Fluid entrapment and reequilibration during subduction and exhumation: A case study from the high-grade Nestos shear zone, Central Rhodope, Greece

A fluid inclusion study on metamorphic minerals of successive growth stages was performed on highly deformed paragneisses from the Nestos Shear Zone at Xanthi (Central Rhodope), in which microdiamonds provide unequivocal evidence for ultrahigh-pressure (UHP) metamorphism. The correlation of fluid inclusion density isochores and fluid inclusion reequilibration textures with geothermobarometric data and the relative chronology of micro- and macro-scale deformation stages allow a better understanding of both the fluid and metamorphic evolution along the P–T–d path. Textural evidence for subduction towards the NE is recorded by the orientation of intragranular NE-oriented fluid inclusion planes and the presence of single, annular fluid inclusion decrepitation textures. These textures occur within quartz “foam” structures enclosed in an earlier generation of garnets with prolate geometries and rarely within recrystallized matrix quartz, and reequilibrated both in composition and density during later stages of exhumation. No fluid inclusions pertaining to the postulated ultrahigh-pressure stage for microdiamond-bearing garnet–kyanite–gneisses have yet been found. The prolate shape of garnets developed during the earliest stages of exhumation that is recorded structurally by (L  S) tectonites, which subsequently accommodated progressive ductile SW shearing and folding up to shallow crustal levels. The majority of matrix kyanite and a later generation of garnet were formed during SW-directed shear under plane-strain conditions. Fluid inclusions entrapped in quartz during this stage of deformation underwent density loss and transformed to almost pure CO₂ inclusions by preferential loss of H₂O. Those inclusions armoured within garnet retained their primary 3-phase H₂O–CO₂ compositions. Reequilibration of fluid inclusions in quartz aggregates is most likely the result of recrystallization along with stress-induced, preferential H₂O leakage along dislocations and planar lattice defects which results in the predominance of CO₂ inclusions with supercritical densities. Carbonic fluid inclusions from adjacent kyanite–corundum-bearing pegmatoids and, the presence of shear-plane-parallel fluid inclusion planes within late quartz boudin structures consisting of pure CO₂-fluid inclusions with negative crystal shapes, bear witness of the latest stage of deformation by NE-directed extensional shear.This study shows that the textures of early fluid inclusions that formed already during the prograde metamorphic path can be preserved and used to derive information about the kinematics of subduction that is difficult to obtain from other sources. The textures of early inclusions, together with later generations of unaltered primary and secondary inclusions in metamorphic index minerals that can be linked to specific deformation stages and even P–T conditions, are a welcome supplement for the reconstruction of a rather detailed P–T–d path.     

Shock-induced formation of kyanite (Al<Subscript>2</Subscript>SiO<Subscript>5</Subscript>) from sillimanite within a dense metamorphic rock from the Ries crater (Germany)

A dense (~3.34 g cm^–3) garnet–sillimanite-rich metamorphic rock from the suevite breccia of the Ries impact crater was studied by scanning-electron microscopy and Raman microprobe spectroscopy. In the strongly shocked rock clast kyanite was formed from sillimanite under momentary high pressures of natural shock waves. Kyanite aggregates were found as thin (~0.3–2.0 m) seams along grain boundaries between, and fractures within, sillimanite grains. Within these seams kyanite c-axes are oriented perpendicular to original grain boundaries and fractures. In addition, larger (up to 10 m) isolated kyanite grains were rarely found within host sillimanite. Filamentary kyanite aggregates and isolated crystals typically show shrinkage cracks due to volume decrease (~10%). Locally, broad interstices between sillimanite crystals are filled with aluminosilicate glass containing a high volume fraction of sub-micrometer-sized euhedral crystals. The silica-rich glass suggests incongruent melting of sillimanite at local post-shock temperatures significantly higher than 1,300°C. The edges of adjacent sillimanite grains are thermally and chemically altered. The local generation of temperature spikes is attributed to strong shock wave interactions due to very high shock impedance contrasts.     

Water diffusion in synthetic iron-free forsterite

The kinetics of hydrogenation of dry synthetic forsterite single crystals was determined by performing experiments under hydrothermal conditions. The experiments were performed at 1.5 GPa, 1000 °C for 3 h in a piston-cylinder apparatus, or at 0.2 GPa, 900–1110 °C, for 3–20 h in TZM cold-seal vessels. The oxygen fugacity was buffered using Fe–FeO or Ni–NiO powders. Polarized Fourier transform infrared spectroscopy was utilized to quantify the hydroxyl distributions in the samples after the experiments. Hydrogenation rates were measured parallel to the three crystallographic axes from profiles of water content as a function of position in the samples. The chemical diffusion coefficients are marginally slower than in natural iron-bearing olivine for the same diffusion process, but the anisotropy of diffusion is the same, with the [001] axis the fastest direction of diffusion and [100] the slowest. Fits of the diffusion data to an Arrhenius law yield similar activation energies for each of the crystallographic axes; a global fit to all the diffusion data gave 211 ± 18 kJ mol^–1, in reasonable agreement with the previous results for natural olivine. Thus hydrogenation most likely occurs by coupled diffusion of protons and octahedrally coordinated metal vacancies. The diffusion rates are fast enough to modify water contents within xenoliths ascending from the mantle, but probably too slow to permit a total equilibration of forsterite or olivine crystals.     

Transition from subduction- to exhumation-related fabrics in glaucophane-bearing eclogites, Oman: evidence from relative fabric chronology and Ar/Ar ages

Controversy over the age of peak metamorphism and therefore the tectonic evolution of the Arabian margin relates to the polydeformed and polymetamorphosed nature of glaucophane-bearing eclogites from the Saih Hatat window beneath the allochthonous Samail ophiolite in NE Oman. These eclogites contain relicts of earlier fabrics, structures and metamorphic assemblages and provide a record of change from subduction to exhumation. The eclogites are part of a mafic layer that was disrupted into boudins up to 0.5 km in length within a lower plate shear zone (As Sifah shear zone). The megaboudins not only preserve the relicts of the highest grade of metamorphism but also an early ENE-trending lineation and sheathlike isoclines enveloped by the flat-lying schistosity. The boudin-bearing layer is isoclinally folded with calc-schist, mafic schist and quartz–mica schist, where the regional folds have axes parallel to the NE-trending stretching lineation (a-type folds). Textural evidence suggests multiple growth events for garnet and clinopyroxene, requiring polymetamorphism of the mafic layers that formed the eclogite megaboudins. The surrounding calc-schist and quartz–mica schist are both intensely deformed with transposition foliation containing an NE-trending lineation in phengite and asymmetric shear indicators such as C′-type shear bands and asymmetric pressure shadows around garnets, that give top-to-the-NE sense of shear. Although consistent ENE-trending lineations in all the boudins suggest that they have largely acted as passive, nonrotating rigid bodies, the presence of NE-vergent asymmetric mesofolds, extensive dynamic recrystallisation, multiple generations of phengites and a range of ⁴⁰Ar–³⁹Ar apparent ages within the megaboudins suggest, however, that they have not acted entirely passively during the later deformation. Phengites isolated from the high-P/low-T fabrics show groupings in ⁴⁰Ar–³⁹Ar apparent ages interpreted as distinct metamorphic/cooling intervals at 140–135, 120–98 and 92–80 Ma. Microstructural relations suggest that age groupings younger than 100 Ma reflect phengite growth during exhumation with the top-to-the-NE shearing. The older ages (120–110 Ma) from fabrics that give top-to-the-S shear sense may reflect growth during the subduction phase. The combination of groupings of apparent argon ages older than the crystallisation age of the Samail Ophiolite, the suggestion of different geothermal gradients, and superposed metamorphism suggest that the eclogites and garnet blueschists formed as a result of underthrusting along a break that was not directly related to the metamorphic sole of the ophiolite. The glaucophane–eclogites are interpreted as having formed at different times under varying pressure–temperature conditions during underthrusting with variations in the rate of underthrusting, allowing thermal equilibration and/or rapid cooling at different crustal levels.     

Origin of quartz clusters in Vinalhaven granite and porphyry,coastal Maine

We use the crystallographic orientations of quartz crystals, as determined with EBSD, to provide new evidence for the formation of clustered quartz crystals during magma crystallization. Vinalhaven is dominated by granite, with minor porphyry that formed when granite remelted during input of coeval basalt. CL zoning suggests that most quartz clusters in granite and porphyry formed by synneusis, the “swimming together” of preformed crystals. In granite, most quartz pairs in clusters have random orientations—only about 10% have parallel or Esterel twin orientations. Porphyry has fewer quartz clusters, and all pairs have approximately parallel or Esterel twin orientations. CL zoning of quartz pairs in porphyry indicates that they attached prior to a major remelting event. Interpretation of the Vinalhaven quartz clusters leads us to propose that oriented synneusis occurs during crystal accumulation on a magma chamber floor. During hindered settling, some quartz crystals should have come into contact along their dipyramidal faces. Once in contact, continued settling and loss of interstitial melt may have rotated some quartz crystals such that lattices on their dipyramidal faces matched—producing parallel and Esterel twin orientations and creating strong bonds between pairs. Only a small proportion of pairs with matched dipyramidal faces formed in the granite and, during rejuvenation to produce porphyry, only these oriented pairs survived. Hence, the presence of oriented synneusis in a plutonic rock may demonstrate a history of crystal accumulation.     

Petrogenesis of extension-related alkaline volcanism in Karaburhan (Sivrihisar–Eskisehir), NW Anatolia, Turkey

Alkaline lavas were erupted as phonolites and trachytes around Karaburhan (Sivrihisar–Eskisehir, NW Anatolia) within the Izmir–Ankara–Erzincan suture zone. These volcanic rocks were emplaced as domes, close and parallel to the ophiolite thrust line. According to ⁴⁰Ar/³⁹Ar geochronological analyses of sanidine crystals from the phonolites, the age of the alkaline volcanics is 25 Ma (Late Oligocene–Early Miocene).The flow-textured phonolites are porphyritic and consist mainly of sanidine, clinopyroxene, and feldspathoid crystals. The clinopyroxenes show compositional zoning, with aegirine (Na_0.82–0.96Fe⁺³_0.68–0.83) rims and aegirine–augite cores (containing calcium, magnesium, and Fe⁺²). Some aegirine–augites are replaced with sodium-, calcium-, and magnesium-rich amphibole (hastingsite). Feldspathoid (hauyne) crystals enriched with elemental Na and Ca have been almost completely altered to zeolite and carbonate minerals. The fine-grained trachytes with a trachytic texture consist of feldspar (oligoclase and sanidine) phenocrystals and clinopyroxene microphenocrystals within a groundmass made up largely of alkali feldspar microlites.Although there are some differences in their element patterns, the phonolites and trachytes exhibit enrichment in LILEs (Sr, K, Rb, Ba, Th) and LREEs (La, Ce, Pr, Nd) and negative anomalies in Nb and Ta. These geochemical characteristics indicate a lithospheric mantle enriched by fluids extracted from the subduction component. In addition, the high ⁸⁷Sr/⁸⁶Sr (0.706358–0.708052) and low ¹⁴³Nd/¹⁴⁴Nd (0.512546–0.512646) isotope concentrations of the alkaline lavas reflect a mantle source that has undergone metasomatism by subduction-derived fluids. Petrogenetic modeling indicates that the alkaline lavas generated from the subduction-modified lithospheric mantle have undergone assimilation, fractional crystallization, and crustal contamination, acquiring high Pb, Ba, Rb, and Sr contents and Pb isotopic compositions during their ascent through the thickened crust in an extensional setting.     

Spatial correlation of deformation and mineral reaction in experimentally deformed plagioclase–olivine aggregates

Shear deformation of hot pressed plagioclase–olivine aggregates was studied in the presence and absence of mineral reaction. Experiments were performed at 900 °C, 1500 MPa, and a constant shear strain rate of 5×10⁻⁵ s⁻¹ in a solid medium apparatus. Whether the mineral reaction between plagioclase and olivine takes place or not is controlled by choosing the appropriate plagioclase composition; labradorite (An₆₀) does not react, anorthite (An₉₂) does. Labradorite–olivine aggregates deformed without reaction are very strong and show strain hardening throughout the experiment. Syndeformational reaction between olivine and anorthite causes a pronounced strain weakening. The reaction produces fine-grained opx–cpx–spinel aggregates, which accommodate a large fraction of the finite strain. Deformation and reaction are localised within a 0.5-mm-wide sample. Three representative samples were analysed for their fabric anisotropy R* and shape-preferred orientation α* (fabric angle with the shear plane) using the autocorrelation function (ACF). Fabric anisotropy can be calibrated to quantify strain variations across the sheared samples. In the deformed and reacted anorthite–olivine aggregate, there is a strong correlation between reaction progress and strain; regions of large shear strain correspond to regions of maximum reaction progress. Within the sample, the derived strain rate variations range up to almost one order of magnitude.     

Graphite–sulfide deposits in Ronda and Beni Bousera peridotites (Spain and Morocco) and the origin of carbon in mantle-derived rocks

This paper describes unusual graphite–sulfide deposits in ultramafic rocks from the Serranía de Ronda (Spain) and Beni Bousera (Morocco). These deposits occur as veins, stockworks and irregular masses, ranging in size from some centimeters to a few meters in thickness. The primary mineral assemblage mainly consists of Fe–Ni–Cu sulfides (pyrrhotite, pentlandite, chalcopyrite and cubanite), graphite and chromite. Weathering occurs in some sulfide-poor deposits that consist of graphite (up to 90%), chromite and goethite. Texturally, graphite may occur as flakes or clusters of flakes and as rounded, nodule-like aggregates. Graphite is highly crystalline and shows light carbon isotopic signatures (δ¹³C≈− 15‰ to − 21‰). Occasionally, some nodule-like graphite aggregates display large isotopic zoning with heavier cubic forms (probably graphite pseudomorphs after diamond with δ¹³C up to − 3.3‰) coated by progressively lighter flakes outwards (δ¹³C up to − 15.2‰).Asthenospheric-derived melts originated the partial melting (and melt–rock reactions) of peridotites and pyroxenites generating residual melts from which the graphite–sulfide deposits were formed. These residual melts concentrated volatile components (mainly CO₂ and H₂O), as well as S, As, and chalcophile elements. Carbon was incorporated into the melts from the melt–rock reactions of graphite-bearing (formerly diamonds) garnet pyroxenites with infiltrated asthenospheric melts. Graphite-rich garnet pyroxenites formed through the UHP transformation of subducted kerogen-rich crustal material into the mantle. Thus, graphite in most of the studied occurrences has light (biogenic) carbon signatures. Locally, reaction of the light carbon in the melts with relicts of ¹³C-enriched graphitized diamonds (probably generated from hydrothermal calcite veins in the subducting oceanic crust) reacted with the partial melts to form isotopically zoned nodule-like graphite aggregates.     

Experiental deformation of single crystals of biotite

Single crystals of biotite have been shortened up to 20% in compression tests parallel to [100], [110] and [010] directions at 3 Kbar confining pressure and temperatures from 300 to 700° C, and at a strain rate of 10^–4 sec^–1. Thick metal constraining sleeves were used and led to a distribution of kinking throughout the crystals. The orientation of kink boundaries, angle of bending and asymmetry of the basal plane across the kink boundaries and the axes of bending were measured. A minor amount of unidentified non-basal slip must have occurred to account for the assymmetry, but basal slip predominates at all temperatures. From the axes of bending, the discrete slip directions [100], [110] and [110] for basal slip are deduced. Increase in temperature mainly leads to a simpler pattern of kinking associated with the kinks being wider and the kinking angle larger, presumably as a result of greater mobility of dislocation walls that form the kink boundaries.In his summary table, Mügge lists these axes as [010] and [130] but the latter seems to be quoted in error, and in conflict with his text, in place of [310]. Borg and Handin (1966) have quoted the [130] indices as given by Mügge in his table. In the analysis of their own observations there has been a confusion between direction indices and plane normal indices. When this is corrected, their results would also indicate [100] and [110] as active slip directions in [001] (Borg, private communication).     

Speleogenetic history of the Hungarian hydrothermal karst

The hydrothermal karst of Hungary displays at least two principal stages of development in two differing environments. Caves of an early stage were formed within a deep zone of low thermal gradient. These caves (vugs) are small (tens of centimeters) and lined with scalenohedral crystals of calcite that are often in association with barite. Calcite yields fluid inclusion temperatures of 55–95°C and is depleted in¹⁸O (–11.2 to –17.6 per mil PDB). The caves were formed by ascending thermal waters charged with CO₂. Solubility of CaCO₃ in such a system gradually increases with the ascent of the fluid (solutional zone) but drops sharply at a depth of –250 m to –500 m below the water surface (depositional zone). Caves formed in the solutional zone may be shifted into the depositional zone due to tectonic uplift, and calcite lines their walls. Large caves (tens to thousands of cubic meters) of a late stage were formed within a shallow zone of high thermal gradient immediately below and above the thermal water table. The calcite of the phreatic crusts has a rhombohedral habit, displays lower fluid inclusion temperatures (35–55°C and less), and a depletion in¹⁸O of –9.5 to –14.6 per mil PDB. Several powerful cave-forming processes may operate there including convection, mixing/cooling corrosion, and condensation corrosion. Due to differences in the rate of tectonic uplift, rate of hydrothermal system decay, and hydrogeologic pattern, these caves were either filled with water for a long period of time (phreatic calcite crusts are formed) or partly dewatered early in their history (waterline and subaerial speleothems are formed). The zones of thermal cave formation recognized in Hungary may have a universal character. Very similar features are found in other hydrothermal karst areas of the world (Kirghizia, Algeria, South Dakota).     

On the optically biaxial character and heterogeneity of anthracites

The results of the study of optical properties of 13 anthracites from different parts of the world are presented in this paper. Measurements of reflectance values were made on non-oriented vitrinite grains for a minimum of 300 points per sample. The reconstruction of Reflectance Indicating Surfaces (RIS) were made by Kilby's method [Kilby, W.E., 1988. Recognition of vitrinite with non-uniaxial negative reflectance characteristics. Int. J. Coal Geol. 9, 267–285; Kilby, W.E., 1991. Vitrinite reflectance measurement — some technique enhancements and relationships. Int. J. Coal Geol. 19, 201–218]. It was found that the use of Kilby's method for strongly anisotropic materials like anthracites did not give unambiguous results. Some improvement in Kilby's method, consisting of the division of the cumulative cross-plot into several elemental components, is suggested. Each elemental cross-plot corresponds to a textural class of anthracite, which is characterized by the values of RIS main axes R_MAX^(k), R_INT^(k) and R_MIN^(k) (k=1,2,…n; n — number of classes). The global texture of anthracite is characterized as a RIS with main axes calculated as the weighted means of , and for each class of this anthracite.The division of cumulative Kilby's cross-plot on elemental components makes possible the calculation of new coefficients H_t and H₁₀ characterizing the heterogeneity of the structure and texture of anthracites. The results of our study show that all anthracites have biaxial negative textures, but their heterogeneity varies in a wide range of H_t and H₁₀ coefficients depending upon the individual coal basin.     

Symplectites in upper mantle peridotites: Development and implications for the growth of subsolidus garnet,pyroxene and spinel

Silicate-oxide symplectites in complex mineral intergrowths are relatively common in upper mantle xenoliths and in xenoliths in the Jagersfontein Kimberlite, South Africa.Harzburgites of olivine and high-Al (1.9–3.6 wt%), Ca (0.6–0.9 wt%) and Cr (0.3–0.9 wt%) enstatite contain symplectites of spinel and diopside, or spinel, diopside and lower-Al (0.8–2.2 wt%), Ca (0.1–0.4 wt%) and Cr (0.02–0.8 wt%) enstatite. From textures and mineral chemistries these symplectites are interpreted to have formed by mineral unmixing and migration from Al–Ca–Cr discrete enstatite to adjoining mineral interfaces.Garnet harzburgites are composed of large (0.5–1 cm) olivine, equally large discrete low-Al (0.6–1.1 wt%), Ca (0.1–0.5 wt%), and Cr (0.1–0.3 wt%) enstatite and smaller interstitial garnet, diopside, and high-Cr and low-Al spinel. Symplectites are composed of either spinel+diopside+garnet, or garnet+spinel. Spinel diopside garnet symplectites have cores of spinel+diopside, resembling symplectites inharzburgites, but surrounded by rims of garnet or garnet+undigested globular spinel. From textures and chemistries we suggest that the spinel+diopside cores formed from Ca-Al-Cr-rich orthopyroxene initially as a nonstoichiometric homogeneous single phase clinopyroxene enriched in Fe, Cr and Al. This was followed by decomposition of the clinopyroxene to diopside+spinel, and subsequent garnet formation in a prograde reaction with olivine or enstatite. In bothharzburgites andgarnet harzburgites the metastable cellular structures may also have formed by the simultaneous precipitation of pyroxene and spinel. In all cases there is a strongly preferred embayment of symplectite bodies into olivine. Olivine appears to have activated adjacent     

Radiaxial fibrous calcite: a replacement after acicular carbonate

Radiaxial fibrous calcite, a common cavity fill in ancient limestones, is characterized by curved twin lamellae, optic axes that converge away from the substrate and subcrystals which diverge in this same direction. The optic axes radiate about three or four axes located in positions between adjacent crystals and orientated parallel with the crystal elongation. The crystals are commonly turbid with inclusions, which may be concentrated along twin lamellae and subcrystal and inter-crystalline boundaries, or form zones parallel to the substrate. Some inclusion patterns reveal the position of former crystal faces. From a consideration of the occurrences, gross morphological characters, the fabrics of the crystals and the inclusion patterns, radiaxial fibrous calcite is interpreted as a replacement of an early diagenetic acicular cement, composed of interfering bundles of radiating crystals. It is suggested that replacement takes place by a solution-precipitation process and the migration of a fluid film through the acicular host, with replacement occurring most rapidly between bundles of acicular carbonate. The optic axis pattern of the fibrous calcite is considered to be inherited from the c-axis orientations of the host acicular crystals. Fibrous calcite intercrystalline boundaries form as fractures after replacement. Some inclusion patterns record the characters of the replaced acicular cement; others, formed of impurities reorganized during the replacement, reveal the form of the replacement front.     

The role of hydrogen bonding in the thermal expansion and dehydration of brushite,di-calcium phosphate dihydrate

The unit-cell and atomic parameters of perdeuterated brushite have been extracted from Rietveld analysis of neutron powder diffraction data within the temperature range 4.2 to 470 K. The thermal expansion of brushite is anisotropic, with the largest expansion along the b axis due principally to the effect of the O(1)···D(4) and O(3)···D(2) hydrogen bonds. Expansion along the c axis, influenced by the Ow1···D(5) interwater hydrogen bond, is also large. The high temperature limits for the expansion coefficients for the unit-cell edges a, b and c are 9.7(5) × 10^–6, 3.82(9) × 10^–5 and 5.54(5) × 10^–5 K^–1, respectively, and for the cell volume it is 9.7(1) × 10^–5 K^–1. The angle displays oscillatory variation, and empirical data analysis results in = 1.28(3) × 10^–6sin(0.0105 T) K^–1, within this temperature range. The evolution of the thermal expansion tensor of brushite has been calculated between 50 T 400 K. At 300 K the magnitudes of the principal axes are ₁₁ = 50(6) × 10^{–6 K–1}, ₂₂ = 26.7(7) × 10^–6 K^–1 and ₃₃ = 7.0(5) × 10^–6 K^–1. The intermediate axis, ₂₂, is parallel to b, and using IRE convention for the tensor orthonormal basis, the axes ₁₁ and ₃₃ have directions equal to (–0.228, 0, –0.974) and (–0.974, 0, 0.228) respectively. Under the conditions of these experiments, the onset of dehydration occurred at temperatures above 400 K. Bond valence analysis combined with assessments of the thermal evolution of the bonding within brushite suggests that dehydration is precipitated through instabilities in the chemical environment of the second water molecule.     

Cation distribution in pentlandites (Fe,Ni)9S8: Dependence on pressure and temperature and kinetics of the cation exchange reaction

This paper describes the distribution of Fe and Ni between the octahedral and tetrahedral sites in pentlandite (Fe,Ni)₉S₈. The dependence of the distribution on pressure and temperature and the activation energy of the cation exchange reaction were determined through annealing experiments. Synthetic crystals were annealed at 433–723 K and pressures up to 4 GPa, and natural crystals were annealed at 423, 448 and 473 K in evacuated silica capillary tubes for various durations. The cation distributions in the synthetic crystals were determined with an X-ray powder method employing the anomalous dispersion effect of CuK. and FeK radiations, while those of natural crystals were calculated from the cell dimensions. The values of U, S and V for the Fe/Ni exchange reaction are –6818 J mol^–1, 20.52 J K^–1 mol^–1, and 6.99 × 10^–6 m³ mol^–1, respectively. The dependence of the Fe/Ni distribution on pressure (Pa) and temperature (Kelvin) was determined as lnK = 2.47+8.20 × 10² T ^–1+8.41 x 10^–7 T ^–1 P, where K = (Fe/Ni)_octahedral /(Fe/Ni)_tetrahedral. The activation energy of the cation exchange reaction was 185 kJ mol^–1.     

Messung des Ultrarot-Pleochroismus von Mineralen. XIV. Der Pleochroismus der OH-Streckfrequenz in Diopsid

Zuammenfassung Klare, hellgrün gefärbte Diopsidkristalle vom Rotkopf, Tirol zeigen bei 3600 cm^–1 eine relativ starke Absorptionsbande, die der OH-Streckfrequenz zugeordnet wird. Die ungefähre Orientierung der OH-Gruppen konnte aus dem UR-Pleochroismus orientiert geschliffener Kristallplatten parallel (100) und (010) abgeleitet werden. Wahrscheinlich wird die O(2)-Punktlage geringfügig durch OH-Gruppen ersetzt, wobei das H-Atom nahe der Verbindungslinie O(2)–O(3)liegt.

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Measurement of the infrared-pleochroism in minerals. XIV. The pleochroism of the OH-stretching frequency in diopside

Summary Clear light-green diopside crystals from Rotkopf, Tyrol show a relatively strong absorption band at 3600 cm^–1, which is assigned to the OH-stretching frequency. The approximate orientation of the OH-groups could be derived from the IR-pleochroism as observed in oriented crystal faces parallel (100) and (010). The O(2) position is apparently partially occupied by OH-groups with the H-atom pointing in the O(3) direction.

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Mit 2 Abbildungen     

Mineralogy and partial melt textures within an ultramafic-mafic body,Greece

Ultramafic-mafic rocks from Makrirrakhi, Central Greece exhibit features of an original ophiolite sequence which contains depleted mantle material, ultramafic containing partial melt textures and possibly the mafic pluton which resulted from the coalescing of these partial melt segregations. Considerable mineralogical variation exists: unzoned olivine crystals range in composition from Fo_78–84 (mafics) to Fo_88–92 (ultramafics), plagioclases An_64–79 (mafics) to An_80–90 (ultramafics) and spinel varies from a chromian spinel (ultramafics) to a more aluminous-titaniferous spinel (mafics). Pyroxenes from the ultramafics display a limited range: En_89–92 Fs_9–8 Wo_0–2 (orthopyroxene) and En_48–54 Fs_1–10 Wo_38–50 (clinopyroxene). Mafic rocks display a greater range being richer in ferrosilite En_36–65 Fs_3–20 Wo_33–51. Pyroxenes from within the partial melt segregations have chemical affinities with those from the gabbrotroctolite series. A model of partial melt within the upper mantle, and, a set of criteria to distinguish partial melt textures from cumulate textures, are developed from analytical data and textural evidence.     

The effect of silica activity on the incorporation mechanisms of water in synthetic forsterite: a polarised infrared spectroscopic study

Pure forsterite crystals were grown from hydrous melts using controlled cooling experiments at 2.0 GPa and varying the bulk Mg/Si ratio from 2.0 to 1.5. Oriented single crystals were then studied by polarised infrared spectroscopy. The spectra of the samples with the lowest silica activity (aSiO₂) contain the main OH bands in the range 3,620–3,450 cm^–1 only. In contrast, the spectra of the samples synthesised with the highest aSiO₂ contain additional pleochroic bands at 3,160, 3,220 and 3,600 cm^–1. The variations are interpreted in terms of protonated silicon vacancies being dominant at low aSiO₂ and Mg vacancies dominant at high aSiO₂. Xenolithic mantle olivines generally do not have the spectrum expected for orthopyroxene buffered conditions, suggesting that they re-equilibrated with their host melts during ascent, but mantle olivine from the Zabargad peridotite massif probably is in equilibrium with the coexisting orthopyroxene.Editorial responsibility: T.L. Grove     

Graphite morphologies from the Borrowdale deposit (NW England,UK): Raman and SIMS data

Graphite in the Borrowdale (Cumbria, UK) deposit occurs as large masses within mineralized pipe-like bodies, in late graphite–chlorite veins, and disseminated through the volcanic host rocks. This occurrence shows the greatest variety of crystalline graphite morphologies recognized to date from a single deposit. These morphologies described herein include flakes, cryptocrystalline and spherulitic aggregates, and dish-like forms. Colloform textures, displayed by many of the cryptocrystalline aggregates, are reported here for the first time from any graphite deposit worldwide. Textural relationships indicate that spherulitic aggregates and colloform graphite formed earlier than flaky crystals. This sequence of crystallization is in agreement with the precipitation of graphite from fluids with progressively decreasing supersaturation. The structural characterization carried out by means of Raman spectroscopy shows that, with the exception of colloform graphite around silicate grains and pyrite within the host rocks, all graphite morphologies display very high crystallinity. The microscale SIMS study reveals light stable carbon isotope ratios for graphite (δ ¹³C = −34.5 to −30.2‰), which are compatible with the assimilation of carbon-bearing metapelites in the Borrowdale Volcanic Group magmas. Within the main mineralized breccia pipe-like bodies, the isotopic signatures (with cryptocrystalline graphite being lighter than flaky graphite) are consistent with the composition and evolution of the mineralizing fluids inferred from fluid inclusion data which indicate a progressive loss of CO₂. Late graphite–chlorite veins contain isotopically heavier spherulitic graphite than flaky graphite. This agrees with CH₄-enriched fluids at this stage of the mineralizing event, resulting in the successive precipitation of isotopically heavier graphite morphologies. The isotopic variations of the different graphite morphologies can be attributed therefore, to changes in the speciation of carbon in the fluids coupled with concomitant changes in the XH₂O during precipitation of graphite and associated hydrous minerals (mainly epidote and chlorite).     

Geochronological and geochemical study of mafic dykes from the northwest Chinese Altai: Implications for petrogenesis and tectonic evolution

Fifteen zircons separated from a mafic dyke in the Chinese Altai give a concordant age population with a weighted mean ²⁰⁶Pb/²³⁸U age of 375.5 ± 4.8 Ma, suggesting a Devonian emplacement. On the basis of their mineralogical compositions and textures, the coeval dykes can be divided into gabbroic and doleritic types. They are both sub-alkaline, tholeiitic, characterized by similarly low SiO₂ contents (45.2–52.7 wt.%) and total alkaline (K₂O + Na₂O = 0.99–4.93 wt.%). Rare earth element patterns of the gabbroic dykes are similar to N-MORB (La/Yb_N = 0.86–1.1), together with their high ε_Nd(t) values (+ 7.6 to + 8.1), indicating that their precursor magma was mainly derived from a N-MORB-type depleted asthenospheric mantle. While the REE patterns of the doleritic dykes resemble that of E-MORB (La/Yb_N = 1.12–2.28), enriched in LILEs and strongly depleted in HFSEs, with relative low ε_Nd(t) values (+ 3.4 to + 5.4) and high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7057–0.7060). The zircon Hf isotopic analysis of the doleritic dykes give ε_Hf(t) values from + 10.7 to + 13.8. These signatures suggest that a depleted mantle wedge metasomatized by slab-derived fluids and/or melts was possibly involved in the generation of the doleritic magma. The refractory peridotite may have been melted with variable degrees caused by upwelling of the hot asthenosphere. The petrogenesis of the mafic dykes suggest a high heat flux as a result of upwelling of the hot asthenosphere and the contrast geochemical signatures can be interpreted by a ridge subduction, which could be an important tectonic control in the accretionary process of the Chinese Altai.