebetween the Ni-NiO and MnO-Mn3O4 oxygen buffers) in the magmachamber. This fO2.102 is supported by the experimentally determinedvariations in hornblende and melt Mg-numbers as functions offO2 A new geobarometer based on the aluminum content of hornblendesin equilibrium with the magmatic assemblage hornblende, biotite,plagioclase, quartz, sanidine, sphene, ilmenite or magnetite,and melt is calibrated experimentally, and yields pressuresaccurate to ?0.5 kb. Total pressure in the Fish Canyon magmachamber is inferred to have been 2.4 kb (equivalent to a depthof 7.9 km) based on the Al-content of natural Fish Canyon hornblendesand this new calibration. This depth is much shallower thanhas been proposed previously for the Fish Canyon Tuff. Variationsin experimental glass (melt) composition indicate that the magmawas water-undersaturated prior to eruption. XH2O in the fluidphase that may have coexisted with the Fish Canyon magma isestimated to have been 0.5 by comparing the An-content of naturalplagioclases to experimental plagioclases synthesized at differentXH2O and Ptotals. This ratio corresponds to about 5 wt.% waterin the melt at depth. The matrix glass chemistry is reproducedexperimentally under these conditions: 760?C, 2.4 kb, XH2O=0.5,and log fo2=NNO+2 log units. The fugacity of SO2 (91 b) is calculatedfrom the coexistence of pyrrhotite and magnetite. Maximum CO2fugacity (2520 b) is inferred assuming the magma was volatilesaturated at 2.4 kb.  相似文献   

18.
Petrologic and geologic arguments suggested that a close approachto chemical equilibrium at a single temperature and pressuremight be found in the rocks near Mt. Grant in central Vermont.An area 2,800 feet by 4,000 feet was selected and studied indetail. The major assemblages are: kyanite-chloritoid-chlorite-quartz-muscovite-paragonite-rutile;garnet-chloritoid-chlorite-quartz-muscovite-paragonite-ilmenite;and garnet-chlorite-biotite-quartz-muscovite-albite-ilmenite.All minerals were separated from a primary sample for each ofthese assemblages and purified, complete gravimetric and spectrographicanalyses performed, and optical properties, X-ray properties,and densities measured. Chlorite, garnet, and chloritoid orbiotite were separated from an additional nine samples of theseassemblages and spectrographic and partial gravimetric analysesperformed. Distribution coefficients of each mineral pair for Mg/Fe andMn/Fe+ Mg and for the minor elements are similar in nearly everysample for a given assemblage. Distribution coefficients forgarnet-chlorite differ in the two assemblages which containthis pair; this difference is attributed to the difference inAl-content of the chlorite from the two assemblages. The smallrange of distribution coefficients, despite a wide range inthe relative proportions of the ferromagnesian phases, is convincingevidence that an equilibrium partition of the major and minorcations between the phases in each sample had been attainedand that the equilibration temperature was the same for eachsample. The coexistence of kyanite+muscovite+quartz and comparison ofthe composition of coexistent Ca-free muscovite and paragoniteand of O18/O10 ratios for coexistent quartz and magnetite withexperimental values indicate that these rocks formed at Ps 11 kb, T 550? C, and aH2o low enough (Pe(H2o) sufficiently lessthan Ps) to depress the pyrophyllite breakdown temperature byabout 40? C. The cations and the ao2 are equilibrated locally(within each sample), but all the samples have equilibratedto a common O18/O16 ratio. A possible explanation is that afluid medium had sufficient oxygen in H2O to control the O18/O16ratio of the rock and its phases, but that the rock system hadsufficient buffer capacity to control the ao2 of the fluid.  相似文献   

19.
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   

20.
Phase relationships on the vapor-saturated liquidus surfacein the system CaO-MgO-CO2-H2O have been deduced from data inthe systems CaO CO2-H2O, CaO-MgO-CO2, and MgO-CO2-H2O, and frompreliminary experiments in the quaternary system. These areillustrated in composition tetrahedra, and in isobaric sectionsthrough the petrogenetic model. The univariant PT curve forthe beginning of melting lies between 625? C and 600? C in thepressure range 10 bars to 4 kilobars, in the presence of a vaporphase rich in H2O. The curve is divided into three sectionsby two invariant points, each section having a different primarymagnesian phase involved in the melting reaction. Periclaseoccurs on the low-pressure section (less than about 1 kilobar),and with increasing pressure first brucite and then dolomitebecome stable on the liquidus. The pressure of the second invariantpoint, above which dolomite is stable on the liquidus, is notknown. The effect of FeO as an additional component is considered.Processes of crystallization resulting from changes in temperature,in pressure, and in the composition of the vapor phase are discussed.These processes are applied to the crystallization and differentiationof carbonatite magmas, and the reverse processes involving fusionare applied to the metamorphism of dolomites. Crystallizationdifferentiation of a carbonatite magma could produce the sequenceof intrusion observed at some carbonatite complexes: calcitics?vite, followed by ankeritic s?vite, and finally sideriticcarbonatite. Partial melting may occur during the thermal metamorphismof dolomites, but melting is unlikely during regional metamorphism.  相似文献   

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