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1.
The phase relations in the Fe2SiO4–Fe3O4 binary system have been determined between 900 and 1200 °C and from 2.0 to 9.0 GPa. At low to moderate pressures magnetite can accommodate significant Si, reaching XFe2SiO4=0.1 and 0.2 at 3.0 and 5.0 GPa respectively, with temperature having only a secondary influence. At pressures below 3.5 GPa at 900 °C and 2.6 GPa at 1100 °C magnetite-rich spinel coexists with pure fayalite. This assemblage becomes unstable at higher pressures with respect to three intermediate phases that are spinelloid polytypes isostructural to spinelloids II, III and V in the Ni-aluminosilicate system. The phase relations between the spinelloid phases are complex. At pressures above ≈8.0 GPa at 1100 °C, the spinelloid phases give way to a complete spinel solid solution between Fe3O4 and Fe2SiO4. The presence of small amounts of Fe3+ stabilises the spinel structure to lower pressures compared to the Fe2SiO4 end member. This means that the fayalite–γ-spinel transition is generally unsuitable as a pressure calibration point for experimental apparatuses. The molar volumes of the spinel solid solutions vary nearly linearly with composition, having a small negative deviation from ideal behaviour described by Wv=−0.15(6) cm3. Extrapolation yields V°(298) = 41.981(14) cm3 for the Fe2SiO4-spinel end member. The cell parameters and molar volumes of the three spinelloid polytypes vary systematically with composition. Cation disorder is an important factor in stabilising the spinelloid polytypes. Each polytype exhibits a particular solid solution range that is directly influenced by the interplay between its structure and the cation distributions that are energetically favourable. In the FeO–FeO1.5–SiO2 ternary system Fe7SiO10 (“iscorite”) coexists with the spinelloid phases at intermediate pressures on the SiO2-poor, or Fe3+-poor side of the Fe2SiO4–Fe3O4 join. On the SiO2 and Fe3+-rich side of the join, orthopyroxene or high-P clinopyroxene coexists with the spinelloids and spinel solid solutions. The assemblage pyroxene+spinel+SiO2 is stable over a wide range of bulk composition. The stability of spinelloid III is of particular petrologic interest since this phase has the same structure as (Mg,Fe)2SiO4–wadsleyite, indicating that Fe3+ can be easily incorporated in this important phase in the Earth's transition zone, in addition to silicate spinel. This has important implications for the redox state of the Earth's transition zone and for the depth at which the olivine to spinel transition occurs in the mantle, potentially leading to a shift in the “410 km” seismic discontinuity to shallower depths depending on the prevailing redox state. In addition, a coupled tetrahedral substitution of Fe3++OH for Si+O could provide a further mechanism for the incorporation of H2O in wadsleyite. Received: 10 January 2000 / Accepted: 12 May 2000  相似文献   

2.
Silicon carbide (SiC) is a particularly interesting species of presolar grain because it is known to form on the order of a hundred different polytypes in the laboratory, and the formation of a particular polytype is sensitive to growth conditions. Astronomical evidence for the formation of SiC in expanding circumstellar atmospheres of asymptotic giant branch (AGB) carbon stars is provided by infrared (IR) studies. However, identification of the crystallographic structure of SiC from IR spectra is controversial. Since >95% of the presolar SiC isolated from meteorites formed around carbon stars, a determination of the structure of presolar SiC is, to first order, a direct determination of the structure of circumstellar SiC. We therefore determined the polytype distribution of presolar SiC from the Murchison CM2 carbonaceous meteorite using analytical and high-resolution transmission electron microscopy (TEM). High-resolution lattice images and electron diffraction of 508 individual SiC grains demonstrate that only two polytypes are present, the cubic 3C (β-SiC) polytype (79.4% of population by number) and the hexagonal 2H (α-SiC) polytype (2.7%). Intergrowths of these two polytypes are relatively abundant (17.1%). No other polytypes were found. A small population of one-dimensionally disordered SiC grains (0.9%), whose high density of stacking faults precluded classification as any polytype, was also observed. The presolar origin of 2H α-SiC is unambiguously established by tens-of-nanometers-resolution secondary ion mass spectroscopy (NanoSIMS). Isotopic maps of a TEM-characterized 2H α-SiC grain exhibit non-solar isotopic compositions of 12C/13C = 64 ± 4 and 14N/15N = 575 ± 24. These measurements are consistent with mainstream presolar SiC thought to originate in the expanding atmospheres of AGB carbon stars. Equilibrium condensation calculations together with inferred mineral condensation sequences predict relatively low SiC condensation temperatures in carbon stars. The laboratory observed condensation temperatures of 2H and 3C SiC are generally the lowest of all SiC polytypes and fall within the predictions of the equilibrium calculations. These points account for the occurrence of only 2H and 3C polytypes of SiC in circumstellar outflows. The 2H and 3C SiC polytypes presumably condense at different radii (i.e., temperatures) in the expanding stellar atmospheres of AGB carbon stars.  相似文献   

3.
A review of published and newly measured densities for 40 hydrous silicate glasses indicates that the room-temperature partial molar volume of water is 12.0 ± 0.5 cm3/mol. This value holds for simple or mineral compositions as well as for complex natural glasses, from rhyolite to tephrite compositions, prepared up to 10–20 kbar pressures and containing up to 7 wt% H2O. This volume does not vary either with the molar volume of the water-free silicate phase, with its degree of polymerization or with water speciation. Over a wide range of compositions, this constant value implies that the volume change for the reaction between hydroxyl ions and molecular water is zero and that, at least in glasses, speciation does not depend on pressure. Consistent with data from Ochs and Lange (1997, 1999), systematics in volume expansion for SiO2–M2O systems (M=H, Li, Na, K) suggests that the partial molar thermal expansion coefficient of H2O is about 4 × 10−5 K−1 in silicate glasses. Received: 30 June 1999 / Accepted: 5 November 1999  相似文献   

4.
 A synthesis technique is described which results in >99% pure NH4-phlogopite (NH4) (Mg3) [AlSi3O10] (OH)2 and its deuterium analogue ND4-phlogopite (ND4) (Mg3) [AlSi3O10] (OD)2. Both phases are characterised using both IR spectroscopy at 298 and 77 K as well as Rietveld refinement of their X-ray powder diffraction pattern. Both NH4 + and ND4 + are found to occupy the interlayer site in the phlogopite structure. Absorption bands in the IR caused by either NH4 + or ND4 + can be explained to a good approximation using Td symmetry as a basis. Rietveld refinement indicates that either phlogopite synthesis contains several mica polytypes. The principle polytype is the one-layer monoclinic polytype (1M), which possesses the space group symmetry C2/m. The next most common polytype is the two-layer polytype (2M 1 ) with space group symmetry C2/c. Minor amounts of the trigonal polytype 3T with the space group symmetry P3112 were found only in the synthesis run for the ND4-phlogopite. Electron microprobe analyses indicate that NH4-phlogopite deviates from the ideal phlogopite composition with respect to variable Si/Al and Mg/Al on both the tetrahedral and octahedral sites, respectively, due to the Tschermaks substitution VIMg2++IVSi4+↔VIAl3++IVAl3+ and with respect to vacancies on the interlayer site due to the exchange vector XII(NH4)++IVAl3+↔XII□+IVSi4+. Received: 30 August 1999 / Accepted: 2 October 2000  相似文献   

5.
156 samples of naturally‐occurring molybdenite from 87 localities in Australia have been examined to determine the distribution of the hexagonal (2H1) and rhombohedral (3R) polytypes. 90% of the specimens examined are 2H1 polytypes, the remainder being 3R and mixtures of 2H1 and 3R polytypes. The 3R and mixed polytypes are more abundant in porphyry copper deposits or as disseminations, in veins, and in garnet‐quartz pipes in granite. 2H1 polytypes are the sole modification observed in pegmatites, quart‐pegmatite pipes, simple quartz veins, and skarns.

The highest average concentrations of rhenium are in molybdenite from porphyry copper deposits, quartz porphyry pipes, skarn deposits, garnet‐quartz pipes, and pegmatites. Low concentrations occur in molybdenite which is disseminated or in quartz veins in granite and in quartz or pegmatitic pipes. Molybdenites in which the 2H1 polytype is dominant have a lower average rhenium content than those with dominant 3R polytype.  相似文献   

6.
Summary Garnet occurs as a significant mineral constituent of felsic garnet-biotite granite in the southern edge of the Třebíč pluton. Two textural groups of garnet were recognized on the basis of their shape and relationship to biotite. Group I garnets are 1.5–2.5 mm, euhedral grains which have no reaction relationship with biotite. They are zoned having high XMn at the rims and are considered as magmatic. Group II garnets form grain aggregates up to 2.5 cm in size, with anhedral shape of individual grains. The individual garnet II grains are usually rimmed by biotite and have no compositional zoning. The core of group I garnets and group II garnets contains 67–80 mol% of almandine, 5–19 mol% of pyrope, 7–17 mol% of spessartine and 2–4 mol% of grossular. Biotite occurs in two generations; both are magnesian siderophyllites with Fe/(Fe + Mg) = 0.50–0.69. The matrix biotite in granites (biotite I) has high Ti content (0.09–0.31 apfu) and Fe/(Fe + Mg) ratio between 0.50 and 0.59. Biotite II forms reaction rims around garnet, is poor in Ti (0.00–0.06 apfu) and has a Fe/(Fe + Mg) ratio between 0.61 and 0.69. The textural relationship between biotite and garnet indicates that garnet reacted with granitic melt to form Ti-poor biotite and a new granitic melt, depleted in Ti and Mg and enriched in Fe and Al. In contrast to the host durbachites (hornblende-biotite melagranites), which originated by mixing of crustal melts and upper mantle melts, the origin of garnet-bearing granites is related to partial melting of the aluminium-rich metamorphic series of the Moldanubian Zone.  相似文献   

7.
Observations by transmission electron microscopy (TEM) of the submicrometer phases present in calcium aluminate cements have shown that Ca-Al-Fe oxides coexist in two forms with brownmillerite (b) and perovskite (p) structures, respectively. Homogeneous single crystals of both brownmillerite and perovskite have been observed but exsolved lamellae also occur on the scale of tens of nanometers. Perovskite lamellae in brownmillerite exhibit coherent interfaces with an almost perfect [1 0 1]b = {1 0 0}p topotactic relationship. Energy-dispersive X-ray spectroscopy (EDXS) measurements show that perovskite lamellae are enriched in Ti and Si relative to the brownmillerite lamellae. The perovskite phase may accommodate up to 0.17 Si atoms per formula unit, but the exsolution process seems mainly to concern the Ti content. It is estimated that the solvus width ranges between concentrations of 0.06 < Ti < 0.13 atoms per formula unit. O K and Fe L 2,3 edges collected by electron energy loss spectrometry (EELS) confirm that both phases are mainly composed of Fe3+, requiring that the perovskite is highly oxygen-deficient. Al K and Si K EELS spectra have features comparable with those of fourfold-co-ordinated Al and Si sites, suggesting that they are probably located close to oxygen-vacant sites. Received: 23 June 1999 / Accepted: 18 February 2000  相似文献   

8.
The nature of the amorphous regions and their recovery processes in two natural metamict zircon samples from Sri Lanka have been studied by high resolution and analytical transmission electron microscopy. Samples untreated and annealed at different temperatures were investigated. Nanoprobe analyses on untreated samples and samples annealed at 1000 K show that within experimental uncertainties, no chemical segregation occurred. In samples annealed at higher temperatures (≥1100 K) recovery occurs in a two-stage process and leads to different microstructures, which depended on the initial amount of metamictization. In highly amorphized samples, recrystallization starts at 1200 K. Randomly oriented ZrO2 grains embedded in a silica-rich matrix are detected. At higher temperature (16 h at 1600 K), the assemblage transforms into a polygonal texture of small zircon grains. Some untransformed zirconia grains and pockets of silica-rich glass are still present, however. In partially metamict samples, recovery starts at 1100 K. The small surviving oriented zircon domains grow at the expense of the surrounding amorphous material. At 1200 K, new zirconia grains nucleate with random orientations. After 1 h annealing at 1400 K, the zircon structure is restored and the microstructure coarse-grained. The proportion of crystalline zirconia and silica-rich glass has dramatically decreased. Received: 15 November 1999 / Accepted: 1 March 2000  相似文献   

9.
New experimental data in CaO-MgO-SiO2-CO2 at 1 GPa define the vapor-saturated silicate-carbonate liquidus field boundary involving primary minerals calcite, forsterite and diopside. The eutectic reaction for melting of model calcite (1% MC)-wehrlite at 1 GPa is at 1100 °C, with liquid composition (by weight) 72% CaCO3 (CC), 9% MgCO3 (MC), and 18% CaMgSi2O6 (Di). These data combined with previous results permit construction of the isotherm-contoured vapor-saturated liquidus surface for the calcite/dolomite field, and part of the adjacent forsterite and diopside fields. Nearly pure calcite crystals in mantle xenoliths cannot represent equilibrium liquids. We recently determined the complete vapor-saturated liquidus surface between carbonates and model peridotites at 2.7 GPa; the peritectic reaction for dolomite (25% MC)-wehrlite at 2.7 GPa occurs at 1300 °C, with liquid composition 60% CC, 29% MC, and 11% Di. The liquidus field boundaries on these two surfaces provide the road-map for interpretation of magmatic processes in various peridotite-CO2 systems at depths between the Moho and about 100 km. Relationships among kimberlites, melilitites, carbonatites and the liquidus phase boundaries are discussed. Experimental data for carbonatite liquid protected by metasomatic wehrlite have been reported. The liquid trends directly from dolomitic towards CaCO3 with decreasing pressure. The 1.5 GPa liquid contains 87% CC and 4% Di, much lower in silicate components than our phase boundary. However, the liquids contain approximately the same CaCO3 (90 ± 1 wt%) in terms of only carbonate components. For CO2-bearing mantle, all magmas at depth must pass through initial dolomitic compositions. Rising dolomitic carbonatite melt will vesiculate and may erupt as primary magmas through cracks from about ˜70 km. If it percolates through metasomatic wehrlite from 70 km toward the Moho at 35–40 km, primary calcic siliceous carbonatite magma can be generated with silicate content at least 11–18% (70–40 km) on the silicate-carbonate boundary. Received: 22 June 1998 / Accepted: 7 July 1999  相似文献   

10.
Pb diffusion in rutile   总被引：16，自引：0，他引：16  
Diffusion of Pb was measured in natural and synthetic rutile under dry, 1 atmosphere conditions, using mixtures of Pb titanate or Pb sulfide and TiO2 as the sources of diffusant. Pb depth profiles were then measured with Rutherford Backscattering Spectrometry (RBS). Over the temperature range 700–1100 °C, the following Arrhenius relation was obtained for the synthetic rutile: D=3.9 × 10−10exp(−250 ± 12 kJ mol−1/RT) m2s−1. Results for diffusion in natural and synthetic rutile were quite similar, despite significant differences in trace element compositions. Mean closure temperatures calculated from the diffusion parameters are around 600 °C for rutile grains of ∼100 μm size. This is about 100 °C higher than rutile closure temperature determinations from past field-based studies, suggesting that rutile is more resistant to Pb loss through volume diffusion than previously thought. Received: 28 June 1999 / Accepted: 29 December 1999  相似文献   

11.
In the Grt-Bt-Sil restitic xenoliths of El Joyazo (Cerro de Hoyazo), hercynitic spinel is a minor phase commonly associated with biotite. The possible reaction relationships among biotite and spinel are studied in reaction textures developed around biotites at their contact with patches of fibrolitic sillimanite and rhyolitic melt. In these textures, resorbed biotite crystals about 1 mm long are rimmed by a layer of glass <200 μm thick containing spinel and ilmenite; the same glass also fills embayments in biotite. Spinel forms euhedral crystals <100 μm in size, and ilmenite occurs as smaller anhedral crystals or needles, often intergrown with spinel. The homogeneous felt-like melt-sillimanite aggregate (“mix”) is richest in glass close to the reaction rim around biotite. Plagioclase and garnet are located >5 mm away from the reaction texture. Biotite is chemically zoned. Cores (Bt 1 ) have XMg=0.35 ± 0.02 and Ti=0.58 ± 0.01 atoms; whereas the outer rims (Bt 2 ) have XMg=0.45 ± 0.01 and Ti up to 0.68 atoms. The hercynite-rich spinel (Spl) has low ZnO content (<0.80 wt%) and XMg=0.26 ± 0.04. The chemical compositions of the mix aggregate represent linear combinations between sillimanite and a silica-rich melt. This melt (melt 1 ) is different from that of the layer around biotite (melt 2 ), which is also richer in Ca and alkalis. Garnet rims (Grt) have low Ca and Mn, and XMg=0.14. Plagioclase is characterized by large homogeneous cores (Pl 1 , An31 ± 2) and more calcic rims (Pl 2 , An49 ± 6). Matrix analysis in the 9-component (Al-Ca-Fe-K-Mg-Mn-Na-Si-Ti), 9-phase (Bt1-Bt2-Grt-Spl-Ilm-melt2-mix-Pl1-Pl2) system provides the mass balance (in mole units):
This relationship is in excellent agreement with the observed textures and hence is considered a good model for the incongruent melting of biotite in the xenoliths. The mass-balance indicates that melt production is dominated by the availability of K from biotite, and that garnet and plagioclase must be involved as reactants, so that the reaction volume is larger than the melt production site. The melting of biotite, constrained at T=900–950°C and P ≥ 5 kbar, is not a terminal reaction, as its variance in the reduced 8-component multisystem is ≥3. Received: 1 June 1999 / Accepted: 8 February 2000  相似文献   

12.
Illite is a dioctahedral K-deficient mica with an interlayer cation content of 0.6–0.85 atoms per formula unit. 1M and 2M1 are the illite polytypes more abundant in nature. Because illite is one of the major component of clays used for the production of traditional ceramics, the understanding of its high temperature transformations is of paramount importance for the knowledge of the structural and microstructural properties of fired ceramic products. To our knowledge, the study of the illite dehydroxylation kinetics has not been attempted to date. Hence, this work presents the investigation of the reaction mechanism of dehydroxylation of illite for the first time. The natural sample investigated in this study is a 1M-polytype from Hungary. Several classical methods of kinetic analysis were used (isoconversional method, Avrami method, direct fit with kinetic expressions, and others) to achieve a complete picture of the dehydroxylation mechanism. The proposed model for the dehydroxylation of illite is a multi-step reaction sequence with (1) condensation of the water molecule in the octahedral layer; (2) one-dimensional diffusion of the water molecules through the tetrahedral ring (rate limiting step of the reaction); (3) two-dimensional diffusion of the water molecules through the interlayer region (rate limiting step of the reaction).  相似文献   

13.
Carbonation and decarbonation of eclogites: the role of garnet   总被引：3，自引：0，他引：3  
Carbonates are potentially significant hosts for primordial and subducted carbon in the Earth's mantle. In addition, the coexistence of carbonate with silicates and reduced carbon (diamond or graphite), allows constraints to be placed on the oxidation state of the mantle. Carbonate-silicate-vapor reactions control how carbonate + silicate assemblages may form from carbon-bearing vapor + silicate assemblages with increasing pressure. In olivine-bearing rocks such as peridotite, considered the dominant rock type in the upper mantle, the lowest-pressure carbonate-forming reactions involve olivine (±clinopyroxene) reacting with CO2 (e.g., Wyllie et al. 1983). In eclogitic rocks, the essential mineral assemblage is omphacitic clinopyroxene + garnet, without olivine. Therefore, alternative carbonate-forming reactions must be sought. The carbonation of clinopyroxene via the reaction dolomite + 2 coesite = diopside + 2 CO2 was studied experimentally by Luth (1995). The alternative possibility that garnet reacts with CO2 is explored here by determining the location of the reaction 3 magnesite + kyanite + 2 coesite = pyrope + 3 CO2 between 5 and 11 GPa in multi-anvil apparatus. At the temperatures ≥1200 °C, carbonation of eclogitic rocks with increasing pressure will proceed initially by reaction with clinopyroxene, because the pyrope-carbonation reaction lies at higher pressures for a given temperature than does the diopside-carbonation reaction. Diluting the pyrope component of garnet and the diopside component of clinopyroxene to levels appropriate for mantle eclogites does not change this conclusion. At lower temperatures, appropriate for “cold” slabs, it is possible that the converse situation will hold, with initial carbonation proceeding via reaction with garnet, but this possibility awaits experimental confirmation. Decarbonation of an eclogite under “normal mantle” geothermal conditions by a decrease in pressure, as in an ascending limb of a mantle convection cell, would be governed by the formation of clinopyroxene + CO2. At higher pressure than this reaction, any CO2 produced by the breakdown of magnesite reacting with kyanite and coesite would react with clinopyroxene to produce dolomite + coesite. Release of CO2 from eclogite into mantle peridotite would form carbonate at sub-solidus conditions and produce a dolomitic carbonate melt if temperatures are above the peridotite-CO2 solidus. Received: 4 May 1998 / Accepted: 23 December 1998  相似文献   

14.
Throughout the ultrahigh-pressure (UHP) metamorphic unit of the Dora-Maira massif, western Alps, pyrope megablasts contain the typical assemblage clinochlore–kyanite–talc–rutile ± phlogopite ± ellenbergerite as prograde inclusions. In the upper part of the UHP unit in Val Gilba, some megablasts (XMg=0.89–0.98) contain in addition polymineralic inclusions consisting of various combinations of enstatite, gedrite, sapphirine, clinochlore, talc, magnesiostaurolite and rare corundum or spinel. We present evidence that these assemblages developed from cracks running across the megablasts, and are therefore of late origin, post-dating the highest-pressure stage. Enstatite (XMg=0.94–0.99) contains 0.7 to, typically, 3 wt% Al2O3, but up to 8.4 wt% in the presence of sapphirine. Sapphirine (XMg=0.96–0.998, Be-free) shows the largest compositional variations, with Si contents ranging from 1.7 to at least 2.1 atoms pfu, thereby clearly exceeding the 2:2:1 stoichiometry. The late-stage talc contains up to 4 wt% Al2O3, 0.35 wt% Na2O and 0.6 wt% F; gedrite 1.1–2.9 wt% Na2O and up to 0.36 wt% F. The successive development within pyrope of alternative hydrous assemblages involving first enstatite plus an Al-rich phase (kyanite, sapphirine, magnesiostaurolite) ± clinochlore, then a gedrite compositionally close to pyrope, then talc plus an Al-rich phase (sapphirine, corundum), is a clear record of decompression. However, the temperature conditions implied under the assumption of high H2O activity are 100 to 150 °C higher than, and so inconsistent with existing constraints on the decompression path. These constraints are in particular the stability of talc + phengite in the matrix assemblage during decompression, and the absence of regional evidence for a granulite-facies event. This inconsistency can only be alleviated if H2O activity inside the garnet megablast was (or became) considerably reduced with respect to that in the matrix. Fluid influx into an opening fracture in garnet, sealing of the fracture by breakdown products of pyrope and continued evolution under closed-system conditions may have led to increasing solute concentration and such low H2O activity within the garnet megablast, driving the microsystem toward fluid-absent conditions. Micrometre-size inclusions of Ca-sulfate and crandallite-type compounds in minerals of these reactive areas may be evidence for such residual brines and suggest that these were phosphate- and sulfate- rather than halide-dominated. This finding is additional evidence for the very local control that fluid composition and H2O activity may have on the occurrence of granulite-facies assemblages, regardless of temperature. It highlights the role of deformation (here fracturing) in triggering reactions in otherwise unreactive systems. It also shows how carefully inclusion- to-host relationships have to be considered, post-growth reaction within the host being more common than hitherto reported. Received: 4 February 1999 / Accepted: 24 August 2000  相似文献   

15.
The recently discovered Hanshan gold deposit in northern Gansu Province, northwestern China, is hosted by a WNW-striking shear zone in Ordovician andesite and basalt. Mineralization consists of surface to near-surface oxidized ore (the yellow sandy gossan type) and three types of primary ore, i.e. early-stage quartz-sericite-pyrite ores in stockworks, early-stage disseminated ore, and the most important late-stage quartz ± calcite-sulfide veins. The ore system is characterized by variable degrees of potassic and silicic alteration. Late-stage gold-related fluid inclusions have homogenization temperatures between 170 to 310 °C, with a peak around 260 °C and low salinities. The ore fluids had high contents of CO2, CH4, and N2. Sulfur isotope measurements of −1.9 to +1.7 per mil for hydrothermal pyrites could be consistent with a hydrothermal fluid source from the mantle, but the oxygen and carbon isotope data from calcite and quartz suggest mixing between mantle and crustal fluid sources. K-Ar ages for hydrothermal sericite from ore zones are 213.9 ± 3.1 and 224.4 ± 3.2 Ma. Due to the arid Cenozoic climate, a yellow gold-bearing gossan developed, which consists of jarosite, gypsum, and relict quartz. It could be a widespread and useful prospecting guide for gold in northwestern China. Received: 1 February 1999 / Accepted: 1 August 1999  相似文献   

16.
A unique sapphirine + orthopyroxene + quartz granulite from Mt. Riiser-Larsen in the Tula Mountains of Enderby Land, East Antarctica, preserves two generations of coarse and texturally equilibrated orthopyroxene and sapphirine coexisting with quartz. Initial subhedral orthopyroxene porphyroblasts retain core compositions enriched in Al2O3 (12.2 ± 0.5 wt%) compared with their rims and finer orthopyroxene (9.6 ± 0.5 wt% Al2O3) that forms granoblastic textures with sapphirine. Sapphirine and quartz also form symplectites on and along cleavage planes within orthopyroxene. These compositional and textural features are consistent with the reaction [2MgAl2SiO6=Mg2Al4 SiO10 + SiO2] leading to the formation of sapphirine + quartz at the expense of aluminous orthopyroxene. Calculations in the MAS and FMAS systems and theoretical considerations involving the phases enstatite, sapphirine, sillimanite, quartz and cordierite indicate that the reaction above progresses from left to right with decreasing temperature in the orthopyroxene + sapphirine + quartz field, at pressures of ca. 8–10 kbar. The temperature difference required to account for the ca. 2.5–3 wt% decrease in Al2O3 in orthopyroxene is at least 60–80 °C, and implies peak temperatures for the initial assemblage of at least 1120 °C if the second granoblastic assemblage equilibrated at 1040 °C, the P–T conditions required by the sapphirine + quartz association and other P–T-sensitive assemblage indicators in the Napier Complex. It is not possible to distinguish whether the two assemblages are simply related by cooling and re-equilibration or reflect a polyphase evolution involving the superposition of a second UHT event on an earlier, even higher temperature, UHT metamorphism. Preliminary thermodynamic modelling of the reaction above incorporating the observed range in orthopyroxene Al2O3 zoning indicates that present estimates for the entropy of high-temperature sapphirine are potentially too high by 15–18% compared with sapphirine entropy estimates that are consistent with MAS system experiments. The Mt. Riiser-Larsen sapphirine–quartz rocks preserve the first definitive record of regional metamorphic temperatures in excess of 1120 °C in the Napier Complex, or indeed any UHT granulite terrain worldwide. Similarly high peak temperatures may be retrieved from detailed studies of sapphirine–quartz granulites from other regions, further expanding the thermal realm of crustal metamorphism, but progress will critically depend on the experimental acquisition of new entropy data for sapphirine. Received: 3 September 1998 / Accepted: 8 November 1999  相似文献   

17.
Experiments have been conducted in a peralkaline Ti-KNCMASH system representative of MARID-type bulk compositions to delimit the stability field of K-richterite in a Ti-rich hydrous mantle assemblage, to assess the compositional variation of amphibole and coexisting phases as a function of P and T, and to characterise the composition of partial melts derived from the hydrous assemblage. K-richterite is stable in experiments from 0.5 to 8.0 GPa coexisting with phlogopite, clinopyroxene and a Ti-phase (titanite, rutile or rutile + perovskite). At 8.0 GPa, garnet appears as an additional phase. The upper T stability limit of K-richterite is 1200–1250 °C at 4.0 GPa and 1300–1400 °C at 8.0 GPa. In the presence of phlogopite, K-richterite shows a systematic increase in K with increasing P to 1.03 pfu (per formula unit) at 8.0 GPa/1100 °C. In the absence of phlogopite, K-richterite attains a maximum of 1.14 K pfu at 8.0 GPa/1200 °C. Titanium in both amphibole and mica decreases continuously towards high P with a nearly constant partitioning while Ti in clinopyroxene remains more or less constant. In all experiments below 6.0 GPa ΣSi + Al in K-richterite is less than 8.0 when normalised to 23 oxygens+stoichiometric OH. Rutiles in the Ti-KNCMASH system are characterised by minor Al and Mg contents that show a systematic variation in concentration with P(T) and the coexisting assemblage. Partial melts produced in the Ti-KNCMASH system are extremely peralkaline [(K2O+Na2O)/Al2O3 = 1.7–3.7], Si-poor (40–45 wt% SiO2), and Ti-rich (5.6–9.2 wt% TiO2) and are very similar to certain Ti-rich lamproite glasses. At 4.0 GPa, the solidus is thought to coincide with the K-richterite-out reaction, the first melt is saturated in a phlogopite-rutile-lherzolite assemblage. Both phlogopite and rutile disappear ca. 150 °C above the solidus. At 8.0 GPa, the solidus must be located at T≤1400 °C. At this temperature, a melt is in equilibrium with a garnet- rutile-lherzolite assemblage. As opposed to 4.0 GPa, phlogopite does not buffer the melt composition at 8.0 GPa. The experimental results suggest that partial melting of MARID-type assemblages at pressures ≥4.0 GPa can generate Si-poor and partly ultrapotassic melts similar in composition to that of olivine lamproites. Received: 23 December 1996 / Accepted: 20 March 1997  相似文献   

18.
 Infiltration of a metabasite sill from Islay, Scotland by an H2O-CO2 fluid caused (1) modification of δ18O and (2) carbonation at the sill margins. Maps of δ18O and reaction progress were constructed from a 20 × 47.7 metre sample grid across the sill. The grid consisted of 300 samples, spaced at m, dm and cm intervals, many of which were analysed for both δ18O and reaction progress. The δ18O was determined by laser fluorination of whole rock silicate powders and reaction progress was determined by rapid field-based measurement of % calcite (“fizz-o-meter”, Skelton et al. 1995). Reaction and isotope fronts outlined tube-like features that emanate from the sill margin and discrete nodes that, although detached from the sill margin in two dimensions, are thought to represent sections through similar tubes in three dimensions. We envisage that these protrusions are the fossil record of metamorphic “fluid pathways” whereby fluid permeated the sill. Isotope and reaction fronts are found to correlate spatially as predicted by a modified form of the chromatographic equation which describes this envisaged geometry, that is where isotopic and reactive transport in the fluid phase are facilitated by advection along specific fluid pathways and transverse diffusion in the surrounding rock. These fluid pathways consist of bundles of anastomosing grain boundary channels or micro-cracks, which are thought to propagate through transient cyclic infiltration, reaction, porosity enhancement and fracturing. This mechanism is self-perpetuating and accentuates random perturbations at the sill margin to form the observed tubes. We argue that this is the earliest stage of the infiltration process which has affected metabasites of the SW Scottish Highlands and that subsequent shear deformation of the reacted rims of these pathways, has caused their re-orientation and juxtaposition to form the reacted sill margins described by Skelton et al. (1995). Received: 17 February 1998 / Accepted: 6 December 1999  相似文献   

19.
The evolution of nephelinitic melts in equilibrium with mica-bearing liquidus assemblages and melting relations have been studied on two silica-undersaturated joins of the KAlSiO4– Mg2SiO4– Ca2SiO4– SiO2– F system at atmospheric pressure by quench runs in sealed platinum capsules. Fluorine has been added to the batch compositions by the direct exchange of fluorine for oxygen (2F− = O2−). The first join is the pseudo-ternary Forsterite – Diopside – KAlSiO3F2 system. Forsterite, diopside, F-phlogopite and leucite crystallisation fields and a fluoride-silicate liquid immiscibility solvus are present on the liquidus surface of the join. Sub-liquidus and sub-solidus phases include akermanite, cuspidine, spinel, fluorite and some other minor fluorine phases. The second system is the pseudo-binary Akermanite – F-phlogopite join that intersects the Forsterite – Diopside – KAlSiO3F2 join. Akermanite, forsterite, diopside, F-phlogopite, leucite and cuspidine are found to crystallise on the join. Forsterite (fo) and leucite (lc) are related to F-phlogopite (phl) by a reaction with the fluorine-bearing liquid: fo + lc + l = phl, and the reaction proceeds until forsterite or leucite are completely consumed. The reaction temperature and resulting phase association depend on batch composition. Thus, leucite is not stable in the sub-solidus of the Akermanite – F-phlogopite join, but is preserved in a part of the Forsterite – Diopside – KAlSiO3F2 system where forsterite reacts out, or does not crystallise at all. The phlogopite-in reaction has an important effect on the composition of the coexisting liquid. The liquids initially saturated in forsterite evolve to extremely Ca rich, larnite-normative residuals. The experimental data show that larnite-normative melilitolites can crystallise from evolved melilititic melts generated from “normal” melanephelinitic parental magmas with no normative larnite. The evolution towards melilitites requires fractionation of phlogopite-bearing assemblages under volatile pressure. Received: 3 June 1997 / Accepted: 5 January 1998  相似文献   

20.
The breakdown of potassium feldspar at high water pressures   总被引：1，自引：0，他引：1  
The equilibrium position of the reaction between sanidine and water to form “sanidine hydrate” has been determined by reversal experiments on well characterised synthetic starting materials in a piston cylinder apparatus. The reaction was found to lie between four reversed brackets of 2.35 and 2.50 GPa at 450 °C, 2.40 and 2.59 GPa at 550 °C, 2.67 and 2.74 GPa at 650 °C, and 2.70 and 2.72 GPa at 680 °C. Infrared spectroscopy showed that the dominant water species in sanidine hydrate was structural H2O. The minimum quantity of this structural H2O, measured by thermogravimetric analysis, varied between 4.42 and 5.85 wt% over the pressure range of 2.7 to 3.2 GPa and the temperature range of 450 to 680 °C. Systematic variation in water content with pressure and temperature was not clearly established. The maximum value was below 6.07 wt%, the equivalent of 1 molecule of H2O per formula unit. The water could be removed entirely by heating at atmospheric pressure to produce a metastable, anhydrous, hexagonal KAlSi3O8 phase (“hexasanidine”) implying that the structural H2O content of sanidine hydrate can vary. The unit cell parameters for sanidine hydrate, measured by powder X-ray diffraction, were a = 0.53366 (±0.00022) nm and c = 0.77141 (±0.00052) nm, and those for hexasanidine were a = 0.52893 (±0.00016) nm and c = 0.78185 (±0.00036) nm. The behaviour and properties of sanidine hydrate appear to be analogous to those of the hydrate phase cymrite in the equivalent barium system. The occurrence of sanidine hydrate in the Earth would be limited to high pressure but very low temperature conditions and hence it could be a potential reservoir for water in cold subduction zones. However, sanidine hydrate would probably be constrained to granitic rock compositions at these pressures and temperatures. Received: 6 May 1997 / Accepted: 2 October 1997  相似文献   

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