Experimental study of a biotite-bearing granitic system under water-saturated and water-undersaturated conditions

Phase relationships in haplogranitic melts containing ferromagnesian minerals were investigated in the temperature range 760–850°C, atP _total=2 kbar, =0.25 1.00, and at NNO buffer conditions. The compositions of coexisting biotite, orthopyroxene, magnetite and granitic melt were determined by electron microprobe. Biotite was the only Fe–Mg mineral in the starting material. It disappeared in water-undersaturated melts at temperatures above 830°C and in melts saturated with water above 810°C. Magnetite was present in all runs. Orthopyroxene became stable between 780 and 800°C. The Mg/(Mg+Fe) ratios of biotite (0,63–0,72) and orthopyroxene (0,63–0,71) are similar in water-saturated and undersaturated melts at comparable temperatures and increase slightly with increasing temperature. The solubility of Mg in haplogranitic melts is very low (0,07–0,24 wt% MgO). It increases slightly with temperature and is independent of the prevailing water activity. The solubility of Fe is low (0,91–1,37 wt% FeO); it also increases with increasing temperature, and it is higher in water-saturated than in water-undersaturated melts.     

Experimental calibration of the partitioning of Fe and Mg between biotite and garnet

The cation exchange reaction Fe₃Al₂Si₃O₁₂ +KMg₃AlSi₃O₁₀(OH)₂ = Mg₃Al₂Si₃O₁₂+KFe₃-AlSi₃ O₁₀(OH)₂ has been investigated by determining the partitioning of Fe and Mg between synthetic garnet, (Fe, Mg)₃Al₂Si₃O₁₂, and synthetic biotite, K(Fe, Mg)₃AlSi₃O₁₀(OH)₂. Experimental results at 2.07 kbar and 550 °–800 ° C are consistent with In [(Mg/Fe) garnet/(Mg/Fe) biotite] = -2109/T(°K) +0.782. The preferred estimates for ¯H and ¯S of the exchange reaction are 12,454 cal and 4.662 e.u., respectively. Mixtures of garnet and biotite in which the ratio garnet/biotite=49/1 were used in the cation exchange experiments. Consequently the composition of garnet-biotite pairs could approach equilibrium values in the experiments with minimal change in garnet composition (few tenths of a mole percent). Equilibrium was demonstrated at each temperature by reversal of the exchange reaction. Numerical analysis of the experimental data yields a geothermometer for rocks containing biotite and garnet that are close to binary Fe-Mg compounds.     

Distribution and significance of cordierite in paragneisses of the Hudson Highlands,southeastern New York

Cordierite occurs locally and sporadically in biotite-quartz-two feldspar paragneisses of the Precambrian Highlands complex in southeastern New York. Cordieritic and associated non-cordieritic gneisses were compared to determine the significance of cordierite for the metamorphic history of the complex.Microprobe analyses of the ferromagnesian phases show the following ranges in Fe/Mg (mol.): cordierite 0.19–0.43; biotite 0.33–0.73; garnet 1.98–3.56. Feldspar compositional ranges are: plagioclase An_25–53; K-feldspar in microperthite Or_62–87Ab_12–37 An_0–1. Garnet and plagioclase associated with cordierite are depleted in Ca relative to those in cordierite-free assemblages.Textural evidence, phase rule considerations and consistent distribution coefficients for FeO and MgO in coexisting garnet, cordierite and biotite from each locality examined suggest that all phases formed in at least local equilibrium during the hornblende-granulite subfacies metamorphism. The assemblages studied limit the conditions of metamorphism to between 700 and 750° C and 3.0 to 5.5. Kb. total pressure, with P _T greater than . Differences in mineral compositions and partitioning coefficients among the sampled areas suggest slight local differences within these ranges.Mineral and modal analyses of cordierite-bearing and cordierite-free gneisses show the latter to be enriched in calcium and potassium and depleted in alumina relative to the former. We conclude that the rarity of cordierite in the Highlands paragneisses reflects a scarcity of parent rocks of suitable composition rather than unfavorable physical conditions.     

The partitioning of Fe and Mg between olivine and carbonate and the stability of carbonate under mantle conditions

We have investigated the effect of Fe on the stabilities of carbonate (carb) in lherzolite assemblages by determining the partitioning of Fe and Mg between silicate (olivine; ol) and carbonates (magnesite, dolomite, magnesian calcite) at high pressures and temperatures. Fe enters olivine preferentially relative to magnesite and ordered dolomite, but Fe and Mg partition almost equally between disordered calcic carbonate and olivine. Measurement of K _d (X _Fe ^carb X _Mg ^ol /X _Fe ^ol X _Mg ^carb ) as a function of Fe/ Mg ratio indicates that Fe–Mg carbonates deviate only slightly from ideality. Using the regular solution parameter for olivine W _FeMg ^ol of 3.7±0.8 kJ/mol (Wiser and Wood 1991) we obtain for (FeMg)CO₃ a W _FeMg ^carb of 3.05±1.50 kJ/mol. The effect of Ca–Mg–Fe disordering is to raise K _d substantially enabling us to calculate W _CaMg ^carb -W _CaFe ^carb of 5.3±2.2 kJ/mol. The activity-composition relationships and partitioning data have been used to calculate the effect of Fe/Mg ratio on mantle decarbonation and exchange reactions. We find that carbonate (dolomite and magnesian calcite) is stable to slightly lower pressures (by 1 kbar) in mantle lherzolitic assemblages than in the CaO–MgO–SiO₂(CMS)–CO₂ system. The high pressure breakdown of dolomite + orthopyroxene to magnesite + clinopyroxene is displaced to higher pressures (by 2 kbar) in natural compositions relative to CMS. CO₂. We also find a stability field of magnesian calcite in lherzolite at 15–25 kbar and 750–1000°C.     

Cordierite gneisses and associated lithologies of the Guri area,Northwest Guayana Shield,Venezuela

Gneisses in the Guri area of the Venezuelan Guayana Shield contain mineral assemblages with cordierite, garnet, sillimanite, hypersthene, biotite and Fe-Ti oxide intergrowths.Analysis of mineral assemblages and compositional relationships in the light of experimental data indicate metamorphic conditions of 725–800° C, 5–6 kb P _T , <P _T for the highest grade rocks and 650–700° C, 5–7 kb P _T , approximating P _T for the lowest grade rocks. Oxygen fugacities in different lithologies ranged between those of the MH and QFM buffers.The distribution coefficient K _D (Mg-Fe) (gar-bio), decreases by 0.006 per atom percent increase in (Mn/Mn+Mg+Fe)gar, falls in the range of K _D typical of the sillimanite-K feldspar zone and granulite facies, and is systematically lower in lower grade rocks-all in accord with observation in other localities. K _D (Mg-Fe) (cord-bio) ranges from 3.0 in the highest grade rocks to 10.0 in the lowest grade rocks, appears independent of FeO/MgO of cordierite or biotite, and varies systematically with grade. In contrast with conclusions based on observation in other localities, data from the Guri area suggest -K_D(cord-bio) may be a sensitive index of grade.A number of mineralogic and geologic observations are difficultly reconciled with existing experimental data.     

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.     

Importance of considerations of mixing properties in establishing an internally consistent thermodynamic database: thermochemistry of minerals in the system Mg2SiO4-Fe2SiO4-SiO2

A thermodynamic solution model is developed for minerals whose compositions lie in the two binary systems Mg₂SiO₄-Fe₂SiO₄ and Mg₂Si₂O₆-Fe₂Si₂O₆. The formulation makes explicit provision for nonconvergent ordering of Fe²⁺ and Mg²⁺ between M1 and M2 sites in orthopyroxenes and non-zero Gibbs energies of reciprocal ordering reactions in both olivine and orthopyroxene. The calibration is consistent with (1) constraints provided by available experimental and natural data on the Fe-Mg exchange reaction between olivine and orthopyroxene ± quartz, (2) site occupancy data on orthopyroxenes including both crystallographic refinements and Mössbauer spectroscopy, (3) enthalpy of solution data on olivines and orthopyroxenes and enthalpy of disordering data on orthopyroxene, (4) available data on the temperature and ordering dependence of the excess volume of orthopyroxene solid solutions, and (5) direct activity-composition determinations of orthopyroxene and olivine solid solutions at elevated temperatures. Our analysis suggests that the entropies of the exchange [Mg(M2)Fe(M1)Fe(M2)Mg(M1)] and reciprocal ordering reactions [Mg(M2)Mg(M1)+ Fe(M2)Fe(M1)Fe(M2)Mg(M1)+Mg(M2)Fe(M1)] cannot differ significantly (± 1 cal/K) from zero over the temperature range of calibration (400°–1300° C). Consideration of the mixing properties of olivine-orthopyroxene solid solutions places tight constraints on the standard state thermodynamic quantities describing Fe-Mg exchange reactions involving olivine, orthopyroxene, pyralspite garnets, aluminate spinels, ferrite spinels and biotite. These constraints are entirely consistent with the standard state properties for the phases-quartz,-quartz, orthoenstatite, clinoenstatite, protoenstatite, fayalite, ferrosilite and forsterite which were deduced by Berman (1988) from an independent analysis of phase equilibria and calorimetric data. In conjunction with these standard state properties, the solution model presented in this paper provides a means of evaluating an internally consistent set of Gibbs energies of mineral solid solutions in the system Mg₂SiO₄-Fe₂SiO₄-SiO₂ over the temperature range 0–1300° C and pressure interval 0.001–50 kbars. As a consequence of our analysis, we find that the excess Gibbs energies associated with mixing of Fe and Mg in (Fe, Mg)₂SiO₄ olivines, (Fe, Mg)₃Al₂Si₃O₁₂ garnets, (Fe, Mg)Al₂O₄ and (Fe, Mg)Fe₂O₄ spinels, and K(Mg, Fe)₃AlSi₃O₁₀(OH)₂ biotites may be satisfactory described, on a macroscopic basis, with symmetric regular solution type parameters having values of 4.86±0.12 (olivine), 3.85±0.09 (garnet), 1.96±0.13 (spinel), and 3.21±0.29 kcals/gfw (biotite). Applications of the proposed solution model demonstrate the sensitivity of petrologic modeling to activity-composition relations of olivine-orthopyroxene solutions. We explore the consequences of estimating the activity of silica in melts forming in the mantle and we develop a graphical geothermometer/geobarometer for metamorphic assemblages of olivine+orthopyroxene+quartz. Quantitative evaluation of these results suggests that accurate and realistic estimates of silica activity in melts derived from mantle source regions,P-T paths of metamorphism and other intensive variables of petrologic interest await further refinements involving the addition of trace elements (Al³⁺ and Fe³⁺) to the thermodynamic formulation for orthopyroxenes.     

Authigenic layer silicate minerals in borehole Elmore 1, Salton Sea Geothermal Field,California, USA

A combined petrographic/X-ray/electron microprobe and energy dispersive system investigation of sandstone cuttings from borehole Elmore # 1 near the center of the Salton Sea Geothermal Field has revealed numerous regular variations in the composition, texture, mineralogy and proportions of the authigenic layer silicate minerals in the temperature interval 185° C (411.5 m depth) to 361° C (2,169 m). At temperatures near 190° C, dolomite/ankerite+calcite-bearing sandstones contain an illite/mixed layer phase with 10% expandable layers (dolomite/ankerite zone). In shale, the percentage of expandable layers in the mixed layer phase changes from 10–15% at 185° C to 5% at 210° C (494 m). In the interval 250° C (620 m) to 325° C (1,135 m), the calcite+pyrite+epidote-bearing sandstones contain a layer silicate assemblage of chlorite and illite (chlorite-calcite zone). In the shallower portions of this metamorphic zone, the illite contains 0–5% expandable layers, while at depths greater than 725 m (275° C) it is completely free of expandable layers. On increasing temperature, the white mica shows regular decreases in Si^IV, Mg and Fe, and increase in Al^IV, Al^VI, and interlayer occupancy, as it changes gradually from fine-grained illite (=textural sericite) to coarse-grained recrystallized phengitic white mica. In the same interval, chlorite shows decreases in Al^VI and octahedral vacancies and an increase in total Mg+Fe. The sandstones range from relatively unmodified detrital-textured rocks with porosities up to 20% and high contents of illite near 250° C to relatively dense hornfelsic-textured rocks with trace amounts of chlorite and phengite and porosities near 5% at 325° C. Numerous complex reactions among detrital (allogenic) biotite, chlorite, and muscovite, and authigenic illite and chlorite, occur in the chlorite-calcite zone.Biotite appears, and calcite disappears, at a temperature near 325° C and a depth of 1,135m. The biotite zone so produced persists to 360° C in sandstone, at which temperature orthoclase disappears and andradite garnet appears at a depth near 2,155 m. Throughout the biotite zone and into the garnet zone, the biotite undergoes compositional changes that are very similar to those observed in illite/phengite in the chlorite-calcite zone, including increases in interlayer occupancy, Al^IV, Al^VI, and Ti, and decreases in F^–, Si^IV, and Mg/Fe_t+Mg, on increasing temperature. Biotite thus changes from a siliceous, K-deficient biotite at the biotite isograd to a typical low-grade metamorphic biotite at temperatures near 360° C. Minor amounts of talc appear with biotite at the biotite isograd in sandstone, while actinolite appears in both sandstone and shale at temperatures near 340° C (1,325 m). Chlorite completely disappears from sandstone at temperatures of approximately 350° C (1,500 m), and diminishes abruptly in amount in the more chloritic shales at the same depth.     

Coexisting cummingtonite and calcic amphibole in amphibolites from the Schneeberg complex,Tyrol, Austria

Summary The formation of cummingtonite in two Ca and Al-poor and Mg-rich amphibolites from the Austroalpine Schneeberg complex occurred at a maximum temperature of about 550°C (5 kb). This is a result of the amphibolite facies Alpine overprint in this part of the Eastern Alps.Textural and chemical relations suggest (Mg_–1Si_–1Al₂)-continuous reactions in the bivariant CMASH-assemblageCam-Cum-Chl ^* followed by the discontinuous reactionCam+Chl+Qu=Cum+Plg+H₂O to be responsible for the formation of cummingtonite in these samples.The Mg–Fe distribution coefficient with values of 0.6–0.7 is similar to cummingtonite-Ca-amphibole pairs from amphibolites with oligoclase+quartz reported in the literature. The Mg/(Mg+Fe) ratio of the calcic amphiboles is lower (0.539–0.555) than the coexisting cummingtonites (0.648–0.662).

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Koexistierende Cummingtonite und Hornblenden in Amphiboliten des Schneeberger Zuges, Tirol, Österreich

Zusammenfassung In zwei Ca- und Al-armen Amphiboliten des nördlichen Schneebergerzuges (Rotmoostal) bildete sich Cummingtonit bei Maximaltemperaturen von 550°C (5 kb) bei der Altalpidschen Metamorphose.Texturelle und chemische Beziehungen lassen vermuten, daß sich Cummingtonite sowohl nach kontinuierlichen Reaktionen (in bezug auf den Tschermak-Vektor Mg_–1Si_–1Al₂) gebildet hat, als auch aus Hornblende und Chlorit nach der diskontinuierlichen ACF-ReaktionCam+Chl+Qu=Cum+Plg+H₂O hervorgegangen ist.Der Mg–Fe-Verteilungskoeffizient zwischen Hornblende und Cummingtonit entspricht den aus der Literatur bekannten Werten. Er beträgt zwischen 0.6–0.7 für die beginnende Amphibolitfazies. Die Mg/(Mg+Fe)-Verhältnisse sind höher in Cummingtonit (0.648–0.662) als in der koexistierenden Hornblende (0.539–0.555).

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With 4 Figures     

An experimental study of Fe-Mg partitioning between garnet and olivine and its calibration as a geothermometer

The partitioning of Fe and Mg between coexisting garnet and olivine has been studied at 30 kb pressure and temperatures of 900 ° to 1,400 °C. The results of both synthesis and reversal experiments demonstrate that K _D (= (Fe/Mg)_gt/(Fe/Mg)_OI) is strongly dependent on Fe/Mg ratio and on the calcium content of the garnet. For example, at 1,000 °C/30 kb, K _D varies from about 1.2 in very iron-rich compositions to 1.9 at the magnesium end of the series. Increasing the mole fraction of calcium in the garnet from 0 to 0.3 at 1,000 ° C increases K _D in magnesian compositions from 1.9 to about 2.5.The observed temperature and composition dependence of K _D has been formulated into an equation suitable for geothermometry by considering the solid solution properties of the olivine and garnet phases. It was found that, within experimental error, the simplest kind of nonideal solution model (Regular Solution) fits the experimental data adequately. The use of more complex models did not markedly improve the fit to the data, so the model with the least number of variables was adopted.Multiple linear regression of the experimental data (72 points) yielded, for the exchange reaction: 3Fe₂SiO₄+2Mg₃Al₂Si₃O₁₂ olivine garnet 2Fe₂Al₂Si₃O₁₂+3Mg₂SiO₄ garnet olivine H ° (30kb) of –10,750 cal and S ° of –4.26 cal deg^–1 mol^–1. Absolute magnitudes of interaction parameters (W _ij ) derived from the regression are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the following differences between solution parameters and these differences are fairly well constrained: W _FeMg ^ol -W _FeMg ^gt 800 cal W _CaMg ^gt -W _CaFe ^gt 2,670 cal.The geothermometer is most sensitive in the temperature and composition regions where K _D is substantially greater than 1. Thus, for example, peridotitic compositions at temperatures less than about 1,300 ° C should yield calculated temperatures within 60 °C of the true value. Iron rich compositions (at any temperature) and magnesian compositions at temperatures well above 1,300 °C could not be expected to yield accurate calculated temperatures.For a fixed K _D the influence of pressure is to raise the calculated temperature by between 3 and 6 °C per kbar.     

Chemical mineralogy of some cordierite-bearing rocks near Yellowknife,Northwest Territories,Canada

Meta-graywacke and meta-argillite of Archean age near Yellowknife contain biotite, cordierite, gedrite and sillimanite isograds towards the Sparrow Lake granite pluton. The chemistry of biotite, cordierite, gedrite and garnet in rocks that up-grade from the cordierite isograd indicate a small range of chemical composition, particularly with reference to Mg, Fe and Mn. The analyses show further that among the coexisting ferromagnesian minerals Fe/Fe+ Mg ratio decreases in the sequence: garnet, gedrite, biotite, cordierite while Mn/Fe+Mg+Mn ratio decreases in the sequence garnet, gedrite, cordierite, biotite. The same order is also observed in the distribution diagrams. The regular distribution of Mg, Fe and Mn among the coexisting phases demonstrate that chemical equilibrium was attained and preserved in these Archean rocks. Mg-Fe distribution between cordierite and biotite appears to be dependent on the temperature of crystallization or metamorphic grade.     

Internally consistent gahnitic spinel-cordierite-garnet equilibria in the FMASHZn system: geothermobarometry and applications

The equilibrium (Mg, Fe, Zn)₃Al₂Si₃O₁₂+2Al₂SiO₅=3(Mg, Fe, Zn)Al₂O₄+5SiO₂ garnet + sillimanite/kyanitc = spinel + quartz was calibrated in the piston-cylinder apparatus between 11 and 30 kbar, and over the temperature range of 950 to 1200°C. Three experimental mixes of Mg no. [100^*MgO/(MgO+FeO)] 40, 47 and 60, in the FeO –MgO–Al₂O₃–SiO₂–ZnO (FMASZn) system were used under low oxygen fugacities and anhydrous conditions. We derive a ternary Fe–Mg–Zn symmetric mixing model for aluminous spinels in equilibrium with garnet, to quantify the increase in gahnitic end-member of spinel with increasing pressure and descreasing temperature. Further experiments in the spinel-cordieritequartz-sillimanite field were combined with garnet-cordierite data from the literature to produce a consistent set of equations describing the exchange reactions in FMASHZn relevant to quartz-sillimanite bearing rocks at granulite facies conditions. As spinel is an important mineral participating in many rocks of aluminous composition at granulite-facies conditions, and as zinc contributes to an enlargement of spinel's stability field towards higher pressures and lower temperatures, the thermobarometric calibrations presented here will be most significant in delineating the prograde and retrograde trajectory of P-T paths.     

Thermodynamics of multicomponent pyroxenes: II. Phase relations in the quadrilateral

The model for the thermodynamic properties of multicomponent pyroxenes (Part I) is calibrated for ortho- and clinopyroxenes in the quadrilateral subsystem defined by the end-member components Mg₂Si₂O₆, CaMgSi₂O₆, CaFeSi₂O₆, and Fe₂Si₂O₆. This calibration accounts for: (1) Fe-Mg partitioning relations between orthopyroxenes and augites, and between pigeonites and augites, (2) miscibility gap features along the constituent binary joins CaMgSi₂O₆-Mg₂Si₂O₆ and CaFeSi₂O₆-Fe₂Si₂O₆, (3) calorimetric data for CaMgSi₂O₆-Mg₂Si₂O₆ pyroxenes, and (4) the P-T-X systematics of both the reaction pigeonite=orthopyroxene+augite, and miscibility gap featurs, over the temperature and pressure ranges 800–1500°C and 0–30 kbar. The calibration is achieved with the simplifying assumption that all regular-solution-type parameters are constants independent of temperature. It is predicated on the assumptions that: (1) the Ca-Mg substitution is more nonideal in Pbca pyroxenes than in C2/c pyroxenes, and (2) entropies of about 3 and 6.5 J/K-mol are associated with the change of Ca from 6- to 8-fold coordination in the M2 site in magnesian and iron C2/c pyroxenes, respectively. The model predicts that Fe²⁺-Mg²⁺ M1-M2 site preferences in C2/c pyroxenes are highly dependent on Ca and Mg contents, with Fe²⁺ more strongly preferring M2 sites both in Ca-rich C2/c pyroxenes with a given Fe/(Fe+Mg) ratio, and in magnesian C2/c pyroxenes with intermediate Ca/(Ca+Fe+Mg) ratios.The proposed model is internally consistent with our previous analyses of the solution properties of spinels, rhombohedral oxides, and Fe-Mg olivines and orthpyroxenes. Results of our calibration extend an existing database to include estimates for the thermodynamic properties of the C2/c and Pbca pyroxene end-members clinoenstatite, clinoferrosilite, hedenbergite, orthodiopside, and orthohedenbergite. Phase relations within the quadrilateral and its constitutent subsystems are calculated for temperatures and pressures over the range 800–1700°C and 0–50 kbar and compare favorably with experimental constraints.     

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.     

Distribution of Mg and Fe in cummingtonite-hornblende and cummingtonite-actinolite pairs from metamorphic assemblages

The atomic fractions Mg/(Mg + Fe) and the Mg-Fe distribution coefficient $$K_{{\text{D}}{\text{.Mg - Fe}}}^{{\text{Ca - am - Cum}}} \left( { = \tfrac{{[{\text{Mg/Fe]}}_{{\text{Ca - am}}} }}{{{\text{[Mg/Fe]}}_{{\text{Cum}}} }}} \right)$$ are calculated for 31 metamorphic cummingtonite-hornblende pairs. Data on 21 pairs are taken from the litterature, and new electron microprobe analyses and structural formulae are presented of nine pairs from Tydal, Sör-Tröndelag, Norway, and of one pair from Cooma, N.S.W., Australia (cf. Kisch, 1969). The electron microprobe methods used are described, particularly the use of mineral standards, and the variation of the mass absorption in substitution series. The hornblendes from the Tydal pairs are markedly pargasitic in composition, and contain minor proportions of the cummingtonite “molecule”. The Mg-Fe distributions in the cummingtonite-hornblende pairs — as plotted on a [Mg/(Mg + Fe)]_Ca-am vs. [Mg/(Mg + Fe)]_Cum diagram (Fig. 3) — differ significantly from the Mg-Fe distribution curve for cummingtonite-actinolite pairs from Quebec (Mueller, 1961). Whereas the actinolites have markedly higher Mg/Fe ratios than the co-existing cummingtonites (K _D.Mg-Fe ^Ca-am-Cum ≈ 1.5–2.0), the cummingtonite-hornblende pairs diverge towards lower values from the distribution coefficient. In most of the metamorphic cummingtonite-hornblende pairs — including the nine pairs from Tydal — the Mg/Fe ratio of the hornblende is lower than in the co-existing cummingtonite, i.e K _D.Mg-Fe ^Ca-am-Cum <1. A relation appears to exist between the Mg-Fe distribution and the Al content of the calcic amphibole phase. This is believed to be due to the non-random distribution of Al^Y among the octahedral lattice sites: in hornblende Al^VI enters the M ₁+M₃ positions, in which Mg is preferred over Fe, rather than M ₂, in which Fe is preferred (Ghose, 1965). Since the cummingtonites remain Al-poor, the over-all Mg/Fe ratio in the hornblende is reduced relative to the co-existing cummingtonite as a result. The variations of the Mg-Fe distribution in the cummingtonite-hornblende pairs can also be related directly to the presence and composition of the plagioclase and other Al-rich phases in the metamorphic mineral assemblage. In any range of Mg/Fe ratios, the cummingtonite-hornblende pairs associated with oligoclase have lower distribution coefficients (0.61–0.81; 12 pairs) than those associated with calcic plagioclase or plagioclase-free assemblages (0.97 to 1.89; 6 pairs); the pairs associated with andesine have intermediate Mg-Fe distributions (0.74–1.15; 6 pairs).     

Shock-induced thermal transformations of ries-biotites

Thermal transformations in biotites depending on increasing postshock temperatures were studied.A decrease of the unit cell volume, caused by oxidation of octahedral Fe²⁺ to Fe³⁺ at temperature rangings of about 400–500° C was observed.In the same temperature range (500° C) the biotite lattice becomes thermodynamically unstable. Intersheet and octahedral layers are decomposed, whereas individual SiO₄-tetrahedra are more resistant to thermal vibrations. The decomposition of the biotite leads to the formation of new minerals and amorphous phases. By X-ray analysesmagnetite, hercynite, pyroxene, feldspar, and quartz were identified.Thermal transformations of biotite induced by shock waves are characterized by states of distinct disequilibrium.     

Effect of grossular-content in garnet on the partitioning of Fe and Mg between garnet and biotite

In contrast to Ferry (1980) (X _Ca)-values in garnet even lower than 0.1 have a significant effect on the calculated equilibrium temperature using the experimental calibration of the Fe and Mg paritioning between garnet and biotite. Garnet compositions and Mg/Fe — distribution coefficients from samples of the Eoalpine staurolite — in zone in the southern Ötztal are related by the quadratic regression equation: InK _D= -1.7500 (±0.0226) + 2.978 (±0.5317)X _Ca ^Gt -5.906(±2.359)(X _Ca ^Gt )² Temperatures derived by the Ferry and Spear (1978) calibration using chemistry — correctedK _D values are petrologically realistic.Analysis of our data supports non ideal mixing of grossular with almandine — pyrope solid solution. The derived excess mixing energies are quite small for the almandine — pyrope solution (W _FeMg= –133 cal/mole) and about +2775 cal/mole for the difference between pyrope-grossular and almandine-grossular solutions (W _MgCa —W _FeCa) at metamorphic conditions of 570° C and 5,000 bar. The mixing parameters proposed by Ganguly and Saxena (1984) are not confirmed by our data as they would result in significantly lower temperatures.     

Hydrothermal alteration of Tertiary igneous rocks from the Isle of Skye,northwest Scotland

Hydrothermal alteration of Tertiary gabbros from Skye involved the reaction of igneous olivine, augite, hypersthene, plagioclase, magnetite, and ilmenite with aqueous fluid primarily to combinations of talc, chlorite, montmorillonite, calcic amphibole, biotite, and secondary magnetite. Lesser amounts of calcite, epidote, quartz, sphene, prehnite, and garnet also developed. During mineralogical alteration of gabbro there was a net addition to rock of K, Na, Sr, and H₂O and a net loss of Mg. Gabbro was oxidized early in the hydrothermal event and later reduced. Iron and silicon were probably initially lost and later added. There is no evidence for significant change in the Al or Ca content of the gabbros. Hydrothermal alteration of Skye gabbro involved not only large-scale migration of ¹⁸O, ¹⁶O, D and H but also of K, Na, Sr, Mg, and probably Fe and Si.Mineral thermometry indicates that pyroxenes in the gabbros crystallized at 1000° C–1150° C and were very resistent chemically as well as isotopically to later hydrothermal alteration. Hypothetical equilibrium between primary and secondary mafic silicates suggests that mineralogical alteration of gabbro occurred at 450°–550° C. The lack of correlation between mineralogical and isotopic alteration of gabbro requires that much isotopic alteration occurred at temepratures above those at which the secondary minerals developed, 550°–1000° C. The chemical alteration of gabbro is correlated with its mineralogical alteration and therefore occurred at 450°–550° C.Measured progress of the mineral-fluid reactions was used to estimate the amount of H₂O fluid that infiltrated the gabbro as primary olivine was converted to talc+magnetite at 525°–550° C. Calculated fluid-rock ratios are in the range 0.2–6 (volume basis) and are smaller than values estimated from isotopic data (fluid/rock 1–10, volume basis). Both isotopic and petrologic data point to pervasive flow of fluid through crystalline rock at elevated temperatures of 500°–1000° C. Isotopic fluid-rock ratios are larger than petrologic fluid-rock ratios because isotopic alteration of cooling gabbro began earlier and at higher temperatures than did the mineralogical alteration.     

Intercrystalline cation partitioning between minerals of the triplite-zwieselite-magniotriplite and the triphylite-lithiophilite series in granitic pegmatites

Minerals of the triphylite-lithiophilite, Li(Fe, Mn)PO₄, and the triplite-zwieselite-magniotriplite series, (Mn, Fe, Mg)₂PO₄F, occur in the late stage period of pegmatite evolution. Unfortunately, neither are the genetic relationships between these phosphates fully understood nor are thermodynamic data known. Consequently, phosphate associations and assemblages from 8 granitic pegmatites — Clementine II, Rubicon II and III, and Tsaobismund (Namibia); Hagendorf-Süd and Rabenstein (Germany); Valmy (France); Viitaniemi (Finland) — have been tested for compositional zoning and intercrystalline partitioning of main elements by electron microprobe techniques. Although the selected pegmatites display varying degrees of fractionation, and the intergrowth textures indicate different genetic relationships between the phosphates, the plots of mole fractions X _Fe=Fe/(Fe+Mn+Mg+Ca), X _Mn=Mn/(Fe+Mn+Mg+Ca), and X _Mg=Mg/(Fe+Mn+Mg+Ca) can be fitted relatively well with smooth curves in Roozeboom diagrams. Their deviations from symmetrical distribution curves are mainly dependent upon X _Mg or X _Ca, and upon non-ideal solutions. Surprisingly small differences between the partition coefficients were detected for intergrowths of different origin. However, the partitioning of shared components among coexisting phases is clearly dependent upon the conditions of formation. Compositional zoning is observed only when both Fe–Mn phosphates are intergrown mutually or with other Fe–Mn–Mg mineral solid-solutios. Thus, the zoning does not seem to be due to continuous crystallization, but to later diffusion processes. The triplite structure has preference for Mn, Mg, and Ca, while Fe prefers minerals of the triphylite series. A quantification of main element fractionation between minerals of the triphylite and the triplite series is possible in the cases where diffusion can be excluded. For the Fe/(Fe+Mn) ratios of core compositions an equation with a high correlation coefficient (R=0.988) was determined: Fe/(Fe+Mn)_Tr=[Fe/(Fe+Mn)_Li]/{2.737-(1.737)[Fe/(Fe+Mn)_Li]} (Tr=triplite series, Li=triphylite series). Consequently, the Fe/(Fe+Mn) ratio of the triplite series can now also be used in the interpretation of pegmatite evolution, just like that of the triphylite series which has been successfully applied in the past.     

Paragenetic relations in gedrite — cordierite — staurolite — biotite sillimanite — kyanite gneisses at Ajitpura,Rajasthan, India

Aluminous parageneses containing gedrite, cordierite, garnet, staurolite, biotite, sillimanite, kyanite, quartz or spinel plus corundum are found as dark colored lenses in the polymetamorphic, multideformed Archean complex at Ajitpura in northwest peninsular India. Staurolite, like kyanite, is a relict phase of earlier metamorphism and is excluded as a paragenetic mineral in view of its incompatibility with quartz and gedrite and its lower X _Mg values than for garnet of the assemblage. Its stability here is attributed to zinc content of up to 3 wt%. The XMg in other ferromagnesian minerals decreases in the order: cordierite, biotite, gedrite, garnet, as found elsewhere in high grade rocks.The textural criteria and systematic partitioning of Fe and Mg in the ferromagnesian phases, excluding staurolite, indicate attainment of equilibrium during the second metamorphism. From tie line configurations in the phase diagrams, X _Mg ratios in the constituent minerals, and other petrographic criteria, it is suggested that gedrite — cordierite-garnet — sillimanite — biotite assemblage has been produced by the reactions: Biotite+Sillimanite+Quartz = Cordierite+Garnet+K-feldspar+Vapor (1) and Biotite+Sillimanite+Quartz = Cordierite +Gedrite+K-feldspar+Vapor (2) which occurred during partial melting of the rocks at fixed P and T conditions.By isothermal P-X(Fe-Mg) sections it has been demonstrated that release of FeO, SiO₂ and other components modified the composition of the reactant biotite presumably by the substitution FeSi2 Al, whereby reaction 1 was replaced by reaction 2. Cordierite with higher X _Mg was produced with gedrite instead of with garnet, whose X _Mg is less than X _Mg of gedrite. Reaction 2 has been tentatively located in T-P space from the intersection of some continuous loops in the P-X(Fe-Mg) diagram at 700°C and also by other constraints. The discontinuous reaction 2 is located about 1–2 kilobars higher than reaction 1, which implies that it is difficult to distinguish between effects of pressure and those of melting on the X _Mg ratios of the reaction phases.The P-T calibrations of garnet — cordierite, garnet — biotite and garnet — plagioclase equilibria and the calibrations from other dehydration curves give temperatures near 700°C and pressure (assuming ) about 6 kilobars.