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1.
Ignimbrites from the central North Island consist mainly of glass or its devitrified product (70–95%); their phenocryst mineralogy is varied and includes plag., hyp., ti-mag., ilm., aug., hblende, biot., san., qtz, ol., with accessory apatite, zircon and pyrrhotite. The Fe-Mg minerals can be used to divide the ignimbrites into four groups with hyp.+aug. reflecting high quench temperatures and biot.+hblende +hyp.+aug., low quench temperatures. Oxygen fugacities lie above the QMF buffer curve and even in ignimbrites with low crystal contents the solid phases apparently buffered fO2. Some ignimbrites contain the assemblage actinolite, gedrite, magnetite and hematite, reflecting post-eruption oxidation. The mineralogy also allows estimation of using pyrrhotite and thence , . The assemblage biotite-sanidine can be used to estimate and thence . Water fugacity is calculated in a variety of ways using both biotite and hornblende as well as the combining reaction . It is high and approaches P total in most ignimbrites (~4kb) but is lower in unwelded pumice breccias. Comparison of temperature estimates using mineral geothermometers for the various phenocryst phases suggests that the ignimbrite magmas showed temperature differences of 60–100 °C and pressure differences of several kilobars. Individual magma chambers therefore, would have extended over several kilometres vertically. The chemical potential of water may have been constant through the magma.  相似文献   

2.
In the Rogers Pass area of British Columbia the almandine garnet isograd results from a reaction of the form: 5.31 ferroan-dolomite+8.75 paragonite+4.80 pyrrhotite+3.57 albite+16.83 quartz+1.97 O2=1.00 garnet+16.44 andesine+1.53 chlorite+2.40 S2+1.90 H2O+10.62 CO2. The coefficients of this reaction are quite sensitive to the Mn content of ferroan-dolomite.Experimental data applied to mineral compositions present at the isograd, permits calculation of two intersecting P, T equilibrium curves. P=29088–39.583 T is obtained for the sub-system paragonite-margarite (solid-solution), plagioclase, quartz, ferroan-dolomite, and P=28.247 T–14126 is obtained for the sub-system epidote, quartz, garnet, plagioclase. These equations yield P=3898 bars and T=638° K (365° C). These values are consistent with the FeS content of sphalerite in the assemblage pyrite, pyrrhotite, sphalerite and with other estimates for the area.At these values of P and T the composition of the fluid phase in equilibrium with graphite in the system C-O-H-S during the formation of garnet is estimated as: bars, bars, bars, bars, bars, bars, bars, bars, , bars, bars.  相似文献   

3.
Two metamorphic isograds cut across graphitic schist near Pecos Baldy, New Mexico. The southern isograd marks the first coexistence of staurolite with biotite, whereas the northern isograd marks the first coexistence of andalusite with biotite. The isograds do not record changes in temperature or pressure. Instead, they record a regional gradient in the composition of the metamorphic fluid phase. Ortega Quartzite, which contains primary hematite, lies immediately north of the graphitic schist. Mineral compositions within the schist change gradually toward the quartzite, reflecting gradients in and . The chemical potential gradients, locally as high as 72 cal/m in and 9 cal/m in , controlled the positions of the two mapped isograds. The staurolite-biotite isograd records where fell below 0.80, at near 10–23 bars; the andalusite-biotite isograd records where fell below 0.25, at near 10–22 bars. Dehydration and oxidation were coupled by graphite-fluid equilibrium.The chemical potential gradients apparently formed during metamorphism, as graphite in schist reacted with hematite in quartzite. Local oxidation of graphite formed CO2 which triggered dehydration reactions along the schistquartzite contact. This process created a C-O-H fluid which infiltrated into overlying rocks. Upward infiltration, local fluid-rock equilibration and additional infiltration proceeded until the composition of the infiltrating fluid evolved to that in equilibrium with the infiltrated rock. This point occurs very close to the staurolite-biotite isograd. Pelitic rocks structurally above this isograd show no petrographic evidence of infiltration, even though calculations indicate that volumetric fluid/rock ratios may have exceeded 15 and thin, rare calc-silicate beds show extensive K-metasomatism and quartz veining.  相似文献   

4.
Iron chlorites with compositions intermediate between the two end-members daphnite (Fe5Al2Si3O10(OH)8) and pseudothuringite (Fe4Al4Si2O10(OH)8) were synthesized from mixtures of reagent chemicals. The polymorph with a 7 Å basal spacing initially crystallized from these mixtures at 300 °C and 2 kb after two weeks. Conversion to a 14 Å chlorite required a further 6 weeks at 550 °C. Shorter conversion times were required at higher water pressures. The products contained up to 20% impurities.The maximum equilibrium decomposition temperature for iron chlorite, approximately 550 °C at 2kb, is at an between assemblages (1) and (2) listed below. Synthetic iron chlorite will break down by various reactions with variable P, T, and fugacity of oxygen. For the composition FeAlSi = 523, the sequence of high temperature breakdown products with increasing traversing the magnetite field for P total = =2kb is: (1) corierite+ fayalite+hercynite; (2) cordierite+fay alite+magnetite; (3) cordierite+magnetite+quartz; (4) magnetite+mullite+quartz. Almandine should replace cordierite in assemblages (1) and (2) but it did not nucleate. The significance of the relationship between iron cordierite and almandine in this system is discussed.At water pressures from 4 to 8.5 kb and at the nickel-bunsite buffer, iron chlorite+quartz break down to iron gedrite+magnetite with temperature 550 to 640 °C along the curve. At temperatures 50 °C greater and along a parallel curve, almandine replaces iron gedrite. For on this buffer curve, almandine is unstable below approximately 4 kb for temperatures to approximately 750 °C.  相似文献   

5.
To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn2SiO4, electrical conductivity () of single crystals was measured along either the [100] or the [010] direction. The experiments were carried out at temperatures T=850–1200 °C and oxygen fugacities atm under both Mn oxide (MO) buffered and MnSiO3 (MS) buffered conditions. Under the same thermodynamic conditions, charge transport along [100] is 2.5–3.0 times faster than along [010]. At high oxygen fugacities, the electrical conductivity of samples buffered against MS is 1.6 times larger than that of samples buffered against MO; while at low oxygen fugacities, the electrical conductivity is nearly identical for the two buffer cases. The dependencies of electrical conductivity on oxygen fugacity and temperature are essentially the same for conduction along the [100] and [010] directions, as well as for samples coexisting with a solid-state buffer of either MO or MS. Hence, it is proposed that the same conduction mechanisms operate for samples of either orientation in contact with either solid-state buffer.The electrical conductivity data lie on concave upward curves on a log-log plot of vs , giving rise to two regimes with different oxygen fugacity exponents. In the low- regime , the exponent, m, is 0, the MnSiO3-activity exponent, q, is 0, and the activation energy, Q, is 45 kJ/mol. In the high regime 10^{ - 7} {\text{atm}}} \right)$$ " align="middle" border="0"> , m=1/6, q=1/4–1/3, and Q=45 and 200 kJ/mol for T<1100 °c=" and=">T>1100 °C, respectively.  相似文献   

6.
The compositions of coexisting pyroxmangites, rhodonites, rhodochrosites and manganese calcites in regional metamorphosed manganese cale-silicate marbles from Val Scerscen and Alagna were analysed by microprobe and permit definition of critical tie lines at metamorphic grades appropriate to temperatures between 400 and 450 °C.Variations in composition of coexisting mineral pairs in one and the same locality are attributed to variations in and not to metamorphic temperatures. From the analysed assemblages isothermal plots (with SiO2 as excess component) were constructed for the system CaO-MnO-SiO2-CO2.  相似文献   

7.
Thermal aureoles surrounding intrusions of the Nain complex, Labrador, contain many unique or unusual mineral assemblages in aluminous gneisses and granulites, ironstones, and ultramafic rocks. Some of the limiting assemblages are (in addition to feldspars±pyrrhotite±ilmenite±graphite ±biotite±magnetite): Ga-Ol-Hy-Sp, Ga-Cd-Hy-Sp, Ga-Hy-Ol-Qz, Cd-Hy-Os-Qz, Ga-Cd-Sp-Si, Cd-Sp-Co-Si, Ga-Ol-Hy-Sp, Ga-Cd-Hy-Sp, Hy-Ol-Qz-Aug, and Ol-Hy-Sp(±Chl±Ca-amph±Aug). On the basis of some of these assemblages are (in addition to feldspars±pyrrhotite±ilmenite±graphite from 3.7–6.6 kbar and temperatures from 645–915 ° C. The paucity of hydrous phases, the preponderance of Ksp-Plag-Qz or even Cd-Ksp-Qz without evidence of a melt at these temperatures, the stable occurrence of osumilite, and the common presence of graphite suggest that was extremely low. The presence of graphite-pyrrhotite, the compositions of ilmenites, the compositions of coexisting Fe-Ti oxides, and other mineralogical data indicate the was relatively low and, despite the wide range in bulk compositions and rock types, may have followed an approximately buffered trend. It is possible that the magmas of the Nain complex may have acted as an external buffer for the aureole rocks. The unique nature of these mineral assemblages appears to be a function of the extremely low .  相似文献   

8.
Near-liquidus phase relationships of a spinel lherzolite-bearing olivine melilitite from Tasmania were investigated over a P, T range with varying , , and . At 30 kb under MH-buffered conditions, systematic changes of liquidus phases occur with increasing ( = CO2/CO2 +H2O+olivine melilitite). Olivine is the liquidus phase in the presence of H2O alone and is joined by clinopyroxene at low . Increasing eliminates olivine and clinopyroxene becomes the only liquidus phase. Further addition of CO2 brings garnet+orthopyroxene onto the liquidus together with clinopyroxene, which disappears with even higher CO2. The same systematic changes appear to hold at higher and lower pressures also, only that the phase boundaries are shifted to different . The field with olivine- +clinopyroxene becomes stable to higher with lower pressure and approaches most closely the field with garnet+orthopyroxene+clinopyroxene at about 27 kb, 1160 °C, 0.08 and 0.2 (i.e., 6–7% CO2+ 7–8% H2O). Olivine does not coexist with garnet+orthopyroxene+clinopyroxene under these MH-buffered conditions. Lower oxygen fugacities do not increase the stability of olivine to higher and do not change the phase relationships and liquidus temperatures drastically. Thus, it is inferred that olivine melilitite 2927 originates as a 5% melt (inferred from K2 O and P2O5 content) from a pyrolite source at about 27kb, 1160 dg with about 6–7% CO2 and 7–8% H2O dissolved in the melt. The highly undersaturated character of the melt and the inability to find olivine together with garnet and orthopyroxene on the liquidus (in spite of the close approach of the respective liquidus fields) can be explained by reaction relationships of olivine and clinopyroxene with orthopyroxene, garnet and melt in the presence of CO2.  相似文献   

9.
Water in microcrystalline quartz of volcanic origin: Agates   总被引：2，自引：0，他引：2  
Agates of volcanic origin, containing the different quartz species, fibrous, length-fast chalcedony (CH), granular fine quartz (FQ), and fibrous, length-slow, to lepidospheric quartzine (QN), have been investigated to evaluate possible relations between microstructure, i.e. crystallite size and texture, refractive indices, densities, contents of trace elements and of water, as well as dehydration behaviour. By means of near infrared spectroscopy, total water contents , could be differentiated quantitatively into contents of molecular water, , and silanole-group water, . Despite the low total water contents of the agates studied ( between 1 and 2 wt.%), near infrared spectroscopy results in reliable data on and .Wall-layering CH consists of fibrous quartz crystals and exhibits higher C-ratios, , than horizontally layered FQ which consists predominantly of granular quartz crystals (C CH=0.45±0.11 (N=6), C FQ=0.36±0.10 (N=4). This result is interpreted to be due to analogy with the behaviour of C-ratios in fluid phase-deposited opals-AN (hyalithe) and liquid phase-deposited opals-AG (non-crystalline opal) or -CT (common opal) (Langer and Flörke 1974).Translucent layers of CH show mostly lower refractive indices, when measured parallel than when measured perpendicular to the axes of the quartz fibers. The same is true for milky layers of CH. Crystallite sizes are smaller in the latter than in the former.For all samples studied, exists a positive correlation between at% (1/2Ca+1/2Mg+Na+K+Li) and at% (Al3++Fe3+). This indicates that at least parts of (A13++ Fe3+) substitute for Si in the quartz structure. The charge is balanced by incorporation of di- and mono-valent cations in structural interstices. When the quantity at % H+, as obtained from , is included into the sum at% (1/2 Me2++Me+), the above correlation is destroyed. This result could be indicative for a strong concentration of the Si-OH groups in the surface of the quartz microcrystallites.  相似文献   

10.
Garnet-pyroxene skarns were formed 90 m.y. B.P. in the Osgood Mountains at or near contacts of grandiorite with calcareous rocks of the Cambrian Preble Formation. The metasomatic replacement followed contact metamorphic recrystallization of the Preble. The sources, temperature, and variation in H2O/CO2 ratios of the metasomatic fluid are interpreted from 269 analyses of oxygen, carbon, hydrogen, and sulfur isotopes in whole rocks, minerals and inclusion fluids.Skarns formed in three mineralogical stages. Oxygen isotope data indicate that temperatures during the crystallization of garnet, pyroxene and wollastonite (Stage I) were least 550 ° C, and that the metasomatic fluid had an 0.035 in the massive skarns, and 0.12 in vein skarns up to 3 cm thick. Pore fluids in isotopic equilibrium with garnet in calc-silicate metamorphic rocks, on the other hand, had 0.15.The metasomatic fluids of Stage I were derived primarily from the crystallizing magma. The isotopic composition of magmatic water was 18O =+9.0, D= –30 to –45. Oxygen isotope temperatures of greater than 620 ° C were determined for the granodiorite. Isotopic and chemical equilibria between mineral surfaces and the metasomatic fluid were approached simultaneously in parts of the skarn several meters or more apart, while isotopic and chemical disequilibria (i.e. zoning) have been preserved between 20 to 40 m-thick zones in grandite garnet. More Fe-, or andradite-rich garnet crystallized in more H2O-rich C-O-H fluids ( 0.01) than present with grossularite-rich garnet ( 0.035).Stage II was marked by the replacement of garnet and pyroxene by quartz, amphibole, plagioclase, epidote, magnetite, and calcite. Many of the replacement reactions took place over a relatively narrow range in temperature (480–550 ° C), as indicated by 18O fractionations between quartz and amphibole. Meteoric water comprised 20 to 50% of the metasomatic fluid during Stage II.Calcite was formed along with pyrite, minor pyrrhotite, and chalcopyrite during Stage III, although the crystallization of pyrite and calcite had begun earlier, during Stages I and II, respectively. Carbon and sulfur isotope compositions of calcite and pyrite indicate a magmatic source for most of the C and S in the metasomatic fluids of Stage III. By the end of Stage III, meteoric water constituted as much as 100% of the metasomatic fluid. Minerals from grandiorite and skarn do not show large depletions in 18O because the oxygen isotope composition of the metasomatic fluid was buffered by the calcareous wall rocks and the grandiorite.Meteoric water in the vicinity of the Osgood Mountains during the Late Crectaceous (18Ocale. –14.0, D = – 107) was slightly enriched in 18O and D relative to present-day meteoric water (18O = 15.9, D = – 117)  相似文献   

11.
The proportions of species in a C-O-H-S fluid in equilibrium with graphite, pyrite and pyrrhotite were calculated for a range of pressure, temperature and conditions, using the equilibrium constants and mass balance method, for ideal and non-ideal mixing in the fluid. Under typical metamorphic conditions, H2O, CO2, CH4 and H2S are the principal fluid species with H2S favored by higher temperatures, lower pressures and lower conditions. The dominance of H2S in the fluid at high temperatures leads to values of becoming significantly less than 1, and causes hydrous minerals to dehydrate at lower temperatures than the case when . The production of H2S-bearing fluids provides a mechanism for the selective transfer of sulfur from a graphite-pyrite-pyrrhotite bearing pelite into a pluton via a fluid phase, without requiring wholesale melting and assimilation of rocks. Such a process is feasible if a magma is intruded by stoping, which allows a significant volume of pelite country rock to be raised rapidly to temperatures approaching that of the magma. H2S-bearing fluids produced from graphite-pyrite-pyrrhotite pelites (due either to magmatic intrusion or regional metamorphism) may also mobilize ore-forming metals as sulfide complexes.  相似文献   

12.
The partition of Ni between olivine and monosulfide-oxide liquid has been investigated at 1300–1395° C, =10–8-9–10–6.8, and =10–2.0–10–0.9, over the composition range 20–79 mol. % NiS. The product olivine compositions varied from Fo98 to Fo59 and from 0.06 to 3.11 wt% NiO. The metal/sulfur ratio of the sulfide-oxide liquid increases with increase in , decrease in , and increase in NiS content. The Ni/Fe exchange reaction has been perfectly reversed using natural olivine and pure forsterite as starting materials. The FeO and NiO contents of olivine from runs equilibrated at the same and form isobaric distributions with NiS content, which, to a first approximation, are dependent at constant temperature and total pressure on a variable term, –0.5 log ( / ). The Ni/Fe distribution coefficient (K D3) exhibits only a weak decrease from 35 to 29 with increase in from the IW buffer to close to the FMQ buffer. At values higher than FMQ, the sulfide-oxide liquid has the approximate composition (Ni,Fe)3±xS2K D358. The present K D3 vs O/(S+O) data define a trend which extrapolates to K D320 at 10 wt% oxygen in the sulfide-oxide liquid. The compositions of olivine and Ni-Cu sulfides associated with early-magmatic basic rocks and komatiites are consistent, at 1400° C, with a value of -log ( / ) of about 7.7, which is equivalent to 0.0 wt% oxygen in the hypothesized immiscible sulfide-oxide liquid. Therefore, K D3 would not be reduced significantly from the 30 to 35 range for sulfide-oxide liquids with low oxygen contents.  相似文献   

13.
Under hydrous conditions the stability field of the assemblage Mg-cordierite+K feldspar+quartz is limited on its low-temperature side by the breakdown of cordierite+K feldspar into muscovite, phlogopite and quartz, whereas the high-temperature limit is given by eutectic melting. The compatibility field of the assemblage ranges from 530° C to 745° C at 1 kbar , from 635 to 725° C at 3 kbars , from 695 to 725° C at 5 kbars and terminates at 5.5 kbars . Most components not considered in the model system will tend to restrict this field even more. However, the condition < P total will increase the range of stable coexistence drastically, making the assemblage common at elevated temperatures from contact metamorphic rocks up to intermediate pressure granulites of appropriate bulk composition.  相似文献   

14.
Microphenocrystic pyrrhotites were observed in the glassy groundmass of two dacite rocks from Satsuma-Iwojima, southwest Kyushu, Japan. It suggests that the dacite magma was saturated with respect to pyrrhotite at the time of eruption, and thus the sulfur contents in the groundmass can be taken as the solubility of sulfur in the dacite magma. The solubility of sulfur in the dacite rocks thus calculated is 65 to 72 ppm sulfur at the estimated conditions of T=900±50°C, and atm.  相似文献   

15.
Redox states of lithospheric and asthenospheric upper mantle   总被引：31，自引：7，他引：24  
The oxidation state of lithospheric upper mantle is heterogeneous on a scale of at least four log units. Oxygen fugacities ( ) relative to the FMQ buffer using the olivine-orthopyroxene-spinel equilibrium range from about FMQ-3 to FMQ+1. Isolated samples from cratonic Archaean lithosphere may plot as low as FMQ-5. In shallow Proterozoic and Phanerozoic lithosphere, the relative is predominantly controlled by sliding Fe3+-Fe2+ equilibria. Spinel peridotite xenoliths in continental basalts follow a trend of increasing with increasing refractoriness, to a relative well above graphite stability. This suggests that any relative reduction in lithospheric upper mantle that may occur as a result of stripping lithosphere of its basaltic component is overprinted by later metasomatism and relative oxidation. With increasing pressure and depth in lithosphere, elemental carbon becomes progressively refractory and carbon-bearing equilibria more important for control. The solubility of carbon in H2O-rich fluid (and presumably in H2O-rich small-degree melts) under the P,T conditions of Archaean lithosphere is about an order of magnitude lower than in shallow modern lithosphere, indicating that high-pressure metasomatism may take place under carbon-saturated conditions. The maximum in deep Archaen lithosphere must be constrained by equilibria such as EMOG/D. If the marked chemical depletion and the orthopyroxene-rich nature of Archaean lithospheric xenoliths is caused by carbonatite (as opposed to komatiite) melt segregation, as suggested here, then a realistic lower limit may be given by the H2O +C=CH4+O2 (C-H2O) equilibrium. Below C –H2O a fluid becomes CH4 rather than CO2-bearing and carbonatitic melt presumably unstable. The actual in deep Archaean lithosphere is then a function of the activities of CO2 and MgCO3. Basaltic melts are more oxidized than samples from lithospheric upper mantle. Mid-ocean ridge (MORB) and ocean-island basalts (OIB) range between FMQ-1 (N-MORB) and about FMQ +2 (OIB). The most oxidized basaltic melts are primitive island-arc basalts (IAB) that may fall above FMQ+3. If basalts are accurate probes of their mantle sources, then asthenospheric upper mantle is more oxidized than lithosphere. However, there is a wide range of processes that may alter melt relative to that of the mantle source. These include partial melting, melt segregation, shifts in Fe3+/Fe2+ melt ratios upon decompression, oxygen exchange with ambient mantle during ascent, and low-pressure volatile degassing. Degassing is not very effective in causing large-scale and uniform shifts, while the elimination of buffering equilibria during partial melting is. Upwelling graphite-bearing asthenosphere will decompress along -pressure paths approximately parallel to the graphite saturation surface, involving reduction relative to FMQ. The relative will be constrained to below the CCO equilibrium and will be a function of . Upwelling asthenosphere whose graphite content has been exhausted by partial melting, or melts that have segregated and chemically decoupled from a graphite-bearing residuum will decompress along -decompression paths controlled by continuous Fe3+-Fe2+ solid-melt equilibria. These equilibria will involve increases in relative to the graphite saturation surface and relative to FMQ. Melts that finally segregate from that source and erupt on the earth's surface may then be significantly more oxidized than their mantle sources at depth prior to partial melting. The extent of melt oxidation relative to the mantle source may be directly proportional to the depth of graphite exhaustion in the mantle source.  相似文献   

16.
Diffusion rates of18O tracer in quartz ( c, 1 Kb H2O) and Amelia albite ( 001, 2 Kb H2O) have been measured, using Secondary Ion Mass Spectrometry (SIMS). A new technique involving hydrothermal deposition of labelled materials has removed the possibility of pressure solution-reprecipitation processes adversely affecting the experiments. Reported diffusion constants are:-quartz ( c), ,Q=98±7 KJ mol–1 (600–825° C, 1 Kb); Amelia albite ( 001), ,Q=85±7 KJ mol–1, (400–600° C, 2 Kb). Measured quartz18O diffusivities decrease discontinuously at the- transition, reflecting strong structural influences. The reported albite data agree with previously recorded studies, but-quartz data indicate significantly lower activation energies. Possible causes of this discrepancy, and some geological consequences, are noted.  相似文献   

17.
Thermodynamic calculations, modified after Nicholls et al. (1971), which relate the activity of silica in a lava to the temperature and pressure conditions at which the lava could be in equilibrium with a mantle mineral assemblage, have been extended to H2O-bearing magmas by using published experimental data to derive the dependence of on the weight fraction of H2O dissolved in a magma. A petrogenetic grid has been calculated which gives the P-T conditions under which a magma with a given at its liquidus at 1 atm could equilibrate with a mantle mineral assemblage containing olivine (ol) and orthopyroxene (opx) for different amounts of H2O in the magma at its source. This grid is in good agreement with the results of experimental studies as summarized by Green (1971) and Brey and Green (1975). The results show that the pressure at which a given magma composition can equilibrate with ol + opx increases for increasing amounts of H2O dissolved in the magma at depth.In addition, experimental data have been used to calculate the effect of olivine crystallization and removal on the in the residual liquid to assess the effect of low-pressure differentiation on . The results show that if 20 % olivine is added to a basalt magma, its calculated pressure of equilibration with ol+opx increases by 4–5 kbar for a given temperature. The calculated effects of olivine removal and H2O addition on are reasonably consistent with the silicate mixing model of Burnham (1975).Thermodynamic calculations of this type may be useful for assessing the internal consistency of certain experimental data, and in extrapolating the results to other magma compositions. The application of these calculations to determining the possible depth of origin of natural lavas appears to be limited primarily by the difficulty in determining in a lava at its liquidus temperature.  相似文献   

18.
APL computer programs for the thermodynamic calculation of devolatilization and solid-solid equilibria operate using stored values for the molar volume and entropy of solids, the free energies of H2O and CO2, and the free energies of formation for 110 geologically-important phases. P-T-X CO 2 calculations of devolatilization equilibria can be made at pressures from 0.2 through 10 kb, and temperatures from 200 through 1,000° C. P-T-X calculations of solid-solid equilibria may be accomplished at pressures to 30 kb and temperatures to 1,000° C. Calculations can be extrapolations from experimental points, or direct calculations from thermochemical data alone. Options are available in these programs to consider effects of: real vs. ideal gas mixing, thermal expansion and compressibility, solid solution, fluid pressure differing from solid pressure, and uncertainties in high-temperature entropies.A collection of thermodynamic data programs accompanies the programs for calculating P-T-X CO 2 equilibria. Over a wide range of physical conditions, the data functions report free energies, entropies, fugacities of H2O and CO2, high temperature entropies of solids, and activities of components in H2O-CO2 mixtures.List of Symbols Activity of H2O and CO2 - Gf Free energy of formation of a phase from elements - Gr Free energy change of reaction - G r o Standard state free energy change of a reaction - Free energies of pure H2O and CO2 - H r o Standard state enthalpy change for a reaction - K Equilibrium constant - R Gas constant - S r o Standard state entropy change of reaction - S s o Standard state entropy change of solids in a reaction - Vs o Standard state volume change of a reaction - Vs o Standard state volume change of solids in a reaction - Mole fraction of H2O and CO2 - Activity coefficient of H2O and CO2  相似文献   

19.
The mineralogy, petrology and geochemistry of the Proterozoic Harney Peak Granite, Black Hills, South Dakota, were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare-element pegmatites. The granite was emplaced at 3–4 kbar as multiple sills and dikes into quartz-mica schists at the culmination of a regional high-temperature, low-pressure metamorphic event. Principally along the periphery of the main pluton and in satellite intrusions, the sills segregated into granite-pegmatite couplets. The major minerals include quartz, K-feldspar, sodic plagioclase and muscovite. Biotite-{Mg No. [Molar MgO/(MgO+FeO)]=0.32-0.38} is the predominant ferromagnesian mineral in the granite's core, whereas at the periphery of the main pluton and in the satellite intrusions tourmaline (Mg No.=0.18–0.48) is the dominant ferromagnesian phase. Almandine-spessartine garnet is also found in the outer intrusions. There is virtually a complete overlap in the wide concentration ranges of SiO2, CaO, MgO, FeO, Sr, Zr, W of the biotite- and tourmaline-bearing granite suites with no discernable differentiation trends on Harker diagrams, precluding the derivation of one suite from the other by differentiation following emplacement. This is consistent with the oxygen isotope compositions which are 11.5 ± 0.6 for the biotite granites and 13.2 ± 0.8 for the tourmaline granites, suggesting derivation from different sources. The concentrations of TiO2 and possibly Ba are higher and of MnO and B are lower in the biotite granites. The normative Orthoclase/Albite ratio is extremely variable ranging from 0.26 to 1.65 in the biotite granites to 0.01–1.75 in the tourmaline granites. Very few sample compositions fall near the high-pressure, watersaturated haplogranite minima-eutectic trend, indicating that the granites for the most part are not minimum melts generated under conditions with =1. Instead, most biotite granites are more potassic than the water-saturated minima and eutectics and in analogy with experimentally produced granitic melts, they are best explained by melting at 6 kbar, <1 and temperatures 800°C. Such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals (Nabelek et al. 1992). Several of the tourmaline granite samples contain virtually no K-feldspar and have oxygen isotope equilibration temperatures 716–775°C. Therefore, they must represent high-temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water-saturated haplogranite minima or during fractionation of intruded melts into granite-pegmatite couplets accompanied by volatile-aided differentiation of the alkali elements. The indicated high temperatures, <1, the relatively high TiO2 and Ba concentrations and the relatively low values of the biotite granites suggest that they were generated by high-extent, biotite-dehydration melting of an immature Archean metasedimentary source. The ascent of the hot melts may have triggered low-extent, muscovite-dehydration melting of schists higher in the crust producing the high-B, low-Ti melts comprising the periphery of the main pluton and the satellite intrusions. Alternatively, the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse, with the low- biotite granites generated at a deeper, hotter region and the high- tourmaline granites at a higher, cooler region of the crust. The low-Ti and high-B concentrations in the high- melts resulted in the crystallization of tourmaline rather than biotite, which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions.  相似文献   

20.
Trace element analyses of 1-atm and high-pressure experiments show that in komatiite and peridotite, the olivine (OL)/liquid (L) distribution coefficient for Al2O3 ( ) increases with pressure and temperature. Olivine in equilibrium with liquid accepts as much as 0.2 wt% Al2O3 in solution at 6 GPa. Convergence to equilibrium compositions at this high level is shown by cation diffusion of Al into synthetic forsterite crystals of low-Al contents in the presence of melt. Convergence to low-Al equilibrium compositions at lower P and T is shown by diffusion of Al out of synthetic forsterite with high initial Al content. Isobaric and isothermal experimental data subsets reveal that temperature and pressure variations both have real effects on . Variation in silicate melt composition has no detectable effect on within the limited range of experimentally investigated mixtures. Least-squares regression for 24 experiments, using komatiite and peridotite, performed at 1 atm to 6 GPa and 1300 to 1960°C, gives the best fit equation: Increase in with increasingly higher-pressure melting is consistent with incorporation of a spinel-like component of low molar volume into olivine, although other substitutions possibly involving more complex coupling cannot be ruled out. High P-T ultrabasic melting residues, if pristine, may be recognized by the high calculated from microprobe analyses of Al2O3 concentrations in residual olivines and estimated Al2O3 concentration in the last liquid removed. In general the low levels of Al in natural olivine from mantle xenoliths suggest that pristine residues are rarely recovered.  相似文献   

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