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1.
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   

2.
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   

3.
The Miocene-Pliocene Macusani volcanics, SE Peru, outcrop in three separate tectonic intermontane basins developed on a Paleozoic-Mesozoic volcano-sedimentary sequence. Several ignimbrite sheets are recognized and K-Ar dates record at least semi-continuous volcanic activity from 10 to 4 Ma in the Macusani field. The volcanics in the Macusani basin comprise crystal-rich (45% crystals) ash-flow tuffs and rare obsidians glasses, both with unusual mineralogy, similar to two-mica peraluminous leucogranites. The mineralogical assemblage (quartz, sanidine Or69–75, plagioclase, biotite, muscovite and andalusite (both coexisting in the entire volcanic field), sillimanite, schörl-rich tourmaline, cordierite-type phases, hercynitic spinel, fluor-apatite, ilmenite, monazite, zircon, niobian-rutile) is essentially constant throughout the entire Macusani field. Two distinct generations of plagioclase are recognized, viz. group I (An10–20) and group II (An30–45). Sillimanite forms abundant inclusions in nearly all phases and is earlier than andalusite which occurs as isolated phenocrysts. Biotite (Al-, Ti-, Fe- and F-rich) shows pronounced deficiencies in octahedral cations. Muscovite is also F-rich and displays limited biotitic and celadonitic substitutions. There is no systematic variation in mineral chemistry with stratigraphic position. The mineralogical data provide a basis for distinction between an early magmatic and a main magmatic stage. The early stage corresponds to the magmatic evolution at or near the source region and includes both restites and early phenocrysts. Some biotites (with textures of disequilibrium melting to Fe — Zn spinel), part of the sillimanite, apatite and monazite, possibly some tourmaline and cordierite-type phases are restites. However, the restite content of the magma was low (5 vol. % maximum). The group II plagioclase are interpreted as early phenocrysts. During this stage, temperatures were as high as 800° C, pressure was no more than 5–7.5 kbar, was intermediate between WM and QFM and was low. The biotite melting textures and the coexistence of restites and early phenocrysts imply fast heating rates in the source region. The transition between the early and the main magmatic stage was abrupt (andalusite crystallization in place of sillimanite, group I vs. group II plagioclases) and suggests rapid ascent of the magma from its source region. During the main crystallization stage, temperature was 650° C or lower at a pressure of 1.5–2 kbar. (calculated from equilibrium between muscovite, quartz, sanidine and andalusite) are around 1, suggesting conditions close to H2O-saturation. f HF is around 1 bar but the ratios are significantly different between samples. ranges between 138 and 225 bar. This study shows that felsic, strongly peraluminous, leucogranitic magmas having andalusite and muscovite phenocrysts may be generated under H2O-undersaturated conditions.CRPG Contribution n 769  相似文献   

4.
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   

5.
The Mogan and Fataga formations on the island of Gran Canaria, Canary Islands, represent a sequence of approximately 30 intercalated pyroclastic and lava flows (total volume about 500 km3 dense-rock equivalent) including subalkaline rhyolitic, peralkaline rhyolitic and trachytic pyroclastic flows, nepheline trachyte lavas and a small volume of alkali basaltic lavas and tephra deposits. The eruption of the intermediate to silicic rocks of the Mogan and Fataga formations follows the roughly 4 Ma duration of basaltic shield volcanism. The most common assemblage in the evolved (Mogan and Fataga) rocks is anorthoclase+ edenitic amphibole+ilmenite+magnetite±augite±hypersthene +apatite+pyrrhotite. A few flows also contain plagioclase, biotite, or sphene. Coexisting Fe-Ti oxides yield equilibrium temperatures between 835 and 930° C and log between –11.2 and –12.6. The lowermost pyroclastic flow of the Mogan formation is zoned from a rhyolitic base (848° C) to a basaltic top (931° C). Unit P1 has an oxygen isotope feldspar-magnetite temperature (850° C) very close to its Fe-Ti oxide temperature. One of the youngest Mogan flows is zoned from a comendite (836° C) at the base to a comenditic trachyte (899° C) at the top. The Fataga formation pyroclastic flows show only slight compositional zonation, and one flow has the same Fe-Ti oxide compositions at top and base.Calculations using the reaction 1/3 magnetite+SiO2 (melt)=ferrosilite+1/6 O2 indicate total pressures of 1–4 (±3) kb for six of the Mogan flows and one of the Fataga flows. For four of the pyroclastic flows, equilibria involving tremolite-SiO2-diopside-enstatite-H2O and phlogopite-SiO2-sanidine-enstatite-H2O imply water contents of 0.9 to 2.6 (±0.5) wt% and between 80 and 610 bars, which indicates that magma within the Tejeda reservoir was H2O-undersaturated throughout the entire history of Mogan to Fataga volcanism. The fluorine contents of amphibole, biotite, and apatite, and chlorine contents of apatite reveal thatf HF/ andf HCl/ high compared to most igneous rocks and are consistent with the peralkaline nature of most of the volcanics. Thef HCl estimate for one flow is 10–2 to 10–1 bars andf HF for six of the flows ranges from about 10–1 to 6 bars. Pyrrhotite compositions yield estimates for log between –1 and –3, log between –2 and 1.5, and log between 0.5 and 3, which fall in the range of most intermediate to silicic systems. The lack of a systematic trend with time for magma composition, Fe-Ti oxide temperatures, water contents, phenocryst abundances, and ferromagnesian phase composition indicate that the Tejeda magmatic system was open and kept at nearly the same conditions by the periodic addition of more primitive melts.The intensive thermodynamic parameters estimated from coexisting phenocryst equilibria are used to constrain the eruption dynamics based on solution of the conservation equations for a vapor plus pyroclast mixture. The estimates of magma reservoir temperature, pressure, and water concentration, when combined with a one-dimensional fluid dynamical model of a pyroclastic eruption, imply that the velocities of the ash flows at the vent exit were on the order of 100 to 200 m s–1, and the mass flow rates were about 107 kg s–1 for an assumed vent radius of 10 m.  相似文献   

6.
The phenocryst assemblage of cummingtonite, orthopyroxene, quartz, titanomagnetite and ilmenite in rhyolites of New Zealand has been used to calculate P total and . The values of P total and depend strongly upon whether an ideal mixing, or an ordered, model is used for the solid-solutions, but in both cases P total.The rhyolite magma contained over 9 per cent water (by weight) when the cummingtonite phenocrysts precipitated, and possibly as much as 12 per cent, so that it is surprising that one of these rhyolites is a coherent lava. The calculated values of P total and are very sensitive to uncertainty in both the composition of the solid-solutions and temperature. Calculations show that >0.7–0.8 P total for cummingtonite to precipitate in rhyolites, and that iron-rich olivine and cummingtonite could only exist in rhyolites over a small temperature range at a pressure near 5 kilobars. Hornblende phenocrysts co-existing with fayalitic olivine in rhyolites accordingly have a very low activity of Mg7Si8O22(OH)2.  相似文献   

7.
Late Pleistocene or Recent lavas from San Quintin, Baja California are basanitoids and alkali basalts. The surface quench temperatures of the lavas average 1 005° C with log =–11.4, as deduced from the groundmass Fe-Ti oxides. Spinel lherzolite xenoliths and megacrysts of augite and andesine have been found in lava flows and cinder deposits. Using analytical data on the rocks and minerals and simple thermodynamic expressions, the pressures and temperatures of equilibration of lavas and xenoliths, megacrysts and phenocrysts have been calculated. The lavas could have been in equilibrium with lherzolite at 1 330–1 410° C and 27.5–31.6 kb, the more silica-poor liquid having the higher values. The basanitoid could have equilibrated with the megacrysts at about 10.5 kb and with phenocrysts at about 1.4 kb and 1130° C. The variation in composition of the lavas may be explained by a rising zone of melting within the mantle, the most silica-poor liquid having the deepest source. The source of the San Quintin basalts is probably related to spreading of the ocean floor in the Gulf of California.  相似文献   

8.
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   

9.
A thermodynamic model is developed that describes the compositional variation of coexisting olivine, orthopyroxene, and ferrite (Fe3O4-MgFe2O4) as a function of and T. The ferrite phase has a cation distribution which varies from nearly inverse to nearly random with increasing T and is described with a model in which the number of sites per formula unit on which mixing occurs varies from 1.67 to 2.0. Given this model and the equilibrium phase composition data for coexisting olivine and ferrite at 1,300° C of Jamieson and Roeder (1984), the ferrite solution is described to an excellent approximation by a symmetric regular solution model with W ft =+14.0 ±0.3 kJ/mole. Orthopyroxene and olivine non-ideality are also considered. The T-dependence of the equilibrium constant for the oxidation reaction 6Fs+2Mt=6Fa+O2 and the two Fe/Mg exchange reactions between olivine-ferrite and olivine-orthopyroxene, are used to calculate the compositional variation of coexisting phases as a function of and T. The results are summarized on an isobaric (1 bar) –1/Tplot with the compositional variation of olivine, ferrite, and orthopyroxene shown by sets of isopleths. The ferrite isopleths intersect those of olivine and orthopyroxene at sufficiently high angles for this assemblage to serve as a sensitive geothermometer and oxygen-barometer. The model is applied to orthopyroxene-ferrite symplectite in coronas around olivine in a metamorphosed gabbro, to olivine-hosted orthopyroxene-ferrite symplectite in unmetamorphosed gabbros and norites and to olivine-hosted orthopyroxene-ferrite symplectites developed within the rims of lherzolite xenoliths.  相似文献   

10.
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   

11.
In pelitic schists of the Sanbagawa metamorphic belt, sphene, rutile, and ilmenite occur as discrete grains, in composite aggregates, and as inclusions in garnets. Textural relationships, disposition of inclusions in garnet, and the compositions of ilmenites suggest that the titanium-bearing accessories stable at the peak metamorphic conditions were as follows: sphene in the chlorite zone, sphene plus rutile in the garnet zone, and ilmenite in the highest grade of the belt, the biotite zone. Rutile appears in the garnet zone as a consequence of Ca incorporation into garnet and a progressive increase in .Retrograde reactions were responsible for the composite aggregates of rutile, sphene and ilmenite and these must be carefully evaluated before prograde relationships between titanium-bearing accessories can be properly understood.  相似文献   

12.
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   

13.
The oxygen fugacity ( ) of a C-O-H fluid in equilibrium with graphite has been determined in the range 10–30 kbar by equilibrating solid -buffer assemblages in graphite capsules containing C-O-H fluid. By using different buffers (FexO-Fe3O4, Ni-NiO, Co-CoO, Mo-MoO2), the of the graphite-saturated fluid is bracketed within a narrow range. This technique produces a calibration for the imposed on a sample contained within a graphite capsule. To achieve a thermodynamically-invariant system at fixed P and T, the was imposed on the system with an external buffer and the double-capsule technique. The experiments were performed in solid-media, high pressure apparatus with 19 mm tale-pyrex assemblies. A series of experiments at 10, 15, 20, 25, and 30 kbar, 800–1600° C, with imposed by the Fe2O3-Fe3O4-H2O equilibrium were conducted. The experimental results have been fitted to the following equation:
  相似文献   

14.
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   

15.
The mineralogy and petrology of volcanic and plutonic rocks from the island of Grenada are described. The volcanic rocks include basanitoids, alkalic and subalkalic basalts, andesites and dacites. Phenocryst phases in the basanitoids and basalts are olivine (Fo90–71), zoned calcic augite, spinel ranging from ferrian pleonaste through chromite to titaniferous magnetite, and plagioclase. Some of the basalts contain pargasitic amphibole. Andesites and dacites generally contain hypersthene and augite, and one pigeonite-hypersthene-augite-bearing andesite was found. Apatite commonly occurs as a phenocryst in the andesites and dacites and quartz is present in some dacites as well as being a possible xenocryst in both alkalic and subalkalic basalts. Plutonic cumulates found as ejected fragments in tuffs and ashes are composed of variable proportions of olivine, magnetite, calcic augite, amphibole and plagioclase. One peridotitic (ol-cpx-opx) fragment was found but spinel or garnet peridotitis are absent. Despite the alkalic nature of the association, calcalkalic characteristics such as calcic plagioclase, restricted Feenrichment in coexisting pyroxenes and generally low TiO2 content relative to oceanic suites are present in Grenada. Estimates of conditions of equilibration of the basanitoids with potential upper mantle materials using the results of high-pressure experiments are compared with estimates from thermodynamic data. Equating and basanitoid with hypothetical garnet peridotite assemblages gives a pressure and temperature of equilibration in the region of 35–38 kbar and 1550–1625 ° K. Experimental results are not supportive of these estimates.  相似文献   

16.
Fluid inclusions occur in a composite xenolith from the Lunar Crater Volcanic Field, Nevada, U.S.A. The xenolith is an amphibole-bearing wehrlite that is cut by an andesine-amphibole vein. The compositions of individual fluid inclusions in both portions of the xenolith have been determined using microthermometry and micro Laser-Raman spectroscopy. Fluids in the host wehrlite are nearly pure CO2 (>99 mol%) whereas those in the vein contain from 8.5 to 12.0 mol % CO in CO2. Chemical modelling shows that the composition of the vein fluids at T room is representative of the composition at the high P, T conditions of trapping. Graphite has not been observed by optical microscopy in any of the fluid inclusions. Graphite is probably absent (although stable at T<800° C) most probably because of the kinetically unfavorable CO decomposition reaction and rapid quenching. By combining the measured fluid compositions with fluid P-V-T data and the chemical equilibrium CO2CO +1/2 O2, we have calculated the oxygen fugacity of the fluid inclusions at 1200° C: log 8.6 (vein) and –6 (host). If the of the fluid in the vein represents that in equilibrium with the magma that crystallized to produce the vein, then the of the basalt magma is near QFM at 1200° C and 10.3 kbar. This is similar to values reported for extrusive basaltic lavas. If the much lower intrinsic oxygen fugacity-values for divines and spinels from alkali basalt nodules are representative of upper mantle conditions, then oxidation of basaltic magmas must occur in the upper mantle prior to ascent to the surface. Implications for the origin of CO2-rich fluids and carbon isotope geochemistry are also discussed.  相似文献   

17.
Aenigmatite, sodic pyroxene and arfvedsonite occur as interstitial minerals in metaluminous to weakly peralkaline syenite patches in alkali dolerite, Morotu, Sakhalin. Aenigmatite is zoned from Ca, Al, Fe3+-rich cores to Ti, Na, Mn, Si-rich rims reflecting the main substitutions Fe2+Ti4+Fe3+, NaSiCaAl and Mn2+Fe2+. Aenigmatite replaces aegirine and ilmenite supporting the existence of a no-oxide field in — T space. In one case aenigmatite has apparently formed by reaction between ilmenite and arfvedsonite. Titanian aegirine (up to 3.0 wt% TiO2) and Fe-chlorite may replace aenigmatite. Sodic pyroxene occurs as zoned crystals with cores of aegirine-augite rimmed by aegirine and in turn by pale green aegirine containing 93 mol% NaFe3+Si2O6. Additional substitution of the type NaAlCaFe2+ is indicated by significant amounts (up to 6 mol%) of NaAlSi2O6. Arfvedsonite is zoned with rims enriched in Na, Fe and depleted in Ca which parallels the variation of these elements in the sodic pyroxenes.The high peralkalinity of the residual liquid from which the mafic phases formed resulted from the early crystallization of microperthite (which makes up the bulk of the syenites) leading to an increase in the Na2O/(Na2O+K2O) and (Na2O+K2O)/Al2O3 ratios of the remaining interstitial liquid which is also enriched in Ti, Fe, and Mn. Bulk composition of the melt, , temperature and volatile content were all important variables in determining the composition and stability of the peralkaline silicates. in the residual liquid appears to have been buffered by arfvedsonite-aegirine and later by the arfvedsonite-aenigmatite and aenigmatite-aegirine equilibria under conditions of a no-oxide field. An increase in , above that of the alkali buffer reactions, is inferred by an increase of Ti and Mn in aenigmatite rims. The latest postmagmatic vapour crystallization stage of the syenites is marked by extremely low which may have been facilitated by exsolution of a gas phase. Low is supported by the replacement of aenigmatite by titanian aegirine, and the formation of rare Ti-rich garnet with a very low (Ti4++Fe3+)/(Ti+Fe) ratio of 0.51, associated with leucoxene alteration of ilmenite.  相似文献   

18.
Laboratory experiments have been conducted with natural minerals to determine the relation of to epidote stability, and to determine stability curves for clinozoisite and epidote. Under oxidizing conditions Fe-epidote decomposes to grandite, anorthite, hematite, and quartz. Under more reducing conditions corundum becomes a stable product instead of quartz, and magnetite, and finally hercynite replace hematite. As conditions change from oxidizing to reducing the temperature of epidote breakdown decreases, epidote becomes more aluminous and the grandite produced increases in grossularite component and, to a lesser extent, in almandine.At 3000 bars under oxidizing conditions epidote is stable up to 694° C, epidote-corundum is stable to 692° C, clinozoisite is stable to 658° C, and clinozoisite-quartz is stable to 628° C. Approximate curves for the fractional decomposition of Al-Fe epidote have been determined as a function of Fe content under oxidizing conditions. Extrapolation of clinozoisite results to an Fe-free composition, and comparison with zoisite stability results suggest that at elevated pressures clinozoisite inverts with increasing temperature to zoisite along a nearly vertical phase boundary at 635±75° C.The stability relations provide an upper limit for epidote mineral stability mainly applicable to calcareous rocks. The epidote composition present in any given rock must be a function largely of bulk composition and . Zoisite replaces Al-clinozoisite in rocks of medium grade and high pressure.  相似文献   

19.
The nature of the near-liquidus phases for a mantle-derived olivine melilitite composition have been determined at high pressure under dry conditions and with various water contents. Olivine and clinopyroxene occur on or near the liquidus and there are no conditions where orthopyroxene crystallizes in equilibrium with the olivine melilitite. We have determined the effect on the liquidus temperature and liquidus phases of substituting CO2 for H2O on a mole for mole basis at 30 kb, using olivine melilitite + 20 wt% H2O at = 0 and = (CO2)/(H2+CO2) (mole fraction) = 0.25, 0.5, 0.75 and 1.0 (i.e. olivine melilitite + 38 wt% CO2). Experiments were buffered by the MH or NNO buffers. At 30 kb, CO2 is only slightly less soluble than water for <0.5 as judged by the slight increase in liquidus temperature on mole-for-mole substitution of CO2 for H2O and at 30 kb, 1200° C, = = 0.5 the olivine melilitite contains 8.8 wt% H2O and 21 wt% CO2 in solution. For 1 the CO2 saturated liquidus is depressed 70 ° C below the anhydrous liquidus and the magma dissolves approx. 17% CO2 at 30kb, 1400 ° C, 1, 0. Infrared spectra of quenched glasses have absorption bands characteristic of CO 3 = and OH- molecules and no evidence for HCO 3 - . The effect of CO 3 = molecules dissolved in the olivine melilitite at high pressure is to suppress the near-liquidus crystallization of olivine and clinopyroxene and bring orthopyroxene and garnet on to the liquidus. We infer that olivine melilitite magmas may be derived by equilibrium partial melting (<5%) of pyrolite at 30 kb, 1150–1200 ° C, provided that both H2O and CO2 are present in the source region in minor amounts. Preferred conditions are 0< <0.5, 0.5< <1, and at low oxygen fugacities (相似文献   

20.
Summary Bastnaesites of Ce and La and their OH-analogs were synthesized and their stability relations were determined atPf = 1 kbar andT = 400 to 900°C in a part of the system (Ce,La)-F-H-C-0. The initial fluid compositions were such that and HF/(HF + H2O) ratios were 0 to 0.172. XRD and IR studies indicate that bastnaesites equilibrated in initial fluids low in HF are all F-enriched. The hydroxylbastnaesite-(La) is stable up to 810°C and the fluorbastnaesite-(La) is stable up to 860°C. Their condensed breakdown products are La2O2CO3 and LaOF, respectively. The stability of Ce bastnaesites is slightly dependent. The hydroxylbastnaesite-(Ce) is stable up to 660°C at the defined by the IQF buffer and up to 640°C by the MH buffer. The fluorbastnaesite-(Ce) is stable up to 800°C at the defined by the IQF and up to 760°C by the MH buffer. The condensed breakdown product for the hydroxyl end-member is simply CeO2 but for the fluorine one is a combination of CeO2, CeF3, and CeOF. Factors, such as OH vs F, , and bulk composition, that affect the stability of individual species are discussed. Petrogenic implications resulting from the present study include that bastnaesites can be stable from hydrothermal to magmatic conditions, that F-enriched species can form in an environment relatively low in F content, and that OH-species are rare and occur only in low-temperature environments essentially devoid of F.
Synthese und Stabilität von Bastndsil in einem Teil des Systems (Ce,La)-F-H-C-O
Zusammenfassung Ce- und La-Bastnäsite, sowie deren OH-Analoga wurden synthetisiert und ihre Stabilitätsbeziehunger beiP f = 1 kbar undT = 400 bis 900°C wurden im System (Ce,La)F-H-C-O bestimmt. Die anfänglichen Flüssigkeitszusammensetzungen waren so, daß und die HF/(HF + H2O)-Verhältnisse 0–0.172 waren. Röntgenpulver- und Ultrarot-Untersuchungen zeigten, daß Bastnäsite, die mit anfänglich HF-armen Flüssigkeiten equilibriert wurden, alle an F angereichert sind. Hydroxilbastndsit-(La) ist bis 810°C und Fluorbastnäsit-(La) bis 860°C stabil. Ihre festen Zersetzungsprodukte sind La2O2O3, bzw. LaOF. Die Stabilität der Ce-Bastnäsite hängt etwas von ab. Hydroxilbastnäsit-(Ce) ist bei des Eisen-Quarz-Fayalit-Puffers bis 660°C stabil und mit Magnetit-Hämatit-Puffer bis 640°C. Das feste Zerfallsprodukt ist für das Hydroxil-Glied nur CeO2, für das Fluor-Glied eine Mischung aus CeO2, CeF3 und CeOF. Faktoren, welche die Stabilität der einzelnen Spezies beeinflussen, werden diskutiert, wie das Verhältnis OH zu F, und die Gesamtzusammensetzung. Petrogenetische Folgerungen aus der vorliegenden Studie schließen ein, daß Bastnäsite von hydrothermalen bis zu magmatischen Bedingungen stabil sein können, daß sich an F angereicherte Glieder in relativ F-armer Umgebung bilden können, und daß OH-Glieder selten sind und nur unter Bildungsbedingungen niedriger Temperatur und weitgehender Abwesenheit von F auftreten.

With 8 Figures  相似文献   

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