Precambrian crustal components,plutonic associations,plate environment of the Hercynian Fold Belt of central Europe: Indications from a Nd and Sr isotopic study

Chichi-jima, Bonin Islands, consists of dominant Eocene submarine volcanic rocks, comprising boninites, andesites and dacites, and subordinate sedimentary rocks. The dacites occur frequently in breccias and pillows overlying a boninite pillow lava sequence. The boninite pillows are intruded by a multiple dike, in which a core boninite is chilled against outer dacites. A density-stratified chamber may have been capped by a dacite magma. The dacites, which can be divided into quartz dacite and quartz-free dacite, are differentiates from the boninite-forming magmas, because they vary continuously in composition from boninites through andesites. The quartz dacites, corresponding to rhyolite in SiO₂, are lower in Na₂O and K₂O than most orogenic dacites. Some of the dacites are characterized by ferropigeonite (Wo_7–16En_23–39Fs_68-54) phenocrysts and are clearly ferrodacite, producing variable amounts of Fs-rich normative pyroxenes. The relation of SiO₂ to total FeO/MgO ratio indicates that many of both types of dacites, with glasses in boninites, are enriched in total FeO despite the strong calc-alkalic affinity of boninites. The crystallization temperature of ferropigeonite with Mg value 30 in a quartz dacite is estimated to be 900° C and that in a quartz-free dacite to be 1050° C, which are unusually high for differentiated silicic rocks. Some Chichi-jima rocks are fresh, having a low ratio of Fe₂O₃ to FeO. On the basis of the experimental study of magmatic ferric-ferrous equilibria at 1 bar, the oxygen fugacities are calculated as 10^–13.6 bars at 900° C for a ferropigeonite quartz dacite and 10^–8.9 bars at 1200° C for a boninite with the lowest Fe³⁺/Fe²⁺. Both values lie below the quartz-fayalite-magnetite buffer line. The boninite series volcanic rocks have preserved low oxygen fugacities as well as high temperatures until the latest differentiation stage. The ferropigeonite phenocrysts have crystallized from the dacite magmas under the conditions of moderately high temperatures, very low oxygen fugacities and high total FeO and SiO₂ concentrations.     

Origin of ultramafic xenoliths containing exsolved pyroxenes from Hualalai Volcano,Hawaii

Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225° to 1350° C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045° and 1090° C. Comparisons of the abundance of K₂O in plagioclase and the abundances of TiO₂ and Fe₂O₃in spinel of xenoliths and mid-ocean ridge basalt, and a single ⁸⁷Sr/ ⁸⁶Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155°–230° C of cooling which took place over about 120 ka — the age of the youngest Hualalai tholeiitic basalt — yield maximum cooling rates of 1.3×10^–3–1.91×10^–3 °C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel.     

Diffusion gradients in an eclogite xenolith from the Roberts Victor kimberlite pipe: (2) kinetics and implications for petrogenesis

In a bimineralic eclogite xenolith (sample JJG41) from the Roberts Victor kimberlite, compositional gradients in clinopyroxene are related to garnet exsolution. Two principal reactions involving clinopyroxene and garnet occur: (i) The net-transfer Al₂Si^-1Mg^-1 which is responsible for garnet growth according to the equation 2Di+Al₂Si^-1Mg^-1=Grossular+MgCa^-1 (reaction 1). This has created substantial compositional gradients in Al, Si and Mg within clinopyroxene. (ii) The exchange of Fe–Mg between garnet and clinopyroxene (reaction 2). During the stage of garnet growth (reaction 1) the lamellae crystallized sequentially as a result of a temperature decrease from around 1400 to 1200° C. This exsolution growth-stage was under the control of Al diffusion in clinopyroxene and at around 1200° C further growth of garnet lamellae became impeded by the sluggishness of Al diffusion in the clinopyroxene host. However, reaction 2 continued during further cooling down to about 1000° C; this temperature being inferred from the constant Fe–Mg partitioning at clinopyroxene-garnet interfaces for the whole set of lamellae. The initial clinopyroxene in JJG41 was probably formed by crystallization from a melt in Archaean time. The cessation of Fe–Mg exchange between garnet and clinopyroxene at about 1000° C may well predate the eruption of the eclogite in kimberlite at around 100 Ma. Kinetic models of reaction are examined for both reactions. Modelling of reaction 1, involving both diffusion and interface migration, allows several means of estimating the diffusion coefficient of Al in clinopyroxene; the estimates are in the range 10^-16-10^-20 cm²/s at 1200° C. These estimates bracket the experimentally determined data for Al diffusion in clinopyroxene, and from these experimental data a preferred cooling rate of about 300° C/Ma is obtained for the period of growth of garnet exsolution lamellae. A geospeedometry approach (Lasaga 1983) suitable for a pure-exchange process (reaction 2) is used to estimate the cooling rate in the later stages of the thermal history (after garnet growth); values 4–40° C/Ma are consistent with the shape of the Fe-diffusion gradients in the clinopyroxene. The extensive thermal history recorded by JJG41, including probable melt involvement at ca. 1400° C, demonstrates the complex evolution of rocks within the mantle. Whilst the notion of formation of mantle eclogites from subducted oceanic crust has become fashionable, it is clear that tracing eclogite geochemical and P-T characteristics backwards from their nature at the time of xenolith eruption, through high-temperature mantle events to the characteristics of the original subducted oceanic crust, will be very complex.     

Thermal behaviour of pyrope at 1000 and 1100 °C: mechanism of Fe<Superscript>2+</Superscript> oxidation and decomposition model

The mechanism of thermally induced oxidation of Fe²⁺ from natural pyrope has been studied at 1000 and 1100 °C using ⁵⁷Fe Mössbauer spectroscopy in conjunction with XRD, XRF, AFM, QELS, TG, DTA and electron microprobe analyses. At 1000 °C, the non-destructive oxidation of Fe²⁺ in air includes the partial stabilization of Fe³⁺ in the dodecahedral 24c position of the garnet structure and the simultaneous formation of hematite particles (15–20 nm). The incorporation of the magnesium ions to the hematite structure results in the suppression of the Morin transition temperature to below 20 K. The general garnet structure is preserved during the redox process at 1000 °C, in accordance with XRD and DTA data. At 1100 °C, however, oxidative conversion of pyrope to the mixed magnesium aluminium iron oxide, Fe-orthoenstatite and cristoballite was observed. During this destructive decomposition, Fe²⁺ is predominantly oxidized and incorporated into the spinel structure of Mg(Al,Fe)₂O₄ and partially stabilized in the structure of orthoenstatite, (Mg,Fe)SiO₃. The combination of XRD and Mössbauer data suggest the definite reaction mechanism prevailing, including the refinement of the chemical composition and quantification of the reaction products. The reaction mechanism indicates that the respective distribution of Fe²⁺and Fe³⁺ to the enstatite and spinel structures is determined by the total content of Fe²⁺ in pyrope.     

An internally consistent model for the thermodynamic properties of Fe−Mg-titanomagnetite-aluminate spinels

A model is developed for the thermodynamic properties of Fe²⁺–Mg²⁺-aluminate-titanate-ferrite spinels of space group Fd3m. The model incorporates an expression for the configurational entropy of mixing which accounts for long-range order over tetrahedral and octahedral sites. Short-range order or departures from cubic symmetry are not considered. The non-configurational Gibbs energy is formulated as a second degree Taylor expansion in six linearly independent composition and ordering variables. The model parameters are calibrated to reproduce miscibility gap constraints, order-disorder phenomena in MgAl₂O₄ and MgFe₂O₄, and Fe²⁺–Mg²⁺ partitioning data between olivine and: (1) aluminate spinels; (2) ferrite spinels; (3) titanate spinels; (4) mixed aluminate-ferrite spinels. This calibration is achieved without invoking non-configurational excess entropies of mixing. The model predicts that the ordering state of FeAl₂O₄ is more normal than that of MgAl₂O₄. It also successfully accounts for heat of solution measurements and activity-composition relations in the constituent binaries. Phase equilibrium constraints require that the structure of Fe₃O₄ is more inverse than random at all temperatures and that Mg²⁺ has a strong tetrahedral site preference with respect to that of Fe²⁺. The analysis suggests that in the titanates short range order on octahedral sites may be significant at temperatures as high as 1300° C. Constraints developed from calibrating the thermodynamic properties of Fe²⁺–Mg²⁺-aluminatetitanate-ferrite spinel solid solutions permit extension of the database of Berman (1988) to include estimates of the end-member properties of hercynite (FeAl₂O₄), ulvöspinel (Fe₂TiO₄), MgFe₂O₄ and cubic Mg₂TiO₄. In constructing these estimates, provision is made for low-temperature magnetic entropy contributions and the energetic consequences of disordering the aluminates and the ferrites. These estimates are consistent with all of the available low-temperature adiabatic calorimetry, high-temperature heat content, and heat of solution measurements on the end-members. The analysis implies that there is a substantial heat capacity anomaly in the range 300°–900° C associated with disordering of the MgAl₂O₄ structure while that in FeAl₂O₄ becomes significant at temperatures above 700° C. The same heat capacity response in the ferrites indicates that the order/disorder transformation is coupled to the antiferromagnetic-paramagnetic transition in MgFe₂O₄ but takes place well above the ferrimagnetic-paramagnetic transition in magnetite. The proposed model is internally consistent with solution theory reported elsewhere for Fe²⁺–Mg²⁺ olivines and orthopyroxenes (Sack and Ghiorso 1989), rhombohedral oxides (Ghiorso 1990a) and the remaining end-member properties of Berman (1988).     

Diffusion of 40Ar in hornblende

Measured radiogenic ⁴⁰Ar loss from two compositionally contrasting hornblendes following isothermal-hydrothermal treatment have provided model diffusion coefficients in the temperature range of 750° C to 900° C. Eight experiments using a hornblende (77–600) with a Mg/(Mg +Fe) ratio of 0.72 yield a linear array on an Arrhenius plot with a slope corresponding to an activation energy of 66.1 kcal-mol^–1 and a frequency factor of 0.061 cm²-sec^–1, assuming spherical geometry for the mineral aggregate. Five experiments undertaken on a hornblende (M Mhb-1) with a Mg/(Mg+Fe) ratio of 0.36 show similar behavior to the Mgrich sample, suggesting that the diffusivity of Ar in hornblendes is not sensitive to the Mg/Fe ratio.These data are consistent with kinetic information obtained from a geological experiment using the thermal effect of a granitoid intrusion. Together these data yield an activation energy of 64.1±1.7 kcal-mol^–1 and a frequency factor of 0.024± _0.011 ^0.053 cm²-sec^–1. For a hornblende with an effective diffusion radius of 80 m, these diffusion parameters predict closure temperatures between 578° C and 490° C for cooling rates in the range 500 to 5° C-Ma^–1.     

An experimental investigation of heulandite-laumontite equilibrium at 1000 to 2000 bar P fluid

The univariant reaction governing the upper stability of heulandite (CaAl₂Si₇O₁₈·6H₂O), heulandite=laumontite+3 quartz+2H₂O (1), has been bracketed through reversal experiments at: 155±6° C, 1000 bar; 175±6° C, 1500 bar; and 180±8° C, 2000 bar. Reversals were established by determining the growth of one assemblage at the expense of the other, using both XRD and SEM studies. The standard molal entropy of heulandite is estimated to be 783.7±16 J mol^–1 K^–1 from the experimental brackets. Predicted standard molal Gibbs free energy and enthalpy of formation of heulandite are –9722.3±6.3 kJ mol^–1 and –10524.3±9.6 kJ mol^–1, respectively. The reaction (1), together with the reaction, stilbite=laumontite+3 quartz+3 H₂O, defines an invariant point at which a third reaction, stilbite=heulandite+ H₂O, meets. By combining the present experimental data with past work, this invariant point is located at approximately 600 bar and 140° C. Heulandite, which is stable between the stability fields of stilbite and laumontite, can occur only at pressures higher than that of the invariant point, for = P _total.These results are consistent with natural parageneses in low-grade metamorphic rocks recrystallized in equilibrium with an aqueous phase in which is very close to unity.     

Paragenesis of sodic pyroxene-bearing quartz schists: implications for the P-T history of the Sanbagawa belt

Sodic pyroxene (jadeite content X _jd=0.1–0.3) occurs locally as small inclusions within, albite porphyroblasts and in the matrix of hematite-bearing quartz schists in the Sanbagawa (Sambagawa) metamorphic belt, central Shikoku, Japan. The sodic, pyroxene-bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene+subcalcic (or sodic) amphibole+phengitic mica+albite+quartz+hematite+titanite±epidote. Spessartine-rich garnet occurs in Mn-rich samples. Sodic pyroxene in epidote-bearing samples tends to be poorer in acmite content (average X _Acm=0.26–0.50) than that in the epidote-free samples (X _Acm=0.45–0.47). X _Jd shows no systematic relationship to metamorphic grade, and is different among the three sampling regions [Saruta-gawa, Asemi-gawa and Bessi (Besshi)]. The average X _Jd of the Saruta-gawa samples (0.21–0.29) is higher than that of the Asemi-gawa (0.13–0.17) and Bessi (0.14–0.23). The P-T conditions of the Asemi-gawa and Bessi regions are estimated at 5.5–6.5 kbar, >360°C in the chlorite zone, 7–8.5 kbar, 440±15°C in the garnet zone and 8–9.5 kbar, 520±25°C in the albite-biotite zone. Metamorphic pressure of the Saruta-gawa region is systematically 1–1.5 kbar higher than that of the Asemi-gawa and Bessi regions, and materials of the Saruta-gawa region have been subducted to a level 3–5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the Asemi-gawa and Bessi regions. Pressure slightly increases toward the north (structurally high levels) through the Sanbagawa belt of central shikoku. Two types of zonal structure were observed in relatively coarse-grained sodic pyroxenes in the matrix. One type is characterized by increasing X _Jd from core to rim, the other type by decreasing X _Jd from core to rim. Both types of zoned pyroxenes show an increase in X _{Fe 2+}[=Fe²⁺/(Fe²⁺+Mg)] from core to rim. The first type of zoning was observed in a sample from the chlorite zone of lowest grade, whereas the latter occurs in the garnet and albite-biotite zones of higher grade. The contrast in zonal structure implies that dP/dT during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher-grade zones. The estimated dP/dT of the prograde stage of the chlorite zone is 3.2 kbar/100°C, and that of the garnet and albite-biotite zones is -1.8 to 0.9 kbar/100°C. The variation of dP/dT at shallow and deep levels of a subduction system probably reflects the difference of heating duration and/or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle.     

Application of arsenopyrite geothermometry and sphalerite geobarometry to the Taebaek Pb-Zn(-Ag) deposit at Yeonhwa I mine,Republic of Korea

The Taebaek Pb-Zn(-Ag) deposit of the Yeonhwa I mine, Republic of Korea, occurs in a broadly folded and reverse-faulted terrain of Paleozoic sedimentary rocks: the Taebaeksan basin. The orebodies consist of several thin tabular orebodies of hydrothermal replacement type where they are hosted by carbonate rocks. The Pb-Zn(-Ag) mineralization can be divided into four distinct stages based upon the mode of occurrence of ore minerals, ore textural relationships and their composition. Based on temperatures inferred from arsenopyrite compositions by means of electron microprobe and fluid inclusions, the estimated temperatures for the stages I, II, III and IV reach 330 to 350 °C, 270 to 340 °C, 230 to 250 °C, and <220 °C, respectively. The sulphur activity (atm) of ore formation at the Taebaek deposit was estimated for each stage as 10^–11 to 10^–11.5, 10^–9.5 to 10^–13, 10^–13.5 to 10^–15, and <10^–15, respectively. Even though application of sphalerite geobarometry is problematic because of the absence of good mineral assemblages, sphalerite coexisting with pyrite but not with pyrrhotite was used to estimate the minimum mineralization pressure (about 1 kbar).     

Diffusion kinetics of Fe and Mg in aluminous spinel: experimental determination and applications

The diffusion coefficients of Fe²⁺ and Mg in aluminous spinel at ∼20 kb, 950 to 1325°C, and at 30 kb, 1125°C have been determined via diffusion couple experiments and numerical modeling of the induced diffusion profiles. The oxygen fugacity, fO₂, was constrained by graphite encapsulating materials. The retrieved self-diffusion coefficients of Fe²⁺ and Mg at ∼20 kb, 950 to 1325°C, fit well the Arrhenian relation, D = D₀exp(−Q/RT), where Q is the activation energy, with D₀(Fe) = 1.8 (±2.8) × 10⁻⁵, D₀(Mg) = 1.9 (±1.4) × 10⁻⁵ cm²/s, Q(Fe) = 198 ± 19, and Q(Mg) = 202 ± 8 kJ/mol. Comparison with the data at 30 kb suggests an activation volume of ∼5 cm³/mol. From analysis of compositional zoning in natural olivine-spinel assemblages in ultramafic rocks, previous reports concluded that D(Fe-Mg) in spinel with Cr/(Cr + Al) ≤0.5 is ∼10 times that in olivine. The diffusion data in spinel and olivine have been applied to the problems of preservation of Mg isotopic inhomogeneity in spinel within the plagioclase-olivine inclusions in Allende meteorite and cooling rates of terrestrial ultramafic rocks.     

Distinctive crystal chemistry and site configuration of the clinopyroxene from alkali basaltic rocks

A crystal chemical investigation of clinopyroxenes from a suite of nepheline-bearing lavas located in the Nyambeni Range of Kenya has delineated the polyhedral site configurations and related intracrystalline relationships. These are distinct from those determined for the clinopyroxene in an analogous suite of leucite-bearing lavas from the Sabatini volcanoes in the Roman Region of Italy (Dal Negro et al. 1985).The Nyambeni clinopyroxene, varying from salite to hedenbergite, preferentially accepts Na in the M2 site to balance increasing Fe²⁺ and Si, respectively, whereas the Sabatini clinopyroxene is confined within the salite field and preferentially accepts Al^iv to balance the effect of increasing (Fe³⁺+Ti⁴⁺+Al^vi+Cr³⁺)_M1.The Fe²⁺/Fe³⁺ and K/Na ratios of the host rocks emerge as significant factors in determining the different polyhedral configurations and evolutions of the clinopyroxene from the two lava suites, respectively. The resulting Mg-Fe²⁺ order-disorder relationships in M1–M2 are also distinct in the two clinopyroxenes. A high degree of MgFe²⁺ order in M1–M2 corresponds to the largest configurational, hence energetic, difference between M1 and M2 in the Nyambeni clinopyroxene, whereas the converse applies to the Sabatini clinopyroxene.In view of the significant crystal chemical differences and distinct evolution trends, it is proposed that salites from alkali volcanic rocks may be referred to as Nyambeni-type or Sabatini-type, respectively.     

Calculation of equilibration conditions for garnet granulite and garnet websterite nodules in African kimberlite pipes

Summary Pressure/temperature estimates based on different combinations of calibrated mineralogical thermometers and barometers and alternative assumptions concerning the Fe²⁺/(Fe²⁺+Fe³⁺) ratios in the mineral phases are compared for a suite of fourteen nodules. PreferredP/T values have been obtained by simultaneous solution of eq. (12) ofWood (1974) for garnet-orthopyroxene equilibria and theEllis andGreen (1979) equation defining theP,T,X dependence of the Fe²⁺–Mg²⁺ partition coefficient for coexisting garnet and clinopyroxene. However, to obtain realistic results it is first necessary to calculate the Fe³⁺ contents in the minerals—conveniently done on a charge balance basis. The favoured meanP/T estimates of 654±36°C and 10.8±3.1 kbs are compatible with a lower crustal origin for this nodule suite.

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Berechnung von Gleichgewichts-Bedingungen von Granat-Granulit und Granat-Websterit-Xenolithen in afrikanischen Kimberliten

Zusammenfassung Druck-Temperatur-Berechnungen auf Grund verschiedener Kombinationen von kalibrierten mineralogischen Thermometern und Barometern sowie alternativer Modelle der Fe²⁺/(Fe²⁺+Fe³⁺)-Verhältnisse in den Mineralen einer Gruppe von 14 Einschlüssen werden verglichen. BevorzugteP/T-Werte ergaben sich durch simultane Lösung der Gl. (12) für Granat-Orthopyroxen-Gleichgewichte vonWood (1974) und der Gleichung vonEllis undGreen (1979), die dieP-T-X-Abhängigkeit des Fe²⁺–Mg²⁺-Verteilungskoeffizienten für koexistierende Granat und Clinopyroxen definiert. Um realistische Ergebnisse zu erhalten, ist es jedoch zuerst notwendig, die Fe³⁺-Gehalte der Minerale zu berechnen, vorzugsweise auf der Grundlage des Ladungsausgleichs. Die bevorzugten Durchschnittswerte von 654°±36°C und 10.8–3.1 kb sind in guter Übereinstimmung mit einer Herkunft aus der unteren Kruste.

     

Phase relations in the CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>–KAlSi<Subscript>3</Subscript>O<Subscript>8</Subscript> join at 6 and 3.5 GPa as a model for formation of some potassium-bearing deep-seated mineral assemblages

The join CaMgSi₂O₆–KAlSi₃O₈ has been studied at 6 GPa (890–1,500°C) and 3.5 GPa (1,000–1,100°C). K-rich melts in the join produce assemblages Cpx + Grt, Cpx + Opx, Cpx + San, and Cpx + Grt + San at 1,100–1,300°C. At N_San^system<~70 mol%, sanidine is unstable on the solidus and appears at the liquidus, if N_San^system>90 mol%. This explains a scarcity of San in mantle Cpx-rich assemblages and its association with high-K aluminosilicate melt inclusions in diamonds. In absence of San, KCpx is the only host for potassium. The K-jadeite content in KCpx systematically increases with decreasing temperature and reaches 10–12 mol% near the solidus. However, KCpx coexists with San at N_San^system>70 mol% and <1,300°C, being formed via reaction San + L=KCpx. The KJd content in KCpx is controlled by the equilibrium San=KJd + SiO₂^L that displaces to the right with increasing pressure and decreasing both the temperature and This equilibrium is considered to be responsible for the formation of San lamellae in natural UHP Cpx. In our experiments at 3.5 GPa, garnet is absent whereas the KJd and Ca-Eskola contents in Cpx are low, and the join CaMgSi₂O₆–KAlSi₃O₈ is close to binary (with the eutectic Cpx + San + L). Different topologies of the join at 6 and 3.5 GPa define a sequence of mineral crystallization from K-rich aluminosilicate melts during cooling and decompression: from KCpx + Grt without San at P>4 GPa to Cpx + San at P<4 GPa. Similar sequence of assemblages is observed in some eclogitic xenoliths from kimberlites and Grt–Cpx rocks of the Kokchetav Complex (Northern Kazakhstan).     

Synthesis and upper stability of paragonite

The mineral paragonite, NaAl₂[AlSi₃O₁₀ (OH)]₂, has been synthesized on its own composition starting from a variety of different materials. Indexed powder data and refined cell parameters are given for both the 1M and 2M₁ polymorphs obtained. The upper stability limit of paragonite is marked by its breakdown to albite + corundum + vapour. The univariant equilibria pertaining to this reaction have been established by reversing the reaction at six different pressures, the equilibrium curve running through the following intervals: 1 kb: 530°–550° C 2 kb: 555°–575° C 3 kb: 580°–600° C 5kb: 625°–640° C 6 kb: 620°–650° C 7 kb: 650°–670° C.Comparison with the upper stability limit of muscovite (Velde, 1966) shows that paragonite has a notably lower thermal stability thus explaining the field observation that paragonite is absent in many higher grade metamorphic rocks in which muscovite is still stable.The enthalpy and entropy of the paragonite breakdown reaction have been estimated. Since intermediate albites of varying structural states are in equilibrium with paragonite, corundum and H₂O along the univariant equilibrium curve, two sets of data pertaining to the entropy of paragonite (S ₂₉₈ ⁰ ) as well as the enthalpy ( H _f,298 ⁰ ) and Gibbs free energy ( G _f,298 ⁰ ) of its formation were computed, assuming (1) high albite and (2) low albite as the equilibrium phase. The values are: (1) (2) S ₂₉₈ ⁰ 67.8±3.9 cal deg^–1 gfw^–1 63.7±3.9 cal deg^–1 gfw^–1 H _f,298 ⁰ –1417.9±2.7 kcal gfw^–1 –1420.2±2.6 kcal gfw^–1 G _f,298 ⁰ –1327.4±4.0 kcal gfw^–1 –1328.5±4.0 kcal gfw^–1.Adapted from a part of the author's Habilitationsschrift accepted by the Ruhr University, Bochum (Chatterjee, 1968).     

Experimentelle Sideritbildung aus Calcit + FeCl2

The stability relations of calcite and siderite in the system Ca²⁺-Fe²⁺-CO ₃ ^2– Cl ₂ ^2– -H₂O have been determined between 150 and 400° C in the pressure range from 250 to 2000 bars.It was found that the composition of the fluid phase coexisting with calcite and siderite is very poor in Fe²⁺ and correspondingly very rich in Ca²⁺ (see Tab. 1 and Fig. 3). The mole proportion Ca²⁺/(Ca²⁺+Fe²⁺) exceeds the value 0.98 at temperatures below 250° C when fluid pressure is 1000 bars. The stability field of calcite narrows with decreasing temperature, increasing pressure, and decreasing concentration of dissolved salts (CaCl₂₊+FeCl₂). In our experiments siderite becomes unstable at about 400° C (see p. 158).The experimental data indicate that siderite will be formed by reaction of calcite with FeCl₂-bearing solutions that have very low concentrations of Fe²⁺ as well as high Ca²⁺/(Ca²⁺+Fe²⁺)-ratios. Solutions coexisting with calcite must be very poor in Fe²⁺, otherwise siderite is formed.

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Herrn Prof. Dr. H. G. F. Wikkleb danke ich sehr für sein förderndes Interesse an dieser Arbeit und für eine kritische Durchsicht des Manuskriptes. Der Deutschen Forschungsgemeinschaft gebiihrt Dank für die Arbeitsmöglichkeit mit den Apparaturen, die Herrn Prof. Dr. Winkler zür Verfugung gestellt wurden.     

Rb-Sr and K-Ar ages on minerals at temperatures of 300°–400° C from deep wells in the Larderello geothermal field (Italy)

Rb-Sr and K-Ar ages have been obtained on six biotites, two muscovites and one hornblende from samples of micaschist, gneiss and amphibolite of Lower Paleozoic to Precambrian age at a depth exceeding 2,000 m in basement rocks of the Larderello-Travale geothermal region. Most of the data cluster in the range 2.5–3.7 Ma, revealing the existence of a Pliocene thermal event to which the origin of the field may be attributed. The resulting duration of the Larderello geothermal field is unexpectedly long. In the basement levels of the two wells examined, unstabilized minimum temperatures of 290° and 380° C were measured. All the biotites show almost complete ⁴⁰Ar and ⁸⁷Sr retention at the measured well temperatures. Petrologic evidence (stilpnomelane stability) and experimental data (activation energies and diffusion coefficients) also favour a closure temperature above 400° C for Rb-Sr and K-Ar in biotites, in agreement with recent direct experimental determinations.For the last 3 Ma mean geothermal gradients of 120°–150° C/km have been evaluated in the first 2–3 km, and 60°–65° C/km in the underlying 2 km. A rough estimate of total cooling in the last 3 Ma gives a value of 120° C at 2,500 m depth and 50° C at 4,000 m depth in Sasso 22 well. A mean uplift rate of about 0.2 mm/year is calculated independently.Research conducted under a collaboration agreement between the Italian National Research Council (CNR) and the Italian National Electricity Board (ENEL)     

A re-examination of the role of hydrogen in Al−Si interdiffusion in feldspars

Recent experimental studies have shown that the rates of Al–Si order-disorder and interdiffusion in alkali feldspars at high pressures under dry conditions increase dramatically in the approximate pressure range 7–14 kb, depending on temperature and feldspar composition (Goldsmith 1987, 1988). Enhancement of Al–Si interdiffusion rates is ascribed to the involvement of hydrogen, but the species of hydrogen involved is undetermined.A simple kinetic analysis of the data of Goldsmith (1987) on disordering of dry albite at 800°–950° C and 6–24 kb in the solid media press is consistent with the NaCl pressure cell acting as a proton donor by enhancing dissociation of water in the pressure medium, generating a high in the experimental environment. The rate constant for disordering of albite is found to increase linearly with the estimated experimental and with the density of aqueous salt solution, implicating H⁺ as the rate-enhancing species.Further experimental studies confirm the importance of . At 16 kb and 850° C, dry albite in sealed Pt capsules in a NaCl cell containing tantalum powder (which reduces H₂O to H₂) remains highly ordered over the same time that complete disordering would occur in the absence of Ta. H₂ cannot therefore be the rate-enhancing species. At 1 kb and 850° C, the extent of Al–Si disorder in albite in direct contact with various NaCl–H₂O solutions increases from partially disordered for pure H₂O to completely disordered for saturated aqueous NaCl solution, giving strong support to the proton model. SIMS scanning ion imaging of albite run products demonstrates conclusively that solution-reprecipitation is not responsible for enhanced disordering rates.Results of disordering experiments in the solid media apparatus cannot be duplicated in Ar gas media internally-heated pressure vessels, even with the same experimental configuration around the albite-bearing capsules, due to the different proton-buffering capacities of the solid and gas media apparatus.     

Melting and subsolidus reactions in the system K2O−CaO−MgO−Al2O3−SiO2−H2O: experiments and petrologic application

Experiments with synthetic starting materials of muscovite, phlogopite, zoisite, kyanite and quartz were performed in the pressure temperature range 10–25 kbar, 640–780° C under water excess conditions. The reaction muscovite+zoisite+quartz+vapor=liquid+kyanite was bracketed at 10.5 kbar/689–700° C, 15.5 kbar/709–731° C and 20 kbar/734–745° C. The equivalent reaction in the Mg-bearing system muscovite_ss +zoisite+quartz+vapor=liquid+kyanite+phlogopite_ss lies at the same temperature around 10 kbar and approximately 10° C higher around 20 kbar, compared with the Mg-free reaction. At slightly higher temperatures formation of melt and tremolite_ss was reversibly observed from the assemblage phlogopite_ss+zoisite +kyanite+quartz around 10.5 kbar/690–710° C, 15.5 kbar/720–750° C and 20.5 kbar/745–760° C. In the subsolidus region, the reaction muscovite_ss+talc_ss+ tremolite_ss=phlogopite_ss+zoisite+quartz+vapor were located in the range 700° C/16.7–19.0 kbar and 740° C/19.7–20.8 kbar. From these data, a wedge shaped stability field of phlogopite_ss+zoisite+quartz appears with a high P, T termination around 21 kbar/755° C. Muscovite+tremolite+talc or kyanite comes in at higher pressures. These phase relations are in qualitative accord with petrographic observations from high pressure metamorphic areas. Formation and crystallization of melts in rocks of a wide compositional range involving zoisite/epidote has been ascribed to relatively high pressures and is consistent with experimentally determined stability fields in the simplified KCMASH system.     

Characterization of vivianite from Catavi,Llallagua Bolivia

Summary Vivianite from Catavi Mine, Llallagua, Bolivia, has a near ideal composition with traces of Mg, Zn and Mn. Total rare-earth elements are < 1,gmg/g. Mössbauer spectroscopy shows Fe^III/(Fe^II + Fe^III) is approximately 0.04.a = 10.030Å,b = 13.434Å,c = 4.714Å, = 102.73dg. The middle-infrared powder spectrum shows H₂O-related bands at 3490, 3290, 3130 cm^–1 (stretch), 1618 cm^–1 (bend), 825 cm^–1 (rock), and at 665 cm^–1 a possible M-OH₂ twist. P0₄ bands occur at 1045-940 cm^–1 (stretch) and 570-450 cm^–1 (bend). Corresponding laser Raman microprobe bands occur at 1051 (ms), 986 (m), 948 (vs), 867 (mw), 828 (w), 568, 532, 453 (m), 442 (mw). Weak Raman bands at about 342, 303, 270 (w), 235 (ms), 227 (sh, ms), 196 (ms), 187 (sh, m), 162 (mw), and 126 (m) may arise from lattice vibrations. Differential thermal responses include a major endotherm from 115–235°C with a shoulder at 170°C and a maximum at 210°C resulting from loss of structural water combined with oxidation of Fe²⁺, and two small exotherms with maxima at 605 and 780°C related to structural transformations.

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Charakterisierung des Vivianits von Catavi, Llallagua, Bolivien

Zusammenfassung Vivianit von der Catavi Mine, Llallagua, Bolivien zeigt annähernd ideale Zusammensetzung mit Spuren von Mg, Zn und Mn. Der gesamte Gehalt an seltenen EvolElementen ist < 1 ppm. Die Mössbauer Spektroskopie liefert ein Fe³⁺/(Fe²⁺ + Fe³⁺) Verhältnis von ungefähr 0.04.a = 10.030,b = 13.434,c = 4.714 Å, = 102.73°. Das Infrarot-Pulverspektrum zeigt dem H₂0 zuzuordnende Banden bei 3490, 3290, 3130 cm^–1 (Streckschwingungen), 1618 cm^–1 (Deformationsschwingung), 825 cm^–1 (Schaukelschwingung) und eine mögliche M-OH₂ Torsionsschwingung bei 665 cm^–1. PO₄ Banden liegen bei 1045-940 cm^–1 (Streckschwingung) und 570-450 cm^–1 (Deformations-schwingung). Entsprechende Banden der Laser Raman Mikrosonde liegen bei 1051 (mst), 986 (m), 948 (sst) 867 (mschw), 828 (schw), 568, 532, 453 (m), 442 (mschw). Raman Banden bei etwa 342, 303, 270 (schw), 235 (mst), 227 (Schulter, mst), 196 (mst), 187 (Schulter, m), 162 (mschw) und 126 (m) können auf Gitterschwingungen zurückgeführt werden. Differential-thermoanalytische Untersuchungen zeigen einen endothermen Bereich von 115–235°C mit einer Schulter bei 170 und einem Maximum bei 210°C, was auf den Verlust von strukturellem Wasser, das an eine Oxidation des Fe²⁺ gebunden ist, zurückzuführen ist; zwei auf strukturelle Transformationen zurückzuführende exotherme Maxima liegen bei 605 und 780°C.

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With 4 Figures     

Relationship between altered pyroxene diorite and the magnetite mineralization in the Chilean Iron Belt,with emphasis on the El Algarrobo iron deposits (Atacama region,Chile)

North of El Algarrobo (one of the four main deposits of the Chilean Iron Belt), the iron-mineralization (magnetite-etrmolite/actinolite-apatite) is related to clinoand orthopyroxene diorite intrusions which have crystallized at shallow depth (4km) under increasing oxygen fugacities. The supercritical fluid phase exsolved during cooling after the consolidation of the plutons (800–900°C), results in a H⁺, Cl^– and sodic enrichment, and in the sequential leaching of Fe (at less than 700°C), then Ca and Mg (between 600 and 500°C) from minerals of the primary magmatic diorite assemblage: titanomagnetite-ilmenite, plagioclase (An_70–40), augite, hypersthene. As a consequence of the cationic leaching, the lower mobility of silica and aluminium and the enrichment in sodium, residual altered dioritic rocks present a retromorphic mineral assemblage evolving down to boundary conditions of the greenchist-amphibolite facies (450°C). Fe, Mg and Ca are carried in cationic form associated with Cl^– anions, toward cooler rocks where they are precipitated. The deposition (between 550 and 450°C) of magnetite, followed iron-mineralization paragenesis, and occurred in fractured zones located both in altered diorites and contact andesites.