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.     

Mikrosondenuntersuchungen von Saussuriten in Abhängigkeit vom Metamorphosegrad und vom geochemischen Milieu

Zusammenfassung In einem Profil ansteigender Metamorphose werden im Stavanger-Gebiet/SW-Norwegen zwei Gesteinstypen, repräsentiert durch 17 Meta-Arkosen und Quarz-Feldspat-Gneise, sowie 19 metamorphe Tuffe und Tuffite, modellhaft auf ihre Saussuritparagenesen hin analysiert.Mikroskopische und mikroanalytische Untersuchungen zeigen eine weitestgehende Eduktabhängigkeit der Saussuritparagenesen. In calcitarmen granitoiden Edukten bilden sich schon im sehr schwach-metamorphen Stadium folgende mit Ab-reichem Plagioklas koexistierende Saussuritmineralien: 4 An-reicher Plagioklas+K-Feldspat+4 Eisenhydroxid + H₂O 2 Fe(III)Al₂-Epidot+Muskovit+Hämatit+2 Quarz. In calcitführenden Meta-Arkosen gewinnt die Mineralreaktion: An-Plagioklas+K-Feldspat+2 Eisenhydroxid+CO₂Calcit+Muskovit+ Hämatit+Quarz an Bedeutung. Im reduzierenden geochemischen Milieu der Meta-Tuffe und -Tuffite tritt in diesem metamorphen Bereich als charakteristische Neubildung Biotit neben Muskovit auf. Auch in den calcitreichen tuffitischen Proben koexistieren die Saussuritkomponenten Calcit, Muskovit, Hämatit, Quarz und Biotit.Die mikroanalytisch untersuchten Saussuritmineralien Plagioklas und Epidot zeigen anhand von annähernd 800 chemischen Vollanalysen in der Entwicklung ihrer Chemismen während der Metamorphose für beide Sedimente gleiche Trends. Im niedrigtemperierten Bildungsstadium der Saussurite können die An-Gehalte der Feldspäte in Abhängigkeit vom Edukt und aufgrund unterschiedlich intensiver Rekristallisation der Sedimente relativ stark schwanken. Die Epidote sind je nach primärem Fe(III)-Angebot mehr oder weniger eisenreich. In Richtung ansteigender Metamorphose stellen sich in den Epidoten beim Übergang zu oligoklasführenden Paragenesen maximale Fe(III)-Gehalte ein, die in Form eines rhythmischen Zonarbaus die wechselnden -Bedingungen im Verlauf der Metamorphose widerspiegeln. Bereits hier setzt der retrograde Saussuritisierungsprozeß ein: es reagieren Fe(III)Al₂-Epidot, Muskovit, Quarz, Hämatit und Albit/Oligoklas zu anorthitreicherem Plagioklas, Biotit und Wasser. Zusätzlich bildet sich durch die Zersetzung des Al-reichen Epidots Mikroklin.Die Untersuchung zur Wechselbeziehung zwischen dem Eisengehalt des Eduktes und dem der Epidote ergibt, daß die Fe(III)-Einbaurate der Epidote in keiner Weise von den Fe(III)-Konzentrationen der Meta-Sedimente beeinflußt wird, vielmehr können eduktspezifische Intervalle für die Eisenanreicherung der Epidote nachgewiesen werden.

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Electron microprobe investigations of saussurites dependent on metamorphic stage and geochemical environment

In a profile of ascending metamorphism, the saussuritic parageneses of two characteristic rock types, 17 acid meta-arkoses and 19 basic meta-tuffs and meta-tuffites, were investigated by microscopical and microanalytical methods.Microscopic studies reveal that the specific saussurite mineral association is dependent on the petrographic substratum. In very low stage metamorphism, granitic rocks show the following saussurite minerals coexisting with albite-rich plagioclase: 4 An-rich plagioclase+K-feldspar +4 ironhydroxide+H₂O2 Fe (III) Al₂-epidote + muscovite + hematite + 2 quartz. Phyllitic and calciferous meta-arkoses exhibit predominantly the following saussurite reaction: An-plagioclase + K-feldspar + 2 ironhydroxide + CO₂ calcite + muscovite + hematite + quartz. In the reducing environment of the meta-tuffs and meta-tuffites, biotite is formed in addition to muscovite.The changing chemical composition of the saussurite minerals (plagioclase and epidote) at different stages of metamorphism was determined by 800 quantitative electron microprobe analyses. The changes found in both rock types are nearly the same. During low metamorphic stage, the An-content of the plagioclases depends on the substratum and the variable intensity of recrystallization. The Fe-content of the epidotes is dependent on the amount of iron available.The epidotes are enriched in Fe toward the transition to oligoclase-bearing parageneses. These Fe-contents cause rhythmical zoning, representing varying conditions during metamorphism. With increasing metamorphic grade the retrograde process of saussuritization is initiated. The reactants are Fe (III) Al₂-epidote, muscovite, quartz, hematite, and albite/ oligoclase. The decomposition of these minerals leads to An-rich plagioclase, biotite, microcline, and H₂O.

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Herrn Prof. Dr. G. Müller, Clausthal, danke ich für viele anregende Diskussionen sowie für die kritische Durchsicht des Manuskriptes.

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Mein Dank gilt auch Herrn Dr. A. Schneider, Geochemisches Institut der Universität Göttingen, der mir freundlicherweise das Rucklidge-Korrekturprogramm für Silikatanalysen zur Verfügung stellte. Zu danken habe ich auch Herrn Dr. F. Wurm, Geologisches Landesamt Baden-Württemberg, für sein großzügiges Entgegenkommen und die tatkräftige Unterstützung bei der ergänzenden Probennahme im Gebiet der nordwestlichen Inseln des Boknfjords.

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Der Deutschen Forschungsgemeinschaft schulde ich Dank für die finanzielle Unterstützung dieser Arbeit.     

The restricted stability of osumilite under hydrous conditions in the system K2O-MgO–Al2O3–SiO2–H2O

Osumilite_ss was synthesized as a single phase product in the model system K₂O-MgO-Al₂O₃-SiO₂ at 800° C/ 0.5 Kbar water pressure and at 800° to 840° C/1.0 Kbar total pressure with 0.3 in the gas phase. The experimentally determined solid solubility range of synthetic osumilites can be expressed by the formula KMg₂(Al_3-xMg_x) (Al_2–xSi_10+x)O₃₀ with 0x0.4. A survey of sixteen chemical analyses of natural osumilites from eleven occurrences shows a solid solubility characterized by 0x0.6. Reversed stability experiments for the synthetic osumilite KMg₂(Al_2.75Mg_0.25)(Al_1.75Si_10.25)O₃₀ determined at water pressure equal to total pressure demonstrate its restriction to water pressures below 0.8 Kbar (at 0.5 Kbar, the stability range is between 765° and 800° C). At the lower thermal stability limit osumilite+H₂O vapor break down to cordierite+K feldspar+phlogopite_ss+quartz, at the higher one to cordierite+K feldspar+phlogopite+liquid. Reduction of water fugacity will expand the stability field largely by shifting the lower and higher thermal stability limits to lower and higher temperatures, respectively. The dependence of osumilite stability on water fugacity makes osumilite a sensitive indicator mineral for dry conditions in rocks formed at total pressures higher than about 0.8 Kbar.     

Relationship between fluid-bearing and fluid-absent invariant points and a petrogenetic grid for a greenschist facies assemblage in the system CaO-MgO-Al2O3-SiO2-CO2-H2O

In the 6 component system CaO-MgO-Al₂O₃-SiO₂-CO₂-H₂ with 9 solid phases (quartz, plagioclase, epidote, tremolite, talc, chlorite, magnesite, calcite, dolomite) and a fluid phase, all 17 possible fluid-absent reactions have been set up and balanced. Using molar entropy and volume data for the solid phases, these reactions are arranged in P-T space about the 8 possible fluid-absent invariant points after the method of Schreinemakers. Field observations in Ordovician greenschist facies basic volcanics at Sofala N.S.W., indicate that neither talc+epidote nor magnesite+calcite are stable under the conditions of metamorphism. Assuming these conditions to apply to the theoretical study here, the fluid-absent invariant points are arranged in a relative fashion with fluid-absent reactions subdividing P-T space into smaller areas.A scheme which permits a fluid of composition (i.e. a fluid containing CO₂ and H₂O together with other components), is modeled by treating H₂O as a mobile component independent of CO₂, and by allowing values that lie off the locus of binary H₂O-CO₂. Taking into account that neither talc+epidote nor magnesite +calcite is to be permitted, the fluid scheme is used to set up and balance all 39 possible fluid-bearing reactions. These are then arranged about 20 valid fluid-bearing invariant points in space after the method of Korzhinskii and Sehreinemakers.A characteristic solid phase assemblage is defined for each P-T area using chemographic relations inherent from the fluid-absent boundary reactions. The fluid-bearing invariant points that have a solid assemblage compatible with the characteristic assemblage in a particular P-T area are stable within the P-T regime of that area. When these stable fluidbearing invariant points are arranged in a relative fashion in space, they outline a fluid grid which can be used to study the possible effects of local variation in X _fluid over the particular P-T regime.Symbols Used U chemical potential - S entropy - V molar volume - n coefficient of a phase in a reaction - X mole fraction - T temperature - P pressure - F number of degrees of freedom - C number of components - p number of phases - s solid - slope of reaction - 1 quartz - 2 plagioclase - 3 epidote - 4 tremolite - 5 talc - 6 chlorite - 7 dolomite - 8 magnesite - 9 calcite     

Stabilitätsbeziehungen zwischen Chlorit,Cordierit und Almandin bei der Metamorphose

The stability relations between cordierite and almandite in rocks, having a composition of CaO poor argillaceous rocks, were experimentally investigated. The starting material consisted of a mixture of chlorite, muscovite, and quartz. Systems with widely varying Fe²⁺/Fe²⁺+Mg ratios were investigated by using two different chlorites, thuringite or ripidolite, in the starting mixture. Cordierite is formed according to the following reaction: $${\text{Chlorite + muscovite + quartz}} \rightleftharpoons {\text{cordierite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} + {\text{H}}_{\text{2}} {\text{O}}$$ . At low pressures this reaction characterizes the facies boundary between the albite-epidotehornfels facies and the hornblende-hornfels facies, at medium pressures the beginning of the cordierite-amphibolite facies. Experiments were carried out reversibly and gave the following equilibrium data: 505±10°C at 500 bars H₂O pressure, 513±10°C at 1000 bars H₂O pressure, 527±10°C at 2000 bars H₂O pressure, and 557±10°C at 4000 bars H₂O pressure. These equilibrium data are valid for the Fe-rich starting material, using thuringite as the chlorite, as well as for the Mg-rich starting mixture with ripidolite. At 6000 bars the equilibrium temperature for the Mg-rich mixture is 587±10°C. In the Fe-rich mixture almandite was formed instead of cordierite at 6000 bars. The following reaction was observed: $${\text{Thuringite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Experiments with the Fe-rich mixture, containing Fe²⁺/Fe²⁺+Mg in the ratio 8∶10, yielded three stability fields in a P,T-diagram (Fig.1):

1. Above 600°C/5.25 kb and 700°C/6.5 kb almandite+biotite+Al₂SiO₅ coexist stably, cordierite being unstable.
2. The field, in which almandite, biotite and Al₂SiO₅ are stable together with cordierite, is restricted by two curves, passing through the following points:
   1. 625°C/5.5 kb and 700°C/6.5 kb,
   2. 625°C/5.5 kb and 700°C/4.0 kb.
3. At conditions below curves 1 and 2b, cordierite, biotite, and Al₂SiO₅ are formed, but no garnet.

An appreciable MnO-content in the system lowers the pressures needed for the formation of almandite garnet, but the quantitative influence of the spessartite-component on the formation of almandite could not yet be determined. the Mg-rich system with Fe²⁺/Fe²⁺+Mg=0.4 garnet did not form at pressures up to 7 kb in the temperature range investigated. Experiments at unspecified higher pressures (in a simple squeezer-type apparatus) yielded the reaction: $${\text{Ripidolite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Further experiments are needed to determine the equilibrium data. The occurence of garnet in metamorphic rocks is discussed in the light of the experimental results.     

Systematic study of liquidas phase relations in olivine melilitite +H2O +CO2 at high pressures and petrogenesis of an olivine melilitite magma

Near-liquidus phase relationships of a spinel lherzolite-bearing olivine melilitite from Tasmania were investigated over a P, T range with varying , , and . At 30 kb under MH-buffered conditions, systematic changes of liquidus phases occur with increasing ( = CO₂/CO₂ +H₂O+olivine melilitite). Olivine is the liquidus phase in the presence of H₂O alone and is joined by clinopyroxene at low . Increasing eliminates olivine and clinopyroxene becomes the only liquidus phase. Further addition of CO₂ brings garnet+orthopyroxene onto the liquidus together with clinopyroxene, which disappears with even higher CO₂. The same systematic changes appear to hold at higher and lower pressures also, only that the phase boundaries are shifted to different . The field with olivine- +clinopyroxene becomes stable to higher with lower pressure and approaches most closely the field with garnet+orthopyroxene+clinopyroxene at about 27 kb, 1160 °C, 0.08 and 0.2 (i.e., 6–7% CO₂+ 7–8% H₂O). Olivine does not coexist with garnet+orthopyroxene+clinopyroxene under these MH-buffered conditions. Lower oxygen fugacities do not increase the stability of olivine to higher and do not change the phase relationships and liquidus temperatures drastically. Thus, it is inferred that olivine melilitite 2927 originates as a 5% melt (inferred from K_{2 O} and P₂O₅ content) from a pyrolite source at about 27kb, 1160 dg with about 6–7% CO₂ and 7–8% H₂O dissolved in the melt. The highly undersaturated character of the melt and the inability to find olivine together with garnet and orthopyroxene on the liquidus (in spite of the close approach of the respective liquidus fields) can be explained by reaction relationships of olivine and clinopyroxene with orthopyroxene, garnet and melt in the presence of CO₂.     

Fe2+-Mg2+ partition between coexisting cordierite and garnet — a discussion of the experimental data

The partition of iron and magnesium between cordierite and garnet depends on as well as temperature. The apparently conflicting experimental data on the values of K _D may be reconciled by considering the pertaining during the different experiments.     

Fe-Mg cordierite stability in high-grade pelitic rocks based on experimental,theoretical, and natural observations

Stability relations of Fe-Mg cordierite with K feldspar have been determined for conditions of muscovite-quartz instability, applicable to highgrade metamorphism of pelitic rocks. Fe cordierite, K feldspar, and water break down to Fe biotite, sillimanite, and quartz at pressures above a line through 640 ° C, 2kbar and 710 ° C, 2.7 kbar. A P-X diagram for the Fe-Mg analogue of this reaction at 675 ° C is consistent with a naturally occuring cordierite-biotite K _D value of 0.53 if Al content of biotite and cordierite water of hydration are taken into account.At higher temperatures Fe cordierite breaks down alone to almandine, sillimanite, quartz and water at pressures above a line through 650 ° C, 3.41 kbar and 760 ° C, 2.9 kbar. For the Fe-Mg reaction, P-X data up to 4 kbar may be extrapolated with use of natural K _D values increasing toward one with increasing temperatures.Lines of constant cordierite composition for the two reactions intersect in an Fe-Mg univariant reaction of sillimanite-biotite-quartz to cordieritealmandine-K feldspar-water which is metastable relative to melt at = P _tot Reduced water pressure and impurities in the garnet and K feldspar greatly reduce the temperature of this reaction so that it becomes a reasonable reaction for upper amphibolite and granulite facies conditions.The results demonstrate that (1) cordierite may be used as a geobarometer if temperature and approximate can be estimated, (2) almandine low in Mn and Ca does not participate in cordierite reactions where muscovite is present, and (3) the reaction which forms cordierite, almandine, and K feldspar is a possible melt-forming reaction which, under reduced , occurs about 50 ° C above the muscovite melting reaction.     

An almandine garnet isograd in the Rogers Pass area,British Columbia: The nature of the reaction and an estimation of the physical conditions during its formation

In the Rogers Pass area of British Columbia the almandine garnet isograd results from a reaction of the form: 5.31 ferroan-dolomite+8.75 paragonite+4.80 pyrrhotite+3.57 albite+16.83 quartz+1.97 O₂=1.00 garnet+16.44 andesine+1.53 chlorite+2.40 S₂+1.90 H₂O+10.62 CO₂. The coefficients of this reaction are quite sensitive to the Mn content of ferroan-dolomite.Experimental data applied to mineral compositions present at the isograd, permits calculation of two intersecting P, T equilibrium curves. P=29088–39.583 T is obtained for the sub-system paragonite-margarite (solid-solution), plagioclase, quartz, ferroan-dolomite, and P=28.247 T–14126 is obtained for the sub-system epidote, quartz, garnet, plagioclase. These equations yield P=3898 bars and T=638° K (365° C). These values are consistent with the FeS content of sphalerite in the assemblage pyrite, pyrrhotite, sphalerite and with other estimates for the area.At these values of P and T the composition of the fluid phase in equilibrium with graphite in the system C-O-H-S during the formation of garnet is estimated as: bars, bars, bars, bars, bars, bars, bars, bars, , bars, bars.     

A calculated petrogenetic grid for ultramafic rocks in the system CaO-FeO-MgO-Al2O3-SiO2-CO2-H2O at low pressures

Calculated phase equilibria among the minerals amphibole, chlorite, clinopyroxene, orthopyroxene, olivine, dolomite, magnesite, serpentine, brucite, calcite, quartz and fluid are presented for the system CaO–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O (CaF-MASCH), with chlorite and H₂O–CO₂ fluid in excess and for a temperature range of 440°C–600°C and low pressures. The minerals chosen in CaFMASCH represent the great majority of phases encountered in metamorphosed ultramafic rocks. The changes in mineral compositions in terms of FeMg_-1 and (Mg, Fe)SiAl_-1Al_-1 are related to variations in the intensive parameters. For example, equilibria at high in the presence of chlorite involve minerals which are relatively aluminous compared with those at low . The calculated invariant, univariant and divariant equilibria are compared with naturally-occurring greenschist and amphibolite facies ultramafic mineral assemblages. The correspondence of sequences of mineral assemblages and the compositions of the minerals in the assemblages is very good.     

Stability of the assemblage cordierite +K feldspar+quartz

Under hydrous conditions the stability field of the assemblage Mg-cordierite+K feldspar+quartz is limited on its low-temperature side by the breakdown of cordierite+K feldspar into muscovite, phlogopite and quartz, whereas the high-temperature limit is given by eutectic melting. The compatibility field of the assemblage ranges from 530° C to 745° C at 1 kbar , from 635 to 725° C at 3 kbars , from 695 to 725° C at 5 kbars and terminates at 5.5 kbars . Most components not considered in the model system will tend to restrict this field even more. However, the condition < P _total will increase the range of stable coexistence drastically, making the assemblage common at elevated temperatures from contact metamorphic rocks up to intermediate pressure granulites of appropriate bulk composition.     

Petrology of garnet — cordierite — sillimanite gneisses from the El Tormes thermal dome,Iberian Hercynian foldbelt (W Spain)

The core of the El Tormes thermal dome, situated in the central part of one of the main metamorphic belts of the Iberian Peninsula, is formed by garnet-cordierite-biotite-sillimanite pelitic gneisses. These rocks, that very often are cut by minor intrusions of Al-rich S-type granites, are metatexitic gneisses in which there exists garnet showing different stages of resorption and transformation into an aggregate of cordierite±plagioclase±biotite. The garnet, mantled and corroded mainly by cordierite, has never been found to occur in contact with the prismatic sillimanite of the matrix, thus indicating that the continuous reaction Gr+Sill+Q = Cd has taken place. The presence of corroded biotite inside the garnet-rimming cordierite of the aggregates as well as inside the cordierite of the matrix, which usually includes remmants of sillimanite, indicates that the continuous reaction Bi+Sill+Q = Cd+FK+H₂O has occurred too. Therefore, a realistic net reaction for these aggretates should be represented by the univariant, at a given , equilibrium: Biotite+Sillimanite+Garnet+Quartz = Cordierite+K-feldspar+H₂O (1)The important garnet resorption near the anatectic granitic veins implies that this process is favoured by a decrease in , this factor being otherwise buffered by the reaction (1) assemblage.The most probable P-T path, assuming these conditions, consistent with the AFM projection of the former (inferred) and present assemblages in the aggregates and in the matrix, implies a decrease in P coeval with a decrease in T (Fig. 4, path 2).The most reliable P-T determination for the final stage of garnet breakdown through reaction (1), based on the coexistence of the seven phase assemblage garnet — cordierite — biotite — sillimanite — plagioclase — potash feldspar — quartz plus melt, gives 695° C, 4.3 kbar, = 0.5, The maximum pressure for this process, obtained from the garnet — plagioclase equilibrium, is 6.5±1 kbar at the same temperature.The estimates of the T for the garnet core-garnet included biotite pairs are consistently lower, ca. 550° C, than those obtained for the garnet rim-biotite in aggregates, ca. 645° C, or garnet rim-adjacent cordierite pairs, ca. 695° C.It may, therefore, be supposed that, during their evolution these rocks underwent first an increase in T and then, during the last stages, as garnet and biotite brokedown, a decrease in P and T. This represents an uplift of the core of El Tormes dome under high grade amphibolite to low pressure granulite facies conditions, accompanied by a process of partial melting with local decrase in . It is suggested, from mineral growth-deformation relationships, that this process took place during the late hercynian deformation phases, P-3 or doming stage.     

Amphibole composition in tonalite as a function of pressure: an experimental calibration of the Al-in-hornblende barometer

The Al-in-hornblende barometer, which correlates Al^tot content of magmatic hornblende linearly with crystallization pressure of intrusion (Hammarstrom and Zen 1986), has been calibrated experimentally under water-saturated conditions at pressures of 2.5–13 kbar and temperatures of 700–655°C. Equilibration of the assemblage hornlende-biotite-plagioclase-orthoclasequartz-sphene-Fe-Ti-oxide-melt-vapor from a natural tonalite 15–20° above its wet solidus results in hornblende compositions which can be fit by the equation: P(±0.6 kbar) = –3.01 + 4.76 Al _hbl ^tot r ²=0.99, where Al^tot is the total Al content of hornblende in atoms per formula unit (apfu). Al^tot increase with pressure can be ascribed mainly to a tschermak-exchange ( ) accompanied by minor plagioclase-substitution ( ). This experimental calibration agrees well with empirical field calibrations, wherein pressures are estimated by contact-aureole barometry, confirming that contact-aureole pressures and pressures calculated by the Al-in-hornblende barometer are essentially identical. This calibration is also consistent with the previous experimental calibration by Johnson and Rutherford (1989b) which was accomplished at higher temperatures, stabilizing the required buffer assemblage by use of mixed H₂O-CO₂ fluids. The latter calibration yields higher Al^tot content in hornblendes at corresponding pressures, this can be ascribed to increased edenite-exchange ( ) at elevated temperatures. The comparison of both experimental calibrations shows the important influence of the fluid composition, which affects the solidus temperature, on equilibration of hornblende in the buffering phase assemblage.     

Aluminum partitioning between olivine and ultrabasic silicate liquid to 6 GPa

Trace element analyses of 1-atm and high-pressure experiments show that in komatiite and peridotite, the olivine (OL)/liquid (L) distribution coefficient for Al₂O₃ ( ) increases with pressure and temperature. Olivine in equilibrium with liquid accepts as much as 0.2 wt% Al₂O₃ in solution at 6 GPa. Convergence to equilibrium compositions at this high level is shown by cation diffusion of Al into synthetic forsterite crystals of low-Al contents in the presence of melt. Convergence to low-Al equilibrium compositions at lower P and T is shown by diffusion of Al out of synthetic forsterite with high initial Al content. Isobaric and isothermal experimental data subsets reveal that temperature and pressure variations both have real effects on . Variation in silicate melt composition has no detectable effect on within the limited range of experimentally investigated mixtures. Least-squares regression for 24 experiments, using komatiite and peridotite, performed at 1 atm to 6 GPa and 1300 to 1960°C, gives the best fit equation: Increase in with increasingly higher-pressure melting is consistent with incorporation of a spinel-like component of low molar volume into olivine, although other substitutions possibly involving more complex coupling cannot be ruled out. High P-T ultrabasic melting residues, if pristine, may be recognized by the high calculated from microprobe analyses of Al₂O₃ concentrations in residual olivines and estimated Al₂O₃ concentration in the last liquid removed. In general the low levels of Al in natural olivine from mantle xenoliths suggest that pristine residues are rarely recovered.     

Die Gneishornfelse um Steinach in der Oberpfalz

Zusammenfassung Bereits vor der Intrusion des Leuchtenberger Granits waren seine Rahmen-gesteine unter den Bedingungen der Amphibolit-Fazies regionalmetamorph eingeformt worden, wobei sich folgende Paragenesen bildeten: Muscovit + Biotit + Granat ± Sillimanit ± Staurolith (+ Quarz + Plagioklas) Muscovit + Biotit ± Kalifeldspat (+ Quarz + Plagioklas).Die Kontaktmetamorphose führt in den äußeren Bereichen der Aureole zur Paragenese Muscovit + Andalusit + Cordierit + Biotit (+ Quarz + Plagioklas), die der Hornblende-Hornfels-Fazies entspricht.In der inneren Kontaktzone werden die p-t-Bedingungen einer höhergradierten Hornfelsfazies erreicht, die sich in der Paragenese ± Kalifeldspat + Sillimanit + Cordierit ± Almandin + Biotit (+ Quarz + Plagioklas) dokumentiert.Die zonale Anordnung der Mineral-Paragenesen im Kontakthof läßt sich durch quantitative Verbreitungskarten anschaulich machen (150 Modalanalysen aus 59 Fundpunkten). Phasenbeziehungen und Mineralreaktionen werden anhand von AKF- und AFM-Diagrammen diskutiert, für deren Aufstellung 19 Mineralanalysen neu ausgeführt wurden.Durch den Vergleich mit derzeit verfügbaren experimentellen Unterlagen lassen sich die p-t-Bedingungen im Steinacher Kontakthof abschätzen. Danach ist ein Druckbereich von 1,5–3 kbar am wahrscheinlichsten. Mit der Bildung der höchst-gradierten Hornfelse waren 550° C sicher überschritten, während nach dem Jaeger-Modell 700° C als alleroberste Temperaturgrenze anzusehen ist. In der innersten Kontaktzone muß man mit geringen O₂-Partialdrucken rechnen.

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The gneiss-hornfelses of Steinach, Oberpfalz (Bavaria)

The southern margin of the Leuchtenberg granite massive is surrounded by a thermal aureole in which banded gneisses and mica schists have been transformed into hornfelses containing andalusite, sillimanite, cordierite, and almandine.Before the intrusion of the granite, the country rocks had undergone regional metamorphism to give mineral assemblages typical of the amphibolite facies: Muscovite + biotite ± K-feldspar + quartz + plagioclase in the banded gneisses and muscovite + biotite + garnet ± sillimanite ± staurolite + quartz + plagioclase in the mica schists.In the outer zone of the aureole, contact metamorphism produced the assemblage: Muscovite + andalusite + cordierite + biotite + quartz + plagioclase, corresponding to the hornblende-hornfels facies.In the inner zone of the aureole, increasing temperatures yielded the high grade assemblages: K-feldspar + sillimanite + cordierite ± almandine + biotite + quartz + plagioclase sillimanite + cordierite ± almandine + biotite + quartz + plagioclase.The zonal arrangement of the mineral assemblages within the Steinach aureole is shown in quantitative distribution maps of andalusite, contact-metamorphic sillimanite, cordierite, and contact-metamorphic garnet respectively, based on 150 modal analyses (from 59 points, Fig. 1). Andalusite (Fig. 10) reaches its maximum at about 200–400 m from the granite border and, toward it, is more and more substituted by sillimanite (Fig. 11). On the other hand, contact metamorphic sillimanite goes to the outer zone of the aureole almost as far as andalusite. This means that the sillimanite isograd passed across the andalusite zone during the contact metamorphism. Cordierite (Fig. 12) shows a constant increase from the margin to the innermost zone of the aureole, as does contact metamorphic garnet (Fig. 13). Phase relations and mineral reactions are discussed in terms of AKP- and APM-diagrams (Figs. 14, 15, 17, 18) based on 19 new mineral analyses (Tables 5–9).With respect to the experimental data so far available, possible P-T-conditions within the Steinach aureole have been discussed. The transition from the low to the high grade assemblages took place very near to the cross-over point of the andalusite/sillimanite equilibrium curve and the breakdown curve of muscovite + quartz in the P-T-diagram (Figs. 16, 19).This gives a minimum H₂O-pressure of 1 kbar. In the innermost contact zone, temperatures must have exceeded 550° C, which is in good agreement with recent oxygen-isotope data as well as with Jaeger's model. As is shown by the ore mineral assemblages, studied in polished sections (Table 10), the oxygen activity in the high grade hornfelses was low.

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Prof. S. Matthes (Würzburg) möchte ich für das lebhafte, fördernde Interesse an dieser Arbeit sowie für die Überlassung von Proben und von Geräten der DFG meinen herz-lichen Dank aussprechen. Wesentlichen Anteil am Gelingen dieser Untersuchung hat auch Prof. B. W. Evans, dem ich für die freundschaftliche Aufnahme in Berkeley, für die ständige Bereitschaft zur Diskussion und für wertvolle Anregungen zu großem Dank verpflichtet bin. Weiter danke ich Prof. W. Schreyer (Bochum) und Prof. F. J. Turner (Berkeley) für die kritische Durchsicht des Manuskripts sowie Mr. L. K. Burns (Berkeley), Dr. A. Peters und Dr. P. Richter (Würzburg) für wichtige Hinweise bei der Analytik. Die Untersuchungen an der Mikrosonde in Berkeley wurden durch ein Stipendium der Deutschen Forschungs-gemeinschaft ermöglicht; das Gerät stand durch die Unterstützung der National Science Foundation (grant GA 500) zur Verfügung. Beiden Organisationen sei für ihre Hilfe gedankt.

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Textlich gekürzte, aber um neue Untersuchungsergebnisse erweiterte Fassung einer Habilitationsschrift, angenommen im Januar 1968 von der Naturwissenschaftlichen Fakultät der Universität Würzburg.     

Antigorite-ophicarbonates: Phase relations in a portion of the system CaO-MgO-SiO2-H2O-CO2

The occurrence of critical assemblages among antigorite, diopside, tremolite, forsterite, talc, calcite, dolomite and magnesite in progressively metamorphosed ophicarbonate rocks, together with experimental data, permits the construction of phase diagrams in terms of the variables P, T, and composition of a binary CO₂-H₂O fluid. Equilibrium constants are given for the 30 equilibria that describe all relations among the above phases. Ophicalcite, ophidolomite, and ophimagnesite assemblages occupy partially overlapping fields in the diagram. The upper temperature limit of ophicalcite rocks lies below that of ophidolomite and ophimagnesite. The fluid phase in ophicarbonate rocks has 0.8$$ " align="middle" border="0"> , and there are indications that during their progressive metamorphism is approximately equal to P _total.     

Water in microcrystalline quartz of volcanic origin: Agates

Agates of volcanic origin, containing the different quartz species, fibrous, length-fast chalcedony (CH), granular fine quartz (FQ), and fibrous, length-slow, to lepidospheric quartzine (QN), have been investigated to evaluate possible relations between microstructure, i.e. crystallite size and texture, refractive indices, densities, contents of trace elements and of water, as well as dehydration behaviour. By means of near infrared spectroscopy, total water contents , could be differentiated quantitatively into contents of molecular water, , and silanole-group water, . Despite the low total water contents of the agates studied ( between 1 and 2 wt.%), near infrared spectroscopy results in reliable data on and .Wall-layering CH consists of fibrous quartz crystals and exhibits higher C-ratios, , than horizontally layered FQ which consists predominantly of granular quartz crystals (C _CH=0.45±0.11 (N=6), C _FQ=0.36±0.10 (N=4). This result is interpreted to be due to analogy with the behaviour of C-ratios in fluid phase-deposited opals-AN (hyalithe) and liquid phase-deposited opals-AG (non-crystalline opal) or -CT (common opal) (Langer and Flörke 1974).Translucent layers of CH show mostly lower refractive indices, when measured parallel than when measured perpendicular to the axes of the quartz fibers. The same is true for milky layers of CH. Crystallite sizes are smaller in the latter than in the former.For all samples studied, exists a positive correlation between at% (1/2Ca+1/2Mg+Na+K+Li) and at% (Al³⁺+Fe³⁺). This indicates that at least parts of (A1³⁺+ Fe³⁺) substitute for Si in the quartz structure. The charge is balanced by incorporation of di- and mono-valent cations in structural interstices. When the quantity at % H⁺, as obtained from , is included into the sum at% (1/2 Me²⁺+Me⁺), the above correlation is destroyed. This result could be indicative for a strong concentration of the Si-OH groups in the surface of the quartz microcrystallites.     

The Machangqing copper-molybdenum deposits, Yunnan, China — an example of Himalayan porphyry-hosted Cu-Mo mineralization

Summary The Machangqing copper-molybdenum deposits occur in the Shanjian fold belt, Yunnan Province, China. Two types of ores are distinguished: (1) Cu-Mo quartz veinlets in magnetite-series granite porphyry; and (2) Cu-Mo skarns occurring at the contact between the Ordovician sedimentary sequence and the granite porphyry. With decreasing temperature and of hydrothermal fluids initially in equilibrium with K-feldspar, the following alteration patterns developed within the porphyry, from the center outwards: silicification, K-silicate, phyllic and argillic alteration. The paragenetic sequence of alteration minerals observed in the Cu-Mo skarns resulted from decreasing temperature and/or increasing of the hydrothermal fluids initially in equilibrium with grandite garnet. Fluid inclusions in quartz suggest boiling during the mineralization. The mineralization temperatures based on filling temperatures and salinities of quartz are in the following ranges: about 265° to 400 °C and 5.0 to 14.6 wt.% NaCl eq. for the Cu-Mo veinlets; and 200° to 500 °C and 10.2 to 42.0 wt.% NaCl eq. for the Cu-Mo skarns. As is evident from log fo₂-pH diagrams, ores of the early stage of mineralization in the Cu-Mo skarns, characterized by the assemblage magnetite + pyrite + rare pyrrhotite + K-feldspar + quartz, were deposited from highly alkaline and high temperature fluids. With decreasing temperature and fo₂, the pH of the ore fluids was shifted towards slightly alkaline to neutral, with the resultant formation of the main stage ores, characterized by the assemblage chalcopyrite + pyrite + molybdenite + sphalerite + K-feldspar +sericite (muscovite) + epidote + uartz. Very minor amounts of ore minerals, including matildite, bismuthinite and electrum, are associated with a late stage of ore formation.In the case of the Cu-Mo veinlets, it can be stated roughly that both fs₂ and fo₂ conditions were in the stability field of pyrite, with pH of the ore fluids buffered by the assemblage sericite + K-feldspar +quartz ± calcite. K-Ar age determinations were made on the granite porphyry, biotite phenocrysts and hydrothermal biotite in the Cu-Mo skarns, giving ages of 42.5 to 34.6 Ma, 52.3 Ma, and 39.2 to 26.4 Ma, respectively.It is concluded that the Cu-Mo mineralization at Machangqing shows a close spatial and temporal association with the Himalayan felsic magmatism of the magnetite-series type.

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Die Machangqing Kupfer-Molybdän-Lagerstätten, Yunnan, China — Ein Beispiel für prophyrische Cu-Mo Vererzung im Himalaya

Zusammenfassung Die Kupfer-Molybdän-Lagerstätten von Machangqing treten im Shanjian Faltengürtel in der Provinz Yunnan, China, auf. Zwei Erztypen sind unterscheidbar: (1) Cu-Mo Quarzgängchen in einem porphyrischen Granit der Magnetit-Serie; (2) Cu-Mo-Skarne am Kontakt zwischen der ordovizischen Sedimentabfolge und dem porphyrischen Granit. Mit Abnahme der Temperatur und des Verhältnisses der hydrothermalen Fluide, die ursprünglich mit Alkalifeldspat im Gleichgewicht waren, entwickelten sich in diesem Porphyrstock vom Zentrum randwärts folgende Alterationszonen: Silizifizierung, K-silikatische, phyllische und argillische Alteration. Die paragenetische Abfolge der Alterationsminerale, die in den Cu-Mo Skamen zu beobachten sind, sind das Resultat abnehmender Temperatur und/oder einer Zunahme des -Verhältnisses der hydrothermalen Fluide, die ursprünglich mit Grandit-Granat im Gleichgewicht waren. Flüssigkeitseinschlüsse in Quarz weisen auf Siedeprozesse während der Mineralisation hin. Die aus den Einschlußtemperaturen und Salinitäten in Quarz bestimmten Mineralisationstemperaturen liegen für die Cu-Mo-Gängchen zwischen 265–400 °C und zwischen 5–14.6 Gew.% NaCI Äquiv. und zwischen 200–500 °C und 10.2–42 Gew.% NaCI Äquiv. für die Cu-Mo Skarne. Wie aus log fo₂-pH Diagrammen hervorgeht, ist die im Frühstadium der Cu-Mo Skamvererzung entstandene Vergesellschaftung von Magnetit +Pyrit+selten Magnetkies + Alkalifeldspat + Quarz durch Ausfällung aus sehr alkalischen und hochtemperierten Fluiden entstanden. Mit Temperatur- und fo₂-Abnahme wurde der pH der Lösungen leicht alkalisch bis neutral. Dies resultierte in der Bildung der Erze des Hauptstadiums, das durch Chalcopyrit + Pyrit + Molybdänit + Sphalerit + Alkalifeldspat + Serizit (Muscovit) + Epidot + Quarz charakterisiert ist. Sehr geringe Mengen an Erzmineralen inklusive Matildit, Bismuthinit und Elektrum sind dem Spätstadium der Vererzung zuzuordnen. Für die Cu-Mo-Gängchen läßt sich etwas verallgemeinernd feststellen, daß fo₂ und fs₂ im Stabilitätsbereich von Pyrit lagen, wobei der pH der Erzfluide durch die Paragenese Serizit+Alkalifeldspat+Quarz+Calcit gepuffert wurde. K-Ar Altersbestimmungen wurden am porphyrischen Granit, an Biotit-Phänokristallen und an hydrothermal gebildetem Biotit der Cu-Mo Skarne durchgeführt. Sie ergaben Alter von 42.5–34.6, 52.3 bzw. 39.2-26.4 Ma.Die Cu-Mo Vererzung von Machangqing zeigt eine räumliche und zeitliche Assoziation mit dem sauren Magmatismus der Magnetit-Serie im Himalaya.

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With 12 Figures     

Some geobarometers involving cordierite in the FeO-Al2O3-SiO2(±H2O) system: refinements,thermodynamic calibration,and applicability in granulite facies rocks

Five geobarometers involving cordierite have been formulated for quantitative pressure sensing in high grade metapelites. The relevant reactions in the FeO-Al₂O₃-SiO₂ (±H₂O) system are based on the assemblages (A) cordierite-garnet-sillimanite-quartz, (B) cordierite-spinel-quartz, (C) cordierite-garnet-spinel-sillimanite, (D) cordierite-garnet-orthopyroxene-quartz and (E) cordierite-orthopyroxene-sillimanite-quartz. Application of the barometric formulations to a large number of granulite grade rocks indicates that the cordierite-garnet-sillimanite-quartz equilibrium is widely applicable and registers pressures which are in good agreement with the “consensus” pressure estimates. The dispersion in the computed P values, expressed as one standard deviation, is within ±1.2 kbar. The geobarometers (B) and (C) also yield pressures which are reasonable and compare well with those computed from equilibrium (A). The estimated pressures from (D) and (E), both involving orthopyroxene, are at variance with these estimates. It has been argued that the discrepancy in pressures obtained from these geobarometers stems from an inadequate knowledge of activity-composition relations and/or errors in input thermodynamic data of aluminous orthopyroxene. The convergence of pressure values estimated from the barometric formulations, especially (A), (B) and (C), implies that the present formulations are more dependable than the existing formulations and are also capable of setting limits on P values in response to varying $$\begin{gathered} {\text{1/2Fe}}_{\text{2}} {\text{Al}}_{\text{4}} {\text{Si}}_{\text{5}} {\text{O}}_{{\text{18}}} \hfill \\ {\text{ = 1/3Fe}}_{\text{3}} {\text{Al}}_{\text{2}} {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} {\text{ + 2/3Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + 5/6SiO}}_{\text{2}} {\text{. (A)}} \hfill \\ {\text{1/2Fe}}_{\text{2}} {\text{Al}}_{\text{4}} {\text{Si}}_{\text{5}} {\text{O}}_{{\text{18}}} {\text{ = FeAl}}_{\text{2}} {\text{O}}_{\text{4}} {\text{ + 5/2SiO}}_{\text{2}} {\text{. (B)}} \hfill \\ {\text{Fe}}_{\text{2}} {\text{Al}}_{\text{4}} {\text{Si}}_{\text{5}} {\text{O}}_{{\text{18}}} {\text{ + FeAl}}_{\text{2}} {\text{O}}_{\text{4}} \hfill \\ = {\text{Fe}}_{\text{3}} {\text{Al}}_{\text{2}} {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} {\text{ + 2Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{. (C)}} \hfill \\ {\text{1/2Fe}}_{\text{2}} {\text{Al}}_{\text{4}} {\text{Si}}_{\text{5}} {\text{O}}_{{\text{18}}} {\text{ + Fe}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{6}} \hfill \\ = {\text{Fe}}_{\text{3}} {\text{Al}}_{\text{2}} {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} {\text{ + 3/2SiO}}_{\text{2}} .{\text{ (D)}} \hfill \\ {\text{1/2Fe}}_{\text{2}} {\text{Al}}{}_{\text{4}}{\text{Si}}_{\text{5}} {\text{O}}_{{\text{18}}} \hfill \\ = 1/2{\text{Fe}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{6}} {\text{ + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + 1/2SiO}}_{\text{2}} .{\text{ (E)}} \hfill \\ \end{gathered}$$ . The present communication addresses the calibration, applicability and reliability of these barometers with reference to granulite facies metapelites.     

Ulvöspinel in native iron-bearing assemblages and the origin of these assemblages in basalts from Ovifak,Greenland, and Bühl,Federal Republic of Germany

Euhedral crystals of ulvöspinel are found in many of the native-iron-bearing xenoliths from the basalt of Bühl near Kassel (West Germany) and Ovifak on Disko Island (West Greenland). The typical assemblage of these xenoliths at both localities is: native Fe, troilite, cohenite, ulvöspinel, ilmenite, olivine, and plagioclase, as well as silicate glass containing droplets of former Fe and troilite melt. The ulvöspinel subsolidus textures and intergrowths also indicate identical cooling histories for the xenoliths in both cases. Ulvöspinel crystallized after the formation of iron, but still above the Fe-FeS eutectic at 988° C. A subsequent strong drop in oxygen fugacity revealed partial breakdown of ulvöspinel according to the reaction .Microprobe analyses of a Bühl xenolith indicate that ulvöspinel contains up to 4.7 w.t.% MnO, while olivine compositions correspond to Fa_64–74Fo_12–24Te_12–15. The entire xenolith contains 1.9 w.t.% MnO. This fact, together with the geological evidence and the occurrence of corroded quartz relicts within some of the xenoliths provides clear evidence for reduction under near-surface conditions in a blast-furnace-like process. The reducing agent was coal from the Tertiary seams cut by the erupting basalt, while the xenolith source material most probably was spherosiderite, which is very common in the coals and would explain the high MnO content. Consequently, the presence of cohenite is not necessarily an indicator of high pressure.The analogies between the Bühl and Ovifak localities and their xenoliths strongly suggest a similar formation through near surface reduction and not derivation from the mantle.