Zoned ternary feldspars in the Klokken intrusion: exsolution microtextures and mechanisms

The microtextures developed during relatively slow cooling as a function of bulk composition in zoned ternary feldspars from syenodiorites and syenites in the Klokken intrusion, described in the preceding paper, were determined by TEM and their origin and evolution deduced. The feldspars normally have a plagioclase core and an alkali feldspar rim; cores become smaller and rims larger and the An content of both decrease with distance from the contact of the intrusion. The following microtextural sequence was observed. The inner plagioclase cores are homogeneous oligoclase-andesine with Albite growth twins only, but are crypto-antiperthitic towards the outer core. At first small platelets of low sanidine a few nanometres thick and up to 10 nm long occur sporadically only on Albite-twin composition planes. With further increase in bulk Or they are homogeneously distributed in the plagioclase. Thicker, through-going plates in platelet-free areas are found, which induce Albite twins in the surrounding plagioclase. The microtextures in the rims are regular cryptomesoperthitic, with (¯601) lenses or lamellae, depending on the bulk Or-content, of low sanidine in Albite-twinned low oligoclase-andesine. Albite and Pericline twins in plagioclase in an M-twin relationship, together with lenticular low sanidine, were found in only one small area. The overall diffraction symmetry of the mesoperthites is monoclinic, showing that exsolution started in a monoclinic feldspar, whereas that of the antiperthites is triclinic. The intermediate zone between the core and rim is more complex and microtextures vary over distances of a few micrometres.The cryptomesoperthites are very regular where Or-rich and probably arose by spinodal decomposition. The platelets in the outer cores arose by heterogeneous nucleation on twin composition planes and by homogeneous nucleation elsewhere. Near the intermediate zone they coarsened to give larger plates which induced Albite-twins in the plagioclase. Because of the zoning, microtextures that were initiated in areas of given composition, can propagate laterally into zones of different composition. A diagram is given showing the relationship between ternary bulk composition and the microtexture developed in coherent perthitic alkali feldspars and plagioclases from slowly-cooled rocks.CRPG contribution 730     

Eight-phase alkali feldspars: low-temperature cryptoperthite, peristerite and multiple replacement reactions in the Klokken intrusion

Eight feldspar phases have been distinguished within individual alkali feldspar primocrysts in laminated syenite members of the layered syenite series of the Klokken intrusion. The processes leading to the formation of the first four phases have been described previously. The feldspars crystallized as homogeneous sodian sanidine and exsolved by spinodal decomposition, between 750 and 600 °C, depending on bulk composition, to give fully coherent, strain-controlled braid cryptoperthites with sub-μm periodicities. Below ~500 °C, in the microcline field, these underwent a process of partial mutual replacement in a deuteric fluid, producing coarse (up to mm scale), turbid, incoherent patch perthites. We here describe exsolution and replacement processes that occurred after patch perthite formation. Both Or- and Ab-rich patches underwent a new phase of coherent exsolution by volume diffusion. Or-rich patches began to exsolve albite lamellae by coherent nucleation in the range 460–340 °C, depending on patch composition, leading to film perthite with ≤1 μm periodicities. Below ~300 °C, misfit dislocation loops formed, which were subsequently enlarged to nanotunnels. Ab-rich patches (bulk composition ~Ab₉₁Or₁An₈), in one sample, exsolved giving peristerite, with one strong modulation with a periodicity of ~17 nm and a pervasive tweed microtexture. The Ab-rich patches formed with metastable disorder below the peristerite solvus and intersected the peristerite conditional spinodal at ~450 °C. This is the first time peristerite has been imaged using TEM within any perthite, and the first time peristerite has been found in a relatively rapidly cooled geological environment. The lamellar periodicities of film perthite and peristerite are consistent with experimentally determined diffusion coefficients and a calculated cooling history of the intrusion. All the preceding textures were in places affected by a phase of replacement correlating with regions of extreme optical turbidity. We term this material ultra porous late feldspar (UPLF). It is composed predominantly of regions of microporous very Or-rich feldspar (mean Ab_2.5Or_97.4An_0.1) associated with very pure porous albite (Ab_97.0Or_1.6An_1.4) implying replacement below 170–90 °C, depending on degree of order. In TEM, UPLF has complex, irregular diffraction contrast similar to that previously associated with low-temperature albitization and diagenetic overgrowths. Replacement by UPLF seems to have been piecemeal in character. Ghost-like textural pseudomorphs of both braid and film parents occur. Formation of patch perthite, film perthite and peristerite occurred 10⁴–10⁵ year after emplacement, but there are no microtextural constraints on the age of UPLF formation.     

Exsolution in ternary feldspars

It is shown that the microstructures of the antiperthites of the South Norwegian anorthosites strongly suggest an exsolution origin. In addition to the normal exsolution reactions responsible for most antiperthites, exsolution of potash feldspar in plagioclase may occur by a discontinuous mode of precipitation.     

Exsolution in ternary feldspars

The equilibrium precipitate formed by exsolution in alkali feldspar containing calcium in solid solution is a two-phase intergrowth of plagioclase and quartz. It is the kinetics of the precipitation, however, which decides whether or not this reaction will take place. It is shown that lamellar growth of plagioclase and quartz is only accomplished by the advancement of an incoherent boundary originating at an alkali feldspar — alkali feldspar boundary or at an interface between alkali feldspar and plagioclase.     

Activity/composition relations in the ternary feldspars

Activity/composition relations are presented for high-structural state feldspars whose bulk compositions lie within the ternary system NaAlSi₃O₈ CaAl₂Si₂O₈-KAlSi₃O₈. The expressions are parameterized from the data for coexisting feldspars of Seck (1971a) using an asymmetric regular solution approximation for the excess Gibbs free energy of mixing and an Al-avoidance model for the configurational entropy of solution. The solution properties of the plagioclase and alkali-feldspar binaries have been made to conform to the recent work of Thompson and Hovis (1979) and Newton et al. (1980). Using the proposed model the ternary feldspar solvus is extrapolated in temperature (up to 1,500° C) and pressure (up to 5kbars). A new two-feldspar geothermometer is presented which provides somewhat more reasonable estimates of crystallization temperatures than the equations and graphs of Stornier (1975), Powell and Powell (1977), Brown and Parsons (1981) and Haselton et al. (1983). In conjunction, some criteria are suggested for establishing the existence of equilibrium tie-lines between coexisting ternary feldspars in rhyolites and trachytes. Calculated values of the activity of KAlSi₃O₈ in plagioclase are examined in some detail. These compare favorably with independent estimates obtained from experimentally grown plagioclases precipitating at liquidus temperatures from igneous rocks of widely varying alkali contents.     

On the mechanisms of cation diffusion processes in ternary feldspars

Cation diffusion processes have been studied in single crystals of intermediate plagioclase and albite composition by tracer-diffusion experiments and optical absorption spectroscopy. Tracer-diffusivities were determined by the residual activity method, using the radioactive isotopes ²²Na, ⁴⁵Ca and ⁵⁹Fe. In most cases, diffusion experiments were performed at 1 bar, at controlled oxygen activity and at temperatures between 750 and 1300°C. The obtained Na-diffusivities for plagioclases were much smaller then previously determined for albite. This indicates a strong composition dependence of Na-diffusion. In contrast, Ca-diffusivity in albite does not differ very much from that in intermediate plagioclases. The relative diffusivities determined for plagioclase of composition An62 at 1200° C (CO/CO₂ =50∶50) were D _infNa ^sup* ∶D _infFe ^sup* ∶D _infCa ^sup* =5000∶10∶1. Despite the an isotropic structure of feldspars, no difference was found for Na-and Ca-diffusion normal to (001) and normal to (010). Water pressure of 2 kbar has no influence on the Na-diffusivity. In contrast to the Ca-diffusion, a dependence on oxyggen activity was found for Na-and Fe-diffusion. Fe-diffusivity increases with decreasing oxygen activity. This can be correlated to changes in oxidation state of iron dissolved in the plagioclases. Optical absorption spectroscopy shows that iron is oxidized in the plagioclases by annealing in air. This effect can be reversed by annealing at reducing conditions. A model is proposed to explain the oxidation of iron by a chemical diffusion process in which A-vacancies are formed by out-diffusion of Na⁺. Preannealing of samples in air gives a temperature independent decrease of Na-diffusivity by a factor of about 2.5. This effect is explained with help of a simple disorder model for A-cations in ternary feldspars. It is concluded that Na⁺ diffuses via interstitials and that the A-vacancy concentration in the plagioclases is controlled extrinsically, probably by dissolved SiO₂.     

Quantitative P-T paths from zoned minerals: Theory and tectonic applications

An analytical approach to the analysis of zoning profiles in minerals is presented that simultaneously accounts for all of the possible continuous reactions that may be operative in a given assemblage. The method involves deriving a system of simultaneous linear differential equations consisting of a Gibbs-Duhem equation for each phase, a set of linearly independent stoichiometric relations among the chemical potentials of phase components in the assemblage, and a set of equations describing the total differential of the slope of the tangent plane to the Gibbs free energy surface of solid solution phases. The variables are the differentials of T, P, chemical potentials of all phase components, and independent compositional terms of solid solution phases. The required input data are entropies, volumes, the compositions of coexisting phases at a reference P and T, and an expression for the curvature of the Gibbs functions for solid solution phases. Results derived are slopes of isopleths (dP/dT, dX/dT or dX/dP) which can be used to contour P-T diagrams with mineral composition.To interpret mineral zoning, T and P can be expressed as functions of n independent composition parameters, where n is the variance of the mineral assemblage. The total differentials of P and T are differential equations that can be solved by finite difference techniques using the derivatives obtained from the analytical formulation of phase equilibria.Results calculated from Zone I and Zone IV garnets of Tracy et al. (1976) indicate that Zone I garnets grew while T increased (T+72° C) and P decreased sharply (P–3 kb). Zone IV garnets zoned in response to decreasing T (T–17° C) and P (P–1 kb). A P-T path calculated for a zoned garnet from the Greinerschiefer series, western Tauern Window, Austria, also indicates growth during decompression (–3kb) and heating (T+15° C). A P-T path calculated for the Wissahickon schist (Crawford and Mark 1982) indicates growth during cooling and compression (T–25 C, P+2.2 kb). The calculated P-T paths differ according to structural environment and can be used to relate mineral growth to tectonic processes.     

Rare earth element partition between sphene,apatite and other coexisting minerals of the Kangerdlugssuaq intrusion,E. Greenland

Cumulus apatite, sphene, feldspar, amphibole and biotite from the pulaskite of the Kangerdlugssuaq alkaline intrusion have been analysed for rare earth elements (REE) by instrumental neutron activation analysis. The apatite is particularly rich in REE, contains 3.6% Ce and shows a steep, light REE-enriched, chondrite-normalised pattern. The other minerals have light REE enrichment but with sphene showing a peak at Ce on a chondrite-normalised plot. REE partition coefficient values show that the light REE are preferentially accommodated by apatite relative to sphene. The differences in these coefficients result from differences in the co-ordination of the REE in the two minerals.     

Partitioning of Pb between volcanic glass and coexisting sanidine and plagioclase feldspars

Pb contents were determined by isotope dilution in separated glass, sanidine, and plagioclase from 18 rocks ranging in composition from basalt to rhyolite. These data indicate that Pb is partitioned into silicate melt relative to plagioclase, but is equally distributed between melt and sanidine. Plagioclase/glass distribution coefficients increase from 0.1 to 0.7 in going from basalt to rhyolite. This relationship suggests that the distribution coefficient is dependent upon bulk composition, temperature, or both. Sanidine/glass distribution coefficients are close to unity in rocks ranging in composition from quartz latite to rhyolite. The variation in Pb contents in a natural magma series from Craters of the Moon National Monument, Idaho, indicates that minerals (olivine, plagioclase, magnetite, apatite and clinopyroxene) fractionated from these magmas all have very low crystal/liquid distribution coefficients for Pb.     

Micro-scale S isotope studies of the Kharaelakh intrusion, Noril’sk region, Siberia: Constraints on the genesis of coexisting anhydrite and sulfide minerals

The coexistence of magmatic anhydrite and sulfide minerals in non-arc-related mafic magmas has only rarely been documented. Likewise the S isotope fractionation between sulfate and sulfide in mafic rocks has infrequently been measured. In the Kharaelakh intrusion associated with the world-famous Noril’sk ore district in Siberia coexisting magmatic anhydrite and sulfide minerals have been identified. Sulfur isotope compositions of the anhydrite-sulfide assemblages have been measured via both ion microprobe and conventional analyses to help elucidate the origin of the anhydrite-sulfide pairs. Magmatic anhydrite and chalcopyrite are characterized by δ³⁴S values between 18.8‰ and 22.8‰, and 9.3‰ and 13.2‰, respectfully. Coexisting anhydrite and chalcopyrite show Δ values that fall between 8.5‰ and 11.9‰. Anhydrite in the Kharaelakh intrusion is most readily explained by the assimilation of sulfate from country rocks; partial reduction to sulfide led to mixing between sulfate-derived sulfide and sulfide of mantle origin. The variable anhydrite and sulfide δ³⁴S values are a function of differing degrees of sulfate reduction, variable mixing of sulfate-derived and mantle sulfide, incomplete isotopic homogenization of the magma, and a lack of uniform attainment of isotopic equilibrium during subsolidus cooling. The δ³⁴S values of sulfide minerals have changed much less with cooling than have anhydrite values due in large part to the high sulfide/sulfate ratio. Variations in both sulfide and anhydrite δ³⁴S values indicate that isotopically distinct domains existed on a centimeter scale. Late stage hydrothermal anhydrite and pyrite also occur associated with Ca-rich hydrous alteration assemblages (e.g., thomsonite, prehnite, pectolite, epidote, xonotlite). δ³⁴S values of secondary hydrothermal anhydrite and pyrite determined by conventional analyses are in the same range as those of the magmatic minerals. Anhydrite-pyrite Δ values are in the 9.1-10.1‰ range, and are smaller than anticipated for the low temperatures indicated by the silicate alteration assemblages. The small Δ values are suggestive of either sulfate-sulfide isotopic disequilibrium or closure of the system to further exchange between ∼550 and 600 °C. Our results confirm the importance of the assimilation of externally derived sulfur in the generation of the elevated δ³⁴S values in the Kharaelakh intrusion, but highlight the sulfur isotopic variability that may occur in magmatic systems. In addition, our results confirm the need for more precise experimental determination of sulfate-sulfide sulfur isotope fractionation factors in high-T systems.     

Sector zoned aegirine from the Ilímaussaq alkaline intrusion,South Greenland

Sector zoned aegirine crystals occur in the interstices of peralkaline nepheline syenites in Ilímaussaq. The crystals have grass-green [001] sectors enriched in Ca and Fe²⁺ (as CaFeSi₂O₆), Mn and Zr; pale green {010} sectors enriched in Al (as NaAlSi₂O₆); blue-green {110} sectors enriched in Ti (as NaTi_0.5Fe _0.5 ²⁺ Si₂O₆); and light green {100} sectors enriched in Fe₃₊ (as NaFe³⁺ Si₂O₆).The crystals grew in the liquid with a rate that did not exceed the diffusion rate of most elements in the liquid. However. Fe³⁺ seems to have had diffusion rates lower than the crystal growth rate, and this probably caused the development of some sectors enriched in acmite and others enriched in the hedenbergite component. For Al, Ti and Zr a crystal structural control is envisaged although a recent structure-based model for sector zoning fails to explain the efficient separation of these elements into different sectors.Three more occurrences of sector zoned aegirine are noted, all from peralkaline nepheline syenites. The phenomenon is probably more widespread than hitherto realised.Contribution to the mineralogy of Ilímaussaq no. 62     

Chemistry of potassium feldspars from three zoned pegmatites,Black Hills,South Dakota: Implications concerning pegmatite evolution

An initial phase of an extensive geochemical study of pegmatites from the Black Hills, South Dakota, indicates potassium feldspar composition is useful in interpreting petrogenetic relationships among pegmatites and among pegmatite zones within a single pegmatite. The $\text{K}\text{Rb}$ and $\text{Rb}\text{Sr}$ ratios and Li and Cs contents of the feldspars within each zoned pegmatite, to a first approximation, are consistent with the simple fractional crystallization of the potassium feldspar from a silicate melt from the wall zone to the core of the pegmatites. Some trace element characteristics (i.e. Cs) have been modified by subsolidus reequilibration of the feldspars with late-stage residual fluid.$\text{K}\text{Rb}$ ratios of the potassium feldspar appear to be diagnostic of the pegmatite mineral assemblage. The relationship between $\text{K}\text{Rb}$ and mineralogy is as follows: Harney Peak Granite (barren pegmatites) > 180; Li-Fe-Mn phosphate-bearing pegmatites = 90?50; spodumene-bearing pegmatites = 60?40; pollucitebearing pegmatites < 30. Although the $\text{K}\text{Rb}$ ratios suggest that the pegmatites studied are genetically related by fractional crystallization to each other and the Harney Peak Granite, overlapping $\text{Rb}\text{Sr}$ ratios and the general increase in Sr and Ba with decreasing $\text{K}\text{Rb}$ indicate the genetic relationship is much more complex and may also be dependent upon slight variations in source (chemistry and mineralogy) material composition and degrees of partial melting.     

Metamorphic fractional crystallization and internal metasomatism by diffusional homogenization of zoned garnets

Metamorphic fractional crystallization involves fractionation of material into the interior of a crystallizing mineral such as garnet with consequent change in the effective bulk composition. In assemblages with thermodynamic variance of 3 or greater, the mineral assemblage, compositions of coexisting phases and the zoning profile preserved in the garnet are a function of the amount of material that is fractionated. Homogenization of zoned garnet by intracrystalline diffusion at elevated temperatures operates on a time scale of millions to tens of millions of years depending on the temperature-time path and radius of the garnet. One effect of homogenization is to drive chemical reactions by a metasomatic mechanism (internal metasomatism), even in the absence of changes in pressure or temperature. This process may affect the appearance or disappearance of minerals, and may, in some rocks, be responsible for the first appearance of kyanite or the disappearance of staurolite in Barrovian terranes.     

Crystallization kinetics of alkali feldspars in cooling and decompression-induced crystallization experiments in trachytic melt

Cooling and decompression experiments have been carried out on trachytic melts in order to investigate crystallization kinetics of alkali feldspar, the effect of the degree of undercooling (ΔT = T _liquidus ? T _experimental) and time on nucleation and crystal growth process. This experimental work gives us new data about crystallization kinetics of trachytic melts, and it that will be useful to better understand the natural system of Campi Flegrei volcanoes. Experiments have been conducted using cold seal pressure vessel apparatus, at pressure between 30 and 200 MPa, temperature between 750 and 855 °C, time between 7,200 and 57,600 s and redox condition close to the NNO +0.8 buffer. These conditions are ideal to reproducing pre- and syn-eruptive conditions of the Campi Flegrei volcanoes, where the “conditions” pertain to the complete range of pressures, temperatures and time at which the experiments were performed. Alkali feldspar is the main phase present in this trachyte, and its abundance can strongly vary with small changes in pressure, temperature and water content in the melt, implying appreciable variations in the textures and in the crystallization kinetics. The obtained results show that crystallization kinetics are strictly related to ΔT, time, final pressure, superheating (?ΔT) and water content in the melt. ΔT is the driving force of the crystallization, and it has a strong influence on nucleation and growth processes. In fact, the growth process dominates crystallization at small ΔT, whereas the nucleation dominates crystallization at large ΔT. Time also is an important variable during crystallization process, because long experiment durations involve more nucleation events of alkali feldspar than short experiment durations. This is an important aspect to understand magma evolution in the magma chamber and in the conduit, which in turn has strong effects on magma rheology.     

Fractional crystallization and zoning in igneous feldspars: ideal water-buffered liquid fractionation lines and feldspar zoning paths

Diagrams giving plagioclase and sanidine fractionation paths and liquid fractionation lines under conditions of ideal water-buffered fractional crystallization in the ternary feldspar system were constructed graphically using topological reasoning, and experimental data and calculated phase relationships from the literature. The liquidus lines and solidus or solvus paths are unique at constant P and a _{H 2}O. The composition of a liquid evolves with time and moves along a fractionation line by removal of successive crystal fractions, whereas the compositions of each of the crystal fractions lie on and define a solidus or solvus path. Most but not all such water-buffered lines and paths differ only slightly from those in which water is free to build up during crystallization and a _{H 2}O to increase, as in many rocks. Liquid compositions lying along liquidus fractionation lines are not normally preserved, unless erupted as aphyric lavas. The solidus or solvus paths may be preserved either as overgrowth zones in crystals (zoning paths) or as a series of crystal fractions in layered intrusions. The topologies of the lines and paths depend mainly on the nature of the two-feldspar boundary line separating the plagioclase and sanidine fields which is a function of P _{H 2}O or a _{H 2}O at constant P; increases in either progressively lower the liquidi and solidi and cause larger intersections of the solidi with the solvus. One-feldspar solidus paths at high P and a _{H 2}O are simple, whereas they are complex and may bend back on themselves at low P _{H 2}O or low a _{H 2}O at high P. Two-feldspar paths may be simultaneous (cotectic) or sequential (peritectic). The former are simple and do not meet at high P and a _{H 2}O, the critical solution line lying in the gap; they are complex and may bend back or overlap at low P _{H 2}O or low a _{H 2}O at high P, the position of the critical solution line being hard to determine. Liquids which have simultaneously fractionated two feldspars may fractionate only one towards the end, crystallization changing from subsolvus to hypersolvus. Sequential paths may involve overgrowth of an early feldspar by a later one, usually sanidine overgrowths on plagioclase, but plagioclase overgrowths on sanidine occur. These complexities explain in part the difficulties of unravelling the textural and compositional relationships of ternary feldspars in water-poor felsic igneous rocks (even in the absence of alteration or complex magma dynamics) and of trying to deduce phase relationships from natural occurrences of feldspars.C.R.P.G. contribution number 948     

西华山钨矿床共生透明矿物与不透明矿物中流体包裹体的对比研究

西华山钨矿床是一个产于燕山期花岗岩中的大脉型钨矿床。笔者利用红外显微镜、冷热台及其他相关设备对矿床中的透明矿物(石英、绿柱石和萤石)与不透明矿物(黑钨矿、黄铁矿)中的流体包裹体进行了对比研究。结果显示,共生透明矿物与不透明矿物之间,在包裹体均一温度等特征上既可基本相同又可出现很大差异。一般来说,黑钨矿能有效地保存原生流体包裹体(t_h=300～420℃;晶洞中为220～290℃),仅有少量次生包裹体(t_h=160～280℃),而与之共生的石英中原生包裹体则几乎被破坏殆尽,现在所见到的包裹体绝大多数是次生的或是在较晚结晶时捕获的(t_h=130～270℃)。只有未经后期应力作用和流体改造的晶洞水晶及与其共生的黑钨矿,二者获得的结果才相同或相似。绿柱石中通常有大量的次生包裹体和原生包裹体。黑钨矿与绿柱石中硅酸盐熔融包裹体的出现,表明西华山钨矿床的成矿作用始于岩浆-热液过渡阶段,其初始成矿流体是一种岩浆-热液过渡性流体,尔后才演变成单一的热水溶液。笔者认为,在进行金属矿床流体包裹体研究时,应强调共生透明矿物与不透明矿物的对比研究,在进行对比研究时,详尽的基础地质研究和包裹体岩相学观察必不可少;当单独利用透明矿物包裹体资料对金属矿床进行地质解释时,需慎之又慎。     

Petrogenetic modelling of in situ fractional crystallization in the zoned Loch Doon pluton,Scotland

The late Caledonian Loch Doon granitic intrusion ranges in composition from hypersthene diorite at the margin, through quartz diorite, granodiorite and granite to cordierite microgranite at its core. Petrogenetic modelling of trace element variations and least squares analysis of major elements indicate that two distinct magmas are involved, each magma controlled by crystallization of plag-opx-cpx-bio. Late stage rocks related to the second magma include the cordierite microgranites and aplites, which are interpreted as the final residue which crystallized rapidly after a build up and loss of volatiles.Analyses of whole rocks and minerals for REE's and other elements of moderate-high ionic potential indicate that these elements are strongly controlled by minor phase crystallization; apatite, zircon, sphene and allanite are dominant at intermediate compositions but other accessory minerals such as monazite and xenotime may also become important at acid compositions.It is probable that within each magma the mechanisms of crystal settling and filter pressing operated, the former being initially dominant, and the latter becoming more important with increasing degree of fractional crystallization.     

Tellurium-bearing minerals in zoned sulfide chimneys from Cu-Zn massive sulfide deposits of the Urals, Russia

Tellurium-bearing minerals are generally rare in chimney material from mafic and bimodal felsic volcanic hosted massive sulfide (VMS) deposits, but are abundant in chimneys of the Urals VMS deposits located within Silurian and Devonian bimodal mafic sequences. High physicochemical gradients during chimney growth result in a wide range of telluride and sulfoarsenide assemblages including a variety of Cu-Ag-Te-S and Ag-Pb-Bi-Te solid solution series and tellurium sulfosalts. A change in chimney types from Fe-Cu to Cu-Zn-Fe to Zn-Cu is accompanied by gradual replacement of abundant Fe-, Co, Bi-, and Pb- tellurides by Hg, Ag, Au-Ag telluride and galena-fahlore with native gold assemblages. Decreasing amounts of pyrite, both colloform and pseudomorphic after pyrrhotite, isocubanite ISS and chalcopyrite in the chimneys is coupled with increasing amounts of sphalerite, quatz, barite or talc contents. This trend represents a transition from low- to high sulphidation conditions, and it is observed across a range of the Urals deposits from bimodal mafic- to bimodal felsic-hosted types: Yaman-Kasy → Molodezhnoye → Uzelga → Valentorskoye → Oktyabrskoye → Alexandrinskoye → Tash-Tau → Jusa.     

Biotite Equilibria and Fluid Circulation in the Klokken Intrusion

Chemical variation in biotite from the KJokken gabbro-syeniteintrusion in the Proterozoic Gardar province in South Greenlandhas been investigated by electron probe and, for F and Li, ionmicroprobe. Most mica occurs in small amounts as fringes onilmenomagnetite or fayalite, rarely as an intercumulus or poikiliticphase. The micas range continuously from Phlog₇₀Ann₃₀ in a gabbro,to Phlog₄Ann₉₆ in the most evolved (slightly persilicic andperalkaline) syenite. In the syenites Fe-Mg partitioning betweenbiotite and olivine can be described by a single distributioncoefficient, K_d = XF X^Biot_Ms/X^Biot_Fe 3, suggesting that thesereactant phases mix ideally at the reaction T. Experimentaldata for Fe-Mg exchange via aqueous chloride solutions (Schulien,1980) imply low T (32Q?C). F was absent in the experiments andmay significantly affect the exchange equilibrium. K_d in thegabbros is 1, consistent with equilibrium via a fluid depletedin F because of crystallization of large amounts of amphibole. Al, Mn, and Ti vary regularly throughout the series and canbe used as markers of cryptic variation in the layered syenites.(Al + Si): 22 O is always in the range 7.7–7.85. A1/(A1+ Si) decreases from 0.31 in gabbros to 0.25 in the most Fe-richmicas. Li is always < 260 ppm w. In the syenite series, Fshows a near-linear inverse relationship with Fe/(Fe + Mg) whichpasses close to OF at Ann₁₀₀ with l.4 wt% F(0–7 F to 44positive charges) at Ann₄₄. Biotites in the gabbro unit (whichforms an outer sheath to the intrusion) have relatively lessF, probably because it was consumed by coexisting amphibole.^I8O is similar for both gabbros and syenites, and it is unlikelythat an envelope fluid was involved in the reactions. G reachesa maximum of 0.3 wt. % in biotite except for that in one syenitesample with 0–7 wt. %. Calculation of relative F-OH fugacitiesfrom the reaction OH-phlogopite + F-annite = F-phlogopite +OH-annite, as calibrated by Munoz (1984), appears to suggestthat each horizon in the layered series was in equilibrium witha slightly different fluid. In view of the intimate interleavingof these lithologies, this is improbable. The equilibrium constantof the exchange reaction, obtained from the experimental data,seems not to be appropriate to the Klokken assemblage, or toother examples of regular F-Fe avoidance. Explanations may includeshort-range Fe-Mg ordering in the natural examples or the effectof additional components in the fluid. F contents are high incomparison with biotites from calc-alkaline complexes; highmagmatic F may account for the igneous layering common in theGardar. Temperatures calculated from reactions involving fayaliteand magnetite show that most biotites grew subsolidus. The F-poorannites grew > 300 ?C subsolidus even when texturally intercumulus.Stable isotope data are consistent with the separation and retentionof a deuteric fluid during the final stage of magmatic crystallization.Klokken was not generally subject to the pervasive, long-range(in both distance and time) dydrothermal interactions demonstratedin calcalkaline and theleiitic intrusions, although more extensivefluid flow is indicated for the more permeable laminated syenites.The biotites preserve chemical variation indicating local equilibriumwith other mafic phases, and halogens provide a useful markerof subsolidus fluid flow.     

Pumpellyites and coexisting minerals in different low-grade metamorphic facies of Liguria, Italy

Pumpellyite from four-phase assemblages (pumpellyite + epidote + prehnite + chlorite; pumpellyite + epidote + actinolite + chlorite; pumpellyite + epidote + Na-amphibole + chlorite, together with common excess phases), considered to be low variance in a CaO-(MgO + FeO)-Al₂O₃-Fe₂O₃ (+Na₂O + SiO₂+ H₂O) system, have been examined in areas which underwent metamorphism in the prehnite-pumpellyite, pumpellyite-actinolite and low-temperature blueschist facies respectively. The analysed mineral assemblages are compared for nearly constant (basaltic) chemical composition at varying metamorphic grade and for varying chemical composition (basic, intermediate, acidic) at constant metamorphic conditions (low-temperature blueschist facies). In the studied mineral assemblages, coexisting phases approached near chemical equilibrium. At constant (basaltic) bulk rock composition the MgO content of pumpellyite increases, and the X_Fe3+ of both pumpellyite and epidote decreases with increasing metamorphic grade, the Fe³⁺ being preferentially concentrated in epidote. Both pumpellyite and epidote compositions vary with the bulk rock composition at isofacial conditions; pumpellyite becomes progressively enriched in Fe and depleted in Mg from basic to intermediate and acidic bulk rock compositions. The compositional comparison of pumpellyites from high-variance (1–3 phases) assemblages in various bulk rock compositions (basic, intermediate, acidic rocks, greywackes, gabbros) shows that the compositional fields of both pumpellyite and epidote are wide and variable, broadly overlapping the compositional effects observed at varying metamorphic grade in low-variance assemblages. The intrinsic stability of both Fe- and Al-rich pumpellyites extends across the complete range of the considered metamorphic conditions. Element partitioning between coexisting phases is the main control on the mineral composition at different P-T conditions.