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Normally, volatiles in silicate melts are ephemeral components that escape as gases when the melt reaches fluid saturation. When fluid saturation occurs at elevated pressure, magmatic fluids may have large amounts of oxide solute dissolved, are less volatile, and may resemble viscous gels. In Cyprus we have the rare case that solutes of a magmatic fluid coexist with H.sub.2O saturated basaltic to boninitic glasses. Quenching of the melts and fluid solutes was induced by fluid segregation. When the fluids exsolved, the liquidus temperature was raised and the melts were left supercooled, while the system temperature remained ± constant. Quenching rates deduced from the morphologies and compositions of quench crystals were high. We analyzed coexisting glasses and fluid solutes for major and trace elements. The fluid mobile trace elements (Rb, K, Pb, Sr) are enriched in both the glasses and fluid solutes. Both endmembers (melt and fluid) have a common parentage and originated within a hydrous mantle source. The glasses have 2.5 ± 0.25 wt.% H.sub.2O and record residual H.sub.2O contents left after fluid exsolution was completed. Water contents in glasses correspond to an H.sub.2O partial pressure (pH.sub.2O) of 65 ± 10 MPa and an emplacement depth on the seafloor of 6500 ± 1000 m, provided equilibrium was reached between the pH.sub.2O imposed by the melts and the seawater column. Following fluid exsolution, the degree of supercooling âT of the melts relative to the dry MgO-in-melt liquidus temperature was - 65 ± 10 °C. The cooling rate âT/t at the time of crystallization of dendritic clinopyroxene crystals can be semi-quantified from the distribution of Al.sub.2O.sub.3 between metastable clinopyroxene dendrites and melt, to at least - 50 °C h.sup.-1. Toward the end of the article we speculate if other cases exist where quenching was triggered by fluid exsolution. A possible example are spinifex textures deep inside komatiite flows where quenching rates by conductive cooling did not exceed 0.3 to 1 °C h.sup.-1. Our proposition assumes that many spinifex-textured komatiites were hydrous, that they contained H.sub.2O in quantities sufficient to reach fluid saturation at emplacement pressure, and that spinifex textures formed as a result of supersaturation by fluid loss., Author(s): Chris Ballhaus [sup.1], Josua Pakulla [sup.2], Richard Wirth [sup.3], Vladimir Roddatis [sup.3], Anja Schreiber [sup.3], Carsten Münker [sup.2], Frank Wombacher [sup.2], Andreas Kronz [sup.4], Raúl O. C. Fonseca [sup.5], [...]

Palladium and Pt are present in magmatic sulfide deposits mainly as discrete platinum-group minerals (PGM) closely associated with base metal sulfides (BMS). It is always debated whether these PGM phases are of magmatic or subsolidus origin. The mechanism by which Pt- and Pd-mineral phases form depends on how Pd and Pt are accommodated in magmatic sulfide phases: as cation substitutions or as stable nano Pd- and Pt-ligand particles. To know how Pd and Pt are hosted in magmatic sulfides and how they behave during cooling, we have investigated magmatic monosulfide solid solution (MSS) (quenched from 950 °C) and low-temperature (slowly cooled from 950 to 25 °C) decomposed MSS, both synthesized from PdSb, PdTe.sub.2, PdBi.sub.2, PtSb.sub.2, PtTe.sub.2-or PtBi.sub.2-saturated CuNiFe-sulfide mixture. Transmission Electron Microscopy (TEM) revealed that at 950 °C, Pd is hosted in MSS as nano Pd-telluride and antimonide melt droplets. Platinum is hosted in MSS as PtTe.sub.2 (moncheite), PtS (cooperite) and PtSb.sub.2 (geversite) nanocrystals. Moncheite and cooperite nanoparticles are aligned along the (0001) plane, and share one crystallographic plane (hkl) with the host, hexagonal MSS. At 25 °C, the Pd-telluride and antimonide melt droplets crystallized to merenskyite (PdTe.sub.2) and Ni-rich sudburyite (Pd(Ni)Sb). The Pt nano phases at 25 °C keep their composition and fabric and show no preference to pyrrhotite and pentlandite. Results imply that Pt and Pd minerals nucleate at magmatic temperature and grow by assembling PGE-ligand nanoparticles, not by exsolution of cationic and anionic metal species from BMS. Results also prove a weak Pd-S chemical affinity at the magmatic stage; Pd atoms are incorporated in MSS and the intermediate solid solution (ISS) when semimetals are not available. During subsolidus transformations of MSS and ISS, Pd preferentially concentrates in pentlandite., Author(s): Hassan M. Helmy [sup.1], Roman Botcharnikov [sup.2], Chris Ballhaus [sup.3], Richard Wirth [sup.4], Anja Schreiber [sup.4], Stephan Buhre [sup.2] Author Affiliations: (1) grid.411806.a, 0000 0000 8999 4945, Department of [...]

Meteorite impact processes are ubiquitous on the surfaces of rocky and icy bodies in the Solar System, including the Moon. One of the most common accessory minerals, zircon, when shocked, produces specific micro-structures that may become indicative of the age and shock conditions of these impact processes. To better understand the shock mechanisms in zircon from Apollo 15 and 16 impact breccias, we applied transmission electron microscopy (TEM) and studied nano-structures in eight lunar zircons displaying four different morphologies from breccias 15455, 67915, and 67955. Our observations revealed a range of shock-related features in zircon: (1) planar and non-planar fractures, (2) 'columnar' zircon rims around baddeleyite cores, (3) granular textured zircon, in most cases with sub-[micro]m-size inclusions of monoclinic ZrO.sub.2 (baddeleyite) and cubic ZrO.sub.2 (zirconia), (4) silica-rich glass and metal inclusions of FeS and FeNi present at triple junctions in granular zircon and in baddeleyite, (5) inclusions of rutile in shocked baddeleyite, (6) amorphous domains, (7) recrystallized domains. In many grain aggregates, shock-related micro-structures overprint each other, indicating either different stages of a single impact process or multiple impact events. During shock, some zircons were transformed to diaplectic glass (6), and others (7) were completely decomposed into SiO.sub.2 and Zr-oxide, evident from the observed round shapes of cubic zirconia and silica-rich glass filling triple junctions of zircon granules. Despite the highly variable effect on textures and Zr phases, shock-related features show no correlation with relatively homogeneous U-Pb or .sup.207Pb/.sup.206Pb ages of zircons. Either the shock events occurred very soon after the solidification or recrystallization of the different Zr phases, or the shock events were too brief to result in noticeable Pb loss during shock metamorphism., Author(s): Monika A. Kusiak [sup.1], Elizaveta Kovaleva [sup.2] [sup.3], Dennis Vanderliek [sup.4], Harry Becker [sup.4], Franziska Wilke [sup.3], Anja Schreiber [sup.3], Richard Wirth [sup.3] Author Affiliations: (1) grid.413454.3, 0000 0001 [...]

Experimental studies on binary, ternary and quaternary Cu-Fe-Ni-S systems are fundamental for the investigation of magmatic sulfide deposits, the main source of Ni, Co and platinum group elements (PGE). Previous experimental studies successfully formulated our general understanding of the evolution of magmatic sulfide systems but, yet, in many cases could not explain some of the key geological, mineralogical and geochemical features of sulfide ore deposits. The challenges are imposed by not well-defined solidus of the Cu-rich sulfide melts, yet poorly constrained phase stability at subliquidus conditions, and poorly resolved subsolidus evolution of magmatic sulfide phases. In this study we aim at better understanding how base metal sulfides crystallize from the evolving sulfide liquid during cooling from superliquidus to room temperatures. We report on controlled cooling (15 °C/day) experiments from 1100 to 25 °C in evacuated silica tubes of a single composition of the quaternary Cu-Ni-Fe-S system similar to the Merensky Reef sulfide ore. Run products were sampled at various temperatures along the cooling path and examined on the microscopic scale by back scattered electron imaging and on the nanometer scale by transmission electron microscopy. The compositions of coexisting phases were analysed using electron microprobe. We show that the sulfide melt (SM) coexists with monosulfide solid solution (MSS) above 950 °C and persists to 700 ± 25 °C before it crystallizes to intermediate solid solution (ISS). The transition to subsolidus state is clearly traced by abrupt change in the Cu/(Fe + Ni) distribution between MSS and Cu-rich phase (either SM or ISS) and in the composition of SM or ISS. The compositional jump at ca. 700 °C is also accompanied by the inverse change in the proportions of coexisting phases indicating significant subsolidus reactions and evolution with decreasing temperature. Pentlandite commences crystallization around MSS grains between 550 and 450 °C and coarsens to a granular-type pentlandite at 450 °C by diffusion of nano pentlandite exsolutions from MSS. Pentlandite exsolves and grows as 'flames' and 'brushes' in both MSS and ISS at lower temperatures (< 250 °C). Mass balance calculations suggest that 32% of pyrrhotite and 7% of pentlandite in magmatic deposits like the Merensky Reef exsolve from ISS. Results imply that magmatic sulfide systems evolve to lower temperatures than previously thought, leading to significant ore metal fractionation and redistribution. Base metal sulfide phases re-equilibrate extremely fast during cooling, ensuring that primary phase compositions and textures are inevitably and completely eradicated during cooling histories even as short as a few days. The sulfide mineral assemblage, texture and modal abundance of magmatic sulfide phases could be used as a proxy for the reconstruction of the parent sulfide liquid evolution in the deposit. The newly established mechanism and timing of base metal sulfides crystallization provides possible explanation of PGE distribution in base metal sulfides., Author(s): Hassan M. Helmy [sup.1], Roman Botcharnikov [sup.2], Chris Ballhaus [sup.3], Anastasia Deutsch-Zemlitskaya [sup.2], Richard Wirth [sup.4], Anja Schreiber [sup.4], Stephan Buhre [sup.2], Tobias Häger [sup.2] Author Affiliations: (1) grid.411806.a, [...]

The metastable paragenesis of corundum and quartz is rare in nature but common in laboratory experiments where according to thermodynamic predictions aluminum-silicate polymorphs should form. We demonstrate here that the existence of a hydrous, silicon-bearing, nanometer-thick layer (called 'HSNL') on the corundum surface can explain this metastability in experimental studies without invoking unspecific kinetic inhibition. We investigated experimentally formed corundum reaction products synthesized during hydrothermal and piston-cylinder experiments at 500-800 °C and 0.25-1.8 GPa and found that this HSNL formed inside and on the corundum crystals, thereby controlling the growth behavior of its host. The HSNL represents a substitution of Al with Si and H along the basal plane of corundum. Along the interface of corundum and quartz, the HSNL effectively isolates the bulk phases corundum and quartz from each other, thus apparently preventing their reaction to the stable aluminum silicate. High temperatures and prolonged experimental duration lead to recrystallization of corundum including the HSNL and to the formation of quartz + fluid inclusions inside the host crystal. This process reduces the phase boundary area between the bulk phases, thereby providing further opportunity to expand their coexistence. In addition to its small size, its transient nature makes it difficult to detect the HSNL in experiments and even more so in natural samples. Our findings emphasize the potential impact of nanometer-sized phases on geochemical reaction pathways and kinetics under metamorphic conditions in one of the most important chemical systems of the Earth's crust., Author(s): Dina S. Schultze [sup.1] [sup.2], Richard Wirth [sup.3], Bernd Wunder [sup.3], Anselm Loges [sup.1] [sup.2], Max Wilke [sup.4], Gerhard Franz [sup.2] Author Affiliations: (1) grid.14095.39, 0000 0000 9116 4836, [...]

In order to characterize the [micro]m-to-nm structures related to operation of pressure solution on phase boundaries in naturally deformed rocks, we have performed a detailed focused ion beam/transmission electron microscopy study in ultramylonite samples from South Armorican Shear Zone (France) that focused on grain boundary scale. We have studied phase boundaries between quartz, K-feldspar and white mica in both 2D and 3D and compare our evidences with theoretical dissolution precipitation models in the current literature. The dissolution (re)precipitation processes lead to the development of different features at different phase boundaries. In both quartz-white mica and quartz-K-feldspar phase boundaries, voids were ubiquitously observed. These voids have different shapes, and the development of some of them is crystallographically controlled. In addition, part of these voids might be filled with vermiculite. Amorphous leached layers with kaolinite composition were observed at the boundaries of K-feldspar-quartz and K-feldspar-white mica. The development of different features along the phase boundaries is mainly controlled by the crystallography of the phases sharing a common interface, together with the presence of fluids that either leaches or directly dissolve the mineral phases. In addition, the local dislocation density in quartz may play an important role during pressure solution. We suggest that the nanoscale observations of the quartz-white mica phase boundaries show direct evidence for operation of island-and-channel model as described in Wassmann and Stockhert (Tectonophysics 608:1-29, 2013 (See CR35)), while K-feldspar-quartz phase boundaries represents amorphous layers formed via interface-coupled dissolution reprecipitation as described by Hellmann et al. (Chem Geol 294-295:203-216, 2012 (See CR13)). Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1186-8) contains supplementary material, which is available to authorized users., Author(s): Zita Bukovska[sup.1] [sup.2] , Richard Wirth[sup.3] , Luiz F. G. Morales[sup.4] Author Affiliations: (1) Faculty of Science, Institute of Petrology and Structural Geology, Charles University in Prague, Albertov 6, [...]

The transition zone of the Earth's upper mantle is characterized by three discontinuities in seismic wave velocity profiles. One of these at about a depth of 520 km is assigned to the transformation of wadsleyite (β-) to ringwoodite [γ-(Mg, Fe).sub.2SiO.sub.4] (e.g., Shearer in J Geophys Res 101:3053-3066, 1996 (See CR32)). The exact location, width, and other properties of that discontinuity are affected by a multitude of parameters. The present study specifically focuses on the effect of water, iron content, and redox environment on the depth of the phase transition. We performed high-pressure experiments in a multi-anvil apparatus at 1200 °C with variation in Mg-Fe compositions (0.10 < x.sub.Fe < 0.24), water contents (0 < [Formula omitted] < 2 wt%), and the redox environment [using different buffers: Fe/FeO (reducing), Re/ReO.sub.2 (oxidizing)]. Run products were investigated using electron microprobe and Fourier transform infrared spectroscopy to obtain the composition including the hydroxyl concentration of coexisting phases. Mossbauer (MB) spectroscopy and electron energy loss spectroscopy as well as single-crystal X-ray diffraction were applied to gain insight into the Fe.sup.3+ content and incorporation mechanisms. Under hydrous and reducing conditions, the wadsleyite-ringwoodite boundary shifts by 0.5 GPa to higher pressures accompanied by a broadening of the region of coexisting wadsleyite and ringwoodite. In contrast, under hydrous and oxidizing conditions, the two-phase field gets narrower and the shift of the two-phase field to higher pressure is amplified. Thus, the stability field of wadsleyite is extended to higher pressure, most likely due to the higher water and Fe.sup.3+ content in the wadsleyite structure compared to ringwoodite. Based on results from MB spectroscopy and single-crystal X-ray diffraction, we infer that Fe.sup.3+ in wadsleyite is incorporated as a spinelloid component and stabilizes wadsleyite to higher pressures. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1163-2) contains supplementary material, which is available to authorized users., Author(s): Maria Mrosko[sup.1] , Monika Koch-Müller[sup.1] , Catherine McCammon[sup.2] , Dieter Rhede[sup.3] , Joseph R. Smyth[sup.4] , Richard Wirth[sup.1] Author Affiliations: (1) Sektion 3.3, Chemie und Physik der Geomaterialien, Deutsches [...]

We investigated rutile needles with a clear shape preferred orientation in garnet from (ultra) high-pressure metapelites from the Kimi Complex of the Greek Rhodope by electron microprobe, electron backscatter diffraction and TEM techniques. A definite though complex crystallographic orientation relationship between the garnet host and rutile was identified in that Rt[001] is either parallel to Grt or describes cones with opening angle 27.6° around Grt. Each Rt[001] small circle representing a cone on the pole figure displays six maxima in the density plots. This evidence together with micro-chemical observations in TEM, when compared to various possible mechanisms of formation, corroborates a precipitate origin. A review of exchange vectors for Ti substitution in garnet indicates that rutile formation from garnet cannot occur in a closed system. It requires that components are exchanged between the garnet interior and the rock matrix by solid-state diffusion, a process we refer to as 'open-system precipitation' (OSP). The kinetically most feasible reaction of this type will dominate the overall process. The perhaps most efficient reaction involves internal oxidation of [Fe.sup.2+] to [Fe.sup.3+] and transfer from the dodecahedral to the octahedral site just vacated by [Ti.sup.4+] : 6 [M.sup.2+.sub.3]TiAl[Al[Si.sub.2]][O.sub.12] + 6 [M.sup.2+.sub.2,5] TiAl[Si.sub.3][O.sub.12] = 10 [M.sup.2+.sub.3.0][Al.sub.1.8][Fe.sub.0.2][Si.sub.3][O.sub.12] + [M.sup.2+] + [2.sup.e-] + 12Ti[O.sub.2]. OSP is likely to occur at conditions where the transition of natural systems to open-system behaviour becomes apparent, as in the granulite and high-temperature eclogite facies. Keywords Exsolution * Rutile needles * Open-system precipitation * Ultrahigh-pressure * Rhodope, Introduction This paper intends to clarify the genesis of rutile needles in garnet that show a clear and very strict shape preferred orientation (SPO) in 3-4 easily observable directions controlled [...]

Xenotime overgrowths on detrital zircon in siliciclastic sediments have been reported in numerous studies. However, in natural samples, solid solution of zircon and xenotime is limited to near-end-member compositions. In order to characterize the interface region between both minerals and to draw inferences on the growth mechanisms of authigenic xenotime, we studied xenotime overgrowths on detrital zircon grains from two Phanerozoic sandstone samples with contrasting post-depositional histories. In one sample, the small (≤ 10 µm), pyramidal xenotime overgrowths are of diagenetic origin and grew without major discontinuity on the detrital zircon grain. The second sample shows up to >50-µm-wide, porous and inclusion-rich, hydrothermal xenotime overgrowths on detrital zircon, whereas the transition zone between both minerals is accompanied by large pore volume. Chemical compositions of the xenotime precipitates from the two samples differ particularly in Y, REE, Th and Sc concentrations, whereas high MREE availability in the diagenetic sample and the presence of Sc in the hydrothermal sample, respectively, appear to have promoted xenotime growth. Transmission electron microscopy on electron-transparent foils cut from the interface region shows that both the diagenetic xenotime and the hydrothermal xenotime are crystalline and grew in optical and crystallographic continuity to their detrital zircon substrata. Only a narrow transition zone ([less than or equal to] 90 nm--diagenetic sample, 200-300 nm--hydrothermal sample) between zircon and xenotime is in part made up of nanometre-scale crystalline domains that are slightly distorted and may have formed from dissolution-re-precipitation processes at the zircon rim along with precipitation from the respective fluid. Keywords Zircon * Xenotime * Post-depositional processes * Interface, Introduction Several studies have shown that overgrowths of xenotime on detrital zircon can be used to successfully date post-depositional processes, such as diagenesis, hydrothermal overprint, metamorphism or deformation in (meta-)sedimentary [...]

Samples of essentially 'dry' high-pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) contain up to 2-mm-large perthitic alkali feldspars with several generations of plagioclase precipitates in an orthoclase-rich host. The first generation takes the form of lenses homogeneous in size, whereas the size of a second generation of very thin albite-rich precipitates is more variable with comparatively high aspect ratios. In the vicinity of large kyanite, garnet or quartz inclusions, the first generation of plagioclase precipitates is significantly less abundant, the microstructure is coarser than in the remainder of the perthitic grain and the host is a tweed orthoclase. The first generation of precipitates formed at around 850°C during the high-pressure stage (16-18 kbar) of metamorphism. Primary exsolution was followed by primary coarsening of the plagioclase precipitates, which still took place at high temperatures (850-700°C). The coarsening was pronounced due to the access of fluids in the outer portions of the perthitic alkali feldspar and in more internal regions around large inclusions. The second generation of albite-rich precipitates was formed at around 570°C. TEM investigations revealed that the interfaces between the second-generation plagioclase lamellae and the orthoclase-rich host are coherent or semi-coherent. During late evolutionary stages of the perthite, albite linings were formed at phase boundaries, and the perthitic microstructure was partially replaced by irregularly shaped precipitates of pure albite with incoherent interfaces. The albitization occurred below 400°C and was linked to fluid infiltration in the course of deuteric alteration. Based on size-distribution analysis, it is inferred that the precipitates of the first generation were most probably formed by spinodal decomposition, whereas the precipitates of the second generation rather were formed by nucleation and growth. Keywords Bohemian Massif * High-pressure granulites * Perthite * Nucleation and growth * Size distribution * Spinodal decomposition, Introduction Perthite is a typical microstructure in alkali feldspar that is generated by exsolution of a more albite-rich phase from alkali feldspar or ternary feldspar of an intermediate composition producing [...]

Chemical zoning according to the substitution Al + (OH,F) = Ti + O is a common phenomenon in natural titanite and can be used to reconstruct its growth conditions. We synthesized titanite, which has a zoning pattern similar to that found in natural titanite that grew at the expense of rutile, visible in element distribution maps as a patchy irregular pattern. Run conditions in the system CaO-[Al.sub.2][O.sub.3]- Ti[O.sub.2]-Si[O.sub.2]-NaCl-[H.sub.2]O(-Ca[F.sub.2]) were 600[degrees]C, 0.4 GPa, 1 to 107 days duration. Natural rutile crystals were placed in a perforated Pt-tube within an outer Au-capsule containing wollastonite, [gamma]-[Al.sub.2][O.sub.3], optionally powdered Ca[F.sub.2], and a NaCl brine, to simulate conditions, where ruffle is transformed into titanite by a hydrous Ca-Si-Al(-F) fluid, driven by a chemical potential gradient between inner and outer capsule. Spontaneous nucleation of titanite is restricted to a small number of crystals on the rutile surface in the inner capsule. Growth proceeds from sparse isolated titanite crystals after 1 day to assemblages of several 100 [micro]m large crystals in a reaction rim in the long-time runs. Titanite is strongly zoned in Al-Ti and shows up to ~0.5 Al per formula unit (pfu) in experiments containing F; without F, substitution is limited to ~0.25 Al pfu. The range of the Ti-Al exchange is already large in short run times and the same heterogeneity is observed in long-time runs. The Al-Ti distribution (with and without F) in compositional domains (up to several tens of micrometers) is patchy and irregular. No relation of the zoning with time (core-rim) or preferred growth directions was observed. In the case of fluid-mediated growth, mineral growth zoning represents a disequilibrium at supersaturated conditions. The concentration of Ti in the fluid is likely the limiting factor for titanite growth considering the low solubility compared to that of Al, Ca, and Si. Titanium is not transported in significant amounts into the outer capsule, indicating low mobility, in contrast to Al, Ca, and Si, which are transported into the inner capsule. Assuming buffered Al concentration from dissolution of the Al-source, we speculate that the Ti/Al of the fluid at the precipitation site was controlled by the local Ti concentration, which varies over the distance from the rutile surface to the growing titanite crystal and produces the irregular patchy zoning. Keywords: Dissolution-precipitation reaction, Al-Ti substitution, mineral zoning, experiments DOI: 10.2138/am.2010.3518

Byline: Bernd Wunder (1), Fabien Deschamps (2), Anke Watenphul (1), Stephane Guillot (2), Anette Meixner (1), Rolf L. Romer (1), Richard Wirth (1) Keywords: Li isotopes; Isotope fractionation; Serpentine phases; Lizardite; Chrysotile; Nanotubes; Antigorite; Subduction zone Abstract: We determined the lithium isotope fractionation between synthetic Li-bearing serpentine phases lizardite, chrysotile, antigorite, and aqueous fluid in the P,T range 0.2--4.0 GPa, 200--500degC. For experiments in the systems lizardite-fluid and antigorite-fluid, 7.sup.Li preferentially partitioned into the fluid and [I.sup.7]Li values followed the T-dependent fractionation of Li-bearing mica-fluid (Wunder et al. 2007). By contrast, for chrysotile-fluid experiments, 7.sup.Li weakly partitioned into chrysotile. This contrasting behavior might be due to different Li environments in the three serpentine varieties: in lizardite and antigorite lithium is sixfold coordinated, whereas in chrysotile lithium is incorporated in two ways, octahedrally and as Li-bearing water cluster filling the nanotube cores. Low-temperature IR spectroscopic measurements of chrysotile showed significant amounts of water, whose freezing point was suppressed due to the Li contents and the confined geometry of the fluid within the tubes. The small inverse Li-isotopic fractionation for chrysotile-fluid results from intra-crystalline Li isotope fractionation of octahedral Li.sup.[6] with preference to 6.sup.Li and lithium within the channels (Li.sup.[Ch]) of chrysotile, favoring 7.sup.Li. The nanotubes of chrysotile possibly serve as important carrier of Li and perhaps also of other fluid-mobile elements in serpentinized oceanic crust. This might explain higher Li abundances for low-T chrysotile-bearing serpentinites relative to high-T serpentinites. Isotopically heavy Li-bearing fluids of chrysotile nanotubes could be released at relatively shallow depths during subduction, prior to complete chrysotile reactions to form antigorite. During further subduction, fluids produced during breakdown of serpentine phases will be depleted in 7.sup.Li. This behavior might explain some of the Li-isotopic heterogeneities observed for serpentinized peridotites. Author Affiliation: (1) German Research Centre for Geosciences GFZ Potsdam, Telegrafenberg, 14473, Potsdam, Germany (2) Laboratoire de Geodynamique des Chaines Alpines, CNRS, Universite Grenoble I, 1381 rue de la Piscine, 38041, Grenoble Cedex 09, France Article History: Registration Date: 06/10/2009 Received Date: 16/07/2009 Accepted Date: 06/10/2009 Online Date: 23/10/2009 Article note: Communicated by J. Hoefs.

Byline: Julia Degi (1,2), Rainer Abart (3), Kalman Torok (4,2), EnikA Bali (5), Richard Wirth (6), Dieter Rhede (6) Abstract: The complex microstructure of kelyphitic rims around garnet in lower crustal garnet granulite xenoliths from the Bakony--Balaton Highland Volcanic Field, Central Pannonian Basin has been studied in order to identify controls on garnet breakdown. Symplectites comprised of a vermicular intergrowth of submicron sized anorthite, orthopyroxene and spinel replace garnet at a sharp reaction front. Based on element distribution maps the transformation of garnet to symplectite is isochemical. Phase diagram calculations indicate that this reaction was induced by a pressure decrease and/or a temperature increase. In site-specific TEM foils prepared by focused ion beam technique and oriented parallel and perpendicular to the reaction front 200 nm wide rods of anorthite and 20 nm wide rods of spinel are identified. The rods are oriented approximately perpendicular to the replacement front and are embedded in an orthopyroxene matrix. The regular spacing of the symplectite phases along the reaction front suggests that their growth is controlled by diffusion. The kinetics of symplectite formation has been modelled based on irreversible thermodynamics. During interaction of the xenolith with the host basalt the microstructure and chemistry of the An--Opx--Spl symplectite was significantly modified and it was partially replaced by an olivine bearing symplectite. In contrast to primary symplectite formation, these processes were metasomatic in nature including addition of sodium, titanium and some trace elements from the basaltic melt and can clearly be discerned from the garnet breakdown. Based on these observations it is inferred that symplectite formation took place within the deep crust during the extension of the Pannonian Basin between 15 and 30 km depth at high temperature (850--1,050degC) prior to the volcanic transport to the surface. Author Affiliation: (1) Metals Research Department, Research Institute of Solid State Physics and Optics of the Hungarian Academy of Sciences, Budapest, Hungary (2) Lithosphere Fluid Research Group, Eotvos University, Budapest, Hungary (3) Freie Universitat Berlin, Berlin, Germany (4) Eotvos Lorand Geophysical Institute of Hungary, Budapest, Hungary (5) Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, Germany (6) GFZ German Research Centre for Geosciences, Helmholtz Centre Potsdam, Potsdam, Germany Article History: Registration Date: 09/07/2009 Received Date: 25/11/2008 Accepted Date: 09/07/2009 Online Date: 07/08/2009 Article note: Communicated by J. Hoefs.

Anhydrous and hydrous wadsleyite were synthesized at 13.3-13.5 GPa and 1150-1200[degrees]C in a multianvil press and investigated by Fourier transform infrared (FTIR) spectroscopy, single-crystal X-ray refinement (SC-XRD), and electron microprobe analyses (EMPA). The FTIR spectra agree with previous data, i.e., the spectra are dominated by a broad band around 3380 [cm.sup.-1], resolvable in three bands 3326 ([v.sub.2]), 3382 ([v.sub.3]), and 3546 ([v.sub.4]) [cm.sup.-1] besides some weaker OH-bands around 3600 [cm.sup.-1]. We confirm that wadsleyite incorporates water in the wt% range and that the concentration strongly increases with decreasing temperature when using secondary ion mass spectrometry (SIMS) and Raman spectroscopy. The quantifications combined with FTIR spectra led us to develop the first IR calibration for water in wadsleyite, i.e., calculating an [[epsilon].sub.i,tot] of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. A SC-XRD determination of hydrous wadsleyite FD0718, bearing 8000 [+ or -] 1000 wt ppm [H.sub.2]O, certifies the presence of Mg vacancies at the M3 sites as previously suggested. Furthermore, we found maxima in the electron density map close to the O atoms O1 and O3 of an M3 octahedron assuming the anhydrous structure. Based on our new data we suggest that the main protonation in wadsleyite occurs along the O1 ... O4 (3.1 [Angstrom]) and O3 ... O4 (3.05 [Angstrom]) edges of a vacant M3 octahedron. H-incorporation seems to be random leading to protonation of either two O1, two O3, or one O1 and one O3 of the vacant M3 octahedra. With this assignment, the observed ambient and high-pressure IR pattern can now be explained. Keywords: Wadsleyite, FTIR spectroscopy, Raman spectroscopy, protonation, SIMS, absorption coefficient for water DOI: 10.2138/am.2010.3267

Complex textures in zircon associated with Ta oxides have been used to assess the processes at the origin of zircon crystallization and associated Ta mineralization in pegmatites, in particular the role of magmatic vs. hydrothermal processes. The Tanco pegmatite is used as an example because its zircon is devoid of post-magmatic alteration and its petrogenesis is well constrained. Zircon in primary units is metamict with high U-Pb-Th contents. By contrast, in secondary assemblages affected by late-magmatic micaceous alteration, zircon is devoid of radiogenic elements, but it may contain abundant [Ta.sub.2][O.sub.5] (up to 4.7 wt%). The incorporation of Ta into zircon may occur through coupled substitution mechanisms involving other minor elements such as P, Al, Mn, or Li. The presence of Ta accounts for the distorted structures in the Ta-rich zircon, as revealed in high-resolution TEM images. Four zircon types occur sequentially in single zircon crystals, which permits a new model for zircon growth and evolution in rare element pegmatites to be advanced: (1) zircon (Z1) is Ta-rich and crystallized possibly before discrete Ta phases; (2) Z1 was overgrown by regularly zoned zircon Z2 and Z3, which show lower Ta and extreme Zr/Hf fractionation (Hf[O.sub.2] up to 38.9 wt%), suggesting crystallization from a highly fractionated melt, possibly at the onset of Ta mineralization; and (3) close to the solidus, the aqueous fluid at the origin of micaceous alteration corroded the distorted structure of Z1, and a low-Ta zircon (Z4) replaced Z1 by dissolution-reprecipitation, whereas Z2 and Z3 resisted this alteration. Tantalum was no longer stable in the zircon structure and crystallized as Ta-oxide inclusions in the reequilibrated zircon (Z4). Keywords: Zircon, tantalum mineralization, hydrothermal alteration, pegmatite, Tanco

Byline: Katharina Hartmann (1), Richard Wirth (1), Gregor Markl (2) Keywords: Feldspar; Exsolution; Pyroxene; Amphibole; Nanotunnel; Fluid pathways; Fluid alteration Abstract: Fluid transport on the grain-scale controls many rock properties and governs chemical exchange. Charnockites from Lofoten indicate fluid penetration into ternary alkali feldspars controlled by their microtextures. In a process of fluid infiltration at granulite-facies conditions ([proportional to]600degC and 8--11 kbar), tiny pyroxenes enclosed in alkali feldspar reacted to amphiboles, which are always spatially connected to perthitic albite. Investigation of these microtextures by TEM imaging of Focused Ion Beam (FIB) prepared foils revealed that pyroxenes in contact with albite lamellae show dissolution features. An amorphous Fe- and Cl-bearing material interpreted to be a residuum of the percolating fluid was found within albite lamellae. Textures and mineral compositions indicate that a Cl-rich aqueous fluid attacked the lamellae, which then provided pathways for further fluid flow. A correspondence was found between feldspar compositions, their microtexture and their degree of alteration as a result of their permeability for fluids at specific temperatures. Hence, in addition to pressure and temperature, small variations of feldspar composition can strongly influence the fluid permeability of feldspathic rocks under lower crustal conditions. Author Affiliation: (1) GeoForschungsZentrum Potsdam, Dept. 4, Telegrafenberg, 14473, Potsdam, Germany (2) Eberhard-Karls Universitat Tubingen, Institut fur Geowissenschaften, Wilhelmstr. 56, 72074, Tubingen, Germany Article History: Registration Date: 25/02/2008 Received Date: 24/07/2007 Accepted Date: 25/02/2008 Online Date: 20/03/2008 Article note: Communicated by J. Hoefs.

To gain insight into hydroxyl solubilities and possible hydration mechanisms of mantle silicates, as well as to test the utility of germanium analog models in studies of water-related defects, our present work is focused on the protonation of germanium analogs of silicates. For this purpose Ge-analogs of ringwoodite, anhydrous phase B (anhB), and for the first time, superhydrous phase B (shyB), were synthesized in a piston cylinder device at 2 GPa and 950-1000 [degrees]C under water-excess conditions. Electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and X-ray diffraction as well as Raman and infrared (IR) spectroscopy were used to characterize the experimental products. Ge-ringwoodite incorporates from 900 to 2200 ppm [H.sub.2]O by weight, which is much less than Smyth et al. (2003) observed for the Si-equivalent synthesized at 22 GPa and 1500[degrees]C, but 200X more than published for [gamma]-[Mg.sub.2]Ge[O.sub.4] by Hertweck and Ingrin (2005). In addition to this discrepancy, the incorporation mechanism of H in Ge-ringwoodite also differs from that of Si-ringwoodite. Ge-anhB, which is currently believed to be anhydrous in the Si-system, contains from 2400 to 5300 ppm water by weight. A hydration model for germanate anhB was constructed based on single-crystal X-ray diffraction analysis and IR spectroscopy, in which OH is incorporated via the hydrogarnet substitution [[[V.sub.Ge] x 4[(OH).sub.O]].sup.x] and via vacant Mg sites [[[V.sub.Mg] x 2[(OH).sub.O]].sup.x]. For Ge-shyB the water concentration and incorporation mechanism obtained in this study are identical to results reported for the silicate phase synthesized at 22 GPa and 1200[degrees]C (Koch-Muller et al. 2005). Thus, germanates are good low-pressure analogs for hydrous mantle silicates in which protonation is controlled by stoichiometry. However, for nominally anhydrous minerals we cannot recommend the use of germanates as high-pressure models in water-related studies. In these Ge-analogs, which are usually synthesized at much lower pressures, i.e., lower water fugacities, OH incorporation seems to differ from the high-pressure silicate equivalents qualitatively and quantitatively, as hydroxyl solubility is governed by other factors such as water fugacity and intrinsic defects. Keywords: Germanates, ringwoodite, nominally anhydrous minerals, FTIR, anhydrous phase B, superhydrous phase B

The mineral eskolaite ([Cr.sub.2][O.sub.3]) has been discovered in association with natural diamond from the Udachnaya pipe in Yakutia, where it is intergrown with an octahedral diamond, mostly as an inclusion in the diamond, but also exposed at its surface. A detailed study was performed on fragments extracted from the outer surface of the diamond, using single-crystal X-ray diffraction (XRD), high-resolution electron microscopy (HRTEM), analytical electron microscopy (AEM), including line-scan and elemental-mapping, EMP, and SIMS. These applied techniques confirmed the nature of the eskolaite with 86.8 wt% [Cr.sub.2][O.sub.3] and notable impurities of Ti[O.sub.2] (3.99 wt%), [Al.sub.2][O.sub.3] (2.00 wt%), [Fe.sub.2][O.sub.3] (5.83 wt%), and MgO (1.11 wt%). Trace elements, including V (4900 ppm), Mn (129 ppm), Zr (56 ppm), and Nb (32 ppm) were also detected. The entire range of REE is just at or below the limits of detection. A small picrochromite inclusion ([X.sub.Mg] 81.2; [Y.sub.Cr] 94.7) was detected in the eskolaite; its chemistry is typical of chromite diamond inclusions. It also contains minute inclusions of perovskite, corundum, and an unidentified Ti-phase. Nano-sized cavities in picrochromite were determined to consist of carbonate and quench products, including Si, Mg, Ca, P, K, and Cl. This may represent relics of the diamond-forming metasomatic fluids. The eskolaite, containing a picrochromite inclusion, was formed at high pressure within the diamond stability field from C-O-H-bearing fluids containing Ca, K, Cl, P, and possibly even peridotitic (U-type) oxides and silicates. Keywords: Diamond association, intergrowth, eskolaite, picrochromite, Udachnaya kimberlite pipe, Yakutia, Russia

Byline: Ralf Milke (1), Ralf Dohmen (2), Hans-Werner Becker (3), Richard Wirth (4) Abstract: The kinetics of (Mg, Fe)SiO.sub.3 pyroxene layer growth within silicate thin films with total thickness Author Affiliation: (1) Institut fur Geologische Wissenschaften, Freie Universitat Berlin, 12249, Berlin, Germany (2) Mineralogie und Geophysik, Ruhr-Universitat Bochum, Institut fur Geologie, 44780, Bochum, Germany (3) Institut fur Physik mit Ionenstrahlen, Ruhr-Universitat Bochum, 44780, Bochum, Germany (4) GeoForschungszentrum Potsdam, Sektion 4.1 Experimentelle Geochemie und Mineralphysik, Telegrafenberg, 14473, Potsdam, Germany Article History: Registration Date: 04/04/2007 Received Date: 22/09/2006 Accepted Date: 04/04/2007 Online Date: 04/05/2007 Article note: Communicated by: J. Hoefs.

Monazite is both partially replaced and overgrown by a ThSi[O.sub.4] phase along grain rims in a series of experiments from 300 to 900 [degrees]C and 200 to 1000 MPa. All experiments consisted of 10 mg of 100-500 [micro]m size, euhedral to subhedral crystals of a natural Th-free monazite-(Ce), 5 mg of Th[(N[O.sub.3]).sub.4]*5[H.sub.2]O, 2.5 mg of Si[O.sub.2], and 5 mg of [H.sub.2]O loaded into 3 mm wide, 1 or 1.3 cm long platinum capsules that were arc welded shut. Experimental conditions were: 300 [degrees]C at 200 and 500 MPa; 300, 400, 500, 600, and 700 [degrees]C at 500 MPa (cold seal hydrothermal autoclave); and 900[degrees]C at 1000 MPa (Catz assembly; pistoncylinder press). Back-scattered electron (BSE) imaging, electron back-scattered diffraction (EBSD) analysis, and transmission electron microscopy (TEM) indicates that in the experiments from 500 to 900[degrees]C, the ThSi[O.sub.4] phase took the form of monoclinic huttonite implying that huttonite, associated with monazite, could exist metastably over a much greater P-T range than previously thought. TEM analysis of a foil cut perpendicular to the monazite-huttonite interface from the 600 [degrees]C, 500 MPa experiment using a focused ion beam (FIB) indicates that the huttonite as well as the interface between the huttonite and monazite is characterized by fluid inclusions. High-resolution TEM analysis indicates that the huttonite-monazite interface is coherent. In the case of replacement of monazite by huttonite, fluid-aided dissolution-reprecipitation is proposed as the most likely mechanism responsible. Keywords: Monazite, huttonite, dissolution-reprecipitation, metastability, metasomatism, EBSD, TEM, pseudomorphism

The release of Zr, U, and Pb from nearly metamict zircon and its recrystallized analog and of Zr from fully crystalline and slightly radiation-damaged zircon in [H.sub.]O + HCl [+ or -] Si[O.sub.2] fluids was investigated in situ at temperatures between 200 and 500[degrees]C using a hydrothermal diamond-anvil cell and time-resolved synchrotron-radiation XRF analyses. Dissolution of nearly metamict zircon proceeded much faster than that of zircon with little or no radiation damage and resulted in a 1.5 to 2 log units higher Zr molality in 6 to 7 m HCl fluids. Extensive recrystallization of the almost fully amorphous material started at 260 to 300[degrees]C in [H.sub.2]O + HCl, and at about 360[degrees]C if quartz was added, and was coupled with a decrease of the Zr concentration in the fluid by more than an order of magnitude. Recrystallization in 7 m HCl had little effect on the aqueous U and Pb concentrations, whereas addition of quartz caused a more sluggish decrease of the Zr concentration in the fluid upon recrystallization and lowered the release of U. The data presented here support the interpretation that enhanced Zr mobility in low-grade metamorphic rocks may be related to dissolution of metamict zircon by aqueous fluids and illustrate the significance of the silica activity on the kinetics of dissolution and recrystallization during zircon-fluid interaction. Keywords: High-temperature studies, zircon-aqueous fluid interactions, hydrothermal diamond-anvil cell, in-situ SR-XRF analysis, dissolution kinetics, order-disorder, metamictization, recrystallization, fluid phase, [H.sub.2]O + HCl [+ or -] Si[O.sub.2], recrystallization kinetics

In the Monte Rosa area, Northern Italy, the assemblage garnet + phengite + quartz was formed in polymetamorphic metapelites from pre-existing biotite and plagioclase during an eclogite-facies Alpine metamorphic overprint. While phengite nucleated within plagioclase, garnet formed l0 to 20 [micro]m wide continuous rims along the original biotite-plagioclase interphase boundaries. Garnet formation involved diffusion of Ca-, Fe-, and Mg-bearing species across the growing rims. The garnet rims show an overall asymmetric compositional zoning and distinct nanometer scale chemical patterns across grain boundaries within the garnet polycrystal. The compositional patterns resulted from the interplay of diffusion along grain and interphase boundaries and volume diffusion. Individual garnet crystals are separated by low-angle grain boundaries, which are predominantly oriented perpendicular to the reaction rim giving rise to an overall palisade microstructure. Grain boundaries contain arrays of closely spaced channels about 2 nm wide, which are filled with an amorphous phase. These channels represent direct links between the garnet-biotite and garnet-plagioclase reaction fronts and provide pathways for fast diffusion. From numerical simulation of the observed chemical patterns, diffusion of Ca and Fe is inferred to have been 5-6 orders of magnitude faster within grain boundaries than through the interior of garnet grains. Although the amorphous grain boundary phase only takes up a small fraction (2-5 per rail) of the total volume of the garnet polycrystal, short-circuit diffusion along grain boundaries contributed substantially to material transfer across the growing garnet rim. Despite the presence of fast pathways, bulk diffusion was too slow to allow for chemical equilibration of the phases involved in garnet rim formation, even on a micrometer scale. Relying on published garnet volume diffusion data, [D.sub.gb] (where gb = grain boundary), is estimated to be on the order of [l0.sup.-(19-20)] [m.sup.2]/s at the estimated reaction conditions of 650 [degrees]C and 12.5 kbar, where [D.sup.Ca.sub.gb] is slower than [D.sup.Fe.sub.gb] by a factor of ten. Keywords: Garnet, high-pressure metamorphism, grain boundary, diffusion, TEM

Byline: Rolf L. Romer (1), Leander Franz (2), Richard Wirth (1) Abstract: Fluid inclusions may provide compositional and isotopic information about fluids from which the host mineral precipitated as long as the host mineral does not react with the fluid. Our transmission electron microscope (TEM) investigation of grain boundaries and of fluid inclusions in zoisite and quartz of high-pressure metamorphic rocks from Dabie Shan (eastern China) demonstrates daughter minerals, such as margarite, muscovite, calcite, and anhydrite. Their precipitation changes (1) the composition of the fluid by selective and mineral-specific removal of CO.sub.2 (carbonates), [H.sub.2]O (sheet silicates, hydration of the walls), or S (gypsum, anhydrite, sulfides), (2) the concentrations and proportions of ions dissolved in the fluid, and (3) the isotopic composition of the fluid because of isotopic fractionation between mineral precipitate and fluid and between unmixed fluids. Fluid leakage from overpressurized fluid inclusions with daughter minerals changes the overall chemical and isotopic composition of the inclusion irreversibly, even when the daughter crystals later redissolve. Such fluid loss yields a wide range of compositionally and isotopically different fluids from a single starting fluid. Depending on the relation between mineral reactions in and fluid loss from the inclusion, the fluid remaining in the inclusion and the fluid lost from the inclusion may appear entirely unrelated. Author Affiliation: (1) GeoForschungsZentrum Potsdam, Telegrafenberg, 14473, Potsdam, Germany (2) Mineralogisch-Petrographisches Institut, Universitat Basel, Bernoullistrasse 30, 4056, Basel, Switzerland Article History: Registration Date: 29/11/2005 Received Date: 27/08/2005 Accepted Date: 16/11/2005 Online Date: 03/01/2006 Article note: Communicated by J. Hoefs

Fluid-bearing microinclusions in diamonds ( Carbonates appear in all assemblages, together with either halides or silicates, indicative of the importance of carbonatitic high-density fluid during diamond growth and fluid evolution. The presence of halide-carbonate or silicate-carbonate assemblages is in agreement with the bulk composition of the microinclusions as detected by EPMA. The high K content of some microinclusions detected by EPMA cannot be accounted for by the solid phases analyzed by TEM. This discrepancy suggests that K is concentrated in the residual fluid that is lost during TEM sample preparation. In addition to microinclusions, large cavities containing amorphous phases were found in the inner parts of one Siberian and one Canadian diamond. An Al-rich phase is the most abundant, and it is accompanied by Ca-rich and Si-rich phases. These phases may be explained by amorphization of crystalline phases. A breakdown of a single melt into three immiscible components is less likely. Keywords: Crystal structure, high-density fluid, high-silica mica, carbonate, halide, secondary minerals, multi-phase assemblage

Byline: Daniel E. Harlov (1), Richard Wirth (1), Hans-Jurgen Forster (1,2) Abstract: In a series of timed experiments, monazite inclusions are induced to form in the Durango fluorapatite using 1 and 2 N HCl and [H.sub.2]SO.sub.4 solutions at temperatures of 300, 600, and 900degC and pressures of 500 and 1,000 MPa. The monazite inclusions form only in reacted areas, i.e. depleted in (Y+REE)+Si+Na+S+Cl. In the HCl experiments, the reaction front between the reacted and unreacted regions is sharp, whereas in the [H.sub.2]SO.sub.4 experiments it ranges from sharp to diffuse. In the 1 N HCl experiments, Ostwald ripening of the monazite inclusions took place both as a function of increased reaction time as well as increased temperature and pressure. Monazite growth was more sluggish in the [H.sub.2]SO.sub.4 experiments. Transmission electron microscopic (TEM) investigation of foils cut across the reaction boundary in a fluorapatite from the 1 N HCl experiment (600degC and 500 MPa) indicate that the reacted region along the reaction front is characterized by numerous, sub-parallel, 10--20 nm diameter nano-channels. TEM investigation of foils cut from a reacted region in a fluorapatite from the 1 N [H.sub.2]SO.sub.4 experiment at 900degC and 1,000 MPa indicates a pervasive nano-porosity, with the monazite inclusions being in direct contact with the surrounding fluorapatite. For either set of experiments, reacted areas in the fluorapatite are interpreted as replacement reactions, which proceed via a moving interface or reaction front associated with what is essentially a simultaneous dissolution--reprecipitation process. The formation of a micro- and nano-porosity in the metasomatised regions of the fluorapatite allows fluids to permeate the reacted areas. This permits rapid mass transfer in the form of fluid-aided diffusion of cations to and from the growing monazite inclusions. Nano-channels and nano-pores also serve as sites for nucleation and the subsequent growth of the monazite inclusions. Author Affiliation: (1) GeoForschungsZentrum Potsdam, Telegrafenberg, 14473, Potsdam, Germany (2) Institute of Earth Sciences, University of Potsdam, 601553, 14415, Potsdam, Germany Article History: Registration Date: 13/07/2005 Received Date: 20/09/2004 Accepted Date: 01/07/2005 Online Date: 24/09/2005 Article note: Editorial Responsibility: F. Poitrasson

Metastable coesite is an important pressure indicator tier ultrahigh-pressure rocks. However, in many cases coesite does not survive exhumation, but reacts back to quartz. Although it was shown experimentally that incorporation of H in coesite increases with increasing pressure, most coesite relics found in nature are essentially dry (i.e., OH concentrations are below detection limit,

For more than 100 years mineralogists, physicists, chemists, geologists, and biologists have discussed the formation of periodic Liesegang patterns observed in natural and experimental systems. Spectacular examples of minerals showing complex periodic patterns are agate, malachite, and sphalerite. Here we report the first observation of Liesegang-like patterns in hydrothermally treated metamict (i.e., amorphous) zircon. The structures observed show curved bands, radial sets of pocket-like wave fronts or irregular curved patterns in both cathodoluminescence and backscattered electron images. They are composed of alternating zones of crystallographically well-aligned, polycrystalline zircon along with remnant amorphous pockets and a phase assemblage of randomly oriented zircon crystallites, monoclinic Zr[O.sub.2], and amorphous Si[O.sub.2], as revealed by transmission electron microscopy. Analyses by secondary ion mass spectrometry and electron microprobe reveal that the latter zones are characterized by higher hydrogen concentrations and higher Zr-Si ratios. Both zones are also distinguishable by a distinctly different crystallite size. We propose a possible pattern-forming mechanism that is based on a feedback of hydrogen diffusion, zircon nucleation, and the displacement of hydrogen atoms from growing crystallites.

A series of clinopyroxenes from the lower crust and upper mantle beneath the Siberian platform was investigated by Fourier-transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM). The IR spectra of all our samples exhibit three groups of absorption bands at (1) 3445-3465, (2) 3500-3540, and (3) 3600-3624 [cm.sup.-1]. Using synchrotron IR radiation, which utilizes a spot size of only 5 x 5 [micro]m, we realized that the intensities of the absorption bands, mostly those of group 3, showed extreme variation within one crystal. TEM as well as polarized and high-pressure IR spectroscopy indicated that the OH groups that cause the bands of group 3 were not intrinsic but due to nm-sized inclusions of sheet silicates. The intensities and peak positions of the bands of group 2 correlate with the amount of tetrahedral [Al.sup.3+] indicating that the charge-deficient substitution of Al for Si is responsible for the bands of group 2. The intensities and peak positions of the bands of group 1 correlate with the concentration of vacancies at M2 indicating that the cation vacancies at M2 control the incorporation of hydroxyl responsible for the bands of group 1. The bands of groups 1 and 2 are caused by the same type of OH dipole, however, occurring in different structural environments. The concentration of the structurally bound water of the omphacitic clinopyroxene is in the range from 31 to 514 ppm [H.sub.2]O (by weight). Surprisingly, the lowest concentration was found in clinopyroxene, that comes from the highest pressure region, i.e., the diamond-bearing eclogite xenoliths of the Mir kimberlite pipe. The highest values were obtained in omphacites of the lower-pressure grospydites of the Zagadochnaya kimberlite pipe and in omphacitic clinopyroxene of the lower pressure granulites of the Udachnaya kimberlite pipe, respectively. The low water content of clinopyroxene from the high-pressure region seems to be controlled by low water activity during crystallization. However, hydrogen loss during the uplift cannot be ruled out.

A suite of 18 zircon gemstones from placers in the Highland/Southwestern Complex, Sri Lanka, were subjected to a comprehensive study of their radiation damages and ages. The investigation included X-ray diffraction. Raman and PL spectroscopy, electron microprobe, PIXE and HRTEM analysis, as well as (U-Th)/He and SHRIMP U-Th-Pb age determinations. Zircon samples described in this study are virtually homogeneous. They cover the range from slightly metamict to nearly amorphous. Generally concordant U-Th-Pb ages averaging 555 [+ or -] 11 Ma were obtained. Late Ordovician zircon (U-Th)/He ages scattering around 443 [+ or -] 9 Ma correspond reasonably well with previously determined biotite Rb-Sr ages for rocks from the HSWC. Slightly to moderately metamict zircon has retained the radiogenic He whereas only strongly radiation-damaged zircon (calculated total fluences exceeding ~3.5 x [10.sup.18] [alpha]-events/g) has experienced significant He loss. When compared to unannealed zircon from other localities, Sri Lanka zircon is about half as metamict as would correspond to complete damage accumulation over a ~555 m.y. lasting self-irradiation period, suggesting significant annealing of the structural radiation damage. Insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon. We suggest to estimate 'effective [alpha]-doses' for Sri Lanka zircon by multiplying total [alpha]-fluences, which were calculated using the zircon U-Th-Pb age, by a correction factor of 0.55. This conversion may be applied to literature data as well, because all gem-zircon samples from Sri Lanka (this work and previous studies) seem to reveal the same general trends of property changes depending on the radiation damage. The use of 'effective [alpha]-doses' for Sri Lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation-damaged zircon.

Aluminum orthoposphate (AlP[O.sub.4]) has polymorphs isostructural with tridymite, cristobalite, and quartz. Berlinite is the low-temperature form that corresponds to [alpha]-quartz. We report berlinite-quartz solid solutions to crystallize from a synthetic P-rich peraluminous granitic melt, similar in composition to the most volatile-rich silicate melt inclusions found in pegmatites. The crystallization took place in experiments performed in cold-seal pressure vessels at 450-700 [degrees]C and 0.1-0.2 GPa [H.sub.2]O pressure. At these conditions, the berlinite-quartz mutual solubility is limited to 5-7 mol% Si[O.sub.2] on the phosphate side of the solvus and to the maximum of 1 mol% AlP[O.sub.4] on the silica-rich side. The mutual solubility appears to decrease with falling temperature. At low T the crystals of berlinitequartz solid solutions are strongly zoned and show complex intergrowths between the P-rich and silica-rich phases. They were studied by electron microprobe, transmission electron microscopy, and Raman spectroscopy. In the light of our new experimental results, the extreme P enrichment reported earlier for some natural quartz-hosted melt inclusions may be explained as a post-entrapment contamination by the berlinite-bearing host.

Electron-microprobe (EMP) U-Th-Pb dating on polyphase and discordant monazites from polymetamorphic granulites of the Andriamena unit (north-central Madagascar) reveals inconsistent chemical ages. To explain these drastic variations, transmission electron microscopy (TEM) foils were prepared directly from thin sections by using the focused ion beam technique. The most important result of the TEM study is the demonstration of the presence of small (~50 nm) Pb-rich domains where large variations in EMP ages occur. We suggest that radiogenic Pb was partially reincorporated in monazite during the recrystallization at 790 Ma. Because the excited volume of EMP is ~4 [micro][m.sup.3], U-Th-Pb dating yielded various apparent older ages without geological significance. In addition, TEM analysis of the foils revealed the presence of an ~150-nm-wide amorphous zone along the grain boundary of monazite and its host quartz. This Fe-Si-Al-rich phase may have formed as a result of fluid activity at 500 Ma, and the phase's amorphous state may be due to the irradiation from U and Th decay in the monazite. This demonstrates for the first time the enormous potential of the TEM investigations on site-specific specimens prepared with the focused ion beam technique for the interpretation of geochronological data. Keywords: monazite, discordance, focused ion beam, transmission electron microscopy, U-Th-Pb ages, Madagascar.

The influence of Fe on the reaction clinochlore + kyanite + magnesiochloritoid + talc (1) was determined experimentally as a function of pressure (1.6-2.6 GPa) and composition at 600 and 540 [degrees] C. Analytical electron microscopy (AEM) was used to determine the compositions of the coexisting phases in these complex multiphase run products. Within the compositional range studied ([X.sub.Fe.sup.bulk]=0.12-0.34), chlorite solid solutions were always richer in magnesium than the coexisting chloritoid solid solutions. Fractionation of Fe into the chloritoid extends the stability field of the assemblage chloritoid plus talc towards lower pressures. However, the extent of the stability field for increasing [X.sub.Fe] is less than one would expect for ideal mixing behaviour in chloritoid and chlorite, and indicates a moderate positive deviation from ideality for chloritoid solid solutions. The thermodynamic data for magnesiochloritoid given in two earlier studies are in good agreement with the present experiments, those of a third study yield pressures which are much too high, at least compared with the experiments at 600 [degrees] C.

Olivine in spinel peridotite xenoliths from the Bismarck Archipelago northeast of Papua New Guinea, which were transported to the surface by Quaternary basalts, shows spinel inclusions up to 25 [micro]m long and 200 nm wide. These inclusions mainly occur as inhomogeneously distributed needles and subordinately as octahedral grains in olivine of veined metasomatic peridotites as well as peridotites without obvious metasomatism. The needles very often occur in swarms with irregular spacing in between them. Similar spinel inclusions in olivine have only previously been reported from ultramafites of meteoritic origin. Composition and orientation of the spinel inclusions were determined by transmission electron microscopy (TEM) and analytical electron microscopy (AEM). Both the needles and the grains display a uniform crystallographic orientation in the host olivine with [[001].sub.Ol]//[[110].sub.Spl] and [(100).sub.Ol]// [(111).sub.Spl]. The needles are elongated parallel [010] in olivine, which is the same in all olivine grains. As these needles have no relation to the metasomatic sections in the peridotite, it is concluded that they are primary features of the rock. Although the composition of the spinel needles is often very similar to the large chromian spinel octahedra in the matrix, the small octahedral spinel inclusions in olivine are in part Mg-rich aluminous spinel and sometimes almost pure magnetite. The spinel needles are suggested to have formed by exsolution processes during cooling of Al- and Cr-rich, high-temperature olivine during the initial formation of the lithospheric mantle at the mid-ocean ridge. The Al-rich spinel octahedra probably formed by the breakdown of an Al-rich phase such as phlogopite or by metasomatism, whereas the magnetite was generated by oxidizing fluids. These oxidizing fluids may either have been set free by dehydration of the underlying, subducted plate or by the Quaternary magmatism responsible for the transport of the xenoliths to the seafloor.

Calcite in former aragonite-dolomite-bearing calc-schists from the ultrahigh-pressure metamorphic (UHPM) oceanic complex at Lago di Cignana, Valtournanche, Italy, preserved different kinds of zoning patterns at calcite grain and phase boundaries. These patterns are interpreted in terms of lattice diffusion and interfacial mass transport linked with a heterogeneous distribution of fluid and its response to a changing state of stress. The succession of events that occurred during exhumation is as follows: As the rocks entered the calcite stability field at T = 530-550 [degrees] C, P ca. 1.2 GPa, aragonite occurring in the matrix and as inclusions in poikilitic garnet was completely transformed to calcite. Combined evidence from microstructures and digital element distribution maps (Mn-, Mg-, Fe- and Ca-K [alpha] radiation intensity patterns) indicates that transformation rates have been much higher than rates of compositional equilibration of calcite (involving resorption of dolomite and grain boundary transport of Mg, Fe and Ca). This rendered the phase transformation an isochemical process. During subsequent cooling to T ca. 490 [degrees] C (where lattice diffusion effectively closed), grains of matrix calcite have developed diffusion-zoned rims, a few hundred micrometres thick, with Mg and Fe increasing and Ca decreasing towards the phase boundary. Composition profiles across concentrically zoned, large grains in geometrically simple surroundings can be successfully modelled with an error function describing diffusion into a semi-infinite medium from a source of constant composition. The diffusion rims in matrix calcite are continuous with quartz, phengite, paragonite and dolomite in the matrix. This points to an effective mass transport on phase boundaries over a distance of several hundred micrometres, if matrix dolomite has supplied the Mg and Fe needed for incorporation in calcite. In contrast, diffusion rims are lacking at calcite-calcite and most calcite-garnet boundaries, implying that only very minor mass transport has occurred on these interfaces over the same T-t interval. From available grain boundary diffusion data and experimentally determined fluid-solid grain boundary structures, inferred large differences in transport rates can be best explained by the discontinuous distribution of aqueous fluid along grain/phase boundaries. Observed patterns of diffusion zoning indicate that fluid was distributed not only along grain-edge channels, but spread out along most calcite-white mica and calcite-quartz two-grain junctions. On the other hand, the inferred non-wetting of calcite grain boundaries in carbonaterich domains is compatible with fluid-calcite-calcite dihedral angles > 60 [degrees] determined by Holness and Graham (1995) for a wide range of fluid compositions under the P-T conditions of interest. Whereas differential stress has been very low at the stage of diffusion zoning (T > 490 [degrees] C), it increased as the rocks were cooling below 440 [degrees] C (at 0.3-0.5 GPa). Dislocation creep and the concomitant increase of strain energy in matrix calcite induced migration recrystallisation of high-angle grain boundaries. For that stage, the compositional microstructure of recrystallised calcite grain boundary domains indicates significant mass transport along calcite two-grain junctions, which at the established low temperatures is likely to have been accomplished by ionic diffusion within a hydrous grain boundary fluid film ('dynamic wetting' of migrating grain boundaries).

Fine irregular glass layers and pockets through mineral interfaces, cracks in olivine, within olivine crystals and in spongy rims of clinopyroxene were found using optical microscopy and transmission electron microscopy on porphyroclastic high temperature spinel peridotite from the Rhon area. The glass has a chemical composition deviating significantly from the composition of the host basanite. Its amorphous nature classifies it as a former melt.

Xenolith samples in basanite host rock from San Carlos show the presence of thin, amorphous intergranular layers along olivine grain boundaries when studied under microscope. The morphology and chemistry of the glass films indicate that partial melting at the interlayers was triggered by decompression and the energy supplied by the host rock during uplift. These results are used to suggest that mixing of melt occurs in melt veins or wider films.

K-feldspar (Kfs) from the Chain of Ponds Pluton (CPP) is the archetypal reference material, upon which thermochronological modeling of Ar diffusion in discrete 'domains' was founded. We re-examine the CPP Kfs using cathodoluminescence and back-scattered electron imaging, transmission electron microscopy, and electron probe microanalysis. [sup.40]Ar/[sup.39]Ar stepwise heating experiments on different sieve fractions, and on handpicked and unpicked aliquots, are compared. Our results reproduce the staircase-shaped age spectrum and the Arrhenius trajectory of the literature sample, confirming that samples collected from the same locality have an identical Ar isotope record. Even the most pristine-looking Kfs from the CPP contains successive generations of secondary, metasomatic/retrograde mineral replacements that post-date magmatic crystallization. These chemically and chronologically distinct phases are responsible for its staircase-shaped age spectra, which are modified by handpicking. While genuine within-grain diffusion gradients are not ruled out by these data, this study demonstrates that the most important control on staircase-shaped age spectra is the simultaneous presence of heterochemical, diachronous post-magmatic mineral growth. At least five distinct mineral species were identified in the Kfs separate, three of which can be traced to external fluids interacting with the CPP in a chemically open system. Sieve fractions have size-shifted Arrhenius trajectories, negating the existence of the smallest 'diffusion domains.' Heterochemical phases also play an important role in producing nonlinear trajectories. In vacuo degassing rates recovered from Arrhenius plots are neither related to true Fick's Law diffusion nor to the staircase shape of the age spectra. The CPP Kfs used to define the 'diffusion domain' model demonstrates the predominance of metasomatic alteration by hydrothermal fluids and recrystallization in establishing the natural Ar distribution among different coexisting phases that gives rise to the staircase-shaped age spectrum. Microbeam imaging of textures is as essential for [sup.40]Ar/[sup.39]Ar hygrochronology as it is for U-Pb geochronology. Keywords Hygrochronology * K-feldspar geochronology * Cathodoluminescence * Electron probe microanalysis * [sup.40]Ar/[sup.39]Ar age spectra * Arrhenius trajectories, Introduction The debate whether mineral ages are controlled exclusively by ambient temperature or are principally influenced by fluid-induced recrystallization is half a century old (e.g., Tilton 1960; Jager 1962; Arnold [...]

The article discusses research which reported the first occurrence of an ultra-high pressure (UHP) mineral inclusion in an Archean ophiolitic podiform chromitite block in an ophiolitic subduction related mélange, from the Neoarchean Central Orogenic Belt (COB), North China. Topics covered include the textures exhibited by the chromitites, and processes such as subduction, melting, volcanism, and the release of volatiles have regulated climate and life on Earth billions of years ago.

In a series of timed experiments, monazite inclusions are induced to form in the Durango fluorapatite using 1 and 2 N HC1 and [H.sub.2]S[O.sub.4] solutions at temperatures of 300, 600, and 900 [degrees]C and pressures of 500 and 1,000 MPa. The monazite inclusions form only in reacted areas, i.e. depleted in (Y+REE)+Si+Na+S+Cl. In the HC1 experiments, the reaction front between the reacted and unreacted regions is sharp, whereas in the [H.sub.2]S[O.sub.4] experiments it ranges from sharp to diffuse. In the 1 N HC1 experiments, Ostwald ripening of the monazite inclusions took place both as a function of increased reaction time as well as increased temperature and pressure. Monazite growth was more sluggish in the [H.sub.2]S[O.sub.4] experiments. Transmission electron microscopic (TEM) investigation of foils cut across the reaction boundary in a fluorapatite from the 1 N HCI experiment (600[degrees]C and 500 MPa) indicate that the reacted region along the reaction front is characterized by numerous, sub-parallel, 10-20 nm diameter nano-channels. TEM investigation of foils cut from a reacted region in a fluorapatite from the 1 N [H.sub.2]S[O.sub.4] experiment at 900[degrees]C and 1,000 MPa indicates a pervasive nano-porosity, with the monazite inclusions being in direct contact with the surrounding fluorapatite. For either set of experiments, reacted areas in the fluorapatite are interpreted as replacement reactions, which proceed via a moving interface or reaction front associated with what is essentially a simultaneous dissolution-reprecipitation process. The formation of a micro- and nano-porosity in the metasomatised regions of the fluorapatite allows fluids to permeate the reacted areas. This permits rapid mass transfer in the form of fluid-aided diffusion of cations to and from the growing monazite inclusions. Nano-channels and nano-pores also serve as sites for nucleation and the subsequent growth of the monazite inclusions.

The nucleation and growth processes of titanian clinohumite (Ti-C1) and titanian chondrodite (TiCh) during hydrous alteration of olivine at 8 GPa, 1300 K are described in detail. We observed two different nucleation sites and orientation relationships for Ti-C1: (1) heterogeneous nucleation at olivine grain or phase boundaries, yielding semicoherent (001) lamellae with (001)[sub.o1]//(001)[sub.Ti-C1] and (010)[sub.o1]//(010)[sub.Ti-C1]; (2) formation of coherent nanometer-sized nuclei inside olivine with the orientation relationship (100)[sub.o1]//(001)[sub.Ti.C1] and (011)[sub.o1]//(020)[sub.Ti.C1]. Only the former, for which the interface with olivine facilitates the transport of OH- and Ti necessary for the formation of the humite-group minerals, survives to large sizes. These results suggest that Ti-C1 should form readily in a Ti- and Hrich environment at mantle temperatures, independent of F content, from shallow environments to those at depths exceeding 250 km, and could play a role in recycling water back into the deep mantle.

Several micro-techniques (confocal laser-Raman microprobe, optical absorption micro-spectroscopy, high-resolution transmission electron microscopy, electron microprobe analysis) were employed in the detailed characterization of radiohaloes in biotites from two Variscan rocks from Germany. The studied biotites are intermediate members of the phlogopite-annite series with Mg/ [Fe.sup.2+] ratios in the range 1.6-1.0. Radiohaloes in biotite resulted from the impact of [sup.4]He cores ([Alpha]-particles) emitted from actinide-bearing inclusions. Monte Carlo simulations yielded [Alpha] ([sup.238]U, [sup.235]U, and [sup.232]Th series) penetration ranges in biotite between 12.5 and 37.3 [micro]m, which are in reasonable agreement with the observed radii of radiohaloes in natural biotites. The coloration pattern of a radiohalo closely correlates with the calculated distribution pattern of point defects generated in displacive events. Calculated point defect densities in the range from [is less than] [10.sup.-5] to at most [10.sup.-2] dpa (displacements per lattice atom) suggest that there are only scattered point defects in a mainly preserved biotite lattice. This is consistent with HRTEM studies that did not reveal any indication for initial volume amorphization in the haloes. However, general Raman band broadening and intensity loss suggest that the short-range order in radiohaloes is significantly disturbed. The darkened color of radiohaloes, when compared with the un-irradiated host biotite, is caused by increased light absorption over the complete visible range due to increased point defect density. No additional color centers were found, and the absorbances of the [sup.VI][Fe.sup.2+], [Fe.sup.2+]-[Fe.sup.3+], and [Fe.sup.2+]-[Ti.sup.4+] centers seem hardly to be changed. Both Raman and optical absorption spectra obtained from radiohaloes retain a clear orientational dependence. The results suggest that the formation of point defects rather than ionization is the main process causing the coloration of radiohaloes in natural biotites. The haloes represent an early stage of structural radiation damage, characterized by significantly disturbed short-range order but still widely preserved long-range order of the structure.

Two samples from the augite syenite unit of the 1.13 Ga alkaline to peralkaline Ilimaussaq intrusion in South Greenland contain olivine ([Fa.sub.75-85]) with micrometer-sized exsolution lamellae of intergrown augite and magnetite. The exsolved olivine contains only about 0.6 wt% CaO, whereas the reintegrated magmatic composition contained about 1.4% CaO corresponding to about 3 mol% of a larnite (La) component. The exsolved olivine bears no measurable Na, Al, or Ti, but the reintegrated magmatic olivine contained up to 0.4 wt% [Na.sub.2]O, 0.2 wt% [Al.sub.2][O.sub.3], and 0.3 wt% Ti[O.sub.2]. Magmatic baddeleyite in the same samples indicates an extremely low silica activity of below 0.4 during the early stages of magmatic crystallization at [is greater than] 900 [degrees] C and 1 kbar. According to the equilibrium [Ca.sub.2]Si[O.sub.4] (La in Ol) + Si[O.sub.2] = 2 CaSi[O.sub.3] (Wo in Cpx), the low silica activity is thought to be responsible for the unusually Ca-rich compositions of the primary olivines, which, in these samples, formed at 875 [+ or -] 15 [degrees] C and at oxygen fugacities between 1.2 and 2.3 log units below the QFM buffer. The composite exsolution lamellae formed during cooling due to an overstep of the schematic reaction Fa + Kst (Ca-rich component in olivine) + 1/3 [O.sub.2] = Hd + 2/3 Mt. Phase equilibria indicate temperatures between 300 and 750 [degrees] C and oxygen fugacities close to QFM for this exsolution process. The lamellae only developed in samples where the olivines were both especially high in Ca and presumably rapidly cooled (although not chilled!), as the lamella-bearing samples were collected at the outermost rim of the augite syenite unit and close to the top of the Ilimaussaq intrusion. Schreinemakers analysis shows that Ca-rich olivine of the kirschsteinite-monticellite solid solution series [Ca(Fe,Mg)Si[O.sub.4]] would exsolve from Ca-bearing olivine of the fayalite-forsterite series only at unusually low silica activities, whereas at higher silica activities, the observed exsolutions of clinopyroxene with magnetite are more stable. Hence, both cooling history and evolution of intensive variables during cooling are important for the formation of olivine exsolution textures.

Abstract: The earliest period of Earth history is the Hadean era (4600–4000 Myr) but to date no rocks are known to be preserved from this time. However, abundant detrital zircons from the Archean Jack Hills conglomerate complex of Western Australia have been found with dates in excess of 4000 Myr and there is a significant literature concerning these zircons from which abundant geochemical and mineralogical information has been obtained. The essence of what has been derived from these zircons and their inclusions concerning conditions of formation is that the zircons crystallized from granitic melts at shallow depths. However, in the last decade two papers were published reporting microdiamond inclusions in a suite of zircons ranging in age from to , implying much greater depths and repeated recycling and re-incorporation of diamonds into zircons over at least 1200 Myr. To complicate the issue, subsequent to this discovery, another study surveyed the inclusion suite in a very large sample (>400) of Jack Hills zircons (JHZs) and found no diamonds but abundant inclusions that are consistent with the previous consensus. It is important, therefore, to verify whether or not the reported JHZ diamonds could be laboratory contamination because diamond abrasive paste was used for sample preparation. Here, we report very-high-resolution re-examination of the original diamond-bearing specimens that were kindly provided to us by the authors (see Acknowledgements). We confirm the presence of diamonds in their zircons but the diamonds we find are fragments of polishing compound. These results combined with the lack of diamonds in any other samples of JHZs strongly suggest that there are no indigenous diamonds in the JHZs. [Copyright &y& Elsevier]

Several macroscopic physical and chemical properties, such as rheology, elasticity, or transport properties are governed by grain boundary processes. An improved understanding of the structure and evolution of grain boundaries has thus become a key challenge in geosciences and material sciences. Here, we report the structure of near Σ5 (210)/[100] grain boundaries in Y3Al5O12 (YAG), which were synthesised by the wafer direct bonding method. The produced grain boundaries were annealed at different temperatures, ranging from 673 to 1,873 K. The grain boundaries annealed at different temperatures are not distinguishable based on their flatness and apparent cohesiveness in high resolution TEM (HRTEM) micrographs, but show a considerable step in their mechanical stability at around 1,273 K, a temperature that corresponds to roughly half the melting temperature of YAG. This study further focuses on the effect of a slight misorientation of the two crystals on the grain boundary structure and we discuss if the boundary can reach a state of minimum energy configuration during annealing. Along the grain boundaries, we observed a long-range strain contrast with a periodicity of 40 nm, which has not been reported for high-angle grain boundaries so far. We conclude that this contrast is caused by faceting along the grain boundary plane, which is needed to achieve minimum energy configuration of the grain boundary plane. [ABSTRACT FROM AUTHOR]