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1.
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
2.
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
3.
Dr. Niranjan D. Chatterjee 《Contributions to Mineralogy and Petrology》1967,14(2):114-122
The reaction 2 epidote+2 calcite+3 wollastonite3 grossular-andraditess+ 2 CO2+1 H2O has been explored by hydrothermal experiments at a total fluid pressure of 1000 bars. For a grossular-andraditess of andradite
25
composition, the isobaric univariant curve passes through the points 458°C: XCO2=0.00; 521°C: XCO2=0.026; 523°C: XCO2=0.052; 526°C: 0.088; 528°C: XCO2=0.104. This curve intersects the isobaric univariant curve of the reaction calcite+quartz+[H2O] wollastonite+CO2+[H2O] at the isobaric invariant point around 528°C and XCO2=0.12. At higher values of XCO2, this reaction is replaced by another one, namely: 2 epidote+5 calcite+3 quartz3 grossular-andraditess+5 CO2+ 1 H2O. It is demonstrated that both the reactions do actually take place during the metamorphism of calcareous rocks. The petrologic significance of contrasted sequence of reactions within this system observed by various workers is also discussed. 相似文献
4.
Experimental data are presented for the solubility of NaNbO3 in the ternary system CaCO3-Ca(OH)2-NaNbO3 (or calcite-portlandite-lueshite) over the temperature range 500-950 °C at 0.1 GPa pressure. Near-liquidus phase relationships are given for the pseudo-binary join ([CaCO3]45[Ca(OH)2]55)100-x-(NaNbO3)x; (0 <x <70 wt%). These data show the presence of a large field of calcite plus liquid and the absence of NaNbO3 as a primary liquidus phase until the melts contain greater than 60 wt% NaNbO3. The primary crystallization fields of calcite and NaNbO3 are separated by a steep thermal valley located at about 55 wt% NaNbO3. Quenched liquids contain calcite, portlandite, sodium carbonates and perovskite-structured calcium niobates. The maximum solubility of Nb2O5 in this system is estimated to be on the order of 48 wt% at 650 °C at 0.1 GPa pressure. The experiments show that perovskite-structured compounds rather than pyrochlore crystallize from these fluorine-free, water-rich melts. 相似文献
5.
Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO2 = 43.7–45.7 wt.per cent, A12O3 = 1.6O–8.21 wt. per cent, CaO = 0.70–8.12wt. per cent, alk = 0.10–0.90 wt. per cent and Mg/(Mg+Fe2+)= 0.94–0.85) have been investigated in the hypersolidusregion from 800? to 1250?C with variable activities of H2O,CO2, and H2. The vapor-saturated peridotite solidi are 50–200?Cbelow those previously published. The temperature of the beginningof melting of peridotite decreases markedly with decreasingMg/(Mg+SFe) of the starting material at constant CaO/Al2O3.Conversely, lowering CaO/Al2O3 reduces the temperature at constantMg/(Mg+Fe) of the starting material. Temperature differencesbetween the solidi up to 200?C are observed. All solidi displaya temperature minimum reflecting the appearance of garnet. Thisminimum shifts to lower pressure with decreasing Mg/(Mg + Fe)of the starting material. The temperature of the beginning ofmelting decreases isobarically as approximately a linear functionof the mol fraction of H2O in the vapor (XH2Ov). The data alsoshow that some CO2 may dissolve in silicate melts formed bypartial melting of peridotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H2O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or co-exist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aHjo conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. Itis suggested that komatiite in Precambrian terrane could formby direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of XH2Ov = 0.5–0.25 (XCO2v= 0.5–0.75). Such activities of H2O result in meltingat depths ranging between 125 and 175 km in the mantle. Thisrange is within the minimum depth generally accepted for theformation of kimberlite. 相似文献
6.
Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO2=43?7–45?7 wt. percent, Al2O3=1?6O–8?21 wt. per cent, CaO=0?70–8?12wt. per cent,alk=0?10–0?90 wt. per cent and Mg/(Mg+Fe2+)=0?94–0?85)have been investigated in the hypersolidus region from 800?to 1250?C with variable activities of H2O, CO2, and H2. Thevapor-saturated peridotite solidi are 50–200?C below thosepreviously published. The temperature of the beginning of meltingof peridotite decreases markedly with decreasing Mg/(Mg+Fe)of the starting material at constant CaO/Al2O3. Conversely,lowering CaO/Al2O3 reduces the temperature at constant Mg/(Mg+Fe)of the starting material. Temperature differences between thesolidi up to 200?C are observed. All solidi display a temperatureminimum reflecting the appearance of garnet. This minimum shiftsto lower pressure with decreasing Mg/(Mg+Fe) of the startingmaterial. The temperature of the beginning of melting decreasesisobarically as approximately a linear function of the mol fractionof H2O in the vapor (XH2O). The data also show that some CO2may dissolve in silicate melts formed by partial melting ofperidotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H2O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or coexist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aH2O conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. It is suggested that komatiite in Precambrian terrane couldform by direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of (). Such activities of H2Oresult in melting at depths ranging between 125 and 175 km inthe mantle. This range is within the minimum depth generallyaccepted for the formation of kimberlite. 相似文献
7.
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
8.
Low melting temperature for calcite at 1000 bars on the join CaCO3‐H2O – some geological implications 
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下载免费PDF全文 Melting experiments of calcite were performed on the join CaCO3‐H2O at a pressure of 1000 bars. The system evolves to the ternary CaO‐H2O‐CO2 system during melting experiments. Our experiments show that partial melting of calcite begins at a low temperature, below 650 °C. Such a low partial melting temperature for carbonates revives the debate about the presence of carbonate melts in the upper crust. More specifically, the conditions for carbonate partial melting are present in carbonate host rocks undergoing contact metamorphism at high temperatures in the presence of water‐rich fluid. The presence of carbonate melts influences physical parameters such as viscosity and permeability in contact aureoles, and, furthermore, decarbonation reactions release massive amounts of CO2. 相似文献
9.
John M. Ferry 《Contributions to Mineralogy and Petrology》1996,124(3-4):235-254
Siliceous dolomites and limestones contain abundant retrograde minerals produced by hydration-carbonation reactions as the
aureole cooled. Marbles that contained periclase at the peak of metamorphism bear secondary brucite, dolomite, and serpentine;
forsterite-dolomite marbles have retrograde tremolite and serpentine; wollastonite limestones contain secondary calcite and
quartz; and wollastonite-free limestones have retrograde tremolite. Secondary tremolite never appears in marbles where brucite
has replaced periclase or in wollastonite-bearing limestones. A model for infiltration of siliceous carbonates by CO2-H2O fluid that assumes (a) vertical upwardly-directed flow, (b) fluid flux proportional to cooling rate, and (c) flow and reaction
under conditions of local equilibrium between peak temperatures and ≈400 °C, reproduces the modes of altered carbonate rocks,
observed reaction textures, and the incompatibility between tremolite and brucite and between tremolite and wollastonite.
Except for samples from a dolomite xenolith, retrograde time-integrated flux recorded by reaction progress is on the order
of 1000 mol fluid/cm2 rock. Local focusing of flow near the contact is indicated by samples from the xenolith that record values an order of magnitude
greater. Formation of periclase, forsterite, and wollastonite at the peak of metamorphism also required infiltration with
prograde time-integrated flux approximately 100–1000 mol/cm2. The comparatively small values of prograde and retrograde time-integrated flux are consistent with lack of stable isotope
alteration of the carbonates and with the success of conductive thermal models in reproducing peak metamorphic temperatures
recorded by mineral equilibria. Although isobaric univariant assemblages are ubiquitous in the carbonates, most formed during
retrograde metamorphism. Isobaric univariant assemblages observed in metacarbonates from contact aureoles may not record physical
conditions at the peak of metamorphism as is commonly assumed.
Received: 19 September 1995 / Accepted: 14 March 1996 相似文献
10.
Olivier Regnault Vincent Lagneau Hubert Catalette Hélène Schneider 《Comptes Rendus Geoscience》2005,337(15):1331-1339
Carbon dioxide sequestration in deep aquifers and depleted oilfields is a potential technical solution for reducing green-house gas release to the atmosphere: the gas containment relies on several trapping mechanisms (supercritical CO2, CO2(sc), dissolution together with slow water flows, mineral trapping) and on a low permeability cap-rock to prevent CO2(sc), which is less dense than the formation water, from leaking upwards. A leakproof cap-rock is thus essential to ensure the sequestration efficiency. It is also crucial for safety assessment to identify and assess potential alteration processes that may damage the cap-rock properties: chemical alteration, fracture reactivation, degradation of injection borehole seals, etc. The reactivity of the host-rock minerals with the supercritical CO2 fluid is one of the potential mechanisms, but it is altogether unknown. Reactivity tests have been carried out under such conditions, consisting of batch reactions between pure minerals and anhydrous supercritical CO2, or a two-phase CO2/H2O fluid at 200?°C and 105/160 bar. After 45 to 60 days, evidence of appreciable mineral-fluid reactivity was identified, including in the water-free experiments. For the mixed H2O/CO2 experiments, portlandite was totally transformed into calcite; anorthite displayed many dissolution patterns associated with calcite, aragonite, tridymite and smectite precipitations. For the anhydrous CO2 experiments, portlandite was totally carbonated to form calcite and aragonite; anorthite also displayed surface alteration patterns with secondary precipitation of fibrous calcite. To cite this article: O. Regnault et al., C. R. Geoscience 337 (2005). 相似文献
11.
The Solubility of Water and Effects of Oxygen Fugacity and Water Content on Crystallization in Mafic Magmas 总被引：6，自引：3，他引：3
The solubility of water in a basaltic and in an andesitic melthas been determined in the pressure range from approximately1,000 to 6,000 bars at 1,100? C. The solubility in basalticmelt is 3.1 weight percent at 1,000 bars and 9.4 weight percentat 6,000 bars; in the andesitic melt it is 4.5 weight percentat 1,000 bars and 10.1 weight percent at 5,300 bars. The temperaturesof appearance of the primary, secondary, and tertiary phasesin the basalt have been determined at 1,000 bars water pressureand at the fo2's of the magnetite+hematite (MH), magnetite+fayalite+quartz(MFQ) and magnetite+w?stite (MW) buffers. Results are as follows: Buffer Pyroxene Plagioclase Ore mineral MH 1,095?C 1,065?C 1,230?C MFQ 1,040? 1,015? 1,010? MW 1,020? 1,010? 995? A comparison of the solubility of water at 1,100?C and up toapproximately 6,000 bars pressure in several silicate melts,ranging in composition from granitic to gabbroic, indicatesthat the spread of solubility is narrower than has been supposed.The marked effect of fo2's on the crystallization sequence inthe Columbia River basalt confirms the importance of this factorin determining liquid lines of descent. In experiments withlow fo2's (MW buffer) and 1,000 bars water pressure, the basaltwas completely liquid at the relatively low temperature of 1,020?C. 相似文献
12.
Mineral paragenescs in the prehnite-pumpellyite to greenschistfades transition of the Karmutsen metabasites are markedly differentbetween amygdule and matrix, indicating that the size of equilibriumdomain is very small. Characteristic amygdule assemblages (+chlorite + quartz) vary from: (1) prehnite + pumpeUyite + epidote,prehnite + pumpellyite + calcite, and pumpellyite + epidote+ calcite for the prehnite-pumpellyite facies; through (2) calcite+ epidote + prehnite or pumpellyite for the transition zone;to (3) actinolite + epidote + calrite for the greenschist facies.Actinolite first appears in the matrix of the transition zone.Na-rich wairakites containing rare analcime inclusions coexistwith epidote or Al-rich pumpellyite in one prehnite-pumpellyitefacies sample. Phase relations and compositions of these wairakite-bearingassemblages further suggest that pumpellyite may have a compositionalgap between 0.10 and 0.15 XFe?. Although the facies boundaries are gradational due to the multi-varianceof the assemblages, several transition equilibria are establishedin the amygdule assemblages. At low Xco2, pumpellyite disappearsprior to prehnite by a discontinuous-type reaction, pumpellyite+ quartz + CO2 = prehnite + epidote + calcite + chlorite + H2O,whereas prehnite disappears by a continuous-type reaction, prehnite+ CO2 = calcite + epidote + quartz-l-H2O. On the other hand,at higher XCO2 a prehnite-out reaction, prehnite + chlorite+ H2O + CO2 = calcite + pumpellyite + quartz, precedes a pumpellyiteoutreaction, pumpellyite + CO2 = calcite + epidote + chlorite +quartz + H2O. The first appearance of the greenschist faciesassemblages is defined at both low and high XCOj by a reaction,calcite + chlorite + quartz = epidote + actinolite+ H2O + CO2.Thus, these transition equilibria are highly dependent on bothXFe3+ + of Ca-Al silicates and XH20 of the fluid phase. Phaseequilibria together with the compositional data of Ca-Al silicatesindicate that the prehnite-pumpellyite to greenschist faciestransition for the Karmutsen metabasites occurred at approximately1.7 kb and 300?C, and at very low Xco2, probably far less than0.1. 相似文献
13.
During an experimental investigation of the metamorphism of siliceous dolomites the equilibrium data of the heterogeneous bivariant reaction 1 $$3{\text{ dolomite + 4 quartz + 1 H}}_{\text{2}} O \rightleftharpoons + 3 calcite + 3 CO_2 $$ were determined for the total fluid pressures of 1,000, 3,000 and 5,000 bars. The equilibrium conditions were found by experiments in which dolomite, quartz and water react to form talc, calcite and CO2, as well as by experiments with reversible reaction direction. Results are shown on the temperature- \(X_{CO_2 } \) -diagram of Fig. 3. The temperature of formation of talc and calcite depends to a considerable extent on the composition of the CO2-H2O-gas phase; this can be read straight off the isobaric (P f =const.) equilibrium curves in Fig. 3. In addition a strong dependence of the equilibrium temperature on the total pressure P f was established (see Fig. 5). At a total gas pressure of 1,000 bars dolomite and quartz can react, according to the composition of the CO2-H2O-gas phase, to talc and calcite over the whole of the temperature range between about 350° and 490° C. This indicates that at low pressures the formation of talc and calcite takes place in the field of the albite-epidote-hornfels facies. At a pressure of 3,000 bars dolomite and quartz are stable up to about 550° C if the fluid phase is rich in carbon dioxide and correspondingly poor in water. Thus, this paragenesis can occur up to the stability field of staurolite [see annotation (5)] if the partial pressure of CO2 is large. At the higher total gas pressure of 5,000 bars dolomite and quartz react even at medium CO2-concentrations only at about 580° C to give talc and calcite. Therefore it is expected that in regional metamorphism at about 5,000 bars pressure or more the paragenesis dolomite plus quartz exists up to and within the stability field of staurolite and reacts only here to form talc and calcite after reaction (1) or tremolite and calcite after the following reaction (2)1: $$5 dolomite + 8 quartz + 1 H_2 O \rightleftharpoons 1 tremolite + 3 calcite + 7 CO_2 $$ . The exact physico-chemical conditions under which dolomite, quartz and water react on the one hand to form talc, calcite and CO2, and on the other hand to form tremolite, calcite and carbon dioxide, will be discussed later when our experimental investigations on the formation of tremolite are completed. First results were already published in a short note by Metz, Puhan and Winkler (1968). 相似文献
14.
Contact-mctamorphic assemblages in ophicarbonate from the Bergellaureole correspond either to model isobaric invariant T-XCO2points [Atg-Cal-Di-Tr-Fo (6 samples) and Atg-Cal-Tr-Fo-Dol (2)]or to isobaric univariant T-XCO2, curves [Tr-Cal-Di-Atg (18),Tr-Dol-Atg-Cal (1), Atg-Cal-Fo-Di (1), and Atg-Cal-Tr-Fo (1)].Calcite-dolomite thermometry and mineral-fluid equilibria inthe invariant assemblages record T=440–540C at P=3•5kbar. Equilibrium metamorphic fluids were very H2O rich withX CO2,=0•001–0•027. In the invariant assemblagesTr + Fo were produced by prograde decarbonation-dehydrationreactions. In contrast, measured modes and reaction texturesin samples with univariant assemblages indicate thai Tr wasproduced by carbonation reactions. The apparent paradox of simultaneousdecarbonation reactions in the model isobaric invariant assemblagesand carbonation reactions in univariant assemblages is resolvedby local mineral-fluid equilibrium and fluid flow through ophicarbohatesin the direction of decreasing temperature as the aureole heated.Time-integrated flux (q) was computed from measured reactionprogress in 28 samples for models of both horizontal and verticaldown-temperature flow. Results are similar, with q decreasingrapidly from (0•2–5•1) 105 cm3 fluid/cm2 rock1•3–1•7 km from the intrusion to 0–0•6105cm3/cm2 at 1•8–4•0 km. The decrease in q ismore consistent with vertical than horizontal flow. Variationsin time-integrated flux of more than an order of magnitude arerecorded by samples from the same outcrop. The absence of carbonatein adjacent metaperidotite indicates that flow was confinedto the ophicarbonate. Channelized, spatially heterogeneous,vertical flow can be explained by the brecciation and strongvertical foliation of the ophicarbonate relative to surroundingmassive metaperidotite. Generation of metamorphicfluids by decarbonation-dehydrationreactions within the ophicarbonates explains larger averageflux 1–2 km from the intrusion compared with more distalpoints. KEY WORDS: Bergell; contact metamorphism; fluid flow; ophicarbonate
*Telephone: (410) 516-8121. Fax: (410) 516-7933 相似文献
15.
The occurrence of talc and tremolite in a temperature gradient was investigated in siliceous calcite-dolomite sediments exposed along a strip in the southeastern part of the Damara Orogen. Five bivariant reactions may lead to the formation of talc and tremolite:
- 3 dolomite+4 quartz+1 H2O ? 1 talc+3 calcite+3 CO2
- 5 talc+6 calcite+4 quartz ? 1 tremolite+6 CO2+2 H2O
- 2 talc+3 calcite ? 1 tremolite+1 dolomite+1 CO2+1 H2O
- 5 dolomite+8 quartz+1 H2O ? 1 tremolite+3 calcite+7 CO2
- 2 dolomite+1 talc+4 quartz ? 1 tremolite+4 CO2.
16.
Summary ?The low-pressure eutectic for the coprecipitation of calcite, portlandite, and periclase/brucite (with H2O-rich vapor) has served as a model for the existence and crystallization of carbonatite magmas. Attempts to determine conditions
for the appearance of dolomite at this eutectic have been unsuccessful. We have discovered a second low-temperature eutectic
for more magnesian liquids which excludes portlandite and includes dolomite (all results are vapor-saturated). Addition of
Ca(OH)2-Mg(OH)2 to CaCO3-MgCO3 at 0.2 GPa depresses the liquidus to temperatures below the crest of the calcite-dolomite solvus; the vapor-saturated liquidus
surface falls steeply, and the field boundary for liquids coexisting with calcite and periclase reaches a peritectic at 880 °C,
where a narrow field for liquidus dolomite begins, extending down to the eutectic at 659 °C for the coprecipitation of calcite,
dolomite and periclase (brucite should replace periclase at slightly higher pressures). The calcite liquidus is very large.
The field boundary for coexistence of calcite and dolomite extends approximately in the direction from CaMg(CO3)2 towards Mg(OH)2. The results illustrate conditions for the formation of mineral-specific cumulates from variable magma compositions. Hydrous
(or sodic) carbonate-rich liquids with compositions from CaCO3 to CaMg(CO3)2 will precipitate calcite-carbonatites first, followed by calcite-dolomite-carbonatites, with the prospect of precipitating
dolomite-carbonatite alone through a limited temperature interval, and with periclase joining the assemblage in the closing
stages. Periclase in the Fe-free system may represent the ubiquitous occurrence of magnetite in natural carbonatites. The
restricted range for the precipitation of dolomite-carbonatites adds credibility to the evidence for primary magnesiocarbonatite
(near-dolomite composition) magmas. Magnesiocarbonatite magmas can precipitate much calcite-carbonatite rock.
Received July 20, 1998;/revised version accepted August 18, 1999 相似文献
Zusammenfassung ?Calciokarbonatitische und magnesiokarbonatitische Gesteine und Magmen im System CaO-MgO-CO 2 -H 2 O bei 0.2 GPa Das Niedrigdruck-Eutektikum der gemeinsamen Ausscheidung von Calcit, Portlandit und Periklas/Brucit (mit H2O-reicher Fluidphase) diente als Modell um die Existenz und Kristallisation karbonatitischer Magmen zu erkl?ren. Versuche die Bedingungen des Auftretens von Dolomit an diesem Eutektikum zu bestimmen blieben bisher ergebnislos. Wir entdeckten ein zweites Niedrigtemperatur-Eutektikum für magnesiumreichere Schmelzen, das Portlandit ausschlie?t, aber Dolomit inkludiert (alle Ergebnisse bei Fluids?ttigung). Die Zugabe von Ca(OH)2-Mg(OH)2 zu CaCO3-MgCO3 bei 0.2 GPa senkt den Liquidus auf Temperaturen unter die Solvus-Schwelle von Calcit-Dolomit. Die fluidges?ttigte Liquidusfl?che verl?uft steil und die Grenzfl?che von Schmelze, die mit Calcit und Periklas koexistiert erreicht ein Peritektikum bei 880 °C. Dort ?ffnet sich ein schmales Feld für Liquidus-Dolomit, das bis zum Eutektikum bei 659 °C reicht, an dem Calcit, Dolomit und Periklas (Brucit sollte Periklas bei geringfügig h?heren Drucken ersetzen) gemeinsam ausgeschieden werden. Der Calcit- Liquidus ist sehr gro?. Die Linie an der Calcit und Dolomit koexistieren erstreckt sich ungef?hr von CaMg(CO3)2 zu Mg(OH)2. Die Ergebnisse zeigen die Bildungsbedingungen für die Bildung mineralspezifischer Kumulate aus unterschiedlichen Magmenzusammensetzungen. Aus w?ssrigen (oder Na-reichen) karbonatreichen Schmelzen mit Zusammensetzungen zwischen CaCO3 und CaMg(CO3)2 werden sich zuerst Calcitkarbonatite und dann Calcit-Dolomitkarbonatite ausscheiden, mit der M?glichkeit Dolomitkarbonatite über ein sehr eingeschr?nktes Temperaturintervall zu bilden und mit Periklas, der zu dieser Vergesellschaftung im Endstadium hinzukommt. Periklas im Fe-freien System k?nnte das weitverbreitete Analog zu Magnetit in natürlichen Karbonatiten sein. Der enge Bereich für die Ausscheidung von Dolomitkarbonatiten untermauert die Existenz prim?rer magnesiokarbonatitischer Magmen (nahe der Zusammensetzung von Dolomit). Magnesiokarbonatitische Magmen k?nnen daher entsprechende Mengen an calcitkarbonatitischen Gesteinen ausscheiden.
Received July 20, 1998;/revised version accepted August 18, 1999 相似文献
17.
We explore the partial melting behavior of a carbonated silica-deficienteclogite (SLEC1; 5 wt % CO2) from experiments at 3 GPa and comparethe compositions of partial melts with those of alkalic andhighly alkalic oceanic island basalts (OIBs). The solidus islocated at 1050–1075 °C and the liquidus at 1415 °C.The sub-solidus assemblage consists of clinopyroxene, garnet,ilmenite, and calcio-dolomitic solid solution and the near solidusmelt is carbonatitic (<2 wt % SiO2, <1 wt % Al2O3, and<0·1 wt % TiO2). Beginning at 1225 °C, a stronglysilica-undersaturated silicate melt (34–43 wt % SiO2)with high TiO2 (up to 19 wt %) coexists with carbonate-richmelt (<5 wt % SiO2). The first appearance of carbonated silicatemelt is 100 °C cooler than the expected solidus of CO2-freeeclogite. In contrast to the continuous transition from carbonateto silicate melts observed experimentally in peridotite + CO2systems, carbonate and silicate melt coexist over a wide temperatureinterval for partial melting of SLEC1 carbonated eclogite at3 GPa. Silicate melts generated from SLEC1, especially at highmelt fraction (>20 wt %), may be plausible sources or contributingcomponents to melilitites and melilititic nephelinites fromoceanic provinces, as they have strong compositional similaritiesincluding their SiO2, FeO*, MgO, CaO, TiO2 and Na2O contents,and CaO/Al2O3 ratios. Carbonated silicate partial melts fromeclogite may also contribute to less extreme alkalic OIB, asthese lavas have a number of compositional attributes, suchas high TiO2 and FeO* and low Al2O3, that have not been observedfrom partial melting of peridotite ± CO2. In upwellingmantle, formation of carbonatite and silicate melts from eclogiteand peridotite source lithologies occurs over a wide range ofdepths, producing significant opportunities for metasomatictransfer and implantation of melts. KEY WORDS: carbonated eclogite; experimental phase equilibria; partial melting; liquid immiscibility; ocean island basalts 相似文献
18.
Field evidence from intrusive and effusive carbonatites supportsthe existence of calciocarbonatite magmas. Published experimentalevidence in the model system Na2O–CaO–Al2O3–SiO2–CO2indicated the formation of nearly pure (99%) CaCO3 immiscibleliquids from a carbonated silicate liquid. This evidence hasbeen used to support interpretations of extremely CaCO3-richcalciocarbonatite magmas, and immiscible liquids with compositionsof almost pure CaCO3 in metasomatized mantle peridotite andeclogite. Detailed phase relationships are constructed in themodel system, based on phase fields intersected by the joinNaAlSi3O8–CaCO3 (Ab-CC) at 1.0, 1.5, and 2.5 GPa between1100 and 1500C, and analyzed immiscible liquids. The miscibilitygap between silicate-rich liquid and carbonate-rich liquid intersectedby the join Ab-CC contracts considerably with decreasing pressure:2.5 GPa, between Ab10CC90 (by wt%) and Ab65CC35 above 1310C;1.5 GPa, betweenAb23CC77 and Ab43CC57 above 1285C; 1.0 GPa,not intersected. The liquidus piercing point between calciteand silicates becomes enriched in CaCO3 with decreasing pressure,from Ab80CC20 at 2.5 GPa to Ab47CC53 at 1.0 GPa. No immiscibleliquid contains more than 80% dissolved CaCO3, and all containat least 5% Na2CO3. A round CaCO3 phase exhibiting morphologysimilar to that displayed by immiscible liquid globules is determinedto be crystalline calcite under experimental conditions. Thetopology of the phase fields and field boundaries illustratesthe kinds of processes and controls existing in magmatic systems.Calciocarbonatite magmas cannot be produced by equilibrium immiscibilityprocess in the mantle. Carbonated silicate magmas in the crustyield residual calciocarbonatite magmas by fractionation alongthe silicate-calcite field boundary, reached either directlyfrom the silicate liquidus or more commonly via the miscibilitygap. Immiscible carbonaterich magmas when freed from the silicateparent cool down a sleep silicate liquidus until they reacha silicate-carbonate field boundary. There is no experimentalevidence for immiscible calciocarbonatite magmas with > 80%CaCO3, and calcite lapilli cannot be formed from 99% CaCO3 magmas.Sovites are surely cumulates. KEY WORDS: carbonatite; join NaAlSi3O8–CaCO3; liquid immiscibility; sovite
* Corresponding author. Telephone: (818)-395–6239. Fax: (818)-568–0935. e-mail: wjl{at}gpi.caltech.edu 相似文献
19.
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
20.
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