Are the light-harvesting I complexes from Rhodospirillum rubrum arranged around the reaction centre in a square geometry?

The basic photosynthetic unit containing the reaction centre and the light-harvesting I complex (RC-LHI) of the purple non-sulphur bacterium Rhodospirillum rubrum was purified and reconstituted into two-dimensional (2D) membrane crystals. Transmission electron microscopy using conventional techniques and cryoelectron microscopy of the purified single particles and of 2D crystals yielded a projection of the RC-LHI complex at a resolution of at least 1.6 nm. In this projection the LHI ring appears to have a square symmetry and packs in a square crystal lattice. The square geometry of the LHI ring was observed also in images of single isolated particles of the RC-LHI complex. However, although the LHI units are packed identically within the crystal lattice, a new rotational analysis developed here showed that the reaction centres take up one of four possible orientations within the ring. This fourfold disorder supports our interpretation of a square ring symmetry and suggests that a hitherto undetected component may be present within the photosynthetic unit.

Lifetime of an ocean island volcano feeder zone: constraints from U-Pb dating on coexisting zircon and baddeleyite, and 40Ar/39Ar age determinations, Fuerteventura, Canary Islands

High-precision isotope dilution - thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 +/- 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. Ar-40/Ar-39 amphibole dating yielded ages from 21.9 +/- 0.6 to 21.8 +/- 0.3, identical within errors to the U-Pb ages, despite the expected 1% theoretical bias between Ar-40/Ar-39 and U-Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i. e., less than the 0.3 to 0.6 Ma 40Ar/39Ar age uncertainties) from closure temperatures (T-c) of zircon (699-988 degrees C) to amphibole (500-600 degrees C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the Ar-40/Ar-39 and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole T-c is 0.8 Ma, suggesting PX1 lifetime of 520 000 to 800 000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 +/- 0.07/0.08/0.15 Ma and 21.58 +/- 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope delta O-18 values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO2-rich carbonatite-derived fluids released during contact metamorphism.

Post-Plutonic Magmatism: from the Source to the Emplacement of an Upper Crustal Dyke Swarm, Adamello Massif, Italy

Magmas of the arc-tholeiitic and calc-alkaline differentiation suites contribute substantially to the formation of continental crust in subduction zones. Different geochemical-petrological models have been put forward to achieve evolved magmas forming large volumes of tonalitic to granitic plutons, building an important part of the continental crust. Primary magmas produced in the mantle wedge overlying the subducted slab migrate through the mantle and the crust. During the transfer, magma can accumulate in intermediate reservoirs at different levels where crystallization leads to differentiation and the heat transfer from the magma, together with gained heat from solidification, lead to partial melting of the crust. Partial melts can be assimilated and mix with more primitive magma. Moreover, already formed crystal cumulates or crystal mushes can be recycled and reactivated to transfer to higher crustal levels. Magma transport in the crust involves fow through fractures within a brittle elastic rock. The solidified magma filled crack, a dyke, can crosscut previously formed geological structures and thus serves as a relative or absolute time marker. The study area is situated in the Adamello massif. The Adamello massif is a composite of plutons that were emplaced between 42 and 29 million years. A later dyke swarm intruded into the southern part of the Adamello Batholith. A fractionation model covering dyke compositions from picrobasalts to dacites results in the cummulative crystallization of 17% olivine, 2% Cr-rich spinel, 18% clinopyroxene, 41% amphibole, 4% plagioclase and 0.1% magnetite to achieve an andesitic composition out of a hydrous primitive picrobasalt. These rocks show a similar geochemical evolution as experimental data simulating fractional crystallization and associated magma differentiation at lower crustal depth (7-10 kbar). The peraluminous, corundum normative composition is one characteristic of more evolved dacitic magmas, which has been explained in a long lasting debate with two di_erent models. Melting of mafic crust or politic material provides one model, whereas an alternative is fractionation from primary mantle derived melts. Amphibole occurring in basaltic-andesitic and andesitic dyke rocks as fractionating cumulate phase extracted from lower crustal depth (6-7.5 kbar) is driving the magmas to peraluminous, corundum normative compositions, which are represented by tonalites forming most of the Adamello Batholith. Most primitive picrobasaltic dykes have a slightly steepened chondrite normalized rare earth elements (REE) pattern and the increased enrichment of light-REE (LREE) for andesites and dacites can be explained by the fractional crystallization model originating from a picrobasalt, taking the changing fractionating phase assemblage and temperature into account. The injection of hot basaltic magma (~1050°C) in a closely spaced dyke swarm increases the surface of the contact to the mainly tonalitic wallrock. Such a setting induces partial melting of the wall rock and selective assimilation. Partial melting of the tonalite host is further expressed through intrusion breccias from basaltic dykes. Heat conduction models with instantaneous magma injection for such a dyke swarm geometry can explain features of partial melting observed in the field. Geochemical data of minerals and bulk rock further underline the selective or bulk assimilation of the tonalite host rock at upper crustal levels (~2-3 kbar), in particular with regard to light ion lithophile elements (LILE) such as Sr, Ba and Rb. Primitive picrobasalts carry an immiscible felsic assimilant as enclaves that bring along refractory rutile and zircon with textures typically found in oceanic plagiogranites or high pressure/low-temperature metamorphic rocks in general. U-Pb data implies a lower Cretaceous age for zircon not yet described as assimilant in Eocene to Oligocene magmatic rocks of the Central Southern Alps. The distribution of post-plutonic dykes in large batholiths such as the Adamello is one of the key features for understanding the regional stress field during the post-batholith emplacement cooling history. The emplacement of the regional dyke swarm covering the southern part of the Adamello massif was associated with consistent left lateral strike-slip movement along magma dilatation planes, leading to en echelon segmentation of dykes. Through the dilation by magma of pre-existing weaknesses and cracks in an otherwise uniform host rock, the dyke propagation and according orientation in the horizontal plane adjusted continuously perpendicular to least compressive remote stress σ3, resulting in an inferred rotation of the remote principal stress field. Les magmas issus des zones de subduction contribuent substantiellement à la formation de la croûte continentale. Les plutons tonalitiques et granitiques représentent, en effet, une partie importante de la croûte continentale. Des magmas primaires produits dans le 'mantle wedge ', partie du manteau se trouvant au-dessus de la plaque plongeante dans des zones de subduction, migrent à travers le manteau puis la croûte. Pendant ce transfert, le magma peut s'accumuler dans des réservoirs intermédiaires à différentes profondeurs. Le stockage de magma dans ces réservoirs engendre, d'une part, la différentiation des magmas par cristallisation fractionnée et, d'autre part, une fusion partielle la croûte continentale préexistante associée au transfert de la chaleur des magmas vers l'encaissant. Ces liquides magmatiques issus de la croûte peuvent, ensuite, se mélanger avec des magmas primaires. Le transport du magma dans la croûte implique notamment un flux de magma à travers différentes fractures recoupant les roches encaissantes élastiques. Au cours de ce processus de migration, des cumulats de cristaux ou des agrégats de cristaux encore non-solidifiés, peuvent être recyclés et réactivés pour être transportés à des niveaux supérieures de la croûte. Le terrain d'étude est situé dans le massif d'Adamello. Celui-ci est composé de plusieurs plutons mis en place entre 42 et 29 millions d'années. Dans une phase tardive de l'activité magmatique liée à ce batholite, une série de filons de composition variable allant de picrobasalte à des compositions dacitiques s'est mise en place la partie sud du massif. Deux modèles sont proposés dans la littérature, pour expliquer la formation des magmas dacitiques caractérisés par des compositions peralumineux (i.e. à corindon normatif). Le premier modèle propose que ces magmas soient issus de la fusion de matériel mafique et pélitique présent dans la partie inférieur de la croûte, alors que le deuxième modèle suggère une évolution par cristallisation fractionnée à partir de liquides primaires issus du manteau. Un modèle de cristallisation fractionnée a pu être développé pour expliquer l'évolution des filons de l'Adamello. Ce modèle explique la formation des filons dacitiques par la cristallisation fractionnée de 17% olivine, 2% spinelle riche en Cr, 18% clinopyroxène, 41% amphibole, 4% plagioclase et 0.1% magnetite à partir de liquide de compositions picrobasaltiques. Ce modèle prend en considération les contraintes pétrologiques déduites de l'observation des différents filons ainsi que du champ de stabilité des différentes phases en fonction de la température. Ces roches montrent une évolution géochimique similaire aux données expérimentales simulant la cristallisation fractionnée de magmas évoluant à des niveaux inférieurs de la croûte (7-10 kbar). Le modèle montre, en particulier, le rôle prépondérant de l'amphibole, une phase qui contrôle en particulier le caractère peralumineux des magmas différentiés ainsi que leurs compositions en éléments en traces. Des phénomènes de fusion partielle de l'encaissant tonalitique lors de la mise en place de _lons mafiques sont observée sur le terrain. L'injection du magma basaltique chaud (~1050°C) sous forme de filons rapprochés augmente la surface du contact avec l'encaissante tonalitique. Une telle situation produit la fusion partielle des roches encaissantes nécessaire à l'incorporation d'enclaves mafiques observés au sein des tonalites. Pour comprendre les conditions nécessaires pour la fusion partielle des roches encaissantes, des modèles de conduction thermique pour une injection simultanée d'une série de filons ont été développées. Des données géochimiques sur les minéraux et sur les roches totales soulignent qu'au niveau supérieur de la croûte, l'assimilation sélective ou totale de l'encaissante tonalitique modifie la composition du liquide primaire pour les éléments lithophiles tel que le Sr, Ba et Rb. Un autre aspect important concernant la pétrologie des filons de l'Adamello est la présence d'enclaves felsiques dans les filons les plus primitifs. Ces enclaves montrent, en particulier, des textures proches de celles rencontrées dans des plagiogranites océaniques ou dans des roches métamorphiques de haute pression/basse température. Ces enclaves contiennent du zircon et du rutile. La datations de ces zircons à l'aide du géochronomètre U-Pb indique un âge Crétacé inférieur. Cet âge est important, car aucune roche de cet âge n'a été considérée comme un assimilant potentiel pour des roches magmatiques d'âge Eocène à Oligocène dans les Alpes Sud Centrales. La réparation spatiale des filons post-plutoniques dans des grands batholites tel que l'Adamello, est une caractéristique clé pour la compréhension des champs de contraintes lors du refroidissement du batholite. L'orientation des filons va, en particulier, indiqué la contrainte minimal au sein des roches encaissante. La mise en place de la série de filon recoupant la partie Sud du massif de l'Adamello est associée à un décrochement senestre, un décrochement que l'on peut lié aux contraintes tectoniques régionales auxquelles s'ajoutent l'effet de la dilatation produite par la mise en place du batholite lui-même. Ce décrochement senestre produit une segmentation en échelon des filons.

Mineral inventory of continuously erupting basaltic andesites at Arenal volcano, Costa Rica: implications for interpreting monotonous, crystal-rich, mafic arc stratigraphies

Except for the first 2 years since July 29, 1968, Arenal volcano has continuously erupted compositionally monotonous and phenocryst-rich (similar to35%) basaltic andesites composed of plagioclase (plag), orthopyroxene (opx), clinopyroxene (cpx), spinel olivine. Detailed textural and compositional analyses of phenocrysts, mineral inclusions, and microlites reveal comparable complexities in any given sample and identify mineral components that require a minimum of four crystallization environments. We suggest three distinct crystallization environments crystallized low Mg# (<78) silicate phases from andesitic magma but at different physical conditions, such as variable pressure of crystallization and water conditions. The dominant environment, i.e., the one which accounts for the majority of minerals and overprinted all other assemblages near rims of phenocrysts, cocrystallized clinopyroxene (Mg# similar to71-78), orthopyroxene (Mg# similar to71-78), titanomagnetite and plagioclase (An(60) to An(85)). The second environment cocrystallized clinopyroxene (Mg# 71-78), olivine (<Fo(78)), titanomagnetite, and very high An (similar to90) plagioclase, while the third cocrystallized clinopyroxene (Mg# 71-78) with high (>7) Al/Ti and high (>4 wt.%) Al2O3, titanomagnetite with considerable Al2O3 (10-18 wt.%) and possibly olivine but appears to lack plagioclase. A fourth crystallization environment is characterized by clinopyroxene (e.g., Mg#=similar to78-85; Cr2O3=0.15-0.7 wt.%), Al-, Cr-rich spinel olivine (similar toFo(80)), and in some circumstances high-An (>80) plagioclase. This assemblage seems to record mafic inputs into the Arenal system and crystallization at high to low pressures. Single crystals cannot be completely classified as xenocrysts, antecrysts (cognate crystals), or phenocrysts, because they often contain different parts each representing a different crystallization environment and thus belong to different categories. Bulk compositions are mostly too mafic to have crystallized the bulk of ferromagnesian minerals and thus likely do not represent liquid compositions. On the other hand, they are the cumulative products of multiple mixing events assembling melts and minerals from a variety of sources. The driving force for this multistage mixing evolution to generate erupting basaltic andesites is thought to be the ascent of mafic magma from lower crustal levels to subvolcanic depths which at the same time may also go through compositional modification by fractionation and assimilation of country rocks. Thus, mafic magmas become basaltic andesite through mixing, fractionation and assimilation by the time they arrive at subvolcanic depths. We infer new increments of basaltic andesite are supplied nearly continuously to the subvolcanic reservoir concurrently to the current eruption and that these new increments are blended into the residing, subvolcanic magma. Thus, the compositional monotony is mostly the product of repetitious production of very similar basaltic andesite. Furthermore, we propose that this quasi-constant supply of small increments of magma is the fundamental cause for small-scale, decade-long continuous volcanic activity; that is, the current eruption of Arenal is flux-controlled by inputs of mantle magmas. (C) 2004 Elsevier B.V. All rights reserved.

Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle

Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder ('melt layer') is overlain by a 'peridotite layer' and a layer of vitreous carbon spheres ('melt trap'). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8 GPa in the temperature range 1,170-1,290 degrees C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting (T (s) similar to 1,260 degrees C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290A degrees C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr(2)O(3) at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260 degrees C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase as a consequence of reactive fractionation. Pervasive percolation of tholeiitic basalt through an upper-mantle thermal boundary layer can thus impose a high-Si 'low-pressure' signature on MORB. This could explain opx + plag enrichment in shallow plagioclase peridotites and prolonged formation of olivine gabbros.

Juxtaposition of melt impregnation and high-temperature shear zones in the upper mantle; field and petrological constraints from the Lanzo peridotite (Northern Italy)

Results of a field and microstructural study between the northern and the central bodies of the Lanzo plagioclase peridotite massif (NW Italy) indicate that the spatial distribution of deformation is asymmetric across kilometre-scale mantle shear zones. The southwestern part of the shear zone (footwall) shows a gradually increasing degree of deformation from porphyroclastic peridotites to mylonite, whereas the northeastern part (hanging wall) quickly grades into weakly deformed peridotites. Discordant gabbroic and basaltic dykes are asymmetrically distributed and far more abundant in the footwall of the shear zone. The porphyroclastic peridotite displays porphyroclastic zones and domains of igneous crystallization whereas mylonites are characterized by elongated porphyroclasts, embedded between fine-grained, polycrystalline bands of olivine, plagioclase, clinopyroxene, orthopyroxene, spinel, rare titanian pargasite, and domains of recrystallized olivine. Two types of melt impregnation textures have been found: (1) clinopyroxene porphyroclasts incongruently reacted with migrating melt to form orthopyroxene plagioclase; (2) olivine porphyroclasts are partially replaced by interstitial orthopyroxene. The meltrock reaction textures tend to disappear in the mylonites, indicating that deformation in the mylonite continued under subsolidus conditions. The pyroxene chemistry is correlated with grain size. High-Al pyroxene cores indicate high temperatures (11001030C), whereas low-Al neoblasts display lower final equilibration temperatures (860C). The spinel Cr-number [molar Cr/(Cr Al)] and TiO2 concentrations show extreme variability covering almost the entire range known from abyssal peridotites. The spinel compositions of porphyroclastic peridotites from the central body are more variable than spinel from mylonite, mylonite with ultra-mylonite bands, and porphyroclastic rocks of the northern body. The spinel compositions probably indicate disequilibrium and would favour rapid cooling, and a faster exhumation of the central peridotite body, relative to the northern one. Our results indicate that melt migration and high-temperature deformation are juxtaposed both in time and space. Meltrock reaction may have caused grain-size reduction, which in turn led to localization of deformation. It is likely that melt-lubricated, actively deforming peridotites acted as melt focusing zones, with permeabilities higher than the surrounding, less deformed peridotites. Later, under subsolidus conditions, pinning in polycrystalline bands in the mylonites inhibited substantial grain growth and led to permanent weak zones in the upper mantle peridotite, with a permeability that is lower than in the weakly deformed peridotites. Such an inversion in permeability might explain why actively deforming, fine-grained peridotite mylonite acted as a permeability barrier and why ascending mafic melts might terminate and crystallize as gabbros along actively deforming shear zones. Melt-lubricated mantle shear zones provide a mechanism for explaining the discontinuous distribution of gabbros in oceancontinent transition zones, oceanic core complexes and ultraslow-spreading ridges.

Stable isotope geochemistry and phase-equilibria of coesite-bearing whiteschists, Dora Maira massif, Western Alps

Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The deltaO-18(SMOW) values of the quartz (after coesite) (delta O-18 = 8.1 to 8.6 parts per thousand, n = 6), phengite (6.2 to 6.4 parts per thousand, n = 3), kyanite (6.1 parts per thousand, n = 2), garnet (5.5 to 5.8 parts per thousand, n = 9), ellenbergerite (6.3 parts per thousand, n = 1) and rutile (3.3. to 3.6 parts per thousand, n = 3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700-750-degrees-C. Minimum pressures are 31-32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc + kyanite = pyrope + coesite + H2O, the a(H2O) must be reduced to 0.4-0.75 at 700 750-degrees-C. The reduced a(H2O) cannot be due to dilution by CO2, as pyrope is not stable at X (CO2) > 0.02 (T = 750-degrees-C; P = 30 kb). In the absence of a more exotic fluid diluent (e.g. CH4 or N2), a melt phase is required. Granite solidus temperatures are approximately 680-degrees-C/30 kb at a(H2O) = 1.0 and are calculated to be approximately 70-degrees-C higher at a(H2O) = 0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f(H2O) estimates for the whiteschist are 34 +/- 2 kb, 700-750-degrees-C and 0.4-0.75. The oxygen isotope fractionation between quartz (deltaO-18 = 11.6%.) and garnet (deltaO-18 = 8.7 parts per thousand) in the surrounding orthognesiss is identical to that in the coesite-bearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (deltaD(smow) = -27 to -32 parts per thousand), on secondary talc and chlorite after pyrope (deltaD = - 39 to - 44 parts per thousand) and on the surrounding biotite (deltaD = -64 parts per thousand) and phengite (deltaD = -44 parts per thousand) gneiss. All phases appear to be in near-equilibrium. The very high deltaD values for the primary hydrous phases is consistent with an initial oceanic-derived/connate fluid source. The fluid source for the retrograde talc + chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar deltaD, but dissimilar deltaO-18 values of the coesite-bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth.

The Gronnedal-Ika carbonatite-syenite complex, South Greenland: Carbonatite formation by liquid immiscibility

The Gronnedal-Ika complex is dominated by layered nepheline syenites which were intruded by a xenolithic syenite and a central plug of calcite to calcite-siderite carbonatite. Aegirine-augite, alkali feldspar and nepheline are the major mineral phases in the syenites, along with rare calcite. Temperatures of 680-910degreesC and silica activities of 0.28-0.43 were determined for the crystallization of the syenites on the basis of mineral equilibria. Oxygen fugacities, estimated using titanomagnetite compositions, were between 2 and 5 log units above the fayalite-magnetite-quartz buffer during the magmatic stage. Chondrite-normalized REE patterns of magmatic calcite in both carbonatites and syenites are characterized by REE enrichment (La-CN-Yb-CN = 10-70). Calcite from the carbonatites has higher Ba (similar to5490 ppm) and lower HREE concentrations than calcite from the syenites (54-106 ppm Ba). This is consistent with the behavior of these elements during separation of immiscible silicate-carbonate liquid pairs. epsilon(Nd)(T = 1.30 Ga) values of clinopyroxenes from the syenites vary between +1.8 and +2.8, and epsilon(Nd)(T) values of whole-rock carbonatites range from +2.4 to +2.8. Calcite from the carbonatites has delta(18)O values of 7.8 to 8.6parts per thousand and delta(13)C values of -3.9 to -4.6parts per thousand. delta(18)O values of clinopyroxene separates from the nepheline syenites range between 4.2 and 4.9parts per thousand. The average oxygen isotopic composition of the nepheline syenitic melt was calculated based on known rock-water and mineral-water isotope fractionation to be 5.7 +/- 0.4parts per thousand. Nd and C-O isotope compositions are typical for mantle-derived rocks and do not indicate significant crustal assimilation for either syenite or carbonatite magmas. The difference in delta(18)O between calculated syenitic melts and carbonatites, and the overlap in epsilon(Nd) values between carbonatites and syenites, are consistent with derivation of the carbonatites from the syenites via liquid immiscibility.

The biogenic origin of needle fibre calcite

Needle fibre calcite is one of the most ubiquitous habits of calcite in vadose environments (caves deposits, soil pores, etc.). Its origin, either through inorganic, indirect or direct biological processes, has long been debated. In this study, investigations at 11 sites in Europe, Africa and Central America support arguments for its biogenic origin. The wide range of needle morphologies is the result of a gradual evolution of the simplest type, a rod. This rod is the elementary brick which, by aggregation and welding, builds more complex needles. The absence of cross-welded needles implies that they are welded in a mould, or under a longitudinal and unidirectional constraint, before being released inside the soil pores. The difference between the lengthening of the needles and the c axis can be explained by the existence of needles observed under a scanning electron microscope in organic sleeves, which can act as a mould during rod growth. Complex morphologies with epitaxial outgrowths on straight rods cannot have grown entirely inside organic microtubes; they must result from soil diagenesis after the release of straight rods in a soil-free medium. Whisker crystals are interpreted as the result of growth and coalescence of euhedral crystals on a rod. Rhomb chains are considered to be the consequence of successive epitaxial growth steps on a needle during variations in growth conditions. Isotopic signatures for needle fibre calcite vary from -16.63[per mille] to +1.10[per mille] and from -8.63[per mille] to -2.25[per mille] for Delta13C and Delta18O, respectively. The absence of high Delta18O values for needle fibre calcite precludes a purely physicochemical origin (evaporative) for this particular habit of calcite. As epitaxial growth cannot precipitate in the same conditions as initial needles, needle fibre calcite stable isotopic signatures should be used with caution as a proxy for palaeoenvironmental reconstructions. In addition, it is suggested that the term needle fibre calcite should be kept for the original biogenic form. The other habit should be referred to as epitaxial forms of needle fibre calcite.

Gabbro, plagiogranite and associated dykes in the supra-subduction zone Evros Ophiolites, NE Greece

The incomplete Evros ophiolites in NE Greece form a NE-SW-oriented discontinuous belt in the Alpine orogen of the north Aegean. Field data, petrology and geochemistry are presented here for the intrusive section and associated mafic dykes of these ophiolites. Bodies of high-level isotropic gabbro and plagiogranite in the ophiolite suite are cross-cut by NE-SW-trending boninitic and tholeiitic-boninitic affinity dykes, respectively. The dykes fill tensile fractures or faults, which implies dyke emplacement in an extensional tectonic regime. The tholeiitic-transitional boninitic gabbro is REE- and HFS-depleted relative to N-MORB, indicating derivation from melting of a refractory mantle peridotite source. Associated boninitic dykes are slightly LREE-enriched, showing mineral and whole-rock geochemistry similar to the gabbro. The plagiogranite is a strongly REE-enriched high-silica trondhjemite, with textures and composition typical for an oceanic crust differentiate. Plagiogranite-hosted tholeiitic and transitional boninitic dykes are variably REE-enriched. Geochemical modelling indicates origin of the plagiogranite by up to 75% fractional crystallization of basaltic magma similar to that producing the associated tholeiitic dykes. All mafic rocks have high LILE/HFSE ratios and negative Ta-Nb-Ti and Ce anomalies, typical for subduction zone-related settings. The mafic rocks show a similar trace-element character to the mafic lavas of an extrusive section in Bulgaria, suggesting they both form genetically related intrusive and extrusive suites of the Evros ophiolites. The field occurrence, the structural context, the petrology and geochemical signature of the studied magmatic assemblage provide evidence for its origin in a proto-arc (fore-arc) tectonic setting, thus tracing the early stages of the tectono-magmatic evolution of Jurassic arc-marginal basin system that has generated the supra-subduction type Evros ophiolites.

Oxygen isotope sector zoning in natural hydrothermal quartz

Oxygen isotope measurements using SIMS and laser-fluorination methods confirm the presence of concentric and sector zoning in low-temperature (200 degrees C to < 400 degrees C) hydrothermal quartz from Alpine veins. While concentric zoning is most readily explained by changes in the chemical composition of the fluid or temperature of crystallization, the reasons for sector zoning are more difficult to explain. Relative enrichment in (18)O for crystallographically different sectors of quartz corresponds to m > r > z. Sector zoning is, however, largely limited to the exterior zones of crystals and/or to crystals with large Al (> 1000 ppm) and trace element contents, probably formed at temperatures < 250 degrees C. Differences in delta(18)O between the prismatic (m) relative to the rhombohedral (r and z) growth sectors of up to 2 parts per thousand can be explained by a combination of a face-related crystallographic and/or a growth rate control. In contrast, isotopic sector zoning of up to about 1.5 parts per thousand amongst the different rhombohedral faces increases in parallel with the trace element content and is likely to represent disequilibrium growth. This is indicated by non-systematic, up to 2 parts per thousand, differences within single growth zones and the irregular, larger or smaller, delta(18)O values (of several permil) of the exterior compared to the inner zones of the same crystals. Disequilibrium growth may be related to the large trace element content incorporated into the growing quartz at lower temperatures (< 250 degrees C) and/or be related to fluid-vapour separation, allowing crystal growth from both a vapour as well as a liquid phase.

Quantification of magmatic and hydrothermal processes in a peralkaline syenite-alkali granite complex based on textures, phase equilibria, and stable and radiogenic isotopes

The Puklen complex of the Mid-Proterozoic Gardar Province, South Greenland, consists of various silica-saturated to quartz-bearing syenites, which are intruded by a peralkaline granite. The primary mafic minerals in the syenites are augite +/- olivine + Fe-Ti oxide + amphibole. Ternary feldspar thermometry and phase equilibria among mafic silicates yield T = 950-750degreesC, a(SiO2) = 0.7-1 and an f(O2) of 1-3 log units below the fayalite-magnetite-quartz (FMQ) buffer at 1 kbar. In the granites, the primary mafic minerals are ilmenite and Li-bearing arfvedsonite, which crystallized at temperatures below 750degreesC and at f(O2) values around the FMQ buffer. In both rock types, a secondary post-magmatic assemblage overprints the primary magmatic phases. In syenites, primary Ca-bearing minerals are replaced by Na-rich minerals such as aegirine-augite and albite, resulting in the release of Ca. Accordingly, secondary minerals include ferro-actinolite, (calcite-siderite)(ss), titanite and andradite in equilibrium with the Na-rich minerals. Phase equilibria indicate that formation of these minerals took place over a long temperature interval from near-magmatic temperatures down to similar to300degreesC. In the course of this cooling, oxygen fugacity rose in most samples. For example, late-stage aegirine in granites formed at the expense of arfvedsonite at temperatures below 300degreesC and at an oxygen fugacity above the haematite-magnetite (HM) buffer. The calculated delta(18)O(melt) value for the syenites (+5.9 to +6.3parts per thousand) implies a mantle origin, whereas the inferred delta(18)O(melt) value of <+5.1parts per thousand for the granitic melts is significantly lower. Thus, the granites require an additional low-delta(18)O contaminant, which was not involved in the genesis of the syenites. Rb/Sr data for minerals of both rock types indicate open-system behaviour for Rb and Sr during post-magmatic metasomatism. Neodymium isotope compositions (epsilonNd(1170 Ma) = -3.8 to -6.4) of primary minerals in syenites are highly variable, and suggest that assimilation of crustal rocks occurred to variable extents. Homogeneous epsilon(Nd) values of -5.9 and -6.0 for magmatic amphibole in the granites lie within the range of the syenites. Because of the very similar neodymium isotopic compositions of magmatic and late- to post-magmatic minerals from the same syenite samples a principally closed-system behaviour during cooling is implied. In contrast, for the granites an externally derived fluid phase is required to explain the extremely low epsilon(Nd) values of about -10 and low delta(18)O between +2.0 and +0.5parts per thousand for late-stage aegirine, indicating an open system in the late-stage history. In this study we show that the combination of phase equilibria constraints with stable and radiogenic isotope data on mineral separates can provide much better constraints on magma evolution during emplacement and crystallization than conventional whole-rock studies.

Silver-base metal epithermal vein and listwaenite types of deposit Crnac, Rogozna Mts., Kosovo. Part I: Ore mineral geochemistry and sulfur isotope study

Pb-Zn-Ag vein and listwaenite types of mineralization in Crnac deposit, Western Vardar zone, were deposited within several stages: (i) the pre-ore stage comprises pyrite, arsenopyrite, pyrrhotite, quartz, kaolinite and is followed by magnetite-pyrite; (ii) the syn-ore stage is composed of galena, sphalerite, tetrahedrite and stefanite; and (iii) the post-ore stage is composed of carbonates, pyrite, arsenopyrite and minor galena. The vein type mineralization is hosted by Jurassic amphibolites and veins terminate within overlying serpentinites. Mineralized listwaenites are developed along the serpentinite-amphibolite interface. The reserves are estimated to 1.7 Mt of ore containing in average 7.6% lead, 2.9% zinc, and 102 g/t silver. Sulfides from the pre- and syn-mineralization assemblage of the vein- and listwaenite-types of mineralization from the Crnac Pb-Zn-Ag deposit have been analyzed using microprobe, crush-leachates and sulfur isotopes. The pre-ore assemblage precipitated under high sulfur fugacities (f(S(2)) = 10(-8)-10(-6) bar) from temperatures ranging between 350 degrees C and 380 degrees C. Most likely water-rock reactions, boiling and/or increase of pH caused an increase of delta(34)S of pyrite toward upper levels within the deposit. The decomposition of pre-ore pyrrhotite to a pyrite-magnetite mixture occurred at a fugacity of sulfur from f(S(2)) = 8.7 x 10(-10) to 9.6 x 10(-9) bar and fugacity of oxygen from f(O(2)) = 2.4 x 10(-30) to 3.1 x 10(-28) bars, indicating a contribution of an oxidizing fluid, i.e. meteoric water during pre-ore stages of hydrothermal activity. The crystallization temperatures obtained by the sphalerite-galena isotope geothermometer range from 230 to 310 degrees C. The delta(34)S values of pre- and syn-ore sulfides (pyrite, galena, sphalerite, delta(34)S = 0.3-5.9 parts per thousand) point to magmatic sulfur. Values of delta(34)S of galena and sphalerite are decreasing upwards due to precipitation of early formed sulfide minerals. Post-ore assemblage precipitated at temperature below 190 degrees C. Based on data presented above, we assume two fluid sources: (i) a magmatic source, supported by sulfur isotopic compositions within pre- and syn-ore minerals and a high mol% of fluorine found within pre- and syn-ore leachates, and (ii) a meteoric source, deduced by coincident pyrite-magnetite intergrowth, sulfur isotopic trends within syn-ore minerals and decrease of crystallization temperatures from the pre-ore stage (380-350 degrees C), towards the syn-ore (310-215 degrees C) and post-ore stages (<190 degrees C). Post-ore fluids are Na-Ca-Mg-K-Li chlorine rich and were modified via water-rock reactions. Simple mineral assemblage and sphalerite composition range from 1.5 to 10.1 mol% of FeS catalog Crnac to a group of intermediate sulfidation epithermal deposit. (C) 2011 Elsevier B.V. All rights reserved.

Trace element distribution among rock-forming minerals from metamorphosed to partially molten basic igneous rocks in a contact aureole (Fuerteventura, Canaries)

Low pressure partial melting of basanitic and ankaramitic dykes gave rise to unusual, zebra-like migmatites, in the contact aureole of a layered pyroxenite-gabbro intrusion, in the root zone of an ocean island (Basal Complex, Fuerteventura, Canary Islands). These migmatites are characterised by a dense network of closely spaced, millimetre-wide leucocratic segregations. Their mineralogy consists of plagioclase (An(32-36)), diopside, biotite, oxides (magnetite, ilmenite), +/-amphibole, dominated by plagioclase in the leucosome and diopside in the melanosome. The melanosome is almost completely recrystallised, with the preservation of large, relict igneous diopside phenocrysts in dyke centres. Comparison of whole-rock and mineral major- and trace-element data allowed us to assess the redistribution of elements between different mineral phases and generations during contact metamorphism and partial melting. Dykes within and outside the thermal aureole behaved like closed chemical systems. Nevertheless, Zr, Hf, Y and REEs were internally redistributed, as deduced by comparing the trace element contents of the various diopside generations. Neocrystallised diopside - in the melanosome, leucosome and as epitaxial phenocryst rims - from the migmatite zone, are all enriched in Zr, Hf, Y and REEs compared to relict phenocrysts. This has been assigned to the liberation of trace elements on the breakdown of enriched primary minerals, kaersutite and sphene, on entering the thermal aureole. Major and trace element compositions of minerals in migmatite melanosomes and leucosomes are almost identical, pointing to a syn- or post-solidus reequilibration on the cooling of the migmatite terrain i.e. mineral-melt equilibria were reset to mineral-mineral equilibria. (C) 2007 Elsevier B.V. All rights reserved.