Anhydrous Partial Melting of a Fertile and Depleted Peridotite from 2 to 30 kb and Application to Basalt Petrogenesis

Reversal experiments have been performed to check the methodof Jaques & Green (1980) to determine equilibrium partialmelts from peridotite compositions. Reversals of the Jaques& Green (1980) calculated equilibrium partial melt (CEPM)compositions have been carried out in two ways: (1) By runningCEPM compositions at original P and T conditions and testingfor multiple saturation in residual phases of the original experiments.(2) By sandwich/mixed experiments using the CEPM compositionplus peridotite (either Hawaiian pyrolite or Tinaquillo lherzolite). The glass (liquid) compositions from the first series of experimentsshow that the CEPM compositions of Jaques & Green (1980)are too olivine-rich. The glass (liquid) compositions from thesecond series of experiments define new olivine+orthopyroxene?clinopyroxenecotectics in a molecular normative tetrahedron. The new cotecticsplot towards the Qz apex of the tetrahedron, away from the cotecticsdefined by the CEPM compositions of Jaques & Green (1980).Partial melt compositions have also been determined at 20 and30 kb, using both the sandwich technique and the approach bymodal analysis and mass balance. The results of the experimental study are used to evaluate thepetrogenesis of mid-ocean ridge basalts, Hawaiian tholentesand primary magmas in intraoceanic convergent margin settings.     

Melting Relations in Part of the System CaO-MgO-Al2O3-SiO2-Na2O-H2O under 5kb Pressure

In the system CaO-MgO-Al₂O₃-SiO₂-Na₂O-H₂O under 5 kb pressurethe invariant equilibrium forsterite-orthopyroxene-Ca-rich clinopyroxene-amphibole-plagioclase-liquid-vapourhas been identified at 960?12 ?C. A similar invariant assemblagewith spinel replacing Ca-rich clinopyroxene exists at 950?8?C. The liquid in the former equilibrium contains 16.5 per cent(wt.) normative quartz and 3 per cent Na₂O; the plagioclaseis more calcic than An₈₇; the pyroxenes contain about 3 percent Al₂O₃ and the amphibole is hypersthene-normative. Two anhydrousthermal maxima, the olivine-Ca-rich clinopyroxene-plagioclaseand the orthopyroxene-Ca-rich clinopyroxene-plagioclase dividezones are not encountered in this system, and nepheline-normativeliquids may crystallize amphibole?olivine?Ca-rich clinopyroxeneto produce quartz-normative residual liquids of andesite-typecomposition. A thermal maximum involving amphibole-olivine-Ca-richclinopyroxene-liquid-vapour exists for liquids containing approximately11 per cent normative nepheline and liquids more undersaturatedthan this will crystallize these phases to produce extremelynephelinitic liquids. Phase diagrams are presented which facilitate the predictionof crystallization sequences and liquid evolution paths forany basic or intermediate composition under the conditions employedhere.     

Phase Relations of Basalts in their Melting Ranges at PH2O = 5 kb. Part II. Melt Compositions

This paper describes the melting relations of three basalts,a Picture Gorge tholeiite, the 1921 Kilauea olivine tholeiite,and the 1801 Hualalai alkali basalt, at 5 kb water pressure,680–1045 °C, at the oxygen fugacities of the quartz-fayalite-magnetite(QFM) and hematite-magnetite (HM) buffer. All melts producedwithin the hornblende stability field are strongly quartzo-feldspathic.All are quartz-normative, including those from the alkali basalt,and all but two of the melts are corundum-normative. Melt compositionshows very little dependence on oxygen fugacity within the hornblendestability field, as MgO and FeO contents are very low. Whenhornblende begins to melt extensively (1000°–1045°C), the TiO₂, FeO, and MgO contents of the melt increaseabruptly. In this range, melts formed on the HM buffer havemuch higher Mg/Fe ratios and lower TiO₂ than melts formed onthe QFM buffer. Melt composition is also quite insensitive to changes in basaltcomposition, within the hornblende stability field. The chiefexception is the Na/Ca ratio, which varies directly with Na/Cain the starting basalt. When projected into the Ab-An-Or-Qzquaternary system, all melts produced follow a rather narrowspiral path through the tetrahedron; they descend from the Ancorner, moving toward Qz at constant Ab/Or, moving toward Oronly when plagioclase± quartz begin to precipitate. The melting behavior of hornblende, plagioclase, and augitein these experiments has been examined closely, with the followingresults: successive partial melts may differ from each otherby compositional increments which are very different in compositionfrom the minerals contributing to the melt in the temperatureinterval under consideration. These increments can almost neverbe expressed solely in terms of members of the one or two mineralsolid solutions from which they are actually derived. In a fewcases the increments cannot be expressed in terms of any reasonablecombination of minerals. This pattern contrasts markedly withthat observed in fractional crystallization, in which the differencebetween successive melts must always correspond to present orpossible phenocryst minerals. The contrast implies that magmaseries generated by any kind of melting process, equilibriumor fractional, should be recognizably different from seriesgenerated by fractional crystallization, if minerals like hornblendeor pyroxene are involved.     

Phase Relations of Basalts in their Melting Range at PH2O = 5 kb as a Function of Oxygen Fugacity: Part I. Mafic Phases

The phase relations of three basalts, the Picture Gorge tholeiite,the 1921 Kilauea olivine tholeiite, and the 1801 Hualalai alkalibasalt, were studied at 5 kb water pressure, 680–1000°C,at the oxygen fugacities of the quartz-fayalite-magnetite (QFM)and hematite-magnetite (HM) buffers. In the range 680–850 °C, the crystalline assemblageon the QFM buffer is dominantly hornblende+ plagioclase, ±ilmenite, magnetite, sphene, fayalitic olivine, and phlogopiticmica. From 875 to 1000 °C the crystalline assemblage ishornblende+ olivine± augite+ ilmenite± magnetite.A melt phase is present from 700 to 1000 °C; a vapor phasewas present in all charges. The hornblendes formed on the QFM buffer range in compositionfrom common green hornblendes at low temperatures to kaersutitichornblendes at 1000 °C. A1(IV) and Ti increase temperature.AI(VI) passes through a maximum near 825 °C, decreasingboth above and below this temperature. AI(IV) is proportionalto the sum A1(VI)+2Ti. There is a positive linear correlationof approximately 3 : 1 between AI(IV) and the number of cationsin the A-site. The most likely explanation for this correlationat present is that the substitution of AI(VI) or Ti⁺⁴for a divalentcation creates local charge imbalances in the amphibole structurewhich can be compensated only by further A-site substitution.There also appears to be a correlation between the a-cell dimensionof hornblende and the A-site occupancy. Above a thresh holdvalue of approxmately 0.5 cations in A, a increases as A-siteoccupancy increases. Phase relations on the hematite-magnetite buffer are considerablysimpler. The hornblendes show relatively little change in compositionas temperature increases, and in the tholelitic compositionsbreak down at or below 970 °C 35–60 °C above thefirst appearance of augite±olivine. The melting of hornblendeis incongruent in all cases. The Fe-Ti oxides are pseudo-brookiteand titanohematite; at 1000 °C these oxides make up 10 percent by weight of the assemblage and contain most of the Tio₂and FeO in the charge. The patterns of hornblende variation observed in this studycompare closely with those reported in a wide range of experimentaland field data. The appearance of high-TiO₂ kaersutitic hornblendesin the tholeities at 1000° C, P_H2O= 5 kb on the QFM bufferimplies that the restricted occurence of kaersutite in nature(where it is associated only with mafic to intermediate alkalicrocks) is controlled by volatile content (H₂O_,F₂)rather thanby differences in condensed bulk composition.     

Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb

The melting relations of five metamorphosed basalts and andesites(greenstones and amphibolites), collected from the late JurassicSmartville arc complex of California, were investigated experimentallyat 800–1000? C and 1, 3, and 6. 9 kb. Dehydration-melting(no water added) experiments contained only the water structurallybound in metamorphic minerals (largely amphiboles). They yieldedmildly peraluminous to metaluminous granodioritic to trondhjemiticmelts (Na/K is a function of starting composition) similar inmajor element composition to silicic rocks in modern oceanicarcs. The dehydration melts are water-undersaturated, with,and coexist with the anhydrous residual solid (restite) assemblageplagioclase + orthopyroxene + clinopyroxene + magnetite ? ilmen-ite,with plagioclase constituting 50% of the restite mode. In thedehydration-melting experiments at 3 kb the onset of meltingoccurred between 850 and 900 ? C, as amphibole and quartz brokedown to yield pyroxenes plus melt. Total pressure is greaterthan in the dehydration-melting experiments and has little effecton melt composition or phase relations. In the water-saturated (water added, so that experiments, meltsformed at 3 kb and above are strongly peraluminous, rich inCa and poor in Fe, Mg, Ti, and K. Their compositions are unlikethose of most silicic igneous rocks. These melts coexist withthe amphibole-rich, plagioclase-poor restite assemblage amphibole+ magnetite ? clinopyroxene ? plagioclase ? ilmenite. The highlyaluminous nature of the melts and the plagioclase-poor natureof the restite both reflect the substantial contribution ofplagioclase (along with quartz) to melts in high-pressure water-saturatedsystems. Water pressure equals P_toul in the water-saturatedexperiments and has a profound effect on both melt compositionand phase relations. At 1 kb, the water-saturated experimentsyielded melt and mineral products with some characteristicsof the dehydration-melting experiments (no amphibole at highT), and some characteristics of the 3-kb, water-saturated experiments(amphibole plus melt coexisting at lower T, elevated Al, loweredFe). As pressure is increased from 3 to 6. 9 kb, the stabilityfields of both plagioclase and clinopyroxene decrease relativeto amphibole and the Al contents of the melts increase. These experiments have important implications for the petrogenesisof low-K silicic rocks in arcs. First, dehydration melting isa viable mechanism for the formation of these rocks; water-saturatedmelting is not. Second, because of the influence of rock compositionon melt composition, low-grade metamorphic and hydrothermalprocesses that alter the alkali contents and Na/ K in arc basementterranes may have a direct impact on the petrogenesis of silicicmagmas in arcs, particularly the formation of extremely low-Ktrondhjemites. Third, the experiments predict that anhydrous,pyroxene- and plagioclase-rich ‘granulitic’ restiteassemblages should develop as a result of partial melting inarc terranes. Such assemblages occur in at least two deeplyeroded arc complexes.     

Hydration and phase relations of grossular-spessartine garnets at P H 2O=2 Kb

The phase relations in the system grossular-spessartine-H₂O were investigated at 2.0 Kb aqueous fluid pressure and at subsolidus temperatures down to 420 ° C. Despite metastable persistence of a compositional gap found in some intermediate members, a complete solid solution between grossular and spessartine exists.Linear relations between the unit cell edge, a ₀, and composition were readily observed down to 620 ° C with a ₀=11.849(2) Å and 11.613(2) Å for grossular and spessartine, respectively. Hydrated garnets began to appear at higher temperature for the Ca-rich members. Grossular and spessartine formed at 420 ° C have a ₀=11.901(2) Å and 11.632(2) Å, indicating the presence of 0.6 and 0.2 mol H₂O, respectively. Intermediate members show varying degrees of hydration. Infrared spectra of the more hydrated members show a major and minor absorption bands at 3,620 cm^–1 and 3,660 cm^–1, respectively, in addition to a broad band around 3,430 cm^–1. All the hydrogarnets formed at 420 ° C were proven to be metastable.The rare occurrence of the intermediate grossular-spessartine garnets may be attributed to the lack of appropriate bulk chemistry of the rock rather than to the P-T conditions to which the rock is subjected. There may be a stability field for hydrogrossular below 420 ° C at 2 Kb, but not for hydrospessartine. Any occurrence of hydrogarnet may be used as a temperature indicator setting the maximum of formation for the hydrogarnet-bearing assemblage below 420 ° C at 2 Kb.     

黑云母片麻岩—H2O系统在0.1—0.2GPa压力下的熔融实验

本文提出了冀东黑云母片麻岩—H2O系统在0.1一0.2GPa压力下熔融买验的相关系。其固相线温度分別为0.1GPa时762℃,0.2GPa时712℃。黑云母消失的温度分別为0.1GPa时787℃,0.2GPa时737℃,石英消失的温度分别为0.1GPa时837.℃,0.2GPa时787.℃。采用Burn-ham模型计算的在液相线温度下岩浆饱和水的含量分別为0.1GPa时3.8%与0.2GPa时5.8%。根据实验结果以及早前寒武纪时冀东陆壳的古地温可知,该区早前寒武纪角阿岩相岩石分布的地区广泛出现的混合岩化作用应主要归因于陆壳岩石(黑云母片麻岩等)的局部熔融作用。由实验结果以及现代冀东陆壳的地温可推知,壳内低速层可能不是由岩石局部熔融所引起,而是由岩石中含有隙间水流体引起。     

Melting relations of some oceanic basalts

The melting relations of a group of oceanic basalts—olivine basalts, tholeiitic to midly alkaline, and high-alumina tholeiites are determined. The liquidi of these rocks follow closely the course of the temperature—iron enrichment relationship of Kilauean thoeliitic basalts, ranging from 1430°C for an olivine enriched tholeiite, to a tholeiite of liquidus temperature of 1190°C, the liquidus phase of the assemblages being either olivine or plagioclase.     

Solid solubility of Al2O3 in enstatite at high temperatures and 1–5 kb water pressure

Solid solubility of Al₂O₃ in orthorhombic enstatite by the substitution AlAl=MgSi is, in the range studied, mainly a function of temperature and not strongly pressure-dependent. Even at 1 kb up to 9 wt.-% Al₂O₃ can be substituted at 1200° C. The thermal stability of the orthorhombic pyroxene phase is strongly increased by the incorporation of Al.In crustal rocks the alumina content of orthopyroxene might be used as a geothermometer but not, as sometimes suggested, as a barometer.     

P CO2 in low-grade metamorphism; zeolite,carbonate, clay mineral,prehnite relations in the system CaO-Al2O3-SiO2-CO2-H2O

Equilibria for several reactions in the system CaO-Al₂O₃-SiO₂-CO₂-H₂O have been calculated from the reactions calcite+quartz=wollastonite+CO₂ (5) and calcite+Al₂SiO₅+quartz=anorthite+CO₂ (19) and other published experimental studies of equilibria in the systems Al₂O₃-SiO₂-H₂O and CaO-Al₂O₃-SiO₂-H₂O.The calculations indicate that the reactions laumontite+CO₂=calcite+kaolinite+2 quartz+2H₂O (1) and laumontite+calcite=prehnite+quartz+3H₂O+CO₂ (3) in the system CaO-Al₂O₃-SiO₂-CO₂-H₂O, are in equilibrium with an H₂O-CO₂ fluid phase having -0.0075 for P _fluid=P _total=2000 bars.These calculations limit the stability of zeolite assemblages to low p CO₂.Using the above reactions as model equilibria, several probelms of p CO₂ in low grade metamorphism are discussed. (a) the problem of producing zeolitic minerals from metasedimentary assemblages of carbonate, clay mineral, quartz. (b) the significance of calcite (or aragonite) associated with zeolite (or lawsonite) in low grade metamorphism and hydrothermal alteration. (c) the reaction of zeolites (or lawsonite) with calcite (or aragonite) to produce dense Ca-Al-hydrosilicates (eg. prehnite, zoisite, grossular).     

Melting and subsolidus reactions in the system K2O-CaO-Al2O3-SiO2-H2O

Beginning of melting and subsolidus relationships in the system K₂O-CaO-Al₂O₃-SiO₂-H₂O have been experimentally investigated at pressures up to 20 kbars. The equilibria discussed involve the phases anorthite, sanidine, zoisite, muscovite, quartz, kyanite, gas, and melt and two invariant points: Point [Ky] with the phases An, Or, Zo, Ms, Qz, Vapor, and Melt; point [Or] with An, Zo, Ms, Ky, Qz, Vapor, and Melt.The invariant point [Ky] at 675° C and 8.7 kbars marks the lowest solidus temperature of the system investigated. At pressures above this point the hydrated phases zoisite and muscovite are liquidus phases and the solidus temperatures increase with increasing pressure. At 20 kbars beginning of melting occurs at 740 °C. The solidus temperatures of the quinary system K₂O-CaO-Al₂O₃-SiO₂-H₂O are almost 60° C (at 20 kbars) and 170° C (at 2kbars) below those of the limiting quaternary system CaO-Al₂O₃-SiO₂-H₂O.The maximum water pressure at which anorthite is stable is lowered from 14 to 8.7 kbars in the presence of sanidine. The stability limits of anorthite+ vapor and anorthite+sanidine+vapor at temperatures below 700° C are almost parallel and do not intersect. In the wide temperature — pressure range at pressures above the reaction An+Or+Vapor = Zo+Ms+Qz and temperatures below the melting curve of Zo+Ms+Ky+Qz+Vapor, the feldspar assemblage anorthite+sanidine is replaced by the hydrated phases zoisite and muscovite plus quartz. CaO-Al₂O₃-SiO₂-H₂O. Knowledge of the melting relationships involving the minerals zoisite and muscovite contributes to our understanding of the melting processes occuring in the deeper parts of the crust. Beginning of melting in granites and granodiorites depends on the composition of plagioclase. The solidus temperatures of all granites and granodiorites containing plagioclases of intermediate composition are higher than those of the Ca-free alkali feldspar granite system and below those of the Na-free system discussed in this paper.The investigated system also provides information about the width of the P-T field in which zoisite can be stable together with an Al₂SiO₅ polymorph plus quartz and in which zoisite plus muscovite and quartz can be formed at the expense of anorthite and potassium feldspar. Addition of sodium will shift the boundaries of these fields to higher pressures (at given temperatures), because the pressure stability of albite is almost 10kbars above that of anorthite. Assemblages with zoisite+muscovite or zoisite+kyanite are often considered to be products of secondary or retrograde reactions. The P-T range in which hydration of granitic compositions may occur in nature is of special interest. The present paper documents the highest temperatures at which this hydration can occur in the earth's crust.     

Melting relations of some ultra alkali volcanics

The melting relations of a group of ultra alkali lavas are determined. They include a range of types rich in melilite and nepheline (melilite nephelinites) from the Nyiragongo volcano of the Congo and a group rich in olivine (olivine nephelinites). The liquidi of these assemblages range from 1100°C (the melilite nephelinite of the lava lake of Nyiragongo) through the Cameroons etindite (clinopyroxene nephelinite, 1118°C to 1300°C for the olivine nephelinites. For the most part the liquidi follow a course dependent on iron-enrichmcnt. Notable departures are examples rich in leucite.     

过氧化钠超强熔矿能力的新认识

过氧化钠熔融能快速分解许多热稳定性高,化学稳定性好,十分难溶（熔）的矿物、金属、合金、碳化物、氮化物和炉渣等。经长期大量熔矿、测试实验研究认为：过氧化钠超强熔矿能力归因于它在520℃～640℃熔融时,释放出原子氧,将物料中许多元素迅速氧化至最高价态;这些高价离子立即与氧形成络合物;过氧化钠熔融物的强碱性,有利于这些高价离子可熔性络合物的形成和稳定,致使物料的晶体构造、结构彻底破坏,而被完全分解。     

Melting of a Hydrous Mantle: III. Phase Relations of Garnet Websterite+H2O at High Pressures and Temperatures

A garnet websterite nodule from the Honolulu volcanic series,Oahu, Hawaii, has been melted in the presence of nearly pureH₂O. The solidus is intermediate between that of peridotiteand gabbro. The curve displays a temperature minimum around20 kb reflecting the breakdown of plagioclase. The Iiquidusis between 1130 ?C and 1150 ?C between 10 and 20 kb vapor pressure.Amphibole (pargasitic hornblende) has an extensive stabilityfield, reaching a maximum temperature about 20 ?C below thegarnet websterite liquidus at 15 kb and a maximum pressure of27.5 kb at 950 ?C. The amphibole-out curve intersects the soliduswith a positive slope. Liquids formed by partial melting of garnet websterite are quartz-normativewithin the stability field of amphibole, but become olivine-normative(tholeiitic) with increasing temperature. Amphibole and clinopyroxeneare enriched in Tschermak's molecule at higher temperatures,pargasite content of amphibole increases with increasing pressure. A garnet websterite-rich upper mantle containing modal olivineyields quartz-normative (13–16 per cent), aluminous (21–4wt. per cent A1₂O₃) melts at 17 P 10 kb and in the presenceof nearly pure H₂O. However, the presence of amphibole controlsthe liquid composition, a situation not found for liquids formedfrom wet peridotite. In contrast to many basalt liquids, liquidof garnet websterite composition cannot fractionate to andesiteby precipitation of amphibole, as amphibole is not a liquidusphase.     

Genesis of Peraluminous Granites I. Experimental Investigation of Melt Compositions at 3 and 5 kb and Various H2O Activities

Melting experiments have been performed on a peraluminous quartzo-feldspathicgneiss from Northern Portugal. This gneiss occurs as xenolithsin the Tourem anatectic complex and is the most probable sourcerock for the surrounding anatectic granites. On the basis offield, petrological, and geochemical data, it can be shown thatanatexis took place in the stability field of cordierite + biotiteand that the evolution of magmas is the result of processesinvolving segregation of partial melt and separation of restiteminerals. Experiments were performed at 700, 750, and 800 ?C, 3 and 5kb, and various H₂O activities (aH₂O) to clarify the influenceof aH₂O and melt fraction on the composition of the generatedmelts. Biotite and cordierite are the two main ferromagnesianphases observed in the run products. Cordierite is formed byincongment melting of biotite. For relatively low melt fractions (< 30–35 wt. %),the partial melts coexisting with quartz, alkali feldspar, andplagioclase have a minimum or near-minimum melt composition.The melts become richer in potassium with decreasing aH₂O. Thisresult using a natural rock as starting material is in goodagreement with results achieved in the synthetic Qz-Ab-Or system.In the stability field of biotite and cordierite, the influenceof aH₂O on melt composition is at least as important as theeffect of changing P and/or T. For higher melt fractions the composition of the melt is stronglycontrolled by the disappearance of alkali feldspar and the meltsbecome richer in An and poorer in Or with increasing degreeof melting. The wide range of compositions (especially for K₂O, which variesfrom 3.5 to 5.4%) observed in the experimental peraluminousmelts demonstrates that a wide variety of granitoid magmas maybe produced from the same source rocks. The application of theexperimental results to the Tourem anatectic complex shows thatmelting occurred at H2O-undersaturated conditions (4–5wt. % H₂O in the melts, corresponding to aH₂O of {small tilde}0.5at 5 kb). Experimental melts similar in composition to the mostleucocratic granite of the Tourem anatectic complex (consideredto approximate the composition of the generated melt) were obtainedaround 800 ?C, suggesting that this temperature was attainedduring the peak of anatexis.     

关于红黏土先期固结压力的探讨

土力学传统理论认为,土的先期固结压力受控于应力历史,与曾经受到的压力有关。然而,通过室内试验发现,红黏土这类区域性特殊土的先期固结压力所表现出的规律与传统理论相悖。研究表明,红黏土的先期固结压力主要受土的成因、结构和成分制约,上覆压力对其影响甚微。笔者分析了土力学传统观点在解释红黏土特性时存在的缺陷,探讨了红黏土先期固结压力的真正含义,提出了对红黏土先期固结压力的新认识。     

Synthesis and stability relations of wairakite,CaAl2 Si4 O12·2H2O

Hydrothermal investigation of the bulk composition CaO·Al₂O₃·4SiO₂ + excess H₂O has been conducted using conventional techniques over the temperature range 200–500° C and 500–5,000 bars P _fluid. The fully ordered wairakite was synthesized unequivocally in the laboratory, probably for the first time.The gradual, sluggish and continuous transformation from disordered to ordered wairakite evidently accounts for failure by previous investigators to synthesize ordered wairakite in runs of week-long duration. The dehydration of metastable disordered wairakite to metastable hexagonal anorthite, quartz and H₂O has been determined; this reaction takes place at temperatures exceeding 400° C, even at fluid pressures of 500 bars or less. The upper P _fluid-T boundary of the disordered phase is equivalent to the maximum temperature curve of synthetic wairakite presented by previous investigators. The hydrothermal breakdown of natural wairakite above its stability limit appears to be a very slow process.The equilibrium dehydration of wairakite to anorthite, quartz and H₂O occurs at 330±5° C at 500 bars, 348±5° C at 1,000 bars, 372±5° C at 2,000 bars and 385±5° C at 3,000 bars. Where fluid pressure equals total pressure, the thermal stability range of wairakite is about 100° C wide. At lower temperatures wairakite reacts with H₂O to form laumontite. Reconnaissance experiments dealing with the effect of CO₂ on stabilities of calcium zeolites suggest that wairakite or laumontite may be replaced by the assemblage calcite + montmorillonite in the presence of a CO₂-bearing fluid phase.The determined P _fluid -T field of wairakite is compatible with field observations in some metamorphic terrains where it is related to the shallow emplacement of granitic magma and with direct pressure-temperature measurements in certain active geothermal areas. Under inferred conditions of higher _CO2/_H2O ratios, essentially unmetamorphosed rocks grade directly into those characteristic of the greenschist facies; moderately high values of _CO2 in carbonate-bearing rocks result in the downgrade extension of the greenschist facies at the expense of zeolite-bearing assemblages.     

Phase relations in the system NaAlSi3O8-KAlSi3O8-CaAl2Si2O8-SiO2 at 1 kilobar water vapour pressure

Crystal-melt relations at a water vapour pressure of 1 kilobar have been determined for planes at 3, 5, 7.5, and 10 weight per cent anorthite in the system NaAlSi₃O₈KAlSi₃O₈-CaAl₂Si₂O₈-SiO₂. The ratio of the silicate components in the liquids which are in univariant equilibrium with plagioclase, alkali feldspar, quartz and gas are Ab₃₁Or₂₈Q₃₈An₃ (weight per cent) at 730°±5–10° C, Ab₂₁Or₃₄Q₄₀An₅ at 745°±5–10° C and Ab₁₀Or₃₉ Q_43.5An_7.5 at 780°±10° C. The univariant curve on which the above compositions lieoriginates on the H₂O-saturated Or-An-Q plane at a composition containing less than 10 weight per cent An and terminates within 1.5 weight per cent An of the H₂O-saturated Or-Ab-Q plane. Experimental data for the synthetic system have been used to illustrate a discussion on the partial melting of metasediments and the possible significance of such a process with respect to the genesis of granitic rocks. Data taken from the literature (Winkler and v. Platen, 1960, 1961a) have been used to illustrate that the normative salic composition of a sediment has a strong influence on the composition of any melt which form when such a rock is subjected to high temperatures and pressures.     

Stability relations of K2Mg5Si12O30, and end member of the merrihueite-roedderite group of meteoritic minerals

The phase K₂Mg₅Si₁₂O₃₀ was synthesized both hydrothermally and dry under a variety of pressures and temperatures, and its stability relations were determined. Under hydrothermal conditions it exhibits a lower stability limit lying at 595°C, 1 kb, and 650°C, 2 kb, due to its breakdown into the hydrous assemblage quartz+KMg_2.5Si₄O₁₀(OH)₂ (a mica phase). Its upper temperature stability under hydrothermal conditions is given by its incongruent melting to MgSiO₃+liquid. Near 820° C at a fluid pressure of approximately 6.5 kb the two univariant curves for these breakdown reactions intersect thus limiting the stability field to lower fluid pressures. — Under anhydrous conditions K₂Mg₅Si₁₂O₃₀ becomes unstable at pressures between approximately 7 and 32.5 kb due to its incongruent melting to the assemblage MgSiO₃+quartz (or coesite)+liquid; this melting curve has a pronounced negative slope. No subsolidus breakdown assemblage was encountered at 32.5 kb down to temperatures as low as 750°C. This behavior is probably due to the instability of other ternary compounds in the system K₂O-MgO-SiO₂ at high pressures and thus to the existence of very low-temperature eutectics involving only binary and unary solid phases plus liquid.It is likely that these stability relations provide a model for those of the natural minerals merrihueite and roedderite which contain Na and Fe₊₂ partly substituting for K and Mg and which were encountered in several meteorites. Therefore, the cosmic events leading to the formation of these minerals must have taken place at relatively low pressures and high temperatures, especially when water was present. The bulk compositions of these minerals appear to be incompatible with average chondritic matter under equilibrium conditions. Hence merrihueite and roedderite are not likely to be found in equilibrated chondrites which contain feldspars instead.     

The stability and phase relations of iron chlorite below 8.5 kb P_{{\text{H}}_{\text{2}} {\text{O}}}

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