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1.
Liquidus phase relationships in the CaAlAl–SiO6–Mg2SiO4–CaMgSi2O6–CaAlSi2O8 portion of the simplified basalt tetrahedron in the CaO–MgO–SiO2–Al2O3 system have been experimentally determined at 20 kbar pressure. The fo+di ss+sp+li univariant curve, that pierces the fo-di-an join and meets the fo+di ss+ enss+sp+li invariant point in the basalt tetrahedron, extends all the way to and pierces the di-fo-CaTs join, the limit of the simplified basalt tetrahedron toward the silica undersaturated portion.An algebraic method, relying on compositions of two successive liquids on a univariant curve and those of the crystalline phases in equilibrium with the respective liquids, is developed to identify the type of reaction that takes place along an isobarically univariant curve and to detect whether there is a temperature maximum on that curve. Use of this method for the di ss+fo+sp+li univariant equilibria shows that a temperature maximum exists on this curve at the composition Fo11Di56An3CaTs30, very close to and slighthly to the SiO2-rich side of the fo-di-CaTs join. The temperature along the univariant curve continuously decreases from the temperature maximum (1500°C) to the invariant point (1475°C) where the univariant curve is terminated by the appearance of e ss as a member of the equilibrium assemblage. Along this part of the curve, a reaction relationship occurs according to the equation fo+li=di ss+ sp. Compositions of di ss in equilibrium with the liquids from the temperature maximum to the fo+di ss+enss+ sp+li invariant point range from Di66En9CaTs25 to Di36En40CaTs24. Because of the reaction relationship of forsterite with liquid, fractional crystallization of a model alkalic basaltic liquid would cause liquids to move off the fo-di ss-sp-li univariant curve onto the sp-di ss divariant surface. Crystallization of di ss and sp would then lead to silica enrichment of residual liquids. Thus at pressures below 30 kbar, at which pressure the Al2O3–CaSiO3–MgSiO3 plane becomes a new thermal divide cutting through both the tholeiitic and alkalic volumes, alkalic liquids will fractionate toward tholeiitic compositions without crossing a thermal divide. This relationship would be expected to persist at pressures down to about 4 kbar where a maximum on the fo-di-an-li boundary line causes a thermal divide near the fo-di-an plane. Strongly SiO2-undersaturated liquids (e.g. nephelinites, basanites), on the other hand, cannot be derived from SiO2-undersaturated basalts (e.g. alkali olivine basalt) by fractional crystallization at 20 kbar. We also found that no gt primary phase volume cuts the wo-en-Al2O3 join at 20 kbar pressure. The wehrlite, the olivine clinopyroxenite, and the Al-augite group lherzolite xenoliths, containing highly aluminous clinopyroxenes (enriched in Ca-Tschermak), can be interpreted as crystal cumulates from alkalic basalts in the light of this experimental study. This is consistent with the mode of origin of these xenoliths proposed from petrographic, mineralogic, and geochemical studies.Abbreviations and notations di CaMgSi2O6 - fo Mg2SiO4 - an CaAl2Si2O8 - CaTs CaAlAlSiO6 - sp MgAl2O4 - en MgSiO3 - wo CaSiO3 - gt Ca3Al2Si3O12–Mg3Al2Si3O12 - qz SiO2 - li Liquid - gl glass - ss Solid Solution - A An mxn matrix - X A column vector - kbar kilobar  相似文献   

2.
In the system CaO-MgO-Al2O3-SiO2, the tetrahedron CaMgSi2O6(di)-Mg2SiO4(fo)-SiO2-CaAl2 SiO6(CaTs) forms a simplified basalt tetrahedron, and within this tetrahedron, the plane di-fo-CaAl2Si2O8(an) separates simplified tholeiitic from alkalic basalts. Liquidus phase relations on this join have been studied at 1 atm and at 7, 10, 15, and 20 kbar. The temperature maximum on the 1 atm isobaric quaternary univariant line along which forsterite, diopside, anorthite, and liquid are in equilibrium lies to the SiO2-rich side of the join di-fo-an. The isobaric quaternary invariant point at which forsterite, diopside, anorthite, spinel, and liquid are in equilibrium passes, with increasing pressure, from the silica-poor to the silica-rich side of the join di-fo-an, which causes the piercing points on this join to change from forsterite+diopside+anorthite+liquid and forsterite +spinel+anorthite+liquid below 5 kbar to forsterite +diopside+spinel+liquid and diopside +spinel+anorthite+liquid above 5 kbar. As pressure increases, the forsterite and anorthite fields contract and the diopside and corundum fields expand. The anorthite primary phase field disappears entirely from the join di-fo-an between 15 and 20 kbar. Below about 4 kbar, the join di-fo-an represents, in simplified form, a thermal divide between alkalic and tholeiitic basalts. From about 4 to at least 12 kbar, alkalic basalts can produce tholeiitic basalts by fractional crystallization, and at pressures above about 12 kbar, it is possible for alkalic basalt to be produced from oceanite by crystallization of both olivine and orthopyroxene. If alkalic basalts are primary melts from a lherzolite mantle, they must be produced at high pressures, probably greater than about 12 kbar.Department of Geosciences, University of Texas at Dallas Contribution No. 327. Hawaii Institute of Geophysics Contribution No. 814.  相似文献   

3.
In the system CaO-MgO-A12O3-SiO2 the tie lines connecting anorthite with other phases are sequentially broken down with increasing pressure according to the following univariant reactions: anorthite+ enstatitess+sillimanite pyrope-grossularss+quartz (3), anorthite+enstatitess pyrope-grossularss+diopsidess+quartz (2), anorthite+pyrope-grossularss+ quartz diopsidess+kyanite (4) and anorthite+diopsidess grossular-pyropess +kyanite+quartz (8). At 1,200 ° C these reactions occur at 14.5± 0.5, 15.5±0.5, 19.5±0.5 and 26.4±1 kilobar and have positive slopes (dP/dT) of 1±0.5, 2.8±0.5, 13.3±0.5 and 24±2bars/°C respectively. An invariant point involving kyanite rather than sillimanite, occurs at 850 °C±25 °C and 14.5±0.5kbar at the intersection of reactions (3), (2) and (4). Reaction(4) exhibits significant curvature with an increase in dP/dT from 13.3±0.5 to 18.5± 0.5 bars/°C between 1,050° and 850° C. The pressure at which the complete grossular-pyrope join is stable with quartz is estimated at 41 ± 1 kbar at 1,200 ° C. The pressure at which garnet appears according to reaction (2) is lowered by 5 kbar for a composition with anorthite and orthopyroxene (En0.5Fs0.5). Enstatite and plagioclase (An0.5Ab0.5) first produce garnet at 2 kbar higher pressure than enstatite and pure anorthite (reaction (2)). The calcium content of garnet in various divariant assemblages is relatively insensitive to temperature but very sensitive to pressure, it is therefore a useful geobarometer. At metamorphic temperatures of 700–850 °C pressures of 8–10 kbar are required for the formation of quartz-bearing garnet granulites containing calcic plagioclase and with (Mg/Mg+Fe) bulk = 0.5.  相似文献   

4.
The effect of Cr on the silicate system has been studied in air at 1 atm by adding a small amount of MgCr2O4 (0.2–0.5 wt.%) to the join Mg2SiO4 (forsterite) — CaAl2Si2O8 (anorthite) — CaMgSi2O6 (diopside), which has been considered to form a thermal divide in the system CaO-MgO-Al2O3-SiO2. The spinel primary field is enlarged compared with that in the Cr-free join at the expense of the anorthite primary field. The piercing points forsterite+anorthite+diopside+liquid and forsterite+anorthite+spinel+liquid approach each other with increasing MgCr2O4, meet at the join with 0.25 wt.% MgCr2O4 (0.20 wt.% Cr2O3) to form the ‘isobaric quaternary invariant point’ forsterite+anorthite+diopside+spinel+liquid, and then separate again as new ‘piercing points’ of diopside+spinel+anorthite+liquid and forsterite+diopside+ spinel+liquid. This process indicates that the join Mg2SiO4-CaAl2Si2O8-CaMgSi2O6 containing more than 0.2 wt.% Cr2O3 cannot be a thermal divide in the basalt tetrahedron. The results of the present study show that the presence of a minor amount of Cr causes a significant effect on the phase relations and therefore, the role of Cr must be taken into account in the formulation of a petrologic model.  相似文献   

5.
Crystal-melt relations at a water vapour pressure of 1 kilobar have been determined for planes at 3, 5, 7.5, and 10 weight per cent anorthite in the system NaAlSi3O8KAlSi3O8-CaAl2Si2O8-SiO2. The ratio of the silicate components in the liquids which are in univariant equilibrium with plagioclase, alkali feldspar, quartz and gas are Ab31Or28Q38An3 (weight per cent) at 730°±5–10° C, Ab21Or34Q40An5 at 745°±5–10° C and Ab10Or39 Q43.5An7.5 at 780°±10° C. The univariant curve on which the above compositions lieoriginates on the H2O-saturated Or-An-Q plane at a composition containing less than 10 weight per cent An and terminates within 1.5 weight per cent An of the H2O-saturated Or-Ab-Q plane. Experimental data for the synthetic system have been used to illustrate a discussion on the partial melting of metasediments and the possible significance of such a process with respect to the genesis of granitic rocks. Data taken from the literature (Winkler and v. Platen, 1960, 1961a) have been used to illustrate that the normative salic composition of a sediment has a strong influence on the composition of any melt which form when such a rock is subjected to high temperatures and pressures.  相似文献   

6.
Amoeboid olivine aggregates (AOAs) in primitive (unmetamorphosed and unaltered) carbonaceous chondrites are uniformly 16O-enriched (Δ17O ∼ −20‰) and consist of forsterite (Fa<2), FeNi-metal, and a refractory component (individual CAIs and fine-grained minerals interspersed with forsterite grains) composed of Al-diopside, anorthite, ±spinel, and exceptionally rare melilite (Åk<15); some CAIs in AOAs have compact, igneous textures. Melilite in AOAs is replaced by a fine-grained mixture of spinel, Al-diopside, and anorthite. Spinel is corroded by anorthite or by Al-diopside. In ∼10% of > 500 AOAs studied in the CR, CV, CM, CO, CH, CB, and ungrouped carbonaceous chondrites Acfer 094, Adelaide, and LEW85332, forsterite is replaced to a various degree by low-Ca pyroxene. There are three major textural occurrences of low-Ca pyroxene in AOAs: (i) thin (<10 μm) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) haloes and subhedral grains around FeNi-metal nodules in AOA peripheries, and (iii) thick (up to 70 μm) continuous layers with abundant tiny inclusions of FeNi-metal grains around AOAs. AOAs with low-Ca pyroxene appear to have experienced melting of various degrees. In the most extensively melted AOA in the CV chondrite Leoville, only spinel grains are relict; forsterite, anorthite and Al-diopside were melted. This AOA has an igneous rim of low-Ca pyroxene with abundant FeNi-metal nodules and is texturally similar to Type I chondrules.Based on these observations and thermodynamic analysis, we conclude that AOAs are aggregates of relatively low temperature solar nebular condensates originated in 16O-rich gaseous reservoir(s), probably CAI-forming region(s). Some of the CAIs were melted before aggregation into AOAs. Many AOAs must have also experienced melting, but of a much smaller degree than chondrules. Before and possibly after aggregation, melilite and spinel reacted with the gaseous SiO and Mg to form Ca-Tschermakite (CaAl2SiO6)-diopside (CaMgSi2O6) solid solution and anorthite. Solid or incipiently melted olivine in some AOAs reacted with gaseous SiO in the CAI- or chondrule-forming regions to form low-Ca pyroxene: Mg2SiO4 + SiO(g) + H2O(g) = Mg2Si2O6 + H2(g). Some low-Ca pyroxenes in AOAs may have formed by oxidation of Si-bearing FeNi-metal: Mg2SiO4 + Si(in FeNi) + 2H2O(g) = Mg2Si2O6 + 2H2(g) and by direct gas-solid condensation: Mg(g) + SiO(g) +H2O(g) = Mg2Si2O6(s) + H2(g) from fractionated (Mg/Si ratio < solar) nebular gas.Although bulk compositions of AOAs are rather similar to those of Type I chondrules, on the projection from spinel onto the plane Ca2SiO4-Mg2SiO4-Al2O3, these objects plot on different sides of the anorthite-forsterite thermal divide, suggesting that Type I chondrules cannot be produced from AOAs by an igneous fractionation. Formation of low-Ca pyroxene by reaction of AOAs with gaseous SiO and by melting of silica-rich dust accreted around AOAs moves bulk compositions of the AOAs towards chondrules, and provide possible mechanisms of transformation of refractory materials into chondrules or chondrule precursors. The rare occurrences of low-Ca pyroxene in AOAs may indicate that either AOAs were isolated from the hot nebular gas before condensation of low-Ca pyroxene or that condensation of low-Ca pyroxene by reaction between forsterite and gaseous SiO was kinetically inhibited. If the latter is correct, then the common occurrences of pyroxene-rich Type I chondrules may require either direct condensation of low-Ca pyroxenes or SiO2 from fractionated nebular gas or condensation of gaseous SiO into chondrule melts.  相似文献   

7.
New equilibrium experiments have been performed in the 20–27 kbar range to determine the upper thermal stability limit of endmember deerite, Fe 12 2+ Fe 6 3+ [Si12O40](OH)10. In this pressure range, the maximum thermal stability limit is represented by the oxygen-conserving reaction: deerite(De)=9 ferrosilite(Fs)+3 magnetite(Mag)+3 quartz(Qtz)+5 H2O(W) (1). Under the oxygen fugacities of the Ni-NiO buffer the breakdown-reduction reaction: De=12 Fs+2 Mag+5 W+1/2 O2 (10) takes place at lower temperatures (e.g. T=63° at 27 kbar). The experimental brackets can be fitted using thermodynamic data for ferrosilite, magnetite and quartz from Berman (1988) and the following 1 bar, 298 K data for deerite (per gfw): Vo=55.74 J.bar-1, So=1670 J.K-1, H f o =-18334 kJ, =2.5x10-5K-1, =-0.18x10-5 bar-1. Using these data in conjunction with literature data on coesite, grunerite, minnesotaite, and greenalite, the P-T stability field of endmember deerite has been calculated for P s=P H 2O. This field is limited by 6 univariant oxygenconserving dehydration curves, from which three have positive dP/dT slopes, the other three negative slopes. The lower pressure end of the stability field of endmember deerite is thus located at an invariant point at 250±70°C and 10+-1.5 kbar. Deerite rich in the endmember can thus appear only in environments with geothermal gradients lower than 10°C/km and at pressures higher than about 10 kbar, which is in agreement with 4 out of 5 independent P-T estimates for known occurrences. The presence of such deerite places good constraints on minimum pressure and maximum temperature conditions. From log f O 2-T diagrams constructed with the same data base at different pressures, it appears that endmember deerite is, at temperatures near those of its upper stability limit, stable only over a narrow range of oxygen fugacities within the magnetite field. With decreasing temperatures, deerite becomes stable towards slightly higher oxygen fugacities but reaches the hematite field only at temperatures more than 200°C lower than the upper stability limit. This practically precludes the coexistence deerite-hematite with near-endmember deerite in natural environments.  相似文献   

8.
Thermodynamic calculations have shown that the dP/dT slope of the reaction 4 margarite+3 quartz5 kyanite +2 zoisite+3 H2O as determined by Storre and Nitsch (1974) is too steep. This reaction has been reinvestigated using synthetic margarite, zoisite, kyanite, and natural quartz in the starting mixtures and using infrared spectroscopy to examine the run products. The experimentally determined dP/dT slope ranges between –2.2 and –17 bars/ K, which is in excellent agreement with predictions based on thermodynamics. An internally consistent set of univariant curves could be fitted to the experimental reversals for the above reaction and for the reactions margarite+ quartz anorthite+kyanite+H2O and 2 zoisite+kyanite +quartz 4 anorthite+H2O investigated by Nitsch et al. (1981) and Goldsmith (1981), respectively. Addition of up to 40 mol % of the component NaAl2(Si3Al) ·O10(OH)2 (paragonite) to margarite will increase the stability of the margarite solid solution plus quartz by 2–3 kbar without significantly affecting the dP/dT slope, making the paragenesis margarite plus quartz a good geobarometer.  相似文献   

9.
Uvarovite (Ca3Cr2Si3O12) forms a complete solid solution series with andradite (Ca3Fe 2 +3 Si3O12) below 1,137±5 ° C at a total pressure of 1 atm. Pure uvarovite decomposes to pseudowollastonite (CaSiO3)+eskolaite (Cr2O3) at 1,385 ± 10 ° C. The incorporation of Ca3Fe 2 +3 Si3O12 component in the uvarovite structure lowers the thermal stability of the garnet. The breakdown assemblage is garnetss (Ca3(Cr,Fe+3 2)Si3O12)+pseudowollastonite (CaSiO3)+hemeskolaitess(Cr,Fe+3O3). Pure andradite decomposes to pseudowollastonite (CaSiO3)+hematite (Fe2O3) at 1,137±5 °C. Andradite thermal stability is increased by incorporation of Ca3Cr2Si3O12 component by 248 °C.At 1,264±5 °C pseudowollastonite+hematite react to liquid defining a thermal minimum of the CaSiO3-Cr2O3-Fe2O3 ternary system. This minimum is located at about 64.5 wt.-% CaSiO3, 0.5 wt.-% Cr2O3, and 35.0 wt.-% Fe2O3. Uvarovite and andradite bulk compositions start to melt at 1,420 °C and 1,265 ±5 °C, respectively.The unit-cell parameter for uvarovite is 11.999 (2) Å, the refractive index 1.866 (2). The substitution of Cr+3 by Fe+3 increases a and n almost linearly toward the andradite end member which displays a unit-cell parameter of 12.059 (3) Å and a refractive index of 1.887 (2).  相似文献   

10.
Summary Chemical compositions of orthopyroxene and clinopyroxene from the Jinchuan ultramafic intrusion have been obtained by electron microprobe analysis. The Mg number (MgO/(MgO + FeO)) for both pyroxenes falls within narrow ranges, 82–87 for clinopyroxene and 81–85.5 for orthopyroxene, suggesting limited magma differentiation in regard to the present igneous body. The Al2O3 content ranges from 2.44 wt.% to 4.43 wt.% and increases with decreasing Mg of the pyroxenes, i.e., with the more evolved magma. This is attributed to the relatively greater effects of Al2O3, TiO2, Cr2O3 and Fe2O3 than that of SiO2 on pyroxene crystallization.Negative linear relationships between Ti4+ and Si4+, and Al3+ and Si4+ characterize the pyroxenes. In clinopyroxene, regression of Si4+ versus Al3+ results in a straight line with a slope of –1.012, indicating that the decrease of Si4+ in the crystal structure is matched by an increase only in tetrahedral Al3+; octahedral Al3+ has remained relatively constant. The negative linear relationship between Ti4+ and Si4+ in clinopyroxene reflects either a greater tendency of Ti4+ to occupy octahedral sites than Al3+, or that replacement of Al3+ for Si4+ demands a more efficient charge balance. The scatter in plots of Ti4+ versus Si4+ for orthopyroxene indicates that charge balance is not as critical as structure symmetry.The crystallization temperature of pyroxene is calculated to be 1108–1229°C usingWood andBanno's (1973) two pyroxene thermometer, and is within 40°C of that calculated fromWells's (1977) thermometer. The distribution coefficient (Kd) for Mg2+ and Fe2+ between clinopyroxene and orthopyroxene is estimated to be 0.86, which is higher than that of the other intrusions and lower than that of mantle nodules, but still falls within their Kd-1/T trend. This suggests that the Kd value of pyroxene is controlled mainly by temperature.
Mineralchemie der Pyroxene der Jinchuan-Intrusion, China
Zusammenfassung Die chemische Zusammensetzung von Orthopyroxenen und Klinopyroxenen aus der ultramafischen Jinchuan Intrusion wurden mit der Mikrosonde bestimmt. Die Mg-Zahl (MgO/(MgO + FeO)) beider Pyroxene liegt innerhalb enger Grenzen, 82–87 für Klinopyroxen und 81–85.5 für Orthopyroxen. Dies weist auf beschränkte magmatische Differentiation der Intrusion hin. Der Al2O3-Gehalt liegt zwischen 2.44 Gew.%. und 4.43 Gew.%. und nimmt mit der abnehmenden Mg-Zahl der Pyroxene ab, d.h. mit dem mehr entwickelten Magma. Dies wird damit erklärt, daß Al2O3, TiO2, Cr2O3 und Fe2O3 einen größeren Einfluß auf die Kristallisation der Pyroxene ausüben als SiO2.Die Pyroxene werden durch negative lineare Beziehungen zwischen Ti4+ und Si4+, sowie Al3+ und Si4+ charakterisiert. In Klinopyroxenen resultiert die Regression von Si4+ gegen Al3+ in einer geraden Linie mit einer Neigung von –1.012. Dies weist darauf hin, daß die Abnahme der Si4+ Gehalte in die Kristallstruktur durch Zunahme von ausschliesslich tetraedrischem Al3+ kompensiert wird; oktaedrisches Al3+ ist relativ konstant geblieben. Die negative lineare Beziehung zwischen Ti4+ und Si4+ in Klinopyroxenen geht entweder auf eine stärkere Tendenz des Ti4O2, oktaedrische Plätze zu besetzen zurück, oder darauf daß ein Ersatz von Al3+ für Si4+ einen effizienteren Ladungsausgleich verlangt. Die unregelmäßige Verteilung der Plots von Ti4+ gegen Si4+ in Orthopyroxenen läßt erkennen, daß Ladungsausgleich hier nicht so kritisch ist wie die Symmetrie der Struktur.Die Kristallisationstemperatur der Pyroxene wurde mit dem Zwei Pyroxenthermometer nachWood undBanno (1973) mit 1108–1229°C bestimmt. Diese Werte liegen innerhalb von 40°C des vonWells (1977) berechneten. Der Verteilungskoeffizient (Kd) für Mg2+ und Fe2+ zwischen Klinopyroxen und Orthopyroxen wird auf 0.86 berechnet; das ist höher als der aus anderen Intrusionen und niedriger als der von Mantelxenolithen, fällt aber immer noch innerhalb des Kd-1/T Trends derselben. Dies legt den Gedanken nahe, daß der Kd Wert der Pyroxene hauptsächlich durch Temperatur bestimmt wird.

With 6 Figures  相似文献   

11.
The univariant reaction governing the upper stability of heulandite (CaAl2Si7O18·6H2O), heulandite=laumontite+3 quartz+2H2O (1), has been bracketed through reversal experiments at: 155±6° C, 1000 bar; 175±6° C, 1500 bar; and 180±8° C, 2000 bar. Reversals were established by determining the growth of one assemblage at the expense of the other, using both XRD and SEM studies. The standard molal entropy of heulandite is estimated to be 783.7±16 J mol–1 K–1 from the experimental brackets. Predicted standard molal Gibbs free energy and enthalpy of formation of heulandite are –9722.3±6.3 kJ mol–1 and –10524.3±9.6 kJ mol–1, respectively. The reaction (1), together with the reaction, stilbite=laumontite+3 quartz+3 H2O, defines an invariant point at which a third reaction, stilbite=heulandite+ H2O, meets. By combining the present experimental data with past work, this invariant point is located at approximately 600 bar and 140° C. Heulandite, which is stable between the stability fields of stilbite and laumontite, can occur only at pressures higher than that of the invariant point, for = P total.These results are consistent with natural parageneses in low-grade metamorphic rocks recrystallized in equilibrium with an aqueous phase in which is very close to unity.  相似文献   

12.
The univariant high-pressure reaction of aluminous enstatite and spinel to pyrope and forsterite in the MgO-Al2O3-SiO2 system has been determined in the temperature range 900 °–1100 °C by hydrothermal reversals in the piston-cylinder apparatus using the low-friction NaCl pressure medium. A mixture of synthetic minerals, including an enstatite with 6 wt% Al2O3, with product and reactant assemblages in nearly equal amounts, was the starting material. The equilibrium pressure of 19.3±0.3 kbar at 1000 ° C and average dP/dT slope of 8.0 bars/ ° C confirm the strong curvature of the equilibrium below 1200 ° C deduced by Obata (1976) from a theoretical study of experimental Al2O3 isopleths of enstatite in the garnet field. His prediction of an absolute minimum pressure near 18 kbar of the garnet peridotite assemblage in the ternary system is undoubtedly correct.Three reversed determinations of the equilibrium Al2O3 content of enstatite in the presence of spinel +forsterite were made at points adjacent to the univariant curve. The points are 5.5 wt% Al2O3 at 950 ° C and 20 kbar, 6.2 wt% at 1000 ° C and 20 kbar and 7.2 wt% at 1080 ° C and 20 kbar. These values are somewhat higher than given by the MacGregor (1974) isopleth set and quite close to those predicted by Fujii (1976) from experimental synthesis data at higher temperatures, using the Wood and Banno (1973) model of ideal solution of the Mg2Si2O6 and MgAl2SiO6 components in enstatite to reduce the data.All of the available spinel-field isopleth data can be systematized with the use of the ideal solution model. A value of H 0 of 9000 cal fits the reduced data well, and is in agreement with the calorimetrically determined value of 8500±1900 calories. An accurate calculation of the dP/dT slope of the univariant equilibrium at 1000 ° C based on calorimetry gives 7±2bars/ °C, also in good agreement with experiment. Thus, all of the available experimental and calorimetric data are consistent with the ideal-solution aluminous enstatite model.The dP/dT slopes of the spinel-field isopleths are too large to permit their use as an accurate geobarometric scale. They do have considerable potential as a thermometric indicator for certain natural peridotites, however. The southwestern Oregon overthrust peridotite masses of Cretaceous age have enstatite of 5.6 wt% Al2O3 with spinel of nearly 80 mole% MgAl2O4. The present reduced isopleth data directly give 930 ° C for the equilibration, assuming 12 kbar pressure. A first order correction based on ideal solution departures from the ternary system, as suggested by Stroh (1976) gives 1000 ° C. Thus, the high temperatures deduced by Medaris (1972) are confirmed. The pressure cannot be deduced independently from the pyroxene Al2O3 contents.  相似文献   

13.
Phase relations for the magnesio-hornblende bulk composition, 2 CaO·4 MgO·Al2O3·7 SiO2+ excess H2O, have been investigated to 10 kb employing hydrothermal and piston-cylinder techniques. The low-temperature limit of amphibole in this system lies at 519° C, 1,000 bars, 541° C, 2,000 bars, and 718° C, 10 kb. The low-T assemblage consists of an+chl+di+tc(+f), and is related to the adjacent high-T equilibrium assemblage, amph+an+chl+f, by the solid-solid reaction (A): 2 di+tc=tr. Small amounts of aluminum, hypothesized to be preferentially dissolved in the cpx (and in the tc) relative to amph, may account for the broad P-T stability range of the di+tc assemblage in the synthetic work relative to systems involving stoichiometric tr, Ca2Mg5Si8O22(OH)2, such as are common in natural, Al-poor calc-silicate parageneses. Alternatively, the low-temperature assemblage produced in the experiments may be metastable. For the investigated bulk composition, synthetic tremolitic-cummingtonitic amphibole contains relatively modest amounts of ts, Ca2Mg3Al2 IVSi6-Al2 IVO22(OH)2; at pressures of 1,000–3,000 bars, solid solution extends from near tremolite only to about cu11tr69ts20, analogous to most analyzed natural magnesio-hornblendic specimens. At 10 kb fluid pressure, the solid solution reaches approximately cu06tr53ts41 for the investigated bulk composition, and appears to be virtually independent of temperature. Amphibole and 14 Å chl react within the amphibole stability field, along curve (B), at about 704° C and 2,000 bars, to produce an, en, fo and f (H=40.9 kcal/ mole); at pressures greater than approximately 7kb, due to the incompatibility of an and fo, the higher temperature assemblage consists of amph, an, en, sp and f. Above P fluid– T curve (B), the amphibole coexists with an+en+fo+f at low pressures; at higher pressures, the amphibole, which is in equilibrium with an+en+sp+f, is relatively more aluminous. The high-T stability limit of aluminous tr+fo lies approximately 20–25° C below the dehydration curve for stoichiometric tremolite on its own bulk composition. Reaction (C), tr+fo=2 di+5 en+f (H = 39.4 kcal/mole), produces an+di+en+f, the highest temperature subsolidus assemblage investigated for the tr50ts50 bulk composition. Hydrous melt is encountered at temperatures at least as low as 900° C at 10 kb, and at that fluid pressure coexists with amphibole over an interval of more than 60° C. Limited solid solution observed between tr and ts in nature (tr100-70) is accounted for by the restricted range of amphibole compositions produced in the present study. Such amphiboles, moreover, appear to have both high- and low-temperature stability limits, as demonstrated by the experimental results.Institute of Geophysics and Planetary Physics Publication No. 2811  相似文献   

14.
Experiments have been conducted in the P-T range 2.5–15 GPa and 850–1,500°C using bulk compositions in the systems SiO2–TiO2–Al2O3–Fe2O3–FeO–MnO–MgO–CaO–Na2O–K2O–P2O5 and SiO2–TiO2–Al2O3–MgO–CaO–Na2O to investigate the Ca-Eskola (CaEs Ca0.5□0.5AlSi2O6) content of clinopyroxene in eclogitic assemblages containing garnet + clinopyroxene + SiO2 ± TiO2 ± kyanite as a function of P, T, and bulk composition. The results show that CaEsss in clinopyroxene increases with increasing T and is strongly bulk composition dependent whereby high CaEs-contents are favoured by bulk compositions with high normative anorthite and low diopside contents. In this study, a maximum of 18 mol% CaEsss was found at 6 GPa and 1,350°C in a kyanite-eclogite assemblage garnet + clinopyroxene + kyanite + rutile + coesite. By comparison, no significant increase in CaEsss with increasing P could be observed. If the formation of oriented SiO2-rods frequently observed in eclogititc clinopyroxenes is due to the retrogressive breakdown of a CaEs-component then these textures are a cooling rather than a decompression phenomenon and are most likely to be found in kyanite-bearing eclogites cooled from temperatures ≥750°C. The presence of clinopyroxene with approx. 4 mol% CaEsss in an experiment conducted at 2.5 GPa/850°C confirms earlier suggestions based on field data that vacancy-rich clinopyroxenes are not necessarily restricted to ultrahigh pressure metamorphic conditions. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

15.
The high-temperature enthalpies of liquid and glassy CaAl2Si2O8 were measured by drop calorimetry using a diphenyl ether drop calorimeter. These data are combined with published values of the high-temperature enthalpy of crystalline anorthite and the enthalpy of vitrification of anorthite to obtain the enthalpy of fusion of anorthite. Analysis of the data yields the following preferred values (enthalpy in kcal/mol, uncertainty limits correspond to two standard deviations):enthalpy of vitrification at 985 K, v H v 985=18.6±0.6; enthalpy of the liquid at 1,830 K, H 1830 l 300 g =130.4±1.2; enthalpy of the glass at 985 K, H 985 g -H 300 g =46.7±0.4; enthalpy of crystalline anorthite between 985 and 1,830 K, H 1830 c -H 985 c =69.9±1.4; calculated enthalpy of fusion of anorthite at 1,830 K, f H 1830= 32.4±2.1.The average heat capacity of supercooled liquid CaAl2Si2O8 between the glass transition (T g 1,086 K) and the melting point (T f7=1,830 K) is 102 ± 2 cal/mol/K. The large difference between the enthalpy of fusion and the enthalpy of vitrification for the minerals anorthite and diopside is emphasized. The practice of assuming fH vH should be discontinued for silicate compounds for which T f T g.  相似文献   

16.
High-pressure, low-temperature metamorphic Mn-rich quartzites from Andros and Evvia (Euboea) islands, Greece, situated in the Eocene blueschist belt of the Hellenides, reveal different Mn-Al-Ca-Mg-silicate assemblages in response to variable metamorphic grade. On Evvia, piemontite- and/or braunite-rich quartzites which are associated with low-grade blueschists (T<400° C, P> 8 kbar) show the principle mineral assemblage quartz + montite + sursassite + braunite + Mg-chlorite + hematite + rutile + titanite. The Mn-Al-silicate sursassite, basically (Mn2+, Ca)4 Al2(Al, Fe3+, Mn3+, Mg)4Si6O21(OH)7, thus far reported as a rare mineral, locally occurs as a rockforming mineral in cm- to m-thick layers. On Andros, higher-grade quartzites (T450–500° C, P>10 kbar) of similar composition contain the assemblage quartz + piemontite + spessartine + braunite + Mg-chlorite+hematite + phengite+ phlogopite + rutile. Rare sursassite is present only as a relict phase. Additional, mostly accessory minerals in quartzites from Evvia and Andros are ardennite, Na-amphibole, acmitic clinopyroxene, albite, apatite, and tourmaline. The chemical composition of the main phases is characterized in detail.Disequilibrium textures and mineral compositions in some samples from Andros and Evvia imply the reactions sursassite + braunite + quartz = spessartine+clinochlore±hematite + H2O + O2 (1) sursassite + braunite + phengite + quartz = spessartine + phlogopite±hematite + H2O + O2 (2) and in braunite-free assemblages sursassite + Mn3+Fe –1 3+ [hematite, piemontite] + hematite + quartz = spessartine + clinochlore + H2O+O2 (3) Reactions (1) to (3) have positive P-T slopes. They are considered to account for the breakdown of sursassite and the formation of spessartine during prograde metamorphism of the piemontite quartzites and related rocks. P-T data from Andros and Evvia and geological data from few other occurrences reported suggest sursassite+ quartz±braunite to be stable at T<400–450° C over a considerable pressure interval at least up to 10 kbar. Theoretical phase relations among Mn3+-Mn2+-silicates in the pseudoquaternary system Al-Mn-Ca-Mg with excess quartz, H2O, and O2 indicate that low-grade assemblages containing sursassite (±braunite±pumpellyite±viridine±piemontite + quartz) are likely precursors of higher-grade assemblages including spessartine, Mg-chlorite, braunite, viridine, and piemontite reported from greenschist-, amphibolite-, and high-grade blueschist-facies rocks of appropriate composition.  相似文献   

17.
Summary Synthesis of Mn-bearing ilvaites, CaFe 2–x 2+ MnxFe3+ [Si2O7/O/(OH)], with 0 x 0.19, have been performed under hydrothermal conditions at 2 and 3 kbars, T = 300 -400°C and at oxygen fugacities defined by the Fe2O3/Fe3O4 - and the Ni/NiO -buffer. As shown by X-ray diffraction, the substitution of Fe2+ by Mn2+ decreases the monoclinic angle and causes a phase transition from monoclinic to orthorhombic at x = 0.19. The Fe-distribution has been determined by Mössbauer spectroscopy.
Synthese und Charakterisierung von Mn-haltigem Ilvait CaFe 2–x 2+ MnxFe3+ [Si2O7/O/(OH)]
Zusammenfassung Mn-haltiger Ilvait CaFe 2–x 2+ Mnx Fe3+ [Si2O7/O/(OH)] wurde unter hydrothermalen Bedingungen bei Drucken von 2 und 3 kbar, Temperaturen zwischen 300 und 400°C und bei Sauerstoff Fugazitäten, die durch Festkörperpuffer (Fe2O3/Fe3O4 und Ni/NiO) kontrolliert wurden, hergestellt. Röntgenbeugungsuntersuchungen zeigen, daß mit steigendem Mn-Einbau der monokline Winkel kleiner wird, und daß bei x = 0.19 ein Phasenübergang von der monoklinen zur orthorhombischen Struktur erfolgt. Die Fe-Verteilung wurde mit Mössbauer-Spektroskopie bestimmt.

With 4 Figures  相似文献   

18.
Spinel-pyroxene-garnet relationships and their dependence on Cr/Al ratio   总被引：2，自引：0，他引：2  
The partitioning of Cr and Al between coexisting spinel and clinopyroxene and the dependence of spinel-cpxgarnet equilibria on Cr/Al ratio have been investigated by a combination of phase equilibrium experiments, high temperature solution calorimetry and thermodynamic calculations.The exchange equilibrium: has a measured enthalpy change for pure phases of –2,100±500 cal at 970 K and 1 atm. Experimental reversals of Cr-Al partitioning between the spinel and clinopyroxene phases yield the following partitioning relationship: where X i j refers to atomic fraction of i in the octahedral sites of phase j. The compositional dependence of partitioning implies that Al-Cr mixing in spinel is nonideal with, on the symmetrical model, a W Cr-Al Sp of 2,700±500 cal/gm. atom. In contrast, aluminum-chromium mixing in clinopyroxene is close to ideal.The measured stability field of knorringite (Mg3Cr2Si2O12) and mixing properties of garnet have been used in conjunction with our experimental data to calculate the influence of Cr/Al ratio on the important reaction: orthopyroxene+clinopyroxene+spinel=olivine+garnetThe stability field of spinel lherzolite increases by about 2.8 Kb for every increase of 0.1 in Cr/(Cr+Al) ratio up to Cr/(Cr+Al) of 0.7. The calculated stabilization is in very good agreement with the experimental results of O'Neill (1981). The partitioning relationships are such that, at the low ratios of Cr/Al (0.07) of primitive lherzolite, clinopyroxene buffers spinel composition and sharpens the spinelgarnet reaction interval from 10 Kb (little or no clinopyroxene) down to about 2 Kb in pyroxene-rich pyrolite.  相似文献   

19.
Quartz and rutile were synthesized from silica-saturated aqueous fluids between 5 and 20 kbar and from 700 to 940°C in a piston-cylinder apparatus to explore the potential pressure effect on Ti solubility in quartz. A systematic decrease in Ti-in-quartz solubility occurs between 5 and 20 kbar. Titanium K-edge X-ray absorption near-edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in quartz at all conditions studied. Molecular dynamic simulations support XANES measurements and demonstrate that Ti incorporation onto fourfold sites is favored over interstitial solubility mechanisms. To account for the P–T dependence of Ti-in-quartz solubility, a least-squares method was used to fit Ti concentrations in quartz from all experiments to the simple expression
RTlnX\textTiO 2 \textquartz = - 60952 + 1.520 ·T(K) - 1741 ·P(kbar) + RTlna\textTiO 2 RT\ln X_{{{\text{TiO}}_{ 2} }}^{\text{quartz}} = - 60952 + 1.520 \cdot T(K) - 1741 \cdot P(kbar) + RT\ln a_{{{\text{TiO}}_{ 2} }}  相似文献   

20.
 The carbonation reaction CaMg(CO3)2 (dolomite)+2SiO2 (coesite)=CaMgSi2O6 (diopside)+2 CO2 (vapor) has been determined experimentally between 3.5 and 6 GPa in a multiple-anvil, solid-media apparatus. This reaction, a candidate for carbonation of eclogites (garnet+clinopyroxene) in the Earth’s mantle, lies at higher pressure for a given temperature than do the carbonation reactions for peridotites (olivine+orthopyroxene±clinopyroxene). A depth interval may exist within the Earth’s mantle under either ‘normal’ or ‘subduction’ thermal regimes where carbonated peridotite could coexist with carbonate-free, CO2-bearing eclogite. Received: 25 May 1994/Accepted: 13 June 1995  相似文献   

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