地下空间形态描述方法研究

地下工程施工、开发地下资源(包括开采地下水)都会引起地面发生沉降,导致位于地面的城市、交通线路及水体随之沉降。这种工程性沉降问题产生的根本原因在于地下出现了空间,而非层状(即不规则形状)空间环境是一种普遍存在的情况,与层状空间环境的地面沉降机理不同。当地下出现非层状空间环境时,目前还没有实用的地面沉降预测模型,否则,把非层状地下空间简化为层状情形进行处理。基于地下出现褶皱层状空间环境时的地面沉降规律,研究非层状地下空间形态的表述和处理方法。     

Mineral Compositions and Equilibria in the Metamorphosed Iron Formation of the Gagnon Region, Quebec, Canada

Forty-seven specimens of the Wabush Iron Formation were collectedfrom ten outcrop areas. Twenty-five specimens contain the assemblage(1), quartz+clinopyroxene+calcite with or without orthopyroxene,grunerite, magnetite, ankerite, and siderite. Five specimenscontain assemblage (2), quartz+clinopyroxene+actinolite+calcite+magnetite+hematite,and two contain assemblage (3), quartz+orthopyroxene+actinolite+magnetite+hematite.In three specimens of assemblage (1), graphite occurs in theabsence of magnetite; pyrrhotite and pyrite occur separatelyor together in specimens with assemblage (1). Thirty-nine clinopyroxenes, 38 orthopyroxenes, 18 grunerites,7 actinolites, 16 calcites, 1 ankerite, and 1 siderite wereanalyzed for iron, manganese, and calcium by X-ray emissionspectrography. Magnesium contents were estimated by assumingstoichiometric proportions. Minerals occurring with hematite show low Fe/(Fe+Mg) ratios,and those in the other assemblages show higher values with awide range of variation. In orthopyroxene, Fe/(Fe+ Mg) rangesfrom 0·17 (with hematite) to 0·77. Regularity in the distributions of Fe, Mn, and Ca between pairsof coexisting minerals shows that equilibrium was attained inmost of the rocks studied. This regularity is also accomplishedin the distribution of Mn between calcite and coexisting silicatesas well as between the silicates themselves. Small differencesin the distributions of Ca and Fe depend on both outcrop areaand mineral assemblage. Phase rule considerations suggest that the specimens with dolomite-ankeriteor magnesitesiderite do not represent equilibrium assemblages.Variations in orthopyroxene compositions in assemblages withpyrite or pyrrhotite, or both, and magnetite indicate non-equilibrationof sulfides with silicates. The presence of the oxygen buffer,magnetite+hematite, attests to the immobility of oxygen duringmetamorphism. Within each outcrop area, over which the temperature and pressureare assumed to have been uniform, variations in the compositionsof the silicates in the sub-assemblages quartz+ orthopyroxene+gruneriteand quartz+orthopyroxene+clinopyroxene+calcite indicate gradientsof µ_H2O µ_CO2 and respectively. As characterizedby the composition of orthopyroxene, both gradients are relativelylow along strike, and high across strike. The direction of gradientsacross strike is almost without reversals, which is consistentwith intergranular diffusion of H₂O and CO₂. Phase rule restrictionsfor a majority of assemblages are not in accord with the simultaneousimposition of µ_H2O and µ_CO2 gradients on the rocks,nor the formation of an H₂O-CO₂ fluid phase during metamorphism.     

Phase Equilibria in the Pyroxene Quadrilateral

Experimental phase equilibrium data on compositions of coexistingpyroxenes in the quadrilateral enstatite-diopside-ferrosilite-hedenbergitehave been used to model pyroxene solid solutions and to formulatepyroxene geothermometers. Each pyroxene is treated as a solidsolution of four quad-components using the Kohler formulation where G_ij^* is the excess free energy of mixing in a binary solutioncalculated with binary mole fractions (e.g. X_i^o = X_i/(X_i+X_j))and X_i is the mole fraction in a multicomponent solution. Thefit to the experimental data is achieved by minimizing the totalGibbs free energy of the assemblage. The following set of thermochemicaldata and simple mixture parameters (W_ij) are found to be bestsuited. Standard (T = 298?15 K) enthalpy and entropy of formationfrom elements for fictive orthohedenbergite are –1416?8kJ and 84?88 J K^–1 mol ^–1 respectively. The heatcapacity is given by 114?67+17?09E-3T–31?40E5T^–2.The W_ij data are: Opx: W₁₂ = W₂₁ = 25 W₁₃ = (13?1–0-015T),W₃₁ = (3?37–0?005T), W₂₃ = 20, W₃₂ = 16, W₂₄ = 5, W₄₂= 7, W₃₄ = 15, W₄₃ = 15; Cpx: W₁₂ = (25?484+0?0812P), W₂₁ =(31?216–0?0061P),W₃₁ = W₁₃ = 0W₁₄ = (93?3–0?045T), W₄₁ = (–20?0+0?028T),W₂₃ = 24, W₃₂ = 15, W₂₄ = 12, W₄₂ = 12, W₃₄ = (16?941+0?00592P),W₄₃ = (20?697–0?00235P). Coexisting pyroxene compositionshave been computed in the temperature range of 700 to 1400?C. Two geothermometers have been constructed, one based on atomicfraction of iron (Fe/(Fe + Mg)) in orthopyroxene and the Fe-Mgdistribution coefficient and the other, based on wollastonitecontent of clinopyroxene. The two scales yield different temperatureswhen applied to the same rock. In igneous pyroxenes, the Catransfer ceased at 150 to 200?C above the closure temperatureof the Fe-Mg ion-exchange. In metamorphic rocks an oppositeeffect seems to have prevailed.     

Material Transfer and Local Equilibria in a Zoned Kelyphite from a Garnet Pyroxenite, Ronda, Spain

A zoned kelyphite after garnet, from a garnet pyroxenite layer,the Ronda peridotite. Spain, has been studied and the mechanismof kelyphite formation is discussed. The kelyphite is an extremelyfinegrained symplectitic mixture of orthopyroxene, spinel, olivine,plagioclase, and ilmenite. It is concentrically zoned, formingthree mineralogical subzones. They are, from adjacent to a garnetgrain toward a clinopyroxene side, zone I (orthopyroxene+spinel+ plagioclase), zone II (olivine+spinel+plagioclase), and zoneIII (olivine+plagioclase). The analysis of phase equilibriashows that this mineralogical zonation can develop stably asa result of the presence of chemical potential gradients. Onthe basis of microprobe chemical analyses for each zone, materialtransfer across the zone that took place during the kelyphitizationwas quantitatively evaluated, and by locating the initial grainboundary between garnet and clinopyroxene grains and by writingmetasomatic reactions for each zone boundary, a simple dynamicmodel for the kelyphite formation is proposed. The kelyphiteformation probably took place when the host Ronda peridotiteascended from the upper mantle to the crust. It involved a co-operativebreakdown of the garnet and aluminous clinopyroxene, being accompaniedby a material transfer across the zone boundaries. By examiningthe Fe-Mg partitioning between olivine, spinel, and orthopyroxenein the kelyphite and by examining the Al content of the orthopyroxene,an attainment of local equilibrium has been confirmed, and thephysical conditions of the kelyphite formation have been estimatedto be 620–700C and 4–8 kbar.     

Phase Equilibria for Calcic Scapolite, and Implications of Variable Al-Si Disorder for P-T, T-XCO2, and a-X Relations

Carbonate scapolite is a potentially powerful mineral for calculatingCO₂ activities in non-calcareous rocks, but an analysis of thethermodynamics and phase equilibria of carbonate scapolite isfirst necessary. This includes an evaluation of Al-Si disorderin meionite, as this has the greatest effect on derived phaserelations. Available experimental data on meionite stability,X-ray diffraction refinements and nuclear magnetic resonancespectra for calcic scapolite do not uniquely constrain the Al-Siordering state of synthetic meionite. However, the data aremost consistent with a high degree of Al-Si disorder and inconsistentwith long-range Al-Si order. An internally consistent thermodynamicdata set was derived and used to calculate P-T and T-X_CO2 equilibriainvolving meionite in the CaO-Al₂O₃-SiO₂-CO₂-H₂O (CASCH) system.The effect of Al-Si disorder is illustrated by calculating thephase equilibria using an ordered, an arbitrary intermediatedisordered, and a completely Al-Si disordered standard statefor meionite. The Gibbs free energy of meionite was calculatedfrom reversals (at 790–815?C, 2–15 kb) on the reaction 3 Anorthite +Calcite =Meionite The _fG?_{m, 298} for each of the standard states is –13 146?6,–13128?8, and –130930kJ/mol, respectively. Becauseof the steep slope of reaction (1) and limited temperature rangeover which it breaks down, meionite used in the experimentsto constrain reaction (1) must possess a limited range of Al-Sidisorder. The P-T slope of reaction (1) increases, and the slopeof meionite decarbonation equilibria changes from positive tonegative in T-X_CO2 and P-T space, as a function of increasingAl-Si disorder. Meionite has a wide stability field at highT in T-X space at 5 and 10 kb (P_Total=P_Fluid), being stableto X_CO2=0?06. Meionite alone breaks down to undersaturated gehleniteand/or corundum-bearing assemblages at 5 kb, and to clinozoisiteat 10 kb. The effect of solid solutions on the T-X stabilityof meionite is similar to that of increasing pressure, stabilizingmeionite to lower temperature. Variable Al-Si disorder doesnot significantly affect the upper limit of meionite stabilityin T-X_CO2 space. Activity-composition relations for meionitein carbonate scapolite were calculated relative to reaction(1) from data on natural scapolite-plagioclase-calcite assemblages.The extent of departure from ideality varies as a function ofAl-Si disorder. Negative deviations from ideality are indicatedfor natural scapolite solid solutions at T<750?C, based ona disordered Al-Si standard state for meionite. This is likelyto reflect a more ordered Al-Si distribution in natural scapolitescompared with the synthetic endmember standard state. Present address: Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794-2100     

Biotite Equilibria and Fluid Circulation in the Klokken Intrusion

Chemical variation in biotite from the KJokken gabbro-syeniteintrusion in the Proterozoic Gardar province in South Greenlandhas been investigated by electron probe and, for F and Li, ionmicroprobe. Most mica occurs in small amounts as fringes onilmenomagnetite or fayalite, rarely as an intercumulus or poikiliticphase. The micas range continuously from Phlog₇₀Ann₃₀ in a gabbro,to Phlog₄Ann₉₆ in the most evolved (slightly persilicic andperalkaline) syenite. In the syenites Fe-Mg partitioning betweenbiotite and olivine can be described by a single distributioncoefficient, K_d = XF X^Biot_Ms/X^Biot_Fe 3, suggesting that thesereactant phases mix ideally at the reaction T. Experimentaldata for Fe-Mg exchange via aqueous chloride solutions (Schulien,1980) imply low T (32Q?C). F was absent in the experiments andmay significantly affect the exchange equilibrium. K_d in thegabbros is 1, consistent with equilibrium via a fluid depletedin F because of crystallization of large amounts of amphibole. Al, Mn, and Ti vary regularly throughout the series and canbe used as markers of cryptic variation in the layered syenites.(Al + Si): 22 O is always in the range 7.7–7.85. A1/(A1+ Si) decreases from 0.31 in gabbros to 0.25 in the most Fe-richmicas. Li is always < 260 ppm w. In the syenite series, Fshows a near-linear inverse relationship with Fe/(Fe + Mg) whichpasses close to OF at Ann₁₀₀ with l.4 wt% F(0–7 F to 44positive charges) at Ann₄₄. Biotites in the gabbro unit (whichforms an outer sheath to the intrusion) have relatively lessF, probably because it was consumed by coexisting amphibole.^I8O is similar for both gabbros and syenites, and it is unlikelythat an envelope fluid was involved in the reactions. G reachesa maximum of 0.3 wt. % in biotite except for that in one syenitesample with 0–7 wt. %. Calculation of relative F-OH fugacitiesfrom the reaction OH-phlogopite + F-annite = F-phlogopite +OH-annite, as calibrated by Munoz (1984), appears to suggestthat each horizon in the layered series was in equilibrium witha slightly different fluid. In view of the intimate interleavingof these lithologies, this is improbable. The equilibrium constantof the exchange reaction, obtained from the experimental data,seems not to be appropriate to the Klokken assemblage, or toother examples of regular F-Fe avoidance. Explanations may includeshort-range Fe-Mg ordering in the natural examples or the effectof additional components in the fluid. F contents are high incomparison with biotites from calc-alkaline complexes; highmagmatic F may account for the igneous layering common in theGardar. Temperatures calculated from reactions involving fayaliteand magnetite show that most biotites grew subsolidus. The F-poorannites grew > 300 ?C subsolidus even when texturally intercumulus.Stable isotope data are consistent with the separation and retentionof a deuteric fluid during the final stage of magmatic crystallization.Klokken was not generally subject to the pervasive, long-range(in both distance and time) dydrothermal interactions demonstratedin calcalkaline and theleiitic intrusions, although more extensivefluid flow is indicated for the more permeable laminated syenites.The biotites preserve chemical variation indicating local equilibriumwith other mafic phases, and halogens provide a useful markerof subsolidus fluid flow.     

Phase Equilibria of Hornblende-Bearing Eclogite in the Western Dabie Mountain,Central China

The high-pressure (HP) eclogite in the western Dabie Mountain encloses numerous hornblendes,mostly barroisite.Opinions on the peak metamorphic P-T condition,PT path and mineral paragenesis of it are still in dispute.Generally,HP eclogite involves garnet,omphacite, hornblendes and quartz,with or without glaucophane,zoisite and phengite.The garnet has compositional zoning with X_(Mg) increase,X_(Ca) and X_(Mn) decrease from core to rim,which indicates a progressive metamorphism.The phase equilibria of the ...     

Phase Equilibria in Three Assemblages of Kyanite-Zone Pelitic Schists, Lincoln Mountain Quadrangle, Central Vermont

Petrologic and geologic arguments suggested that a close approachto chemical equilibrium at a single temperature and pressuremight be found in the rocks near Mt. Grant in central Vermont.An area 2,800 feet by 4,000 feet was selected and studied indetail. The major assemblages are: kyanite-chloritoid-chlorite-quartz-muscovite-paragonite-rutile;garnet-chloritoid-chlorite-quartz-muscovite-paragonite-ilmenite;and garnet-chlorite-biotite-quartz-muscovite-albite-ilmenite.All minerals were separated from a primary sample for each ofthese assemblages and purified, complete gravimetric and spectrographicanalyses performed, and optical properties, X-ray properties,and densities measured. Chlorite, garnet, and chloritoid orbiotite were separated from an additional nine samples of theseassemblages and spectrographic and partial gravimetric analysesperformed. Distribution coefficients of each mineral pair for Mg/Fe andMn/Fe+ Mg and for the minor elements are similar in nearly everysample for a given assemblage. Distribution coefficients forgarnet-chlorite differ in the two assemblages which containthis pair; this difference is attributed to the difference inAl-content of the chlorite from the two assemblages. The smallrange of distribution coefficients, despite a wide range inthe relative proportions of the ferromagnesian phases, is convincingevidence that an equilibrium partition of the major and minorcations between the phases in each sample had been attainedand that the equilibration temperature was the same for eachsample. The coexistence of kyanite+muscovite+quartz and comparison ofthe composition of coexistent Ca-free muscovite and paragoniteand of O¹⁸/O¹⁰ ratios for coexistent quartz and magnetite withexperimental values indicate that these rocks formed at P_s 11 kb, T 550? C, and aH₂o low enough (Pe(H₂o) sufficiently lessthan P_s) to depress the pyrophyllite breakdown temperature byabout 40? C. The cations and the ao₂ are equilibrated locally(within each sample), but all the samples have equilibratedto a common O¹⁸/O¹⁶ ratio. A possible explanation is that afluid medium had sufficient oxygen in H₂O to control the O¹⁸/O¹⁶ratio of the rock and its phases, but that the rock system hadsufficient buffer capacity to control the ao₂ of the fluid.     

Anhydrous Phase Equilibria in Earth's Upper Mantle

Phase equilibrium relations are computed in five different chemicalcompositions appropriate for a primitive upper mantle (estimated),a peridotite, an olivine-basalt, an olivine-tholeiite, and aquartz-tholeiite in the pressure range of 1 bar to 100 kb andin the temperature range of 500 to 1500 ?C. The mineralogicalphase assemblages change from peridotite and lherzolite to garnetgranulites, eclogites and kyanite-quartz-eclogites. The smallpressure-temperature field of stability of plagioclase-garnet-granulitein the primitive compositions extends at 600 ?C from less than3 kb to more than 15 kb in the quartz-tholeiite compositionwith intermediate pressure values in other basalts. Continentalcrust with composition similar to quartz-tholeiite remains asplagioclase-garnet-granulite changing to eclogite only belowa depth of 50 km. Calculated adiabats in the peridotite compositions show thatdiapirs must ascend from a depth of at least 300 km to becomepartially molten in the low-velocity zone. Various shallowerdepths of partial melting are possible for material ascendingfrom depths corresponding to the oceanic geothermal gradient. Actual pressure-temperature densities of rocks have been calculatedfrom the mineral densities computed for the equilibrium assemblageson the oceanic geothermal gradient. While there are no importantchanges found in the peridotite density down to a pressure of100 kb, there are important inflections in the pressure-densitycurve in quartz-tholeiite which relate to the increasing conversionof plagjoclase to jadeite component in clinopyroxene and a sharprise in density due to the quartz-coesite phase transformation.     

Phase Equilibria of Margarite-Bearing Schists and Chloritoid + Hornblende Rocks from Western New Hampshire, USA

A variety of uncommon garnet-grade assemblages have been foundin rocks from three outcrops in the western part of centralNew Hampshire, and include the associations Grt+MrgCld, Grt+Bt+CldMrg,and Mrg+Cld+HblGrt (all rocks contain Ms, Chl, Ilm, and Qtz).These unusual rocks coexist with more typical Grt+Bt+Chl+Plmetapelites and amphibolites. Rim P–T conditions are {smalltilde}49035C and 5•751•25 kbar. Projection of the assemblages from Qtz, H₂O, and Ilm into theCa–Al'–Na–(Fe+Mg) tetrahedron, and from Qtz,Ilm, H₂O, and Chl into the Ca–Al'–Fe'–Mn tetrahedronindicates that Ca/(Ca+Na) and Mn differ among the assemblagesin a systematic fashion. Common Grt+Bt+Chl+Pl assemblages arerestricted to relatively high Mn and low Ca/(Ca+Na) values,whereas Cld+Bt+Mrg and Cld+Hbl+Mrg assemblages are stable atlow Mn and high Ca/(Ca+Na). These data suggest that at thisgrade Cld+Bt is more stable than Grt+Chl in the KFMASH system,whereas in the Ca—KFMASH system, Hbl+Cld assemblages arestable. Composition space analysis using the singular value decompositionmethod indicates that compositions of minerals from individualsamples are consistent with local equilibrium, but that differentoutcrops may not have all equilibrated at the same P–T–a_H2Oconditions. Thermodynamic analysis suggests that a garnet-zoneprograde sequence of ferromagnesian associations for these bulkcompositions would be Hbl+Cld+Grt+ChlBt+Cld+Grt+ChlBt+Grt+Chl. Staurolite-grade rocks from the same stratigraphic units areexposed across strike, and contain the assemblage Grt+StBtPl(all rocks contain Ms, Qtz, Chl, and Ilm). Margarite is commonlypresent as inclusions in the cores of garnets, but is absentas inclusions near garnet rims and from the matrix; conversely,staurolite inclusions are present towards the rims of the garnets,but are absent from the cores. These inclusion relations suggestthat margarite may react to form staurolite and garnet withincreasing grade via a reaction such as chlorite+margarite=staurolite+garnet+H₂O. Biotite is common in the matrix but is not typically abundant,and appears to have been the last phase to join the assemblage.Biotite is inferred to have joined the Grt+St+Chl assemblagesafter margarite breakdown through the reaction Grt+Chl+Ms=St+Bt+H₂O. Thus, uncommon margarite assemblages may evolve into commonGrt+Bt+St+Chl assemblages. ^* Present address: Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706.     

Seasonal Variability of Adsorption and Exchange Equilibria in Soil Waters

Chemical analyses for major ions have been conducted on waters,collected on an approximately weekly basis over the period April, 1993 toNovember, 1996, that drain three small experimental ecosystems(sandboxes) at Hubbard Brook, New Hampshire. One sandbox is planted withpine trees, another with grass, and the third is left bare (actually itis covered sporadically by bryophytes and lichens). Results show linearcorrelations, independent of discharge, between the concentrations ofdissolved Na⁺ and K⁺ on the one hand andCa⁺⁺ and Mg⁺⁺ on the other for all threesandboxes. No correlations between singly charged and doubly chargedcations were found. These correlations are interpreted to represent cationexchange equilibria between soil waters and clay minerals plus soil organicmatter. The correlation slope, representing the exchange constant, for Na vsK is different for the pine-covered sandbox than for the other two whereasfor Ca vs Mg the correlation is independent of the presence or absence oftrees. We interpret this as representing a shift of cation exchangeequilibria in the pine sandbox by the activities of growing trees.Concentrations of Na, K, Ca, Mg, and H₄SiO₄from the barren and grass-lined sandboxes were found to vary seasonally witha marked sinusoidal pattern which was independent of the discharge from eachsandbox. (The discernment of a similar pattern in the tree lined sandbox wasdifficult due to a lack of discharge over much of the year.) Concentrationmaxima occurred in August and minima in February, and there is a closeparallelism with soil temperature. We interpret this as representingtemperature induced variations in cation exchange equilibria and silicaadsorption. Independence from highly varying water discharge, e.g.,. thataccompanying severe rainstorms, indicates rapidly re-attained equilibrium.Variations in the concentrations of cations are likely due to exchange withunmeasured cations, probably H⁺ or dissolved Al species, as aresult of possible seasonal changes in internal acid production and externalinput of acid rain to the sandboxes. Internal production may represent aresponse to seasonal changes in respiration rate as it responds toseasonally varying temperature. Added to this is the effect of temperatureon exchange equilibrium. Seasonal variations in dissolved silica are mostlikely due to the dependence of adsorption/desorption equilibria ontemperature. The temperature dependence of a number of silica-consumingreactions are consistent with the measured values.     

Heterogeneous Equilibria in Saturated Aqueous Solutions of Uranophane

Heterogeneous equilibria in the system Ca(HSiUO₆)₂ · 5H2O_(c)–aqueous solution were studied over broad ranges of pH, ionic strength, and ionic composition of the solution, and the pH range of stability of Ca uranyl silicate is determined. Hydrolysis products of Ca uranyl silicate are identified, and their solubility is determined. The equilibrium constant of the dissolution reaction and the standard Gibbs function of formation of Ca(HSiUO₆)₂ · 5H₂O are calculated from experimental data, and solubility curves of uranophane and equilibrium speciation diagrams for U(VI), Si(IV), and Ca(II) in coexisting aqueous solutions and solid phases are calculated.     

Nepheline--Alkali Feldspar Equilibria: I. Experimental Data and Thermodynamic Calculations

Nepheline-alkali feldspar equilibria with alkali chloride aqueoussolutions have been determined for the temperature range 400to 700 °C at 1000 bars pressure. Nepheline-alkali feldsparequilibria with alkali chloride melts have been determined forthe temperature range 800 to 1100 °C at approximately 6bars pressure. (1) NaAlSiO₄ + KCl(aq) = NaCl(aq) + KAlSiO₄ (2) NaAlSiO₄ + KCl(melt) = NaCl(melt) + KAlSiO₄ (3) NaAlSi₃O₈(high) + KCl(aq) = NaCl(aq) + KAlSi₃O₈(San) (4) NaAlSi₃O₈(low) + KCl(aq) = NaCl(aq) + KAlSi₃O₈(Mic) (5) NaAlSi₃O₈(high) + KCl(melt) = NaCl(melt) + KAlSi₃O₄(San) (6) NaAlSi₃O₈(low) + KCl(melt) = NaCl(melt) + KAlSi₃O₈(Mic) From these, two diagrams of phase relationships were derivedfor the following exchange equilibria: (7) NaAlSiO₄ + KAlSi₃O₈(San) = NaAlSi₃O₈(high) + KAlSiO₄; (8) NaAlSiO₄ + KAlSi₃O₈(Mic) = NaAlSi₃O₈(low) + KAlSiO₄. The effect of pressure on these equilibria has been determinedby comparing the experimental data for 1000 and 5000 bars (t= 500 °C) and thermodynamic calculations. It has also beenshown that the effect of excess silica in nepheline solid solutionon the K—Na distribution between nepheline and alkalifeldspar is substantial and opposite to that of temperature.In the high temperature region an increase in silica contentin nepheline of 2 wt. per cent eliminates the effect on theredistribution of a temperature increase of 100 °C. Thesecation exchange data and unit cell data for the crystal phasesare used to calculate thermodynamic mixing properties of nephelinesolid solution and alkali feldspar solid solution for a widerange of temperature and pressure.     

Failure Mechanical Characteristics of Rock with Different Hole Shapes

Geotechnical and Geological Engineering - Due to the role of geological structure, the underground rock contains a large number of holes. In order to study the influence of the hole on the...     

Quantification of Magmatic and Hydrothermal Processes in a Peralkaline Syenite-Alkali Granite Complex Based on Textures, Phase Equilibria, and Stable and Radiogenic Isotopes

The Puklen complex of the Mid-Proterozoic Gardar Province, SouthGreenland, consists of various silica-saturated to quartz-bearingsyenites, which are intruded by a peralkaline granite. The primarymafic minerals in the syenites are augite ± olivine +Fe–Ti oxide + amphibole. Ternary feldspar thermometryand phase equilibria among mafic silicates yield T = 950–750°C,a_SiO2 = 0·7–1 and an f_O2 of 1–3 log unitsbelow the fayalite–magnetite–quartz (FMQ) bufferat 1 kbar. In the granites, the primary mafic minerals are ilmeniteand Li-bearing arfvedsonite, which crystallized at temperaturesbelow 750°C and at f_O2 values around the FMQ buffer. Inboth rock types, a secondary post-magmatic assemblage overprintsthe primary magmatic phases. In syenites, primary Ca-bearingminerals are replaced by Na-rich minerals such as aegirine–augiteand albite, resulting in the release of Ca. Accordingly, secondaryminerals include ferro-actinolite, (calcite–siderite)_ss,titanite and andradite in equilibrium with the Na-rich minerals.Phase equilibria indicate that formation of these minerals tookplace over a long temperature interval from near-magmatic temperaturesdown to     

Fractionation and Assimilation Processes in the Alkaline Augite Syenite Unit of the Ilimaussaq Intrusion, South Greenland, as Deduced from Phase Equilibria

The early augite syenite unit in the 1·13-Ga-old Ilímaussaqintrusive complex, South Greenland, consists of a magmatic assemblageof ternary alkali feldspar + fayalitic olivine + augite + titanomagnetite+ apatite + baddeleyite ± nepheline ± quartz ±ilmenite ± zircon. Feldspar, nepheline and QUILF thermometryyield T = 1000–700°C, at P = 1 kbar, which is derivedfrom fluid inclusion data from other parts of the complex. Ternaryfeldspar was the first major liquidus phase. It crystallizedat temperatures between 950 and 1000°C from a homogeneousmagma with a_SiO2 = 0·8 and f_O2 about 1·5–2log units below the fayalite–magnetite–quartz (FMQ)buffer. Later, closed system fractionation produced nepheline-bearingassemblages with a_SiO2 = 0·4 and log f_O2 = FMQ –3 to FMQ – 5. Assimilation of wall rocks produced localvariations of melt composition. Four traverses through the unitwere sampled parallel to the assumed direction of crystallization.They exhibit significant differences in their mineral assemblagesand compositions. The chemical zoning and calculated intensiveparameters of four sample suites reflect both closed systemfractional crystallization and local assimilation of wall rocks. KEY WORDS: alkaline magmatism; assimilation; fractionation; redox equilibria; QUILF     

Phase Equilibria Constraints on the Chemical and Physical Evolution of the Campanian Ignimbrite

The Campanian Ignimbrite is a > 200 km³ trachyte–phonolitepyroclastic deposit that erupted at 39·3 ± 0·1ka within the Campi Flegrei west of Naples, Italy. Here we testthe hypothesis that Campanian Ignimbrite magma was derived byisobaric crystal fractionation of a parental basaltic trachyandesiticmelt that reacted and came into local equilibrium with smallamounts (5–10 wt%) of crustal rock (skarns and foid-syenites)during crystallization. Comparison of observed crystal and magmacompositions with results of phase equilibria assimilation–fractionationsimulations (MELTS) is generally very good. Oxygen fugacitywas approximately buffered along QFM + 1 (where QFM is the quartz–fayalite–magnetitebuffer) during isobaric fractionation at 0·15 GPa ( 6km depth). The parental melt, reconstructed from melt inclusionand host clinopyroxene compositions, is found to be basaltictrachyandesite liquid (51·1 wt% SiO₂, 9·3 wt%MgO, 3 wt% H₂O). A significant feature of phase equilibria simulationsis the existence of a pseudo-invariant temperature, 883 °C,at which the fraction of melt remaining in the system decreasesabruptly from 0·5 to < 0·1. Crystallizationat the pseudo-invariant point leads to abrupt changes in thecomposition, properties (density, dissolved water content),and physical state (viscosity, volume fraction fluid) of meltand magma. A dramatic decrease in melt viscosity (from 1700Pa s to 200 Pa s), coupled with a change in the volume fractionof water in magma (from 0·1 to 0·8) and a dramaticdecrease in melt and magma density acted as a destabilizingeruption trigger. Thermal models suggest a timescale of 200kyr from the beginning of fractionation until eruption, leadingto an apparent rate of evolved magma generation of about 10^–3km³/year. In situ crystallization and crystal settling in density-stratifiedregions, as well as in convectively mixed, less evolved subjacentmagma, operate rapidly enough to match this apparent volumetricrate of evolved magma production. KEY WORDS: assimilation; Campanian Ignimbrite; fractional crystallization; magma dynamics; phase equilibria     

Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral-Fluid Equilibria and Petrogenetic Grids

The role of clinopyroxene in producing grandite garnet is evaluatedusing data from an ultrahigh-temperature metamorphosed calc-silicategranulite occurrence in the Eastern Ghats Belt, India. ‘Peak’pressure–temperature conditions of metamorphism were previouslyconstrained from associated high Mg–Al granulites as c.0·9 GPa, >950°C, and the rocks were near-isobaricallycooled to c. 750°C. Grandite garnet of variable compositionwas produced by a number of reactions involving phases suchas clinopyroxene, scapolite, plagioclase, wollastonite and calcite,in closely spaced domains. Compositional heterogeneity is preservedeven on a microscale. This precludes pervasive fluid fluxingduring either the peak or the retrograde stage of metamorphism,and is further corroborated by computation of fluid–rockratios. With the help of detailed textural and mineral compositionalstudies leading to formulation of balanced reactions, and usingan internally consistent thermodynamic dataset and relevantactivity–composition relationships, new petrogenetic gridsare developed involving clinopyroxene in the system CaO–Al₂O₃–FeO–SiO₂–CO₂–O₂in T–aCO₂–fO₂ space to demonstrate the importanceof these factors in the formation of grandite garnet. Two singularcompositions in garnet-producing reactions in this system arededuced, which explain apparently anomalous textural relations.The possible role of an esseneite component in clinopyroxenein the production of grandite garnet is evaluated. It is concludedthat temperature and fO₂ are the most crucial variables controllinggarnet composition in calc-silicate granulites. fO₂, however,behaves as a dependent variable of CO₂ in the fluid phase. Externalfluid fluxing of any composition is not necessary to producechemical heterogeneity of garnet solid solution. KEY WORDS: grandite garnet; role of clinopyroxene; internal buffering; oxidation–decarbonation equilibria     

Phase Equilibria in the System CaCO3-MgCO3-FeCO3

Experimental data on phase relations in the CaCO₃-MgCO₃-FeCO₃ternary by Goldsmith et al. (1962) are inconsistent with thoseof Rosenberg (1967). These inconsistencies cannot be reconciledby the pressure or temperature differences between the two setsof experiments. Available reversed experiments on the binarysystems have been re-evaluated to yield consistent binary solvi.These data have been combined with analyses of natural carbonatesto yield approximate ternary phase diagrams at 250,400,550,and 700 ?C, and 4 to model ternary activity/composition relationsfor calcite- and dolomite-structure carbonates. An empiricalmodel for thermometry with calcite-ferroan dolomite was derivedfrom the ternary solvus by fitting an empirical equation tothe calcite limb of the ternary solvus. This model appears toextend reliably to higher iron contents than the models of Bickle* Powell (1977) and Powell et al. (1984).     

Natural Disturbance Shapes Benthic Intertidal Macroinvertebrate Communities of High Latitude River Deltas

Unlike lower latitude coastlines, the estuarine nearshore zones of the Alaskan Beaufort Sea are icebound and frozen up to 9 months annually. This annual freezing event represents a dramatic physical disturbance to fauna living within intertidal sediments. The main objectives of this study were to describe the benthic communities of Beaufort Sea deltas, including temporal changes and trophic structure. Understanding benthic invertebrate communities provided a baseline for concurrent research on shorebird foraging ecology at these sites. We found that despite continuous year-to-year episodes of annual freezing, these estuarine deltas are populated by a range of invertebrates that represent both marine and freshwater assemblages. Freshwater organisms like Diptera and Oligochaeta not only survive this extreme event, but a marine invasion of infaunal organisms such as Amphipoda and Polychaeta rapidly recolonizes the delta mudflats following ice ablation. These delta sediments of sand, silt, and clay are fine in structure compared to sediments of other Beaufort Sea coastal intertidal habitats. The relatively depauperate invertebrate community that ultimately develops is composed of marine and freshwater benthic invertebrates. The composition of the infauna also reflects two strategies that make life on Beaufort Sea deltas possible: a migration of marine organisms from deeper lagoons to the intertidal and freshwater biota that survive the 9-month ice-covered period in frozen sediments. Stable isotopic analyses reveal that both infaunal assemblages assimilate marine and terrestrial sources of organic carbon. These results provide some of the first quantitative information on the infaunal food resources of shallow arctic estuarine systems and the long-term persistence of these invertebrate assemblages. Our data help explain the presence of large numbers of shorebirds in these habitats during the brief summer open-water period and their trophic importance to migrating waterfowl and nearshore populations of estuarine fishes that are the basis of subsistence lifestyles by native inhabitants of the Beaufort Sea coast.