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Layoutgetreues Digitalisat der Ausg.: Dillenburg : Verl. d. Waysenhauses, 1789. - Standort: Universität Marburg, Bibliothek Evangelische Theologie [381] (Gesangbuchsammlung) Signatur: PTh Dp Dil 1789 sekr. - Bemerkungen: In Fraktur. - Digitalisiert 2017

This volume accompanies an EMU School intended to bring contemporary research on mineral reaction kinetics to the attention of young researchers and to put it into the context of recent developments in related disciplines. A selection of topics, methods and concepts, which the contributors deem currently most relevant and instructive, is presented.

Mineral coronas and reaction rims are frequent features in manymetamorphic rocks commonly interpreted as having been controlled by solid-state diffusion or by diffusion in an undefined medium. In material science the term ‘interlayer growth’ is often used for such processes. However, the terms ‘reaction bands’, ‘corona structures’, or ‘rim structures’ are commonly used as descriptive terms in petrology and we will use these below. The formation of reaction bands, coronas and reaction rims between incompatible phases requires that one or more chemical components are mobile. Knowledge of the transport mechanism and relative mobility of the distinct chemical components is of prime interest for the interpretation of reaction band sequences, growth rates, and subsequently forming fabrics and textures. Recent experimentation at high pressures and temperatures has shown that even minute traces of water are decisive in changing reaction mechanisms in silicate systems. In this chapter we present avenues of experimentation classified as water-rich, waterpoor and water-absent. All of them are important in order to gain deeper understanding of mineral-reaction kinetics in the Earth in different environments, and of the formation of texture and zoning patterns in rock assemblages of the Earth or Earth-like planets. Experimental simulation and parameterization of variables relevant for mineralreaction kinetics and the development of microstructures and textures requires a specific experimental approach. This consists of (1) application of time (or temperature) series; (2) miniaturization of experimental setups; and (3) experiments with perfectly defined geometries of the phases involved. This chapter specifies experimental setups for investigating mineral-reaction kinetics at high P-T, including setups for time-resolved, real-time monitoring of mineral nucleation and growth between incompatible phases at high T. We address mainly the experimental strategies in elucidating the controls of mineral-reaction kinetics rather than responding to results of particular studies, which can be found in many other chapters in this volume and in previous review articles (e.g. Dohmen and Milke, 2010; Watson and Dohmen, 2010). This holds also for the applied analytical methods as many experimental setups are specifically designed for subsequent analytical procedures.

In the late 20th century advances in experimentation and in material characterization greatly fostered the development of internally consistent thermodynamic data. Together with the development of thermodynamic modelling tools this enhanced our ability to analyse phase equilibria in rocks and to obtain accurate quantitative information on the conditions of magmatic and metamorphic crystallization. This gave an unprecedented boost to mineralogy, petrology and geochemistry and helped illuminate long-standing questions in geodynamics as well as in geo- and cosmochemistry. Attainment of thermodynamic equilibrium among the phases constituting a rock and metastable preservation of equilibrium phase relations, which are indispensible pre-requisites for application of equilibrium thermodynamics, could be demonstrated or, in many cases, were tacitly assumed.

Many studies dealing with the synthesis of tourmaline report a sharp intragranular chemical gradient and extensive porosity in the core zones of the crystals, both of which still lack a reliable explanation. Using the example of olenitic tourmaline, we show that these features are likely explained by the occurrence of a precursor phase during tourmaline formation. Time-dependent piston–cylinder synthesis experiments were performed in the system SiO2–Al2O3–B2O3–NaCl–H2O at 700 °C/40 kbar with run durations of 0.5 h, 2.5 h and 216 h, starting from quartz–Al2O3–H3BO3 solid mixtures and NaCl solutions. Sharply zoned olenitic tourmaline ([4]B-rich cores, [4]B-poor rims) formed in all experiments, with its abundance increasing with increasing run duration. The amount of the additional solid product phases coesite and jeremejevite decreased with time. Extensive porosity is recognized in jeremejevite and in the cores of early grown acicular tourmaline. Textural relationships indicate that olenitic tourmaline grows at the expense of jeremejevite which acts as a crystalline precursor in this system. A possible reaction is: ![Formula][1] The transformation likely proceeds via a dissolution/re-precipitation mechanism, which triggers the sharp chemical zonation in olenite. Based on Rayleigh fractionation modelling, we estimate a minimum B isotope fractionation between jeremejevite and fluid with Δ11Bjer-fluid ≈ −2.8‰ at 700 °C/40 kbar. Due to the progressive dissolution of jeremejevite, the fluids δ11B values continuously decrease with increasing run duration. Hence, olenite growth concomitant with the dissolution of jeremejevite will produce scattered or inverse boron isotope patterns (heavy cores, light rims) in tourmaline, which cannot result from simple Rayleigh fractionation. Similar reactions involving jeremejevite or other precursor phases might explain chemical zonation and porous textures in tourmaline core zones reported in many experimental studies. The occurrence of natural tourmaline overgrowing jeremejevite in pegmatites of the Erongo Mountains, Namibia, gives rise to the assumption that jeremejevite might also act as a precursor for tourmaline formation in natural systems. [1]: /embed/mml-math-1.gif

Much of the Earth’s dynamics is related to mineral reactions in the solid-state. Classically, this is referred to as metamorphic crystallization (Kretz, 1994). Based on the chemical compositions of the phases involved in a metamorphic mineral reaction, two basic reaction types may be distinguished. Reactions that involve only structural re-arrangements, while the compositions of the reactant and product phases are identical, are referred to as partitionless and ‘polymorphic phase transformations’. If, in contrast, one or more reactant phases are replaced by one or more product phases with different compositions, this implies that chemical components are supplied to or removed from the reaction interfaces separating the reactants from the product phases. In the absence of advective transport via a fluid or melt, the necessary chemical mass transport can occur only by diffusion. Accordingly, this reaction type is partitioning and is referred to as ‘diffusive phase transformation’. Some treatments of the kinetics of mineral reactions are based on partitionless polymorphic phase transformations and are reviewed only briefly in this chapter. However, because most metamorphic mineral reactions are partitioning diffusive phase transformations, the following discussion will focus mainly on this reaction type. In this chapter, three types of reactions that play a key role in metamorphic crystallization are addressed. During prograde metamorphism continuous supply of aqueous fluid by dehydration reactions may facilitate relatively rapid intercrystalline diffusion so that a state close to chemical equilibrium on the scale of mineral grains and beyond may be attained resulting in ‘porphyroblastic mineral growth’. Interface-reaction controlled and diffusion-controlled growth are two endmember models in the kinetics of porphyroblastic growth and differ in terms of the spatial extent of chemical equilibration and its influence on the distribution and compositional zoning of porphyroblasts. The first section of this chapter may serve as a review of some of the key works in metamorphic petrology addressing the factors that control the abundance and size distribution of porphyroblasts and their chemical zoning patterns.

Layoutgetreues Digitalisat der Ausg.: Herborn : Regelein, 1752 Standort: Universität Marburg, Bibliothek Religionswissenschaft (510) Signatur: 583 Bemerkungen: (Bibliothek Wittgenstein) Digitalisiert 2015

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In this study, we address volume diffusion of ytterbium in yttrium aluminum garnet (YAG) using thin-film single crystal diffusion couples. We employ analytical transmission electron microscopy (ATEM) as a tool for combined microstructural and microchemical analysis and compare the results to Rutherford backscattering (RBS) analysis. Given the high spatial resolution of the method, we focus on microstructural changes of the thin-film diffusant source during the diffusion anneal. We evaluate the potential influence of the associated changes in its transport properties on the evolution of concentration profiles in the single crystal substrate. This approach allows us to test the reliability of determination of volume diffusion coefficients from thin-film diffusion experiments. We found that for the chosen experimental setting, the influence of thin-film re-crystallization is small when compared with the experimental uncertainty and good estimates for the volume diffusion coefficients of Yb in YAG can be obtained using standard assumptions. Both Yb-concentration profiles analyzed with ATEM and with RBS give similar results. At 1,450°C and 1 bar, we infer log D Yb (m2/s) values of −19.37 ± 0.07 (TEM) and −19.84 ± 0.02 (RBS). Although the change in thin-film transport properties associated with successive crystallization during the diffusion anneal does not play a major role for our experimental setup, this effect cannot generally be ignored.

In the pseudobinary system CaMgSi 2 O 6 –(NH 4 )M 3+ Si 2 O 6 , with M = Cr or Al, and NH 4 OH in excess, multi-anvil experiments at 9.5 to 12.8 GPa, 725 to 750 °C produced NH 4 -bearing diopside. Incorporation mainly follows the coupled substitution (Ca 2+ ) M2 + (Mg 2+ ) M1 ⇔ (NH 4 + ) M2 + (M 3+ ) M1 . Ammonium was identified and quantified by IR spectroscopy. In Cr-bearing diopside we found maximum concentrations in the range of 500 to 1000 ppm of NH 4 . The storage capacity of mantle clinopyroxenes for ammonium turns them to potential candidates for the nitrogen reservoir in the Earth's upper mantle, and this mechanism also contributes to its water budget. While nitrogen is transported into the mantle via cold slabs through NH 4 inherited from sedimentary material, and stored in K-bearing minerals and successor high-pressure phases, nitrogen output from the mantle is through degassing of N 2 . A probable mechanism for that is that nitrogen is kept as NH 4 in clinopyroxene in the Earth's mid- and lower mantle, whereas in the upper part, it is lost due to oxidation to molecular nitrogen. It is most likely that clinopyroxene plays a major role in the long-time, large-scale nitrogen cycle between surface and deep mantle of the Earth.

Evidence of fluid immiscibility in metamorphic rocks comes, in the best of all worlds, from fluid inclusion observations. In fact, fluid immiscibility should be anticipated over the full range of metamorphic conditions provided that appropriate bulk fluid compositions are present. In a review paper entitled “ Metamorphic Fluids: the Evidence from Fluid Inclusions ,” Crawford and Hollister (1986) summarized: “The role of fluid immiscibility in metamorphism was only recognized after serious study of fluid inclusions in metamorphic rocks was underway in the late 1970s and has yet to attract the attention of more than a handful of petrologists studying metamorphic rocks.” Since then, tremendous progress has been made. A huge amount of fluid inclusion studies have convincingly shown that immiscible fluids can be present in rocks of all bulk compositions and at all P-T conditions. Thermodynamic databases of minerals, solid solutions and fluid mixtures along with the development of Gibbs free energy minimization programs now allow for detailed prediction of the evolution of phase assemblages and mineral compositions in P-T space. Significant progress has also been made in understanding reaction kinetics, diffusional and convective mass transport and the interpretation of fluid-rock interaction during metamorphism. Given the vast fluid inclusion evidence of immiscibility it would appear, however, that phase petrology and mass transport in any specific rock where fluids are involved are not correctly interpreted if possible or proven immiscibility is not considered. Relatively few studies directly link mineral reactions with immiscible fluids. Therefore, this review aims at working out the effects of fluid immiscibility on the progress of metamorphic reactions and fluid transport over a wide range of conditions. Questions arise directly from the fluid inclusion record in metamorphic rocks. This chapter, however, is organized the other way round. At first it summarizes phase relations of fluid systems pertinent to …

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Due to the similar ionic radius of K + and NH 4 + , K-silicates can incorporate a significant amount of NH 4 . As tourmaline is able to accommodate K in its crystal structure at high and ultrahigh pressure, we test if this also holds true for NH 4 . Piston-cylinder experiments in the system (NH 4 ) 2 O-MgO-SiO 2 -Al 2 O 3 -B 2 O 3 -H 2 O at 4.0 GPa, 700 °C, with B 2 O 3 and NH 4 OH in excess produce an assemblage of tourmaline, phengite, and coesite. The tourmaline crystals are up to 10 × 40 μm in size. EMP analyses indicate that the tourmalines contain 0.22 (±0.03) wt% (NH 4 ) 2 O and are solid solutions mainly along the magnesio-foitite and “NH 4 -dravite” join with the average structural formula X [(NH 4 ) 0.08(1) □ 0.92(1) ] Y [Mg 2.28(8) Al 0.72(8) ] Z [Al 5.93(6) Si 0.07(6) ] T [Si 6.00(5) O 18 ](BO 3 ) 3 (OH) 4 . NH 4 incorporation is confirmed by characteristic stretching and bending modes in the IR-spectra of single crystals on synthetic tourmaline. Further evidence is the increased unit-cell parameters of the tourmaline [ a = 15.9214(9) A, c = 7.1423(5) A, V = 1567.9(2) A 3 ] relative to pure magnesio-foitite. Incorporation of NH 4 in natural tourmaline was tested in a tourmaline-bearing mica schists from a high- P /low- T (>1.2 GPa/550 °C) metasedimentary unit of the Erzgebirge, Germany, rich in NH 4 . The NH 4 -concentrations in the three main NH 4 -bearing phases are: biotite (~1400 ppm) > phengite (~700 ppm) > tourmaline (~500 ppm). This indicates that tourmaline can act as important carrier of nitrogen between the crust and the deep Earth, which has important implications for a better understanding of the large-scale light element cycle.

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Tourmaline was synthesized in the system MgO–Al2O3–B2O3–SiO2–KCl–NaCl–H2O from an oxide mixture and excess fluid at 500–700 °C and 0.2–4.0 GPa to investigate the effect of pressure, temperature, and fluid composition on the relative incorporation of Na and K in dravitic tourmaline. Incorporation of K at the X-site increases with pressure, temperature, and KCl concentration; a maximum of 0.71 K pfu (leaving 0.29 X-vacant sites pfu) was incorporated into K-dravite synthesized at 4.0 GPa, 700 °C from a 4.78 m KCl, Na-free fluid. In contrast, Na incorporation depends predominately on fluid composition, rather than pressure or temperature; dravite with the highest Na content of 1.00 Na pfu was synthesized at 0.4 GPa and 700 °C from a 3.87 m NaCl and 1.08 m KCl fluid. All synthesized crystals are zoned, and the dominant solid solution in the Na- and K-bearing system is between magnesio-foitite [□(Mg2Al)Al6Si6O18(BO3)3(OH)3OH] and dravite [NaMg3Al6Si6O18(BO3)3(OH)3(OH)], with the dravitic component increasing with the concentration of Na in the fluid. In the K-bearing, Na-free system, the dominant solid solution is between magnesio-foitite and K-dravite [KMg3Al6Si6O18(BO3)3(OH)3(OH)], with the K-dravitic component increasing with pressure, temperature, and the concentration of K in the fluid. The unit-cell volume of tourmaline increases with K incorporation from 1555.1(3) to 1588.1(2) A3, reflecting the incorporation of the relatively large K+ ion. Comparison of our results to the compositional data for maruyamaite (K-dominant tourmaline) from the ultrahigh-pressure rocks of the Kokchetav Massif in Kazakhstan suggests that the latter was formed in a K-rich, Na-poor environment at ultrahigh-pressure conditions near the diamond-stability field.

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Fractional crystallization of peraluminous F- and H(2)O-rich granite magmas progressively enriches the remaining melt with volatiles. We show that, at saturation, the melt may separate into two immiscible conjugate melt fractions, one of the fractions shows increasing peraluminosity and the other increasing peralkalinity. These melt fractions also fractionate the incompatible elements to significantly different degrees. Coexisting melt fractions have differing chemical and physical properties and, due to their high density and viscosity contrasts, they will tend to separate readily from each other. Once separated, each melt fraction evolves independently in response to changing T/P/X conditions and further immiscibility events may occur, each generating its own conjugate pair of melt fractions. The strongly peralkaline melt fractions in particular are very reactive and commonly react until equilibrium is attained. Consequently, the peralkaline melt fraction is commonly preserved only in the isolated melt and mineral inclusions. We demonstrate that the differences between melt fractions that can be seen most clearly in differing melt inclusion compositions are also visible in the composition of the resulting ore-forming and accessory minerals, and are visible on scales from a few micrometers to hundreds of meters.

The cation exchange equilibria Muscovite + NH 4 Cl ⇔ Tobelite + KCl KAl 2 AlSi 3 O 10 ( OH ) 2 + NH 4 Cl aq ⇔ NH 4 Al 2 AlSi 3 O 10 ( OH ) 2 + KCl aq K - feldspar + NH 4 Cl ⇔ Buddingtonite + KCl KAlSi 3 O 8 + NH 4 Cl aq ⇔ NH 4 AlSi 3 O 8 + KCl aq are determined by hydrothermal experiments at 400, 500, 600 °C at 400 MPa, and piston cylinder experiments at 500, 600 °C at 1500 MPa along the entire K–NH4 compositional range. The composition of solid phases are determined by XRD, EMP and FTIR-methods, NH4–K ratios in coexisting 2–3 molal chloride solutions by ion chromatography. Muscovite–tobelite and K-feldspar–buddingtonite form complete solid solutions series at all conditions. Consistent phase relations and mass balances indicate equilibrium between solid solutions and fluids. In both the (K,NH4)-muscovite and (K,NH4)-feldspar-fluid systems, NH4 strongly fractionates into the fluid. In the range from 400 to 600 °C, 400 to 1500 MPa, fractionations are slightly temperature and pressure-dependent. The distribution coefficient K D solid - fluid = X NH 4 solid X K solid X K fluid X NH 4 fluid varies within the experimental range from 0.10 at XKbulk=0.81 to 0.46 at XKbulk=0.20 for muscovite-fluid, and from 0.05 at XKbulk=0.91 to 0.70 at XKbulk=0.15 for feldspar-fluid. NH4 is preferentially incorporated into feldspar relative to mica with KDfsp-ms ranging between 1.36 and 2.0 at 400 MPa, and between 1.13 and 1.5 at 1500 MPa as XKbulk varies from 0.15 to 0.89. Results are evaluated thermodynamically assuming equal mixing of NH4Cl and KCl in the fluid. Mixing energies of solids were estimated using multidimensional regression and a regular solution model. For the muscovite–tobelite equilibrium, values of Δs°=−11.7 J K−1 mol−1, Δv°=−2.1 J MPa−1 mol−1, and W=4.6 kJ mol−1, and for the K-feldspar–buddingtonite equilibrium, Δs°=−8.8 J K−1 mol−1, Δv°=−1.1 J MPa−1 mol−1, and W=5.4 kJ mol−1 were derived. Results are used to calculate partition coefficients D among phases at very low NH4 bulk concentrations. Between 400 and 600 °C, DNH4fluid-ms ranges from 7 to 8 at 400 MPa and is 5 at 1500 MPa. DNH4fluid-ms is 6 to 7 at 400 MPa, and about 5 at 1500 MPa. DNH4fsp-ms is between 1 and 1.2 at all conditions. Partition coefficients are valid for NH4 contents of up to several hundred ppm in mica and feldspar, well within the concentration range observed in many rocks. Combining the data with that of K–NH4-partitioning between phlogopite and fluid results in DNH4phl-ms≈3.5 and DNH4phl-ms≈3 at 550 °C, 200 to 400 MPa. NH4 concentrations in coexisting muscovite, biotite, and K-feldspar from a variety of rocks show near-equilibrium distributions. NH4–K-partitioning between major K-bearing minerals and saline fluids allows for assessment of the dehydration history of metamorphic rocks. During prograde metamorphism, water is progressively produced by dehydration reactions and expelled along rock's P–T path. Nitrogen is subsequently removed due to preferred fractionation of NH4 into fluids. The remaining NH4 is continuously redistributed among muscovite and biotite, and at higher grades, K-feldspar with biotite as the main solid phase carrier of ammonium. The large fractionation effect of NH4 among most phases highlights its potential as tracer of devolatilization processes and fluid–rock interactions.

The trace element composition of individual fluid inclusions was investigated by using synchrotron radiation X-ray fluorescence for analysis and Monte Carlo simulations for quantification. To validate the input parameters of the Monte Carlo simulation code for the applied spectrometer, area scan measurements on synthetic standard reference glasses (NIST 612 and NIST 610) were performed. Results yielded an accuracy of generally better than 20 % for all elements with Z between 37 and 92 at a standard deviation of less than 7 %. The analytical procedure was then applied to synthetic inclusions in quartz: single point analysis yielded an accuracy between 10 and 30 % with a reproducibility of ± 20 % and a repeatability of ± 7 % for the elements Cu, Rb, Sn and Cs. Ten trace elements (Mn, Fe, Cu, Zn, As, Rb, Nb, Sn, Sb, Cs) with concentrations between 2 and 10500 ppm were determined in fluid inclusions of hydrothermal quartz hosts from the Ehrenfriedersdorf Complex, Germany. Inclusions of three evolutionary stages of the Ehrenfriedersdorf Complex were studied. Vapour-rich inclusions of the pegmatite stage have low concentrations of all elements with Z > 20. Inclusions of the hydrothermal stage record progressive enrichment and differentia- tion in trace elements. Fluids of the early hydrothermal stage are enriched in Sn, Cu and As. Subsequently, they evolve to Fe-, Mn-, Zn- and Cs-rich fluids.

Detailed analyses of melt and fluid inclusions combined with an electron-microprobe survey of boron-bearing minerals reveal the evolution of boron in a highly evolved peraluminous granite-pegmatite complex and the associated high- and medium-temperature ore-forming hydrothermal fluids (Ehrenfriedersdorf, Erzgebirge, Germany). Melt inclusions in granite represent embryonic pegmatite-forming melts containing about 10 wt% H2O and 1.8 wt% B2O3. These melts are also enriched in F, P, and other incompatible elements such as Be, Sn, Rb, and Cs. Ongoing differentiation and volatile enrichment drove the system into a solvus, where two pegmatite-forming melts coexisted. The critical point is at about 712 °C, 100 MPa, 20 wt% H2O and 4.1 wt% B2O3. Cooling and concomitant fractional crystallisation from 700 to 500 °C induced development of two conjugate melts, an H2O-poor (A-melt) and an H2O-rich melt (B-melt) along the opening solvus. Boron is a major element in both melts and is preferentially partitioned into the H2O-rich melt. Temperature-dependent distribution coefficients $$ D_{{\rm{boron}}}^{{\rm{B - melt/A - melt}}} $$ are 1.3 at 650 °C, 1.5 at 600 °C, and 1.8 at 500 °C. In both melts, boron concentrations decreased during cooling because of exsolution of a boron-rich hypersaline brine throughout the pegmatitic stage. Boromuscovite containing up to 8.5 wt% was another sink for boron at this stage. The end of the melt-dominated pegmatitic stage was attained at a solidus temperature of around 490 °C. Fluid inclusions of the hydrothermal stage reveal trapping temperatures of 480 to 370 °C, along with varying densities and highly variable B2O3 contents ranging from 0.20 to 2.94 wt%. A boiling system evolved, indicating a complex interplay between closed- and open-system behaviour. Pressure switched from lithostatic to hydrostatic and back, generating hydrothermal convection cells where meteoric waters were introduced and mixed with magmatic fluids. Boron-rich solutions originated from magmatic fluids, whereas boron-depleted fluids were mainly of meteoric origin. This highlights the potential of boron for discriminating fluids of different origin. Tin is continuously enriched during the evolution because tin and boron are cross-linked by formation of boron-, fluorine- and tin-fluorine-bearing complexes and is finally deposited within quartz-cassiterite veins during the transition from closed- to open-system behaviour. Boron does not only trace the complex evolution of the Ehrenfriedersdorf complex but exerts, together with H2O, F and P, an important control on the physical and chemical properties of pegmatite-forming melts, and particularly on the formation of a two-melt solvus at low pressure. We discuss this with respect to experimental results on H2O solubility and the critical behaviour of the haplogranite-water system which contained variable concentrations of volatiles.

The Thorium distribution between monazite and xenotime has been determined experimentally using the coupled substitution Th + Si=REE + P. Experiments have been conducted in standard cold seal hydrothermal and internally heated pressure vessels at 200MPa in the range of 600-1100°C. Starting mixtures were prepared from gels composed of equal amounts of CePO4 and YPO4 with addition of 10, 20 and 50 mol%ThSiO4. The grain sizes of the run products were in the range of a few microns. Analytical electron microscopy (AEM) methods were applied to obtain reliable chemical compositions of the reaction products. Lattice parameters of run products were determined using Rietveld analysis. For runs with 10 and 20 mol% ThSiO4 component in the bulk the ThSiO4 component distributes almost exclusively into monazite at all temperatures. The amount of the YPO4 component in monazite increases relative to the Th-free system if significant amounts of ThSiO4 are present within the structure. ThSiO4 favours incorporation of YPO4 resulting in a shift of the monazite limb and the shrinkage of the monazite-xenotime miscibility gap in the CePO4-YPO4-ThSiO4 ternary diagram. Thermometric calculations based on monazite-xenotime equilibria must be corrected for this effect. For runs with 50 mol% ThSiO4 in the bulk, thorite formed as an additional phase at 600 to 900°C but was absent at higher temperatures. At high Xbulk ThSiO4 and low T, the system is three-phase. The three-phase stability field strongly shrinks with increasing temperature. A tentative phase diagram of the ternary system CePO4-YPO4-ThSiO4 is proposed and discussed in the light of monazite-xenotime-thorite-bearing assemblages in natural rocks.

The healing of radiation damage in natural monazite has been experimentally studied in annealing experiments using XRD, TEM, Raman microprobe and cathodoluminescence analysis. The starting material was a chemically homogeneous monazite from a Brazilian pegmatite with a concordant U–Pb age of 474 ± 1 Ma and a U–Th/He age of 479 Ma. The monazite shows nm-scale defects induced by radioactive decay. The Xray pattern of the unheated starting material revealed two distinct monazite ''phases'' A and B with slightly different lattice parameters. Monazite A shows sharp reflections of high amplitudes and slightly expanded lattice parameters (1% in volume) compared to a standard monazite. Phase B exhibits very broad reflections of low amplitudes. Two sets of experiments were performed. First, dry monazite powder was annealed at 500, 800 and 1000 °C for 7 days. Each run product was analysed by X-ray diffractometry. Second, monazite grains were hydrothermally annealed at temperatures from 500 to 1200°C for 5 to 15 days. TEM observations show that partial healing of the monazite lattice already occurred at 500°C and increased gradually with temperature, so that after 10 days at 900°C complete healing was achieved. The observations are interpreted accordingly: the starting material has a mosaic structure consisting of two domains, A and B, which are basically two monazite crystals with different lattice parameters. We suggest that the A domains correspond to wellcrystallised areas where helium atoms are trapped. The accumulation of He causes expansion of the A monazite lattice. Diffraction domains B are interpreted as a helium-free distorted monazite crystal lattice, which can be referred to old alpha-recoil tracks. These B domains are composed of ''islands'' with an expanded lattice, induced by the presence of interstitials, and ''islands'' of a compressed monazite lattice, induced by presence of vacancies. Both the islands will pose stress on the lattice in the vicinity of the islands. The broadening of the B reflections is due to the expanded or compressed diffraction domains and to the different amount of the distortion. With increasing temperature the unit-cell volume of monazite A decreases, i.e. the position of the A reflections shifts towards smaller dhkl values. This was interpreted as a relaxationof the monazite lattice due to helium diffusion out of the monazite lattice. Simultaneously, the nm-sized defect domains B are healed. At 900–1000°C only a monazite with well-crystallised lattice and minimum unit-cell volume is observed.

The composition of synthetic amphiboles was investigated experimentally along the tremolite-tschermakite join in the system CaO-MgO-Al 2 O 3 -SiO 2 -H 2 O-Br 2 . Compositions of these amphiboles were studied within the phase assemblages: amphibole-anorthite-quartz-diopside (I), amphiboleanorthite-quartz-talc (II), amphibole-anorthite-quartz-enstatite (III), amphibole-anorthite-talcclinochlore (IV), amphibole-zoisite-talc-quartz (V), and amphibole-zoisite-talc-clinochlore (VI). Assemblages were synthesized from oxide-hydroxide mixtures in the presence of a CaBr 2 -bearing solution between 600-800 °C and 200-2000 MPa. Solid phases were investigated using SEM, HRTEM, EMP, and XRD techniques. EMP data show that the amphiboles produced are solid solutions of the ternary system tremolite-tschermakite-cummingtonite. Enstatite, diopside, talc, and clinochlore showed small deviations from their respective end-member compositions due to incorporation of some Al. The thermodynamic properties of the tschermakite end-member and the mixing properties along the tremolite-tschermakite join were extracted from corresponding exchange reactions of the unreversed synthesized phase assemblages I-VI. Various ideal mixing models were tested for Al-Mg and Al-Si substitution at octahedral M2 and M3 sites and at tetrahedral T1-sites. Best fits were obtained for a two-site coupled model, resulting in Δ f H 0 t s =-12528.3 ′ 11.7 kJ/mol and S 0 t s = 556.5 ′ 12.0 J/(mol.K) for the tschermakite end-member. Similar calculations were carried out for magnesiohornblende, and values of Δ f H 0 M g h b =-12418.7 ′ 5.9 kJ/mol and S 0 M g h b = 562.8 ′ 6.1 1 J/(mol.K) were extracted. Calculated phase relations and amphibole compositions agree well with experimental data if the derived thermodynamic data of tschermakite and a two-site mixing model for Al incorporation in amphibole solid solutions are applied.

The distribution of Rb-Na and Rb-K between richterite and a 2-molal aqueous (Na, K, Rb)-chloride solution has been investigated with hydrothermal experiments at 800∘C and 200 MPa. Experiments were performed as syntheses in which amphiboles grew in the presence of an excess fluid containing the exchangeable cations Na+-Rb+ or Na+-K+-Rb+. The obtained amphiboles were large enough (up to 20 m in width) for reliable EMP analysis. They were chemically homogeneous and HRTEM investigations showed that they were structurally well ordered. The Rb, Na, K, Ca and Mg concentrations in coexisting fluids were measured by ICP-AES. According to the possible incorporation of Na, K and Rb on the A-site, solid solutions in the ternary Na(NaCa) Mg5[Si8O22/(OH)2] (richterite)-K(NaCa)Mg5[Si8O22/(OH)2] (K-richterite)-Rb(NaCa)Mg5[Si8O22/(OH)2] (Rb-richterite) were expected. However, Rb-rich richterites always had significant amounts of A-site vacancy concentrations (X □ amph=□ A /(RbA+KA +NaA+□A) of up to 0.42 in the K-free (Na,Rb)-richterites and of up to 0.67 in the (Na, K, Rb)-richterites which corresponds to the same content of tremolite+cummingtonite-component. Amphiboles containing practically only Rb besides vacancies and no Na and/or K on the A-site were also synthesized, however. The Rb-Na and Rb-K exchange coefficients between fluid and richterites are similar. Rubidium always fractionated strongly into the fluid phase. For low Rb-concentrations in richterite (X Rb amph

Solid solutions of the REE+Y orthophosphates monazite and xenotime were synthesized hydrothermally between 300°C and 800°C at 2 and 5 kbar using REE+Y-oxides and H3PO4 as starting materials. The REE bulk composition was similar to those found in metapelitic rocks. All experiments produced two solid phases, monoclinic monazite and tetragonal xenotime, in volumetric ratios similar to relative abundances in metapelites. REE concentrations of coexisting monazite–xenotime pairs were determined by EMP analysis, and REE distribution coefficients DREE=XREEmonazite/XREExenotime were derived for the entire REE+Y suite. Values of DREE depend strongly on ionic radii of the REEs and on temperature. The log DREE vs. ionic radius curve shows a nearly linear decrease from La to Lu and comprises almost five orders of magnitude from DLa≈102.5 to DLu≈10−2. Between 450° and 800°C, most DREE values vary continuously by a factor of about 2 to 3. DNd decreases from 48 to 16 and DSm from 9 to 5; DTb increases from about 0.2 to about 0.4, DDy from 0.1 to 0.2 and DY from 0.06 to 0.12 along this temperature range. Experimentally determined values of DREE are in close agreement with those derived from natural monazite–xenotime pairs equilibrated between 400° and 700°C. REE partitioning between monazite and xenotime is appropriate for geothermometry. The most useful application is that equilibrium and disequilibrium partitioning of REE in metamorphic rocks are easily recognized. In the case of equilibrium distributions, U–Pb chronometry of single monazites or xenotimes provides a direct link to their growth conditions and information on temperature–time paths. Equilibrium distributions of Sm and Nd, expressed in molar fractions, result in KD=(XSmmonaziteXNdxenotime/(XSmxenotimeXNdmonazite)=0.2 to 0.3 from 600° to 800°C, and Sm–Nd chronometry should provide reasonable two-mineral isochrons for such monazite–xenotime pairs.

Our knowledge of the identity and functional significance of the pathogenic mechanisms responsible for restenosis and arteriosclerosis in man is still limited. Among others, phenotypic conversion, migration, and proliferation of smooth muscle cells have been suggested to lead to hypercellular neointima. In the present study, we examined intimal cell numbers and cell types in tissue of 23 postangioplasty lesions biopsied by directional atherectomy. using histology and transmission electron microscopy. Comparative tissue analysis was performed for 53 primary lesions. Tissue specimens obtained from coronary (n = 32) and peripheral lesions (n = 44) of 69 symptomatic patients were analyzed. Histological assessment of cell density showed intra- and interlesional variability. A markedly (P 514 cells/mm2) was observed in 12/23 lesions (52%), but hypocellularity (25th percentile; < 76 cells/mm2) in none. In contrast, primary lesions were more variable, with hypercellularity in 7/53 plaques (13%), and hypocellularity in 19/53 (36%). Transmission electron microscopic analysis of subcellular features revealed hypocellular plaques to have an extensive build-up of extracellular matrix, with only sparsely embedded smooth muscle cells (SMCs). These SMCs have a range of intermediate to microfilament-rich contractile phenotypes, thereby indicating only marginal metabolic activity. In contrast, hypercellular plaque regions contained predominantly organelle-filled SMCs, irrespective of postangioplasty or primary origin. In conclusion, increased SMC density was observed predominantly in most renarrowed lesions encompassing classical restenoses (2 to 6 months post angioplasty) as well as late recurrent lesions (7 to 30 months postangioplasty). Concordantly, primary lesion hypercellularity is suggested to be related to the formation and progression of native arteriosclerosis.

Solid solutions of (Ce,Y)PO4 were synthesized hydrothermally between 300 8C and 1000 8C at pressures of 2, 5, 10, and 15 kbar. Experiment products were analyzed by electron microprobe, and their lattice parameters were refined by Rietveld analysis of XRD powder patterns. Over a wide range of bulk compositions, two immiscible phases formed: monazite with a P21/n structure (REEO9 polyhedron) and xenotime with an I41/amd structure (REEO8 polyhedron). Variations of the unit-cell dimensions are directly correlated with variations in composition. The boundaries of the asymmetric miscibility gap are strongly dependent on temperature. At 2 kbar and from 300‐1000 8C, maximum Y concentrations in monazite coexisting with xenotime increase from 3‐16 mol%. With increasing pressure, this limb is shifted to higher Y contents so that at 15 kbar it ranges from 6 to about 25 mol% for the same temperature range. In contrast, Ce concentrations in xenotime do not exceed 3 mol% at 1000 8C over the entire pressure range. Our experimentally determined boundaries fit surprisingly well with empirical calibrations of the (LREE-HREE,Y)PO 4-solvus boundaries derived from a suite of low pressure metapelites (3 to 5 kbar) that equilibrated at peak temperatures ranging from 400‐700 8C. The behavior of the natural (REE,Y)PO4 system is probably well described by the simple (Ce,Y)PO4 binary, at least for metapelitic compositions. Many natural samples of monazite coexisting with xenotime show prograde growth zonation with respect to the incorporation of HREE 1 Y. The combination of our thermometer along with U-Pb and Sm-Nd age determinations of high spatial resolution may thus provide information about prograde branches of metamorphic PTt paths.

Natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their UPb isotope systematics measured before and after the runs. Runs were carried out with monazite grain size fractions ranging from 40 to 125 μm in diameter, and on 207 Pb 206 Pb ages close to 380 Ma. U concentrations remained always unchanged. All recrystallized monazites from the powdered starting material define a discordia with intercept ages of 379.3 ± 2.3 and −7 ± 18 Ma, the latter indicating the hydrothermally induced Pb loss at the time of the experiment. SEM observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces. Run products from monazite powders showed recrystallization and grain growth induced by a dissolution-precipitation mechanism. Increasing temperature resulted in a distinct enlargement of grain sizes at 650° and 750°C. A dissolution-precipitation process in presence of a fluid phase rather than Pb volume diffusion acts as a very efficient mechanism for the resetting of the UPb isotope system of monazites, even at low temperatures. This process is most important in shear zones where channelized fluid flow may occur.

Rare earth element (REE) and yttrium concentrations of coexisting monazite and xenotime were determined from a suite of seven metapelites from the Variscan fold belt in NE Bavaria, Germany. The metapelites include a continuous prograde, mainly low-P (3–5 kbar) metamorphic profile from greenschist (c. 400 °C) to lower granulite facies conditions (c. 700 °C). The LREE (La–Sm) are incorporated preferentially in monoclinic monazite (REO9 polyhedron), whereas the HREE plus Y are concentrated in tetragonal xenotime (REO8 polyhedron). The major element concentrations of both phases in all rocks are very similar and do not depend on metamorphic grade. Monazite consists mainly of La, Ce and Nd (La0.20–0.23, Ce0.41–0.45, Nd0.15–0.18)PO4, all other elements are below 6 mol%. Likewise, xenotime consists mainly of YPO4 with some Dy and Gd solid solutions (Y0.76–0.80, Dy0.05–0.07, Gd0.04–0.06). In contrast, the minor HREE concentrations in monazite increase strongly with increasing metamorphic grade: Y, Dy and Gd increase by a factor of 3–5 from greenschist to granulite facies rocks. Monazite crystals often show zonation with cores low in HREE and rims high in HREE that is interpreted as growth zonation attained during prograde metamorphism. Similarly, Sm and Nd in xenotimes increase by a factor of 3–4 with increasing metamorphic grade. Prograde zonation in single crystals of xenotime was not observed. The XHREE+Y in monazite and XLREE in xenotime of the seven rocks define two limbs along the strongly asymmetric miscibility gap from c. 400 °C to 700 °C. The empirical calibration of the monazite miscibility gap limb coexisting with xenotime is appropriate for geothermometry. Due to its contents of U and Th, monazite has often been used for U–Pb age determination. The combination of our empirical thermometer on prograde zoned monazite along with possible age determination of zoned single crystals may provide information about prograde branches of temperature–time paths.