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1.
Almandine, although decomposing in the presence of metallic iron into the anhydrous subsolidus assemblage fayalite + ferrocordierite + hercynite solid solution at low pressures, melts incongruently to hercynitess + quartz + liquid at 10 kb. At pressures between about 12 and 20 kb the products of incongruent melting are hercynitess + liquid only, and at still higher pressures almandine melts congruently. For the intermediate pressures between 2 and 10 kb not investigated a sequence of probable breakdown and melting relations involving the phases ferrocordierite, fayalite, hercynitess, quartz, and liquid is derived through Schreinemakers' analyses.The lower temperature stability limit of almandine in the presence of water at low oxygen fugacities and pressures of 15 to 20 kb lies between 550° and 600° C as at low pressures. It is marked, however, by the breakdown to a hydrous assemblage involving chloritoid and the new phase aluminous deerite. Since the anhydrous melting at these pressures occurs between 1300° and 1400° C, the thermal stability range of almandine increases drastically with pressure. Its upper breakdown limit shows in principle a similar behavior as those of other garnet end members.  相似文献   

2.
The phase K2Mg5Si12O30 was synthesized both hydrothermally and dry under a variety of pressures and temperatures, and its stability relations were determined. Under hydrothermal conditions it exhibits a lower stability limit lying at 595°C, 1 kb, and 650°C, 2 kb, due to its breakdown into the hydrous assemblage quartz+KMg2.5Si4O10(OH)2 (a mica phase). Its upper temperature stability under hydrothermal conditions is given by its incongruent melting to MgSiO3+liquid. Near 820° C at a fluid pressure of approximately 6.5 kb the two univariant curves for these breakdown reactions intersect thus limiting the stability field to lower fluid pressures. — Under anhydrous conditions K2Mg5Si12O30 becomes unstable at pressures between approximately 7 and 32.5 kb due to its incongruent melting to the assemblage MgSiO3+quartz (or coesite)+liquid; this melting curve has a pronounced negative slope. No subsolidus breakdown assemblage was encountered at 32.5 kb down to temperatures as low as 750°C. This behavior is probably due to the instability of other ternary compounds in the system K2O-MgO-SiO2 at high pressures and thus to the existence of very low-temperature eutectics involving only binary and unary solid phases plus liquid.It is likely that these stability relations provide a model for those of the natural minerals merrihueite and roedderite which contain Na and Fe+2 partly substituting for K and Mg and which were encountered in several meteorites. Therefore, the cosmic events leading to the formation of these minerals must have taken place at relatively low pressures and high temperatures, especially when water was present. The bulk compositions of these minerals appear to be incompatible with average chondritic matter under equilibrium conditions. Hence merrihueite and roedderite are not likely to be found in equilibrated chondrites which contain feldspars instead.  相似文献   

3.
The univariant pressure temperature curve of the reaction aluminous enstatite solid solution+sillimanitesapphirine solid solution+quartz was determined experimentally in the pressure range between 12 and 20 kb. It is defined by four reversals at 15 kb, 1140°+10°C; at 1190°C, 15.9±1 kb; at 1300°C, 17.2±1 kb; and at 1400°C, 18±1 kb. Among the coexisting phases the Al-content of the enstatites increases strongly, with rising temperatures and pressures, up to values approaching that of pyrope composition, whereas the Al-content of the sapphirine solid solution appears to increase only slightly. Concomitantly, the sillimanites, most probably of invariant composition, exhibit growing Al/Si-disorder. These compositional and structural variations, in addition to the changing stoichiometry of the reaction equation, cause the progressively decreasing positive dP/dT slope of the equilibrium curve. — The assemblage sapphirine + quartz found in natural granulites is indicative of conditions of water pressure much lower than total pressure.  相似文献   

4.
Deerite, Fe 12 2+ Fe 6 3+ [Si12O40](OH)10, thus far known from ten localities in glaucophane schist terranes, was synthesized at water pressures of 20–25 kb and temperatures of 550–600 °C under the of the Ni/NiO buffer. The X-ray powder diagram, lattice constants and infrared spectrum of the synthetic phase are closely similar to those of the natural mineral. A solid solution series extends from this ferri-deerite end member to some 20 mole % of a hypothetical alumino-deerite, Fe 12 2+ Al 6 3+ [Si12O40](OH)10. The upper temperature breakdown of ferri-deerite to the assemblage ferrosilite +magnetite+quartz+water occurs at about 490 °C at 15 kb, and 610 °C at 25 kb fluid pressure for the of the Ni/NiO buffer. Extrapolation of these data to lower water pressures indicates that deerite can be a stable mineral only in very low-temperature, high-pressure environments.  相似文献   

5.
Experiments were conducted at 6–30 kb and 875–1200°C on two garnet pyroxenite xenoliths from the Bullenmerri and Gnotuk Maars of western Victoria, Australia. The (garnet + clinopyroxene + plagioclase + spinel) assemblage of DR9734 was stable between 10 and 12.5 kb, and 950 and 1,050°C. The compositions of its natural mineral phases were most closely approximated in experiments at 12.5 kb and 1,000–1,050°C. The (garnet + spinel + clinopyroxene + orthopyroxene + amphibole) assemblage of DR10165 was stable at pressures > 8 kb and temperatures > 950°C. However, differences between natural and experimental mineral compositions indicate that the mineral assemblage of this xenolith persisted metastably after cooling below 950°C with chemical exchange continuing down to approximately 850–900°C. When the experimental data for DR9734 and DR10165 are applied to mineralogical data for other mafic and ultramafic xenoliths from the Bullenmerri and Gnotuk Maars, they indicate that previous pressure and temperature estimates for individual xenoliths are 2–3 kb and 50°C too high. These corrections increase average temperatures for the geotherm beneath western Victoria by about 50°C over a depth range of 30–45 km and confirm its perturbed (high-temperature) character.This paper is a contribution to IGCP Project 304 (Lower Crustal Processes)  相似文献   

6.
The MgAl surinamite end member, (Mg3Al3)[6]O[AlBeSi3O15], was synthesized in the requisite system with and without water. The new phase is monoclinic, space group P2/n, with a=9.881(1)Å; b=11.311(1) Å; c=9.593(1) Å; =109.52(2)°. Refractive indices are n x=1.7015(20); n y=1.7035(20); n z=1.7055(20). The infrared spectrum shows characteristic differences against the structurally related and optically extremely similar phase sapphirine.Using the seeding technique, the preliminary stability field for MgAl surinamite was found to lie at high temperatures (650 °C) and high pressures (4 kbar). At lower temperatures breakdown takes place to hydrous assemblages of chlorite, talc, and chrysoberyl with kyanite or yoderite; at lower pressures chrysoberyl forms parageneses with sapphirine and cordierite. In crystal chemical terms the underlying principle for the stability of surinamite versus that of the low-pressure assemblages is the higher proportion of octahedrally coordinated Al in surinamite (75%). Following the same principle surinamite itself decomposes at still higher pressures to a paragenesis, in which all Al enters octahedral coordination (pyrope+a chrysoberyl-type phase and some unidentified X-ray peaks).The stability field of synthetic MgAl surinamite is in good agreement with P, T-estimates of some 8–12 kbar, 800°–950° C as taken from the literature for the few occurrences of natural, Fe-bearing surinamite in granulite and upper amphibolite facies environments. The incorporation of iron in surinamite must be limited, because this mineral is known to coexist with its more iron-rich breakdown assemblage almandine-rich garnet+chrysoberyl. As the minimum melting curve of granite under hydrous conditions lies outside the surinamite field up to a water pressure of about 20 kbar, the absence of surinamite in normal granitic pegmatites can already be explained by physical constraints. However, there are probably also chemical constraints in the generally high Fe/Mg bulk chemistry of the pegmatite environments.Now at Institut für Kristallographie, Technische Hochschule, Templergraben 55, D-5100 Aachen, FRG  相似文献   

7.
Pyrope and quartz are stable with respect to aluminous enstatite and sillimanite at 1400 °C, 20 kb and at 1100 °C, 16 kb. The phase boundary limiting the coexistence of pyrope and quartz towards lower pressures is probably slightly curved. A slope of 15 bars/°C at 1400° and of 10 bars/°C at 1000 °C has been estimated from the experimental data. Between 1050 and 1100 °C the curve is intersected by the kyanite-sillimanite phase boundary. The calculated slope of the reaction aluminous enstatite + kyanite pyrope + quartz is negative (ca. 18–25 bars/°C). The existence of a negative slope has been demonstrated experimentally. Experimental evidence indicates that the assemblage aluminous enstatite and sillimanite is metastable with respect to sapphirine + quartz at high temperature. The invariant point involving the phases pyrope-sapphirine-aluminous enstatite-sillimanite-quartz is estimated to occur at 1125°±25 °C and 16±1 kb. A model phase diagram for the silicasaturated part of the system MgO-Al2O3-SiO2 has been constructed. The position of three invariant points in this system has been estimated on the basis of presently available data.  相似文献   

8.
Garnet in silicic liquids and its possible use as a P-T indicator   总被引：1，自引：0，他引：1  
Melting experiments on a model pelitic composition yield low-spessartine garnet as an important residual phase at pressures above 7 kb. The K D values for distribution of iron and magnesium between coexisting garnet and liquid in the pelitic composition are mainly sensitive to temperature, but also have a small pressure dependence. At temperatures above 950 ° C garnet has a higher value than coexisting liquid, but below 950 ° C the garnet value is lower than that of the coexisting liquid. Thus at temperatures below 950 ° C silicic magmas may fractionate garnet and produce more magnesian derivative liquids.Reconnaissance experiments with added MnO content in the model pelite demonstrate that spessartine-rich garnets are stable in silicic liquids to pressures as low as 3 kb. The MnO and CaO contents of the experimentally crystallized garnets show an antipathetic relation. Also, the grossular content of near-liquidus garnets crystallizing from a range of compositions increases with increasing pressure. The spessartine and grossular contents of most natural garnets in eastern Australian granitic rocks suggest that these garnets formed at pressures greater than 5 kb. Increased spessartine content allows crystallization of garnet in equilibrium with a silicic magma well within the pressure limit of stability of cordierite, provided the garnet contains 10 mol.% spessartine. Thus the depth range over which garnet and cordierite may coexist in a silicic melt is broadened, subject to the availability of MnO. The effect of increased Mn content on the low-pressure stability limit of garnet may also explain the lack of resorption of some garnets in granitic magmas, as these magmas rise to shallower levels. These euhedral garnets characteristically show zoning from an Mn-poor core (typically <4 % MnO) to an Mn-richer rim (typically >4 % MnO) and may reflect continued growth of the garnet in a low pressure regime, stabilized by Mn concentrated in the residual liquid fractions of the crystallizing magma.  相似文献   

9.
Hydrothermal investigation of the bulk composition CaO·Al2O3·4SiO2 + excess H2O has been conducted using conventional techniques over the temperature range 200–500° C and 500–5,000 bars P fluid. The fully ordered wairakite was synthesized unequivocally in the laboratory, probably for the first time.The gradual, sluggish and continuous transformation from disordered to ordered wairakite evidently accounts for failure by previous investigators to synthesize ordered wairakite in runs of week-long duration. The dehydration of metastable disordered wairakite to metastable hexagonal anorthite, quartz and H2O has been determined; this reaction takes place at temperatures exceeding 400° C, even at fluid pressures of 500 bars or less. The upper P fluid-T boundary of the disordered phase is equivalent to the maximum temperature curve of synthetic wairakite presented by previous investigators. The hydrothermal breakdown of natural wairakite above its stability limit appears to be a very slow process.The equilibrium dehydration of wairakite to anorthite, quartz and H2O occurs at 330±5° C at 500 bars, 348±5° C at 1,000 bars, 372±5° C at 2,000 bars and 385±5° C at 3,000 bars. Where fluid pressure equals total pressure, the thermal stability range of wairakite is about 100° C wide. At lower temperatures wairakite reacts with H2O to form laumontite. Reconnaissance experiments dealing with the effect of CO2 on stabilities of calcium zeolites suggest that wairakite or laumontite may be replaced by the assemblage calcite + montmorillonite in the presence of a CO2-bearing fluid phase.The determined P fluid -T field of wairakite is compatible with field observations in some metamorphic terrains where it is related to the shallow emplacement of granitic magma and with direct pressure-temperature measurements in certain active geothermal areas. Under inferred conditions of higher CO2/H2O ratios, essentially unmetamorphosed rocks grade directly into those characteristic of the greenschist facies; moderately high values of CO2 in carbonate-bearing rocks result in the downgrade extension of the greenschist facies at the expense of zeolite-bearing assemblages.  相似文献   

10.
The phase relations in the system grossular-spessartine-H2O were investigated at 2.0 Kb aqueous fluid pressure and at subsolidus temperatures down to 420 ° C. Despite metastable persistence of a compositional gap found in some intermediate members, a complete solid solution between grossular and spessartine exists.Linear relations between the unit cell edge, a 0, and composition were readily observed down to 620 ° C with a 0=11.849(2) Å and 11.613(2) Å for grossular and spessartine, respectively. Hydrated garnets began to appear at higher temperature for the Ca-rich members. Grossular and spessartine formed at 420 ° C have a 0=11.901(2) Å and 11.632(2) Å, indicating the presence of 0.6 and 0.2 mol H2O, respectively. Intermediate members show varying degrees of hydration. Infrared spectra of the more hydrated members show a major and minor absorption bands at 3,620 cm–1 and 3,660 cm–1, respectively, in addition to a broad band around 3,430 cm–1. All the hydrogarnets formed at 420 ° C were proven to be metastable.The rare occurrence of the intermediate grossular-spessartine garnets may be attributed to the lack of appropriate bulk chemistry of the rock rather than to the P-T conditions to which the rock is subjected. There may be a stability field for hydrogrossular below 420 ° C at 2 Kb, but not for hydrospessartine. Any occurrence of hydrogarnet may be used as a temperature indicator setting the maximum of formation for the hydrogarnet-bearing assemblage below 420 ° C at 2 Kb.  相似文献   

11.
Viridine containing the highest amounts of Mn2O3 detected thus far (up to 20.5 mol % “Mn2SiO5”) coexists in a metasedimentary hornfels with spessartine, Mn-phlogopite (mangan-ophyllite), Mn-phengite (alurgite), hematite, quartz and probably some primary braunite. In layers poorer in viridine spessartine is absent but piemontite appears as an additional phase. Microprobe analyses of all these phases are presented which indicate very strong fractionation of Mg and Mn in coexisting phlogopite and garnet, and of Fe and Mn in coexisting hematite and braunite. Sericitic aggregates consisting of phengitic muscovite and braunite are interpreted as retrograde alteration products of viridine, but might partly be pinitic alterations of a former Mg-rich cordierite. Due to the occurrence of the assemblage spessartine-viridine-quartz Mn-cordierite cannot have been a stable phase prior to retrograde alterations. In general the stability field of viridine is extended towards higher temperatures as compared to that of pure andalusite, Al2SiO5. Due to the coexistence of phlogopite and muscovite (phengite) the temperature of contact metamorphism cannot have exceeded some 550°–650° C depending on the prevailing water pressure.  相似文献   

12.
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   

13.
Osumilitess was synthesized as a single phase product in the model system K2O-MgO-Al2O3-SiO2 at 800° C/ 0.5 Kbar water pressure and at 800° to 840° C/1.0 Kbar total pressure with 0.3 in the gas phase. The experimentally determined solid solubility range of synthetic osumilites can be expressed by the formula KMg2(Al3-xMgx) (Al2–xSi10+x)O30 with 0x0.4. A survey of sixteen chemical analyses of natural osumilites from eleven occurrences shows a solid solubility characterized by 0x0.6. Reversed stability experiments for the synthetic osumilite KMg2(Al2.75Mg0.25)(Al1.75Si10.25)O30 determined at water pressure equal to total pressure demonstrate its restriction to water pressures below 0.8 Kbar (at 0.5 Kbar, the stability range is between 765° and 800° C). At the lower thermal stability limit osumilite+H2O vapor break down to cordierite+K feldspar+phlogopitess+quartz, at the higher one to cordierite+K feldspar+phlogopite+liquid. Reduction of water fugacity will expand the stability field largely by shifting the lower and higher thermal stability limits to lower and higher temperatures, respectively. The dependence of osumilite stability on water fugacity makes osumilite a sensitive indicator mineral for dry conditions in rocks formed at total pressures higher than about 0.8 Kbar.  相似文献   

14.
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   

15.
The stability of chloritoid, FeAl2SiO6H2O, was investigatedat fluid pressures less than 10 kb. At oxygen fugacities definedby the Ni-NiO buffer, chloritoid reacts to Fe-cordierite andhercynite spinel at 550 and 575 °C at 1 and 2 kb fluid pressure.At pressures between 2.5 and 3.5 kb the assemblage aluminousferro-anthophyllite, staurolite and hercynite spinel appears.The breakdown of chloritoid to this assemblage takes place at625, 650, and 675 °C at 5.5, 7.0, and 8.7 kb, respectively.The aluminous ferro-anthophyllite assemblage is stable onlyover 50 °C, reacting with increasing temperature to almandine,staurolite, and hercynite spinel. Under the QFM buffer, thesame equilibria are displaced to higher temperatures and thealuminous ferro-anthophyllite bearing field is further restrictedwith respect to temperature. The 7 Å chamosite assemblage,previously considered to be the metastable equivalent of chloritoidat low pressures, is shown to be unstable and chloritoid canbe synthesized at pressures as low as 1 kb. An analysis of the equilibria and related experimental datapermits the construction of a schematic P-T grid which outlinesthe stability limits of several important mineral assemblagesin this system. Although the experimental and natural systemsare not strictly analogous, there is an excellent degree ofcorrespondence between the defined upper limit of chloritoidstability and previous estimates of the facies boundaries itserves to define.  相似文献   

16.
Hydrothermal experiments utilizing the seeding technique show conclusively that the lower temperature stability limit of Mg cordierite so far accepted (Schreyer and Yoder, 1964) represents metastable equilibria throughout the pressure range 1–7 kb. The stable equilibrium curve generally lies at somewhat higher temperatures and is not subdivided by an invariant point involving the phase pyrophyllite within this pressure range. Instead of pyrophyllite+andalusite+chlorite, hitherto assumed to be the breakdown assemblage of cordierite below 5 kb water pressure, the paragenesis Al silicate+chlorite+quartz was found to be stable over the entire pressure range investigated. The andalusite/kyanite transition, as determined by Richardson et al. (1969), is confirmed by a minor change of slope of the cordierite breakdown curve. Direct breakdown of cordierite into pyrophyllite-bearing assemblages must be limited to conditions below about 1 kb and 450° C. This gives indirect support to Kerrick's (1968) stability data on pyrophyllite, as opposed to those of Althaus (1969) and others. These new results offer an explanation for the common assemblage chlorite+andalusite (or kyanite)+quartz occuring in spotted slates, greenschists, and also as retrograde alteration products of natural cordierites.We thank Dr. K. Langer, Bochum, for partial analyses of some of the natural starting materials. Financial support of this work by Deutsche Forschungsgemeinschaft, Bad Godesberg, is gratefully acknowledged.  相似文献   

17.
Six crystalline mixtures, picrite, olivine-rich tholeiite, nepheline basanite, alkali picrite, olivine-rich basanite, and olivine-rich alkali basalt were recrystallized at pressures to 40 kb, and the phase equilibria and sequences of phases in natural basaltic and peridotitic rocks were investigated.The picrite was recrystallized along the solidus to the assemblages (1) olivine+orthopyroxene+ clinopyroxene +plagioclase+spinel below 13 kb, (2) olivine+orthopyroxene+clinopyroxene+spinel between 13 kb and 18 kb, (3) olivine+orthopyroxene+clinopyroxene+ garnet+spinel between 18 kb and 26 kb, and (4) olivine+clinopyroxene+garnet above 26 kb. The solidus temperature at 1 atm is slightly below 1,100° and rises to 1,320° at 20 kb and 1,570° at 40 kb. Olivine is the primary phase crystallizing from the melt at all pressures to 40 kb.The olivine-rich tholeiite was recrystallized along the solidus into the assemblages (1) olivine+ clinopyroxene+plagioclase+spinel below 13 kb, (2) clinopyroxene+orthopyroxene+ spinel between 13 kb and 18 kb, (3) clinopyroxene+garnet+spinel above 18 kb. The solidus temperature is slightly below 1,100° at 1 atm, 1,370° at 20 kb, and 1,590° at 40 kb. The primary phase is olivine below 20 kb but is orthopyroxene at 40 kb.In the nepheline basanite, olivine is the primary phase below 14 kb, but clinopyroxene is the first phase to appear above 14 kb. In the alkali-picrite the primary phase is olivine to 40 kb. In the olivine-rich basanite, olivine is the primary phase below 35 kb and garnet is the primary phase above 35 kb. In the olivine-rich alkali basalt the primary phase is olivine below 20 kb and is garnet at 40 kb.Mineral assemblages in a granite-basalt-peridotite join are summarized according to reported experimental data on natural rocks. The solidus of mafic rock is approximately given by T=12.5 P Kb+1,050°. With increasing pressure along the solidus, olivine disappears by reaction with plagioclase at 9 kb in mafic rocks and plagioclase disappears by reaction with olivine at 13 kb in ultramafic rocks. Plagioclase disappears at around 22 kb in mafic rocks, but it persists to higher pressure in acidic rocks. Garnet appears at somewhat above 18 kb in acidic rocks, at 17 kb in mafic rocks, and at 22 kb in ultramafic rocks.The subsolidus equilibrium curves of the reactions are extrapolated according to equilibrium curves of related reactions in simple systems. The pyroxene-hornfels and sanidinite facies is the lowest pressure mineral facies. The pyroxene-granulite facies is an intermediate low pressure mineral facies in which olivine and plagioclase are incompatible and garnet is absent in mafic rocks. The low pressure boundary is at 7.5 kb at 750° C and at 9.5 kb at 1,150° C. The high pressure boundary is 8.0 kb at 750° C and 15.0 kb at 1,150° C. The garnet-granulite facies is an intermediate high pressure facies and is characterized by coexisting garnet and plagioclase in mafic rocks. The upper boundary is at 10.3 kb at 750° C and 18.0 kb at 1,150° C. The eclogite facies is the highest pressure mineral facies, in which jadeite-rich clinopyroxene is stable.Compositions of minerals in natural rocks of the granulite facies and the eclogite facies are considered. Clinopyroxenes in the granulite-facies rocks have smaller jadeite-Tschermak's molecule ratios and higher amounts of Tschermak's molecule than clinopyroxenes in the eclogite-facies rocks. The distribution coefficients of Mg between orthopyroxene and clinopyroxene are normally in the range of 0.5–0.6 in metamorphic rocks in the granulite facies. The distribution coefficients of Mg between garnet and clinopyroxene suggest increasing crystallization temperature of the rocks in the following order: eclogite in glaucophane schist, eclogite and granulite in gneissic terrain, garnet peridotite, and peridotite nodules in kimberlite.Temperatures near the bottom of the crust in orogenic zones characterized by kyanitesillimanite metamorpbism are estimated from the mineral assemblages of metamorphic rocks in Precambrian shields to be about 700° C at 7 kb and 800° C at 9 kb, although heat-flow data suggest that the bottom of Precambrian shield areas is about 400° C and the eclogite facies is stable.The composition of liquid which is in equilibrium with peridotite is estimated to be close to tholeiite basalt at the surface pressure and to be picrite at around 30 kb. The liquid composition becomes poorer in normative olivine with decreasing pressure and temperature.During crystallization at high pressure, olivine and orthopyroxene react with liquid to form clinopyroxene, and a discontinuous reaction series, olivine orthopyroxene clinopyroxene is suggested. By fractional crystallization of pyroxenes the liquid will become poorer in SiO2. Therefore, if liquid formed by partial melting of peridotite in the mantle slowly rises maintaining equilibrium with the surrounding peridotite, the liquid will become poorer in MgO by crystallization of olivine, and tholeiite basalt magma will arrive at the surface. On the other hand, if the liquid undergoes fractional crystallization in the mantle, the liquid may change in composition to alkali-basalt magma and alkali-basalt volcanism may be seen at a late stage of volcanic activity.Publication No. 681, Institute of Geophysics and Planetary Physics, University of California, Los Angeles.  相似文献   

18.
Osumilite-sapphirine-quartz granulites from Enderby Land, Antarctica (Ellis et al. 1980) were metamorphosed at 8–10 kb pressure, 900°-980° C under very low conditions. Retrograde mineral coronas in these rocks record subsequent cooling from the peak of metamorphism at approximately constant pressure. The inferredP-T cooling-uplift path differs markedly from that evident in many other granulite terrains.Present garnet-cordierite geothermometers imply equilibration at temperatures of 500°–600° C, well within the kyanite stability field. These temperatures are inconsistent with the presence of sillimanite and the high temperature stability fields of the actual mineral assemblages. Examination of available garnetcordierite experimental data suggests a possible large increase in the Gt-Cd Fe-MgK D with increasingX Mg of the cordierite (and pressure). New experiments designed to test this possibility were inconclusive because of the failure to attain satisfactory equilibrium, even at 1,000° C.Possible reasons for the exposure of these unusual granulites in Enderby Land are considered. Although they formed at much higher temperatures than other granulites exposed on a regional scale, suchP-T conditions are not exceptional for the base of the crust. Instead, the unusual isobaric cooling to low temperatures followed by uplift to the surface which these granulites are inferred to have undergone is considered of importance. The unusual tectonic conditions are reflected in the disctinctive type of mineral reaction coronas found in these rocks. The common occurrence elsewhere of mineral reaction during uplift, and the role of anatexis during uplift in obliterating such high temperature assemblages elsewhere in the world are considered.  相似文献   

19.
The Violet Town Volcanics are a 373 Ma old, comagmatic, S-type volcanic sequence mainly comprising crystal-rich intracaldera ignimbrites. Rock types vary from rhyolites to rhyodacites, all containing magmatic cordierite and garnet phenocrysts. Variation in the suite is primarily due to fractionation of early-crystallized quartz, plagioclase and biotite (plus minor accessory phases) in a high-level magma chamber prior to eruption. Early magmatic crystallization occurred at around 4 kb and 850° C with melt water contents between 2.8 and 4 wt.%. This high-temperature, markedly water-undersaturated, restite-poor, granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain, leaving a granulite facies residue.Table of Less Common Abbreviations Used Pkb pressure in kilobars - T° C temperature in degrees Celsius - mole fraction of water in the fluid - aH2O activity of water - Bi biotite - Cd cordierite - Gt garnet - Py pyrope - Gr grossular - Alm almandine - Sp spessartine - He hercynite - Ilm ilmenite - Kfs potassium feldspar - Opx orthopyroxene - Pl plagioclase - An anorthite - Q quartz - Sill sillimanite - Ap apatite  相似文献   

20.
A garnet websterite nodule from the Honolulu volcanic series,Oahu, Hawaii, has been melted in the presence of nearly pureH2O. The solidus is intermediate between that of peridotiteand gabbro. The curve displays a temperature minimum around20 kb reflecting the breakdown of plagioclase. The Iiquidusis between 1130 ?C and 1150 ?C between 10 and 20 kb vapor pressure.Amphibole (pargasitic hornblende) has an extensive stabilityfield, reaching a maximum temperature about 20 ?C below thegarnet websterite liquidus at 15 kb and a maximum pressure of27.5 kb at 950 ?C. The amphibole-out curve intersects the soliduswith a positive slope. Liquids formed by partial melting of garnet websterite are quartz-normativewithin the stability field of amphibole, but become olivine-normative(tholeiitic) with increasing temperature. Amphibole and clinopyroxeneare enriched in Tschermak's molecule at higher temperatures,pargasite content of amphibole increases with increasing pressure. A garnet websterite-rich upper mantle containing modal olivineyields quartz-normative (13–16 per cent), aluminous (21–4wt. per cent A12O3) melts at 17 P 10 kb and in the presenceof nearly pure H2O. However, the presence of amphibole controlsthe liquid composition, a situation not found for liquids formedfrom wet peridotite. In contrast to many basalt liquids, liquidof garnet websterite composition cannot fractionate to andesiteby precipitation of amphibole, as amphibole is not a liquidusphase.  相似文献   

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