Potassium-rich Rocks: Phase Analysis and Heteromorphic Relations

A large number of potassium-rich igneous rocks can be displayedas a contiguous set of mineral assemblages in a modified normativetetrahedron kalsilite-forsterite-larnite-quartz. Pairs of phasesin subtetrahedra undergo reaction relations that result in newsubtetrahedra representative of observed igneous rock assemblages.Some of the heteromorphic relations recognized are: Jumillite-ShonkiniteMissourite-Absarokite Alnoite-ItaliteVenanzite-Melilite Ugandite Leucite Katungite-Melilite UganditeMelilite Mafurite-LeuciteMafurite Alnoite-VeseciteMonticellite Peridotite-Monticellite Melilitite Other subtetrahedra are generated by the breakdown of phlogopite,leucite, and monticellite. The relationships of the various magma types are outlined ina flow sheet for 1 atm, and some of the critical changes inthermal barriers are described for elevated pressures. The high-potassiumcontent remains the principal problem; however, the shift ofinvariant points with pressure suggests generation by partialmelting at high pressures. The complex mineralogy and diversity of the potassium-rich rocksare attributed mainly to incomplete reactions, with or withoutvolatiles, between critical phases of limited range of stability.     

钠长花岗岩—H2O—HF体系相关系及含黄玉花岗质岩石的成因

在Ｐ＝１００ＭＰａ,ｔ＝８４０－４５０℃条件下,通过钠长花岗岩Ｈ２Ｏ－ＨＦ体系相关系实验获得;（１）随体系Ｆ含量的增加,固相线温度显著下降。（２）石英和黄玉的温度稳定域上限升高,碱性长石的稳定域上限降低;在Ｆ≤４％时,体系能在固线性之上结晶出典型的黄玉花岗岩矿物组合;在Ｆ＝６％时,体系能在固相线之上结晶出典型的黄玉云英矿物组合;（２）含氟浅色花岗质熔体具有能分异出极端富Ｆ残余熔体的趋势。     

Rare Earth and 3d Transition Element Geochemistry of Peridotitic Rocks: II. Ligurian Peridotites and Associated Basalts

Peridotites and associated basalts of the External (EL) andInternal (IL) Ligurian terrains of the Northern Apennines havebeen investigated geochemically. The REE and 3d transition elementcompositions of IL peridotites reflect partial fusion of a spinelperidotite protolith probably in the spinel-plagioclase transitionzone. EL peridotites underwent only incipient melting underclosed system conditions. Basalts associated with EL and ILperidotites have petrography and chemistry akin to MORB. Basedon REE, they can be regarded as the products of fractional melting(IL to some extent at higher F with respect to EL). PositiveEu anomalies in most of the studied IL basalts are due to plagioclasefractional crystallization; also, on the basis of Sc, Co, Nicompositions, fractional crystallization of olivine can be recognizedfor both EL and IL lavas. IL basalts are possibly cogeneticwith IL peridotites, but the same cannot be said for EL lavas.     

H2O and Ultrahigh-Pressure Subsolidus Phase Relations for Mafic and Ultramafic Systems

Experimental studies show that aqueous fluid-mediated mineralogic solution/redeposition mechanisms are orders faster than solid-solid transformations; hence the presence of a separate aqueous fluid markedly enhances reaction rates, whereas its total absence impedes mineralogic transformations. Where does this volatile component come from? For typical subduction-zone P-T trajectories, amphibole constitutes the major OH-bearing phase in most deep-seated metamorphic rocks of basaltic composition; other hydrous minerals are of minor abundance. Clinoamphiboles dehydrate at pressures of-2.0 to 2.4 GPa, but devolatilization may be delayed slightly by pressure overstepping; thus mafic blueschists and barroisitic amphibolites expel H₂O at arc melt-generation depths of ～100 km, and commonly achieve the stable eclogitic phase configuration. Serpentinized mantle beneath the oceanic crust devolatilizes at comparable conditions. Only where metagabbroic rocks are completely dry and coarse grained are low-pressure assemblages metastably preserved. For realistic subduction-zone geothermal gradients, white micas ± biotites remain stable in sialic crust to pressures exceeding 3.5 GPa. Accordingly, under conditions attending descent to great depths, mica-rich quartzofeldspathic schists and gneisses that constitute the continental crust fail to evolve substantial amounts of H₂O, and transform incompletely to stable eclogite-facies assemblages. The deep underflow of partly hydrated oceanic lithosphere thus generates most of the deep-seated volatile flux—and consequent partial melting to produce the calc-alkaline suite along and above a subduction zone; where large volumes of micaceous sialic materials are carried down to extreme depths, volatile flux severely diminishes.     

Rare Earth and 3d Transition Element Geochemistry of Peridotitic Rocks: I. Peridotites from the Western Alps

Trace element analyses have been obtained employing RNAA andINAA techniques for 23 bulk-rock specimens and for five pairsof mechanically separated opx and cpx from Western Alpine peridotites.Investigated rocks include 5 garnet lherzolites from Alpe Arami,and spinel (+plagioclase) lherzolites from Finero (2), Balmuccia(7), Baldissero (6) and Lanzo (3). Three pyroxene pairs wereanalysed from Balmuccia and two from Baldissero. All rocks exhibit marked LREE depletions relative to chondriticabundances except for the two Finero samples which appear tobe HREE depleted. Separated minerals also show LREE depletionsand HREE enrichments relative to chondrites. However, intermediaterare earths are markedly depleted in opx whereas they are enrichedin coexisting cpx. Higher overall concentrations and patternssimilar to those of the bulk rocks indicate that REE distributionsin lherzolites are dominated by clinopyroxene chemistry. Incontrast, both opx and cpx appear to contribute equally to the3d transition element geochemistry of the investigated peridotites. Most of the investigated rocks show the effects of early partialmelting of a pre-existing mantle source material characterizedby ‘chondritic’ REE fractionation and by a 3d transitionelement composition near the estimated values of Jagoutz etal. (1979). The melting process probably developed in a closed system (equilibriummelting) and at temperatures which, for the spinel peridotiteprotolith, seemingly were compatible with estimates of Presnallel al. (1979) for the ‘melting at the cusp’ process(T = 1200–1250 °C). In some cases the residual rocksunderwent a further contamination event. This is particularlyevident for the Lanzo peridotites, but possibly also for singleBaldissero and Balmuccia specimens. During ascent to the surface, the rocks underwent subsolidusannealing which occurred at temperatures around 900–1000°C under more or less closed system conditions.     

Stability Relations of the Pyroxenes and Olivine in Certain High Grade Metamorphic Rocks

The stability relations of mineral assemblages consisting ofvarious combinations of Capyroxene, orthopyroxene, olivine,quartz, dolomite, calcite, and graphite are treated by analyticalmeans with emphasis on reactions of the following type: MgSiO₃CaCO₃SiO₂CaMgSi₂O₆CO₂ (1) 3MgSiO₃CaCO₃ Mg₂SiO₄CaMgSi₂O₆CO₂ (2) and the associated reactions involving ferrous iron. In particular,reaction (1) is discussed in detail in terms of uncertaintiesinherent in the thermochemical data and the limits placed onits equilibrium constant by the geological conditions of theoccurrence of the observed assemblages. It is shown from theequation of equilibrium of (1) that the composition of orthopyroxeneis a linear function of the carbon dioxide fugacity and thatthe latter is lowered by the presence of the FeSiO₃ component.The latter conclusion is compatible with the rarity of the associationcalcite-orthopyroxcne and its apparent confinement to iron-richrocks. An analysis of the same assemblage involved in reaction (1)indicates that when graphite coexists with the iron-rich pyroxene,the fugacity of oxygen falls near the center of the magnetitefield for a wide range of CO₂ fugacities and temperatures. It is shown that reaction (2) is also displaced strongly tothe right under most metamorphic conditions, which is compatiblewith fairly common occurrence of the association of Capyroxeneand olivine. Finally, a critique is presented on the possibility of the useof the pyroxene-bearing assemblages as indicators of metamorphicgrade and it is concluded that this is not rigorously possiblesince the systems almost invariably possess too many degreesof freedom.     

Phase Relations on the Actinolite-Pargasite Join

Phase relations along the join Ca₂Mg₄Fe²⁺Si₈O₂₂ (OH)₂ (Actinolite)-NaCa₂Mg_3?2Fe_0?8²⁺AlSi₆Al₂O₂₂(OH)₂ (Pargasite) have been studied at P_H2O = 1 kb andthe oxygen fugacities defined by the iron-wustite(IW) buffer. Actinolite and bornblende are separated by a solvus and thefield of actinolite+hornblende+vapor is present in the regionbetween Ac₈₅Pa₁₅ and Ac₅₅ Pa₄₅ at 680 ?C. Complete miscibilityis achieved at 720 ?C. At temperatures higher than the solvusthere is a continuous solid solution series between the twoend members. The stability field of amphibole solid solutiongradually increases with increasing pargasite content in actinolite.The phase assemblages at temperatures higher than those of asolid solution series between the two end members change withincreasing pargasite content in the bulk composition as follows;Act+Cpx+Qz+V, ActHbl+Cpx+Opx+Qz+V, Hbl+Cpx+Opx+Pl+V and Hbl+Cpx+Pl+Ol+V. In comparison with the Fe-free system, the extent of the miscibilitygap between actinolite and hornblende is reduced by an increasein the Fe²⁺ content. The present study should provide an adequatebasis for the interpretation of actinolite-hornblende pairsin metamorphic rocks.     

Fe-Ni-Co-O-S Phase Relations in Peridotite-Seawater Interactions

Serpentinization of abyssal peridotites is known to produceextremely reducing conditions as a result of dihydrogen (H₂,aq)release upon oxidation of ferrous iron in primary phases toferric iron in secondary minerals by H₂O. We have compiled andevaluated thermodynamic data for Fe–Ni–Co–O–Sphases and computed phase relations in fO₂,g–fS₂,g andaH₂,aq–aH₂S,aq diagrams for temperatures between 150 and400°C at 50 MPa. We use the relations and compositions ofFe–Ni–Co–O–S phases to trace changesin oxygen and sulfur fugacities during progressive serpentinizationand steatitization of peridotites from the Mid-Atlantic Ridgein the 15°20'N Fracture Zone area (Ocean Drilling ProgramLeg 209). Petrographic observations suggest a systematic changefrom awaruite–magnetite–pentlandite and heazlewoodite–magnetite–pentlanditeassemblages forming in the early stages of serpentinizationto millerite–pyrite–polydymite-dominated assemblagesin steatized rocks. Awaruite is observed in all brucite-bearingpartly serpentinized rocks. Apparently, buffering of silicaactivities to low values by the presence of brucite facilitatesthe formation of large amounts of hydrogen, which leads to theformation of awaruite. Associated with the prominent desulfurizationof pentlandite, sulfide is removed from the rock during theinitial stage of serpentinization. In contrast, steatitizationindicates increased silica activities and that high-sulfur-fugacitysulfides, such as polydymite and pyrite–vaesite solidsolution, form as the reducing capacity of the peridotite isexhausted and H₂ activities drop. Under these conditions, sulfideswill not desulfurize but precipitate and the sulfur contentof the rock increases. The co-evolution of fO₂,g–fS₂,gin the system follows an isopotential of H₂S,aq, indicatingthat H₂S in vent fluids is buffered. In contrast, H₂ in ventfluids is not buffered by Fe–Ni–Co–O–Sphases, which merely monitor the evolution of H₂ activitiesin the fluids in the course of progressive rock alteration.The co-occurrence of pentlandite–awaruite–magnetiteindicates H₂,aq activities in the interacting fluids near thestability limit of water. The presence of a hydrogen gas phasewould add to the catalyzing capacity of awaruite and would facilitatethe abiotic formation of organic compounds. KEY WORDS: serpentinization; ODP Expedition 209; sulfide; oxygen fugacity; sulfur fugacity; hydrothermal system; metasomatism; Mid-Atlantic Ridge     

The Habit of Apatite in Synthetic Systems and Igneous Rocks

Apatite crystals co-existing with liquid or with vapour in thesystem CaO-CaF₂-P₂O₆-H₂O-CO₂ are equant whereas those precipitatedfrom a liquid during a quench are greatly elongated parallelto the c-axis and they exhibit skeletal forms. Basic rocks inthe Karroo volcanic succession of Southern Rhodesia containapatite crystals with forms very similar to the synthetic quenchapatites. Acicular apatite crystals may prove to be of valueas a criterion for the former existence of a liquid phase inrocks whose origin is debatable.     

Tetrahedral Plots of the Phase Relations for Basalts

The phase relations of quaternary systems are generally represented by projections onto ternary compositional planes. Such projections often obscure relationships that would only be evident in a three-dimensional tetrahedral plot. The tetrahedral plot requires that compositions of the minerals and melts be transformed into Cartesian coordinates. It is shown here how this transformation is carried out. The application is demonstrated by tetrahedral plots of experimental melt compositions of partially molten lherzolite. Furthermore, the plot can be used to evaluate whether or not a particular basaltic composition represents a primary melt. The methods are applicable to any four-component system.     

An Experimental Study of the Influence of Oxygen Fugacity on Fe-Ti Oxide Stability, Phase Relations, and Mineral--Melt Equilibria in Ferro-Basaltic Systems

Equilibrium crystallization experiments at atmospheric pressureand over a range of oxygen fugacity (fO₂) have been carriedout on a ferro-basaltic composition similar to liquids proposedto have been parental to much of the exposed portion of theSkaergaard intrusion. Before Fe-Ti oxide saturation the liquidline of descent is little affected by fO₂. However, the appearancetemperatures of the magnetite-ulvspinel solid solution (Mt)and the ilmenite-haematite solid solution (Ilm) depend stronglyon fO₂. Above the fayalite-magnetite-quartz (FMQ) buffer Mtis the first oxide phase to appear on the liquidus, but belowthe FMQ buffer Ilm is the first oxide to crystallize. The appearancetemperature of Mt is 1100C at FMQ and the Mt liquidus slopeis 30C/log fO₂ unit between FMQ–;2 and FMQJ+1. The Ilmliquidus is at 1100C between FMQ and FMQ–2, but movesto lower temperature at higher fO₂ where Mt is the first oxidephase. The results indicate that the ferric iron content ofMt-saturated melts varies linearly with inverse temperature,and that Ilm saturation is closely related to melt TiO₂ content.Mt saturation produces an immediate enrichment of SiO₂ and depletionin FeO^* in the melt phase, whereas Ilm saturation produces similarenrichment in SiO², but inn enrichment may continue for 10Cbelow the ilmenite liquidus. The experimental liquids reacha maximum of 18 wt% FeO^*, at 48 wt% SiO₂ for ilmenite-saturatedmelts at low f_O2, more differentiated melts having lower ironand higher silica. Cotectic proportions, derived from mass balancecalculations, are in good agreement with data from natural samplesand other experimental studies. Olivine resorption is inferredat all fO₂, with the onset of resorption occurring 10C higherthan the appearance of magnetite. The effect of fO₂ on silicatemineral compositions, and partitioning of elements between coexistingmineral-melt pairs, is small. Thermodynamic considerations suggestthat variations of Fe-Mg partitioning between the iron-richolivines, pyroxenes and melts produced in this study may beexplained by known non-idealities of Fe-Mg mixing in the crystallinephases, rather than nonidealities in the coexisting melts. Theseexperiments also provide insights into many features commonto natural tholeiitic series of volcanic and plutonic rocks,and provide experimental data required for modelling of fractionalcrystallization and crystallization closed to oxygen, processeswhich are not easily investigated experimentally. KEY WORDS: ferro-basalt; Fe-Ti oxides; oxygen fugacity; Skaergaard intrusion; iron enrichment ^*Corresponding author. Present address: Bayerisches Geoinstitut, Univerritt Bayreuth, D-95440 Bayreuth, Germany     

政和地区变质岩石的变质结晶作用与变形作用的关系

政和地区变质岩石在晋宁期遭受了３期构造变形作用（Ｄ）。以这３期构造变形作用为相对时间标志，讨论了变质砂泥质岩石中各种不同产状、不同种类变质矿物的生长期，确定了基质主要加粗期为Ｄ２－Ｄ３变形期间的静态期。进而将晋宁期变质作用划分为早期、主期和晚期３个变质阶段，变质作用的主期为Ｄ２变形期和Ｄ２－Ｄ３变形期间静态期。     

Minerogenic Model of Chrysotile Deposits in Ultramafic Rocks

Most chrysotile deposits occur in ultramafic rocks of the ophiolite suite. The chrysotile deposits dis-cussed in the present paper were formed through metasomatism and infilling-crystallization in a continentalserpentinization environment after plate convergence, where ultramafic rocks were replaced byhydrothermal solutions consisting mainly of deep-circulating heated water derived from atmospheric precip-itation. The critical state for the formation of asbestos in ultramafic rock bodies might be reached bysuperposition of multiple stages of serpentinization. Favourable fracture systems and relatively stable geo-logical environment are important conditions for forming chrysotile deposits. Three subtypes of chrysotiledeposits could be formed in different tectonic settings and under different minerogenic geochemical condi-tions.     

Theoritical and Experimental Studies on the Phase Equilibria and Phase Diagrams on the Salt-Water Systems

正1 Introduction Salt lakes are widely distributed in the world,and salt lakes in China are mainly located in the area of the Qinghai-Xizang(Tibet),and the Autonomous Regions of Xinjiang and Inner Mongolia.There are more than 700salt lakes,each with an area larger than 1 km2,in the     

Phase Relations of Peralkaline Silicic Magmas and Petrogenetic Implications

The phase relationships of three peralkaline rhyolites fromthe Kenya Rift have been established at 150 and 50 MPa, at oxygenfugacities of NNO - 1·6 and NNO + 3·6 (log fO₂relative to the Ni–NiO solid buffer), between 800 and660°C and for melt H₂O contents ranging between saturationand nominally anhydrous. The stability fields of fayalite, sodicamphiboles, chevkinite and fluorite in natural hydrous silicicmagmas are established. Additional phases include quartz, alkalifeldspar, ferrohedenbergite, biotite, aegirine, titanite, montdoriteand oxides. Ferrohedenbergite crystallization is restrictedto the least peralkaline rock, together with fayalite; it isreplaced at low melt water contents by ferrorichterite. Riebeckite–arfvedsoniteappears only in the more peralkaline rocks, at temperaturesbelow 750°C (dry) and below 670°C at H₂O saturation.Under oxidizing conditions, it breaks down to aegirine. In themore peralkaline rocks, biotite is restricted to temperaturesbelow 700°C and conditions close to H₂O saturation. At 50MPa, the tectosilicate liquidus temperatures are raised by 50–60°C,and that of amphibole by 30°C. Riebeckite–arfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40–100°C compared with the solidusof the metaluminous granite system, as a result of the abundanceof F and Cl. Low fO₂ lowers solidus temperatures by at least30°C. Comparison with studies of metaluminous and peraluminousfelsic magmas shows that plagioclase crystallization is suppressedas soon as the melt becomes peralkaline, whatever its CaO orvolatile contents. In contrast, at 100 MPa and H₂O saturation,the liquidus temperatures of quartz and alkali feldspar arenot significantly affected by changes in rock peralkalinity,showing that the incorporation of water in peralkaline meltsdiminishes the depression of liquidus temperatures in dry peralkalinesilicic melts compared with dry metaluminous or peraluminousvarieties. At 150 MPa, pre-eruptive melt H₂O contents rangefrom 4 wt % in the least peralkaline rock to nearly 6 wt % inthe two more peralkaline compositions, in broad agreement withprevious melt inclusion data. The experimental results implymagmatic fO₂ at or below the fayalite–quartz–magnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H₂O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria     

Peridotitic and gabbroic rocks associated with the shield-forming lavas of réunion

The peridotitic and gabbroic rocks described occur a) as a tectonically emplaced layered body in Piton des Neiges volcano, b) as blocks in basaltic agglomerate, Piton des Neiges, and c) as nodular inclusions in lavas of both Piton des Neiges and Piton de la Fournaise volcanoes. All are associated with the olivine basalts of the early shield-forming growth stages and not later alkaline lavas, thereby contrasting with the Hawaiian situation. Rock-types include dunite, clinopyroxenite, wehrlite, feldspathic wehrlite, olivine eucrite, allivalite, (bytownite) anorthosite and gabbro. The peridotites and most of the gabbroic rocks are inferred to be cumulates formed in floored magma chambers occurring at depths from 30 km upwards. The inclusion suite is probably derived from repetitive layered units consisting predominantly of ol + sp cumulates with sporadic development of ol + cpx±sp and ol + cpx + plag cumulate horizons.     

复杂橄榄岩捕虏体:反演岩石圈演化过程的罕见而重要的样品

地幔和／或下地壳中存在2类“复杂”橄榄岩捕虏体:一类为“混杂”,即“polymict”橄榄岩捕虏体;另一类为相对常见的“复合”,即“composite”橄榄岩捕虏体．复合橄榄岩捕虏体通常由橄榄岩和辉石岩／麻粒岩或由橄榄岩夹辉石岩脉2部分或多部分组成．岩相学上易于辨认,有时手标本上即可辨认．而混杂橄榄岩捕虏体为各种橄榄岩和／或辉石岩来源的矿物和熔体混杂在一起组成的构造岩,岩相学上很难辨认,只有通过矿物组成分析来判断．混杂橄榄岩捕虏体仅见于南非Kaapvaal 克拉通金伯利岩岩管中,该类捕虏体来源于地幔塑性剪切带,记录了流／熔体参与下的地幔变形和交代作用的全过程．复合橄榄岩捕虏体较常见,出现在全球各类玄武岩和金伯利岩中．复合橄榄岩捕虏体有多种成因,如地幔变形、壳幔过渡带样品、交代作用和堆积成因等等．这些复合橄榄岩捕虏体同样记录了岩石圈地幔和／或下地壳交代作用／改造过程的中间状态．因此,这些罕见而重要的复杂橄榄岩捕虏体为大陆岩石圈动力学演化过程提供了最直接的样品．     

右江裂谷区三叠系岩石物性特征及其与金矿化的关系

本文阐述了右江裂谷区微细粒浸染型金矿床控矿层位三叠系百蓬组泥质粉砂岩及砂质泥岩的粒度，比表面、孔隙度及孔隙类型和渗透率等物性特征，讨论了与其成矿流体的循回、水岩反应及其有效浸取围岩矿物质的关系。在此基础邮成矿流体和矿矿物质主要来自于地层；由于热液躲避低势能方向迁移，造成断理解发的盆地边缘区和水下隆起周围的环状断裂带为金矿床沉淀的场所。并通过形成金矿所需热液量的估算得出，该区金矿床的形成，是由于热流     

Phase Equilibria in the Metamorphic Rocks of St. Paul Island and Cape North, Nova Scotia

A suite of schists—one from the garnet zone, 19 from thestaurolite zone, 2 from the kyanite isograd, and one from thekyanite zone—were separated into their constituent minerals.Chemical analyses of one chlorite and of 23 sets of coexistingbiotites and garnets were carried out by photometric and titrametricprocedures. Plots of garnet-biotite tie-lines from divariantassemblages on appropriate phase diagrams result in intersectingtie-lines which cannot be ascribed to experimental error. Theoreticalconsiderations argue that at equilibrium, at the same pressureand temperature or at constant pressure and varying temperature,tie-lines of divariant assemblages should not intersect. Possibleexplanations require that diffusion equilibrium of Fe and Mgbe restricted to volumes smaller than that of a hand specimenor that P as well as T varies considerably. Emission spectrographicdeterminations of Fe and Mg in biotite indicate that the Fe/Mgratio varies among biotites little more than a centimeter apart.Such a variation would argue more in favor of a lack of diffusionequilibrium.     

An Orthotropic Cosserat Elasto-Plastic Model for Layered Rocks

Summary Modelling the behaviour of rock masses consisting of a large number of layers is often necessary in mining applications (e.g. coal mining). Such a modelling can be carried out in a discontinuum manner by explicit introduction of joints. When the number of rock layers is large, it is advantageous to devise a continuum-based model in which case the joints are considered to be virtually smeared across the mass. In this study, a fully elasto-plastic equivalent continuum model suitable for describing the behaviour of such layered rock masses is considered. The model is based on the Cosserat continuum theory and incorporates the moment stresses in its formulation. In contrast to the earlier Cosserat models, the possibility of rock layer plasticity is considered. The accuracy of the developed Cosserat model is verified against analytical and experimental results. Received October 15, 2000; accepted July 30, 2001 Published online August 2, 2002