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Despite the first-order importance of crystallisation-differentiation for arc magma evolution, several other processes contribute to their compositional diversity. Among them is the remelting of partly crystallised magmas, also known as cumulate melting or 'petrological cannibalism'. The impact of this process on the plutonic record is poorly constrained. We investigate a nepheline-normative dyke suite close to the Blumone gabbros, a large amphibole-gabbro unit of the Tertiary Southern Alpine Adamello igneous complex. The compositions of the studied dykes are characterised by low SiO.sub.2 (43-46 wt. %), MgO (5.0-7.2 wt. %), Ni (18-40 [mu]g/g), and high Al.sub.2O.sub.3 (20.2-22.0 wt. %) contents. Phenocrystic plagioclase in these dykes exhibits major, trace, and Sr isotope compositions similar to Blumone cumulate plagioclase, suggesting a genetic link between the nepheline-normative dykes and the amphibole-gabbro cumulates. We tested this hypothesis by performing saturation experiments on a nepheline-normative dyke composition in an externally heated pressure vessel at 200 MPa between 975 and 1100 °C at fO.sub.2 conditions close to the Ni-NiO buffer. Plagioclase and spinel are near-liquidus phases at and above 1050 °C, contrasting with the typical near-liquidus olivine ± spinel assemblage in hydrous calc-alkaline basalts. The alkaline nature of the dykes results from the abundance of amphibole in the protolith, consistent with melting of amphibole-gabbro cumulates. We modelled the heat budget from the repeated injection of basaltic andesite into a partly crystallised amphibole-gabbro cumulate. The results of this model show that no more than 7% of the cumulate pile reaches temperatures high enough to produce nepheline-normative melts. We propose that such nepheline-normative dykes are a hallmark of hydrous cumulate melting in subvolcanic plumbing systems. Therefore, ne-normative dykes in arc batholiths may indicate periods with high magma fluxes., Author(s): Manuel Pimenta Silva [sup.1], Felix Marxer [sup.1] [sup.2], Tobias Keller [sup.1], Andrea Giuliani [sup.1], Peter Ulmer [sup.1], Othmar Müntener [sup.3] Author Affiliations: (1) grid.5801.c, 0000 0001 2156 2780, Institute [...]

The Seve Nappe Complex (SNC) comprises continental rocks of Baltica that were subducted and exhumed during the Caledonian orogeny prior to collision with Laurentia. The tectonic history of the central SNC is investigated by applying in-situ zircon and monazite (Th-)U-Pb geochronology and trace element analysis to (ultra-)high pressure (UHP) paragneisses in the Avardo and Marsfjället gneisses. Zircons in the Avardo Gneiss exposed at Sippmikk creek exhibit xenocrystic cores with metamorphic rims. Cores show typical igneous REE profiles and were affected by partial Pb-loss. The rims have flat HREE profiles and are interpreted to have crystallized at 482.5 ± 3.7 Ma during biotite-dehydration melting and peritectic garnet growth. Monazites in the paragneiss are chemically homogeneous and record metamorphism at 420.6 ± 2.0 Ma. In the Marsfjället Gneiss exposed near Kittelfjäll, monazites exhibit complex zoning with cores enveloped by mantles and rims. The cores are interpreted to have crystallized at 481.6 ± 2.1 Ma, possibly during garnet resorption. The mantles and rims provide a dispersion of dates and are interpreted to have formed by melt-driven dissolution-reprecipitation of pre-existing monazites until 463.1 ± 1.8 Ma. Depletion of Y, HREE, and U in the mantles and rims compared to the cores record peritectic garnet and zircon growth. Altogether, the Avardo and Marsfjället gneisses show evidence of late Cambrian/early Ordovician partial melting (possibly in (U)HP conditions), Middle Ordovician (U)HP metamorphism, and late Silurian tectonism. These results indicate that the SNC underwent south-to-north oblique subduction in late Cambrian time, followed by progressive north-to-south exhumation to crustal levels prior to late Silurian continental collision., Author(s): Christopher J. Barnes [sup.1], Michal Bukala [sup.1], Riccardo Callegari [sup.2] [sup.3], Katarzyna Walczak [sup.2], Ellen Kooijman [sup.4], Melanie Kielman-Schmitt [sup.4], Jaroslaw Majka [sup.2] [sup.3] Author Affiliations: (1) grid.413454.3, 0000 [...]

The problem of the observed very rapid advection of heat into metamorphic thrust stacks is reviewed. Conductive models relying on the thermal relaxation of a thickened crust will not produce the observed Barrovian (medium temperature, medium pressure) assemblages within some short-lived orogens (e.g., western Ireland and Timor). Studies of the rate and timing of metamorphic mineral growth suggest that this is commonly faster than predicted by thermal relaxation. Barrovian assemblages are localised in some orogens (e.g., the Alps) but extensive in others (e.g., the Himalayas). Metamorphic mineral growth brackets deformation; consequently, slow growth is inconsistent with the rapid uplift of many orogens. Thus, no single mechanism can account for the development of Barrovian assemblages during collisional orogeny. The only mechanisms that can supply large amounts of heat for regional metamorphism quickly ( Key words: Barrovian metamorphism, thermal relaxation, advection, collisional orogeny. Le probleme de l'advection tres rapide de chaleur observee dans les empilements de nappes de charriage metamorphiques est passe en revue. Les modeles de conduction reposant sur la relaxation thermique d'une croute epaissie ne peuvent pas produire les assemblages barroviens (temperature et pression intermediaries) observes dans certains orogenes de courte duree (p. ex. ouest de l'Irlande, Timor). Des etudes du taux et du moment de la croissance de mineraux metamorphiques donnent a penser qu'elle est souvent plus rapide que ce que predit la relaxation thermique. Les assemblages barroviens sont localises dans certains orogenes (p. ex. les Alpes), mais de grande etendue dans d'autres (p. ex. Himalaya). La croissance des mineraux metamorphiques encadre la deformation, de sorte qu'une croissance lente ne concorde pas avec le soulevement rapide de nombreux orogenes. Ainsi, aucun mecanisme ne peut a lui seul expliquer le developpement d'assemblages barroviens durant les orogeneses de collision. Les seuls mecanismes pouvant fournir de grandes quantites de chaleur rapidement pour le metamorphisme regional (>). Le rechauffement visqueux et par frottement produit un rechauffement rapide local, mais non un rechauffement regional rapide. L'expansion arriere-arc ou toute forme d'expansion lithospherique accroit le gradient geothermique et le flux thermique, mais ne rechauffe pas les roches. Nous suggerons que l'advection magmatique de chaleur associee a l'amincissement lithospherique ou le charriage de type <> d'un arc/d'une ophiolite sont les principales sources de chaleur dans les orogenes de courte duree. [Traduit par la Redaction] Mots-cles: metamorphisme barrovien, relaxation thermique, advection, orogene de collision., Introduction A great problem in tectonics is the rapid advection of heat into supracrustal thrust stacks forming Barrovian metamorphic assemblages on the lower, refrigerated plate during collisional orogeny. The typical [...]

Infiltration-driven metamorphism has produced the widespread development of forsterite in the siliceous dolomites of the Alta, Utah contact aureole. SIMS (secondary ion mass spectrometry) [delta].sup.18O analyses show that in most of the middle to outer forsterite zone samples, forsterite, calcite and dolomite are homogenous in [delta].sup.18O at the grain-scale, but forsterite and calcite exhibit minor intergranular heterogeneities of < 1â° on the thin-section to hand sample-scales. In contrast, within ~ 300 m of the igneous contact (inner forsterite and periclase zones), grain-scale [delta].sup.18O heterogeneities in forsterite are as large as 3.0â°, but forsterite crystals do not exhibit systematic center-to-edge decreases in [delta].sup.18O due to skeletal growth and infilling. With one exception, forsterite, calcite and dolomite in all samples analyzed yield disequilibrium fractionations. The oxygen isotope disequilibrium commonly recorded among forsterite, calcite and dolomite may reflect the rapid growth of forsterite from increased reaction overstepping as temperature increased, or results from .sup.18O/.sup.16O depletion in the matrix carbonate minerals during continued fluid infiltration after forsterite crystallization was complete, during the initial stages of cooling in the forsterite zone. In a traverse across the boundary between forsterite- and periclase-bearing marble layers in the periclase zone, forsterite and calcite SIMS [delta].sup.18O profiles combined with the trend in forsterite textures indicate that these layers were not infiltrated simultaneously, nor continuously during both prograde reactions responsible for forsterite and periclase. Rather, the texture and SIMS data require infiltration and formation of forsterite first in the forsterite marble layer, followed by infiltration and formation of periclase in the periclase marble layer. As a consequence, the [delta].sup.18O profile recorded by forsterite formed earlier than the [delta].sup.18O profile recorded by calcite, but both profiles are prograde features-that is, both profiles formed in response to prograde reactions forming first forsterite and then periclase. These grain-scale [delta].sup.18O profiles further suggest that these layers experienced significant changes in permeability that are linked to their sequential infiltration-reaction history. The asynchronous growth of these index minerals at the outcrop-scale during prograde heating may be more common during contact metamorphism of carbonate lithologies than currently recognized., Author(s): Carl J. Beno [sup.1], John R. Bowman [sup.1], Kouki Kitajima [sup.2], John W. Valley [sup.2], Patrick C. Loury [sup.3], Lorraine M. Tapanila [sup.4] Author Affiliations: (1) grid.223827.e, 0000 0001 [...]

Fluid flow in crystalline rocks in the absence of fractures or ductile shear zones dominantly occurs by grain boundary diffusion, as it is faster than volume diffusion. It is, however, unclear how reactive fluid flow is guided through such pathways. We present a microstructural, mineral chemical, and thermodynamic analysis of a static fluid-driven reaction from dry granulite to 'wet' eclogite. Fluid infiltration resulted in re-equilibration at eclogite-facies conditions, indicating that the granulitic protolith was out of equilibrium, but unable to adjust to changing P-T conditions. The transformation occurred in three steps: (1) initial hydration along plagioclase grain boundaries, (2) complete breakdown of plagioclase and hydration along phase boundaries between plagioclase and garnet/clinopyroxene, and (3) re-equilibration of the rock to an eclogite-facies mineral assemblage. Thermodynamic modelling of local compositions reveals that this reaction sequence is proportional to the local decrease of the Gibbs free energy calculated for 'dry' and 'wet' cases. These energy differences result in increased net reaction rates and the reactions that result in the largest decrease of the Gibbs free energy occur first. In addition, these reactions result in a local volume decrease leading to porosity formation; i.e., pathways for new fluid to enter the reaction site thus controlling net fluid flow. Element transport to and from the reaction sites only occurs if it is energetically beneficial, and enough transport agent is available. Reactive fluid flow during static re-equilibration of nominally impermeable rocks is thus guided by differences in the energy budget of the local equilibrium domains., Author(s): Sascha Zertani [sup.1] [sup.2], Timm John [sup.1], Caroline Brachmann [sup.1], Johannes C. Vrijmoed [sup.1], Oliver Plümper [sup.3] Author Affiliations: (1) grid.14095.39, 0000 0000 9116 4836, Institute of Geological Sciences, [...]

The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of prime importance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in the southern Dora-Maira Massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now in the other parts of the massif. We report the discovery of a new UHP unit in the northern Dora-Maira Massif (Western Alps), named Chasteiran Unit. It is only a few tens of metres thick and consists of graphite-rich, garnet-chloritoid micaschists, whose protoliths may be black shales of Silurian age. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P-T evolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusions in the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record of growth during the prograde increase in P and T (25-27 kbar 470-500 °C) (stage 1), up to the coesite stability field (27-28 kbar 510-530 °C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to 15 kbar 500-515 °C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, began to develop during this stage, and was then overprinted by chlorite-ilmenite (stage 4). The Chasteiran Unit is discontinuously exposed in the immediate hangingwall of the Pinerolo Unit, and it is located far away from, and without physical links to the classic UHP Brossasco-Isasca Unit. Moreover, it records a different, much colder, P-T evolution, showing that different slices were detached from the downgoing subduction slab. The Chasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P-T conditions are comparable to the ones of the Tian Shan coesite-chloritoid-bearing rocks., Author(s): Paola Manzotti [sup.1], Federica Schiavi [sup.2], Francesco Nosenzo [sup.1], Pavel Pitra [sup.3] [sup.4], Michel Ballèvre [sup.3] Author Affiliations: (1) grid.10548.38, 0000 0004 1936 9377, Department of Geological Sciences, Stockholm [...]

The geometry and composition of deformation-related low-angle boundaries in naturally deformed olivine were characterized by electron backscattered diffraction (EBSD) and atom probe tomography (APT). EBSD data show the presence of discrete low-angle tilt boundaries, which formed by subgrain rotation recrystallisation associated with the (100)[001] slip system during fluid-catalysed metamorphism and deformation. APT analyses of these interfaces show the preferential segregation of olivine-derived trace elements (Ca, Al, Ti, P, Mn, Fe, Na and Co) to the low-angle boundaries. Boundaries with 2°), the interfaces become more ordered and linear enrichment of trace elements coincides with the orientation of dislocations inferred from the EBSD data. These boundaries show a systematic increase of trace element concentration with disorientation angle. Olivine-derived trace elements segregated to the low-angle boundaries are interpreted to be captured and travel with dislocations as they migrate to the subgrain boundary interfaces. However, the presence of exotic trace elements Cl and H, also enriched in the low-angle boundaries, likely reflect the contribution of an external fluid source during the fluid-present deformation. The observed compositional segregation of trace elements has significant implications for the deformation and transformation of olivine at mantle depth, the interpretation of geophysical data and the redistribution of elements deep in the Earth. The observation that similar features are widely recognised in manufactured materials, indicates that the segregation of trace elements to mineral interfaces is likely to be widespread., Author(s): Tommaso Tacchetto [sup.1] [sup.2], Steven M. Reddy [sup.1] [sup.2], David W. Saxey [sup.2], Denis Fougerouse [sup.1] [sup.2], William D. A. Rickard [sup.2], Chris Clark [sup.1] Author Affiliations: (1) grid.1032.0, [...]

Plagioclase hosted, oriented magnetite micro-inclusions are a frequently observed phenomenon in magmatic and metamorphic rocks. Understanding the orientation relationships between these inclusions and the plagioclase host is highly relevant for interpreting paleomagnetic measurements. The systematics of the shape and crystallographic orientation relationships between needle- and lath-shaped magnetite micro-inclusions and their plagioclase host from oceanic gabbro were investigated using optical microscopy including universal stage measurements, scanning electron microscopy, and crystal orientation analysis by electron backscatter diffraction. The magnetite inclusions show preferred shape orientations following six well-defined directions and with specific crystallographic orientation relationships to the plagioclase host. These relationships are rationalized based on angular and dimensional similarities between the crystal structures of magnetite and plagioclase, which favor the parallel alignment of oxygen layers with similar lattice spacing in both phases. The parallel alignment of oxygen layers in plagioclase and magnetite can be traced back to the oriented nucleation of magnetite, which occurs by the accommodation of FeO.sub.6 octahedra in six-membered rings of SiO.sub.4 and AlO.sub.4 tetrahedra of the plagioclase structure. The orientation systematics of the magnetite micro-inclusions is related to four orientation variants for placing the FeO.sub.6 octahedra into the plagioclase structure., Author(s): Olga Ageeva [sup.1] [sup.2], Ge Bian [sup.1], Gerlinde Habler [sup.1], Alexey Pertsev [sup.2], Rainer Abart [sup.1] Author Affiliations: (1) grid.10420.37, 0000 0001 2286 1424, Department of Lithospheric Research, University [...]

The 156-138 Ma Chidliak kimberlites on the Eastern Hall peninsula (EHP) of Baffin Island entrained mantle xenoliths interpreted to have been a part of the Archean North Atlantic Craton (NAC) lithospheric mantle. We studied 19 Chidliak eclogite xenoliths that comprise 10 bimineralic, 5 rutile-bearing, 3 orthopyroxene-bearing and 1 kyanite-bearing eclogites. We report major and trace element compositions of the minerals, calculated bulk compositions, pressures and temperatures of the rock formation and model melt extraction from viable protoliths. The eclogite samples are classified into three groups of HREE-enriched, LREE-depleted and metasomatized based on their reconstructed whole-rock REE patterns. PT parameters of the eclogites were calculated by projecting garnet-clinopyroxene temperatures onto the local P-T arrays for 65 Chidliak peridotite xenoliths. All Chidliak eclogites are equilibrated in the diamond P-T field and cluster in two groups, low-temperature (n = 5, 840-990 °C at 4.1-5.0 GPa) and high-temperature (n = 11, T > 1320 °C at P > 7.0 GPa). The reconstructed Mg-rich major element bulk compositions and trace elements patterns are similar to Archean basalts from the North Atlantic and Superior cratons and the oceanic gabbros. The LREE-depleted Chidliak eclogites could be residues after 15-55% partial melting of Archean basalt at the eclogite facies of metamorphism that led to extraction of a tonalite-trondhjemite-granodiorite melt from the EHP. The HREE-depleted eclogites may have experienced a lower degree (95%. The former are more magnesian, less ferrous and calcic, contain more magnesian and less calcic garnets, and lower proportions of group C eclogites. The contrast may relate to the stronger NAC metasomatism by silicate-carbonate melt observed in Chidliak peridotitic mantle, or to the different formation ages of the eclogites beneath the two cratons., Author(s): Vedran Pobric [sup.1], Nester Korolev [sup.1] [sup.2], Maya Kopylova [sup.1] Author Affiliations: (1) grid.17091.3e, 0000 0001 2288 9830, Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, [...]

We report the results of kinetic experiments of graphitization at various pressures (0.5-8.0 GPa) and durations (1 s to 24 h) at 1200 °C. The natural carbonaceous material in sedimentary rocks from the Shimanto accretionary complex and the Hidaka metamorphic belt, Japan, underwent systematic changes in crystallinity and morphology with increasing pressure. To assess the pressure dependence of graphitization, we adopted three approaches to formulating the graphitization kinetics using a power law rate model, a Johnson-Mehl-Avrami-Kolmogorov model, and a superposition method. Activation volumes of - 21.7 ± 3.0 to - 45.7 ± 4.5 cm.sup.3 mol.sup.-1 and - 0.7 ± 0.2 to - 16.8 ± 1.8 cm.sup.3 mol.sup.-1 were obtained for pressures from 0.5 to 2.0 GPa and 2.0 to 8.0 GPa, respectively. Such large negative activation volumes might arise from structural modification and compression in the primary carbonaceous material. We applied the experimental data to the Arrhenius-type equation of graphitization, extrapolated to geological P-T-t conditions. Our model predicts that carbonaceous material undergoing metamorphism for ~ 10 Myr at pressures of 0.5-3.0 GPa will begin to crystallize at around 350-420 °C and transform fully to ordered graphite at around 450-600 °C, depending on the peak pressure. Thus, natural graphitization might proceed much more rapidly than previously estimated, owing to the large negative activation volumes for the reaction rate. This indicates that subducted carbonaceous materials will completely convert to fully ordered graphite by rapid recrystallization and metamorphic devolatilization before reaching sub-arc depths (< 100 km)., Author(s): Yoshihiro Nakamura [sup.1], Takashi Yoshino [sup.2], Madhusoodhan Satish-Kumar [sup.3] Author Affiliations: (1) grid.466781.a, 0000 0001 2222 3430, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, [...]

This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subduction-related metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluid-rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 [mu]g/g B) occur in peak metamorphic phengite from a garnet-phengite quartzite. The B isotopic composition is variable ([delta].sup.11B = - 10.3 to - 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid-rock interaction during peak metamorphic conditions. This interpretation is supported by fluid-rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in [delta].sup.11B and [B]. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive [delta].sup.11B values ([delta].sup.11B = 20 ± 5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 [mu]g/g and [delta].sup.11B values that cluster around 0% ([delta].sup.11B = - 5.0 to + 3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid-rock interaction with low-to-moderate (< 3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones., Author(s): Ralf Halama [sup.1], Matthias Konrad-Schmolke [sup.2], Jan C. M. De Hoog [sup.3] Author Affiliations: (1) 0000 0004 0415 6205, grid.9757.c, School of Geography, Geology and the Environment, Keele University, [...]

We applied interface-controlled kinetics, including interface-controlled Ostwald ripening and non-hydrostatic dissolution-precipitation, to metamorphic zircon and phengite growth in a high-pressure (high-P) metamorphic complex. This kinetic modeling yields the growth duration of metamorphic zircon, based on the assumption that dissolution and precipitation of zircon is much slower than that of phengite. The model was applied to zircon and phengite growth in the Nishisonogi unit of the high-P Nagasaki metamorphic complex, western Kyushu, Japan. Given that detrital zircons that are tens of microns in size remain after metamorphism, our model assumption is justified. Our results show that the duration of metamorphic zircon growth in an individual rock ranges from 10 to 20 Myr. In general, the duration of metamorphic zircon growth in the whole Nishisonogi unit is ca. 30 Myr. This prolonged duration implies that high-P metamorphism is sustained by continuous subduction of hydrated oceanic crust and overlying trench-fill sediments. The continuous subduction and accretion of these materials may supply the metamorphic fluid into the high-P metamorphic domain in the deeper part of the accretionary prism, thereby contributing to interface-controlled kinetics in the metamorphic complex., Author(s): Kazuhiro Miyazaki [sup.1], Kenshi Suga [sup.2], Yasushi Mori [sup.3], Hideki Iwano [sup.4], Koshi Yagi [sup.5], Miki Shigeno [sup.3], Tadao Nishiyama [sup.6], Tohru Danhara [sup.4], Takafumi Hirata [sup.7] Author Affiliations: [...]

Skarn garnets in the Mineral King roof pendant of the south-central Sierra Nevada within Sequoia National Park, California, USA reveal variable fluid chemistry with a significant component of meteoric water during metasomatism in the Early Cretaceous Sierra Nevada Batholith. We focus on andradite garnet associated with Pb-Zn mineralization in the White Chief Mine. Laser fluorination oxygen isotope analyses of [delta].sup.18O of garnet ([delta].sup.18O(Grt)) from sites along the skarn show a large range of values (- 8.8 to + 4.6â° VSMOW). Ion microprobe (SIMS) analyses elucidate that individual andradite crystals are strongly zoned in [delta].sup.18O(Grt) (up to 7â° of variation). Total rare-earth element concentrations ([summation]REE) across individual garnets show progressive depletion of skarn-forming fluids in these elements during garnet growth. These findings support a skarn model of earliest red high-[delta].sup.18O grandite garnet consistent with a magmatic-dominated equilibrium fluid ([delta].sup.18O(H.sub.2O) as high as [almost equal to] + 8â°). Later, green andradite crystallized in equilibrium with a low-[delta].sup.18O fluid indicating a significant influx of meteoric fluid ([delta].sup.18O(H.sub.2O) [almost equal to] - 6 to - 5â°), following a hiatus in garnet growth, associated with late-stage Pb-Zn mineralization. Latest orange overprint rims have higher [delta].sup.18O values ([delta].sup.18O(H.sub.2O) [almost equal to] 0-2â°), and depleted total REEs, suggesting influx of high-[delta].sup.18O, trace-element depleted fluid derived from regional metamorphism of the carbonate host. Remarkably, low [delta].sup.18O(Grt) values in the White Chief canyon skarn require a significant proportion of meteoric fluid available during > 400 °C andradite-forming metasomatism. Fluid flow was channelized at the pluton-wallrock contact, evidenced by the narrow extent of skarn., Author(s): J. Ryan-Davis [sup.1] [sup.2], J. S. Lackey [sup.2], M. Gevedon [sup.3], J. D. Barnes [sup.3], C-T. A. Lee [sup.4], K. Kitajima [sup.5], J. W. Valley [sup.5] Author Affiliations: (1) [...]

Integrated mapping, structural analysis, and U-Pb geochronology of the Committee Bay area, Nunavut, establish a record of Neoarchean crustal growth followed by penetrative Paleoproterozoic deformation. Supracrustal rocks include a lower ca. 2.73 Ga mafic-ultramafic volcanic-dominated sequence, a middle, economically significant 2.71 Ga intermediate volcanic-bearing sequence with intercalated sulphidized, gold-bearing iron formation, and an upper Une cartographie integree, une analyse structurale et de la geochronologie U-Pb de la region de la baie Committee, au Nunavut, ont permis de determiner la croissance de la croute au Neoarcheen, laquelle a ete suivie d'une deformation penetrante au Paleoproterozoique. Les roches supracrustales comprennent une sequence inferieure, vers 2,73 Ga, mafiqueultramafique dominee par des volcaniques, une sequence mediane, d'interet economique, vers 2,71 Ga, de roches volcaniques intermediaires avec interstratification d'une formation sulfurisee de fer aurifere et une sequence superieure clastique ± komatiite-quartzique,, Introduction Archean supracrustal belts are the most significant hosts of mineral resources within the Precambrian Canadian Shield. As such, their stratigraphic, structural, and chronological characteristics elucidate the context for interpreting [...]

The internal energies and entropies of 21 well-known minerals were calculated using the density functional theory (DFT), viz. kyanite, sillimanite, andalusite, albite, microcline, forsterite, fayalite, diopside, jadeite, hedenbergite, pyrope, grossular, talc, pyrophyllite, phlogopite, annite, muscovite, brucite, portlandite, tremolite, and CaTiO.sub.3-perovskite. These thermodynamic quantities were then transformed into standard enthalpies of formation from the elements and standard entropies enabling a direct comparison with tabulated values. The deviations from reference enthalpy and entropy values are in the order of several kJ/mol and several J/mol/K, respectively, from which the former is more relevant. In the case of phase transitions, the DFT-computed thermodynamic data of involved phases turned out to be accurate and using them in phase diagram calculations yields reasonable results. This is shown for the Al.sub.2SiO.sub.5 polymorphs. The DFT-based phase boundaries are comparable to those derived from internally consistent thermodynamic data sets. They even suggest an improvement, because they agree with petrological observations concerning the coexistence of kyanite + quartz + corundum in high-grade metamorphic rocks, which are not reproduced correctly using internally consistent data sets. The DFT-derived thermodynamic data are also accurate enough for computing the P-T positions of reactions that are characterized by relatively large reaction enthalpies (> 100 kJ/mol), i.e., dehydration reactions. For reactions with small reaction enthalpies (a few kJ/mol), the DFT errors are too large. They, however, are still far better than enthalpy and entropy values obtained from estimation methods., Author(s): Artur Benisek [sup.1], Edgar Dachs [sup.1] Author Affiliations: (Aff1) 0000000110156330, grid.7039.d, Chemistry and Physics of Materials, University of Salzburg, , Jakob-Haringerstr. 2a, 5020, Salzburg, Austria Introduction Investigating the standard [...]

High-pressure and ultrahigh-pressure (UHP) eclogite-bearing metamorphic assemblages in the North Tianshan of Kyrgyzstan are known from the Aktyuz and Makbal areas, where eclogites and garnet amphibolites are associated with continental rocks such as granitoid gneisses in Aktyuz and shallow-water clastic (passive margin?) metasediments in Makbal. We present the first Lu-Hf isotope data for an eclogite and two garnet amphibolite samples from the two metamorphic terranes which, combined with petrological analysis, tightly constrain the age of high-pressure metamorphism in the Kyrgyz North Tianshan. A fivepoint isochron for an Aktyuz eclogite sample provides a LuHf age of 474.3 ± 2.2 Ma, and a four-point isochron on a Makbal sample corresponds to 470.1 ± 2.5 Ma. A prograde, subduction-related path is inferred for both samples with peak P-T conditions ranging from 1.4 to 1.6 GPa and 610-620°C. A further Makbal sample provided a significantly older Lu-Hf age of 486 ± 5.4 Ma, most likely due to late alteration in the sample (late addition of unradiogenic Hf). We conclude that garnet growth in all three samples occurred around ca. 474 Ma and that these rocks likely experienced UHP metamorphism contemporaneously. Our results support previous geochronological evidence for an Early Ordovician collision belt in the North Tianshan and allow refinement of a tectonic model involving subduction of thinned continental crust to considerable depth along the margin of a small microcontinent. Keywords Lu-Hf * Mafic dike * Aktyuz * Makbal * Northern Tianshan * Kyrgyzstan, Introduction The Tianshan orogen of north-western China, Kyrgyzstan, Kazakhstan and Uzbekistan is part of the Central Asian Orogenic Belt (CAOB; Fig. 1) which evolved over some 800 Ma from the [...]

The chemical composition of fluid inclusions in quartz crystals from Alpine fissure veins was determined by combination of microthermometry, Raman spectroscopy, and LA-ICPMS analysis. The veins are hosted in carbonate-bearing, organic-rich, low-grade metamorphic metapelites of the Bundnerschiefer of the eastern Central Alps (Switzerland). This strongly deformed tectonic unit is interpreted as a partly subducted accretionary wedge, on the basis of widespread carpholite assemblages that were later overprinted by lower greenschist facies metamorphism. Veins and their host rocks from two locations were studied to compare several indicators for the conditions during metamorphism, including illite crystallinity, graphite thermometry, stability of mineral assemblages, chlorite thermometry, fluid inclusion solute thermometry, and fluid inclusion isochores. Fluid inclusions are aqueous two-phase with 3.7-4.0 wt% equivalent NaCl at Thusis and 1.6-1.7 wt% at Schiers. Reproducible concentrations of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, B, Al, Mn, Cu, Zn, Pb, As, Sb, Cl, Br, and S could be determined for 97 fluid inclusion assemblages. Fluid and mineral geothermometry consistently indicate temperatures of 320 ± 20°C for the host rocks at Thusis and of 250 ± 30°C at Schiers. Combining fluid inclusion isochores with independent geothermometers results in pressure estimates of 2.8-3.8 kbar for Thusis, and of 3.3-3.4 kbar for Schiers. Pressure-temperature estimates are confirmed by pseudosection modeling. Fluid compositions and petrological modeling consistently demonstrate that chemical fluid-rock equilibrium was attained during vein formation, indicating that the fluids originated locally by metamorphic dehydration during near-isothermal decompression in a rock-buffered system. Keywords Fluid inclusions * LA-ICPMS * Low-grade metamorphism * Accretionary wedge * Veins * Fluid-rock equilibrium * Graphitic metasediments, Introduction The chemical composition of metamorphic fluids is of fundamental importance for predicting mineral stability, evaluating the pressure-temperature history of rocks, and assessing rates and scales of mass transfer during [...]

The evolution of chlorite composition with temperature (and pressure) serves as basis to a number of chlorite chemical thermometers, for which the oxidation state of iron has been recognised as a recurrent issue, especially at low temperature (T). A new chlorite geothermometer that does not require prior [Fe.sup.3+] knowledge is formulated, calibrated on 161 analyses with well-constrained T data covering a wide range of geological contexts and tested here for low-T chlorites (T < 350°C and pressures below 4 kbar). The new solid-solution model used involves six end-member components (the Mg and Fe end-members of 'Al-free chlorite S', sudoite and amesite) and so accounts for all low-T chlorite compositions; ideal mixing on site is assumed, with an ordered cationic distribution in tetrahedral and octahedral sites. Applied to chlorite analyses from three distinct low-T environments for which independent T data are available (Gulf Coast, Texas; Saint Martin, Lesser Antilles; Toyoha, Hokkaido), the new pure-[Fe.sup.2+] thermometer performs at least as well as the recent models, which require an estimate of [Fe.sup.2+] content. This relief from the ferric iron issue, combined with the simple formulation of the semi-empirical approach, makes the present thermometer a very practical tool, well suited for, for example, the handling of large analytical datasets--provided it is used in the calibration range (T < 350°C, P < 4 kbar). Keywords Chlorite * Geothermometry * Diagenesis * Low-grade metamorphism * Solid solution, Introduction Chlorites are ubiquitous in most diagenetic and metamorphic rocks (Foster 1962; Cathelineau 1988; Laird 1988; De Caritat et al. 1993; Vidal et al. 2001). Their wide compositional variations, through [...]

Isotopic analyses of ancient mantle-derived magmatic rocks are used to trace the geochemical evolution of the Earth's mantle, but it is often difficult to determine their primary, initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration. We present in situ analyses by LA-MC-ICPMS for the Pb isotopic compositions of igneous plagioclase ([An.sub.75-89]) megacrysts and the Hf isotopic compositions of BSE-imaged domains of zircon grains from two mantle-derived anorthosite complexes from south West Greenland, Fiskenaesset and Nunataarsuk, which represent two of the best-preserved Archean anorthosites in the world. In situ LA-ICPMS U-Pb geochronology of the zircon grains suggests that the minimum crystallization age of the Fiskenaesset complex is 2,936 ± 13 Ma (2σ, MSWD = 1.5) and the Nunataarsuk complex is 2,914 ± 6.9 Ma (2σ, MSWD = 2.0). Initial Hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 [ζ.sub.Hf] units. In terms of Pb isotopic compositions of plagioclase, both anorthosite complexes share a depleted mantle end member yet their Pb isotopic compositions diverge in opposite directions from this point: Fiskenaesset toward a high-μ, more radiogenic Pb, crustal composition and Nunataarsuk toward low-μ, less radiogenic Pb, crustal composition. By using Hf isotopes in zircon in conjunction with Pb isotopes in plagioclase, we are able to constrain both the timing of mantle extraction of the crustal end member and its composition. At Fiskenaesset, the depleted mantle melt interacted with an Eoarchean (~3,700 Ma) mafic crust with a maximum [sup.176]Lu/[sup.177]Hf ~0.028. At Nunataarsuk, the depleted mantle melt interacted with a Hadean (~4,200 Ma) mafic crust with a maximum [sup.176]Lu/[sup.177]Hf ~0.0315. Evidence from both anorthosite complexes provides support for the long-term survival of ancient mafic crusts that, although unidentified at the surface to date, could still be present within the Fiskenasset and Nunataarsuk regions. Keywords Archean * Anorthosites * LA-MC-ICPMS * Pb-Pb isotopes * Lu-Hf isotopes, Introduction Long-lived radiogenic isotope systems (e.g. Sm-Nd, Lu-Hf, Pb-Pb) are powerful tracers of geochemical processes. They have been used extensively to track complementary processes such as the geochemical depletion of [...]

The Southern Marginal Zone of the Limpopo Complex is composed of granite-greenstone cratonic rocks reworked by a Neoarchean high-grade tectono-metamorphic event. Petrographic and mineral chemical characterization of an Al-Mg granulite from this zone is presented here. The granulite has a gneissic fabric with distinct Al-rich and Si-rich layers, with the former preserving the unusual lamellar (random and regular subparallel) intergrowths of corundum and symplectic intergrowth of spinel with orthopyroxene. The Al-rich layer preserves mineral assemblages such as rutile with orthopyroxene + sillimanite ± quartz, Al-rich orthopyroxene (~ 11 wt%), spinel + quartz, and corundum in possible equilibrium with quartz, while the Si-rich layer preserves antiperthites and orthopyroxene + sillimanite ± quartz, all considered diagnostic of ultrahigh-temperature metamorphism. Application of Al-in-opx thermometry, ternary feldspar thermometry and construction of suitable pressure-temperature phase diagrams, compositional and model proportion isopleth results indicate P-T conditions as high as ~1,050-1,100°C, and ~10-12 kbars for the Al-Mg granulite. Our report of ultrahigh-temperature conditions is significant considering that the very high temperature was reached during decompression of an otherwise high-pressure granulite complex (clockwise P-T path), whereas most other ultrahigh-temperature granulites are linked to magma underplating at the base of the crust (counterclockwise P-T path). Keywords Orthopyroxene + sillimanite ± quartz * High-Al orthopyroxene * Antiperthite * Spinel * quartz * Ultrahigh-temperature metamorphism * Al-Mg granulite * Clockwise P-T path * Southern Marginal Zone * Limpopo complex, Introduction Rocks that preserve mineral assemblages that are diagnostic of ultrahigh-temperature metamorphism, such as Mg-Al granulites with highly aluminous and magnesian bulk compositions, are volumetrically rare in nature and occur [...]

Byline: Ria Mukherjee (1), Sisir K. Mondal (1,2,3), Minik T. Rosing (2), Robert Frei (2,4) Keywords: Chromite; Ferritchromit; Komatiite; Boninite; Subduction zone; Archean greenstone belt; Dharwar Craton; India Abstract: The chromite deposits in the Archean Nuggihalli schist belt are part of a layered ultramafic--mafic sequence within the Western Dharwar Craton of the Indian shield. The 3.1-Ga ultramafic--mafic units occur as sill-like intrusions within the volcano-sedimentary sequences of the Nuggihalli greenstone belt that are surrounded by the tonalite--trondhjemite--granodiorite (TTG) suite of rocks. The entire succession is exposed in the Tagdur mining district. The succession has been divided into the lower and the upper ultramafic units, separated by a middle gabbro unit. The ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite-bearing serpentinites. The chromitite bodies occur in the form of pods and elongated lenses (~60--500 m by ~15 m). Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (~50% modal chromite) and serpentinite (~2% modal chromite) throughout the entire ultramafic sequence. However, the primary composition of chromite is preserved in the massive chromitites (~60--75% modal chromite) from the Byrapur and the Bhaktarhalli mining district of the Nuggihalli schist belt. These are characterized by high Cr-ratios (Cr/(Cr + Al) = 0.78--0.86) and moderate Mg-ratios (Mg/(Mg + Fe.sup.2+) = 0.38--0.58). The compositional variability occurs due to sub-solidus re-equilibration in the accessory chromite in the serpentinite (Mg-ratio = 0.01--0.38 Cr-ratio = 0.02--0.99) and in silicate-rich chromitite (Mg-ratio = 0.06--0.48 Cr-ratio = 0.60--0.99). In the massive chromitites, the sub-solidus re-equilibration for chromite is less or absent. However, the re-equilibration is prominent in the co-existing interstitial and included olivine (Fo.sub.96--98) and pyroxene grains (Mg-numbers = 97--99). Compositional variability on the scale of a single chromite grain occurs in the form of zoning, and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite. In the zoned grains, the composition of the core is modified and the rim is ferritchromit. In general, ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains. In this case, ferritchromit formation is not very extensive. This indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization. In some occurrences, the ferritchromit rim is very well developed, and only a small relict core appears to remain in the chromite grain. However, complete alteration of the chromite grains to ferritchromit without any remnant core is also present. The regular, well-developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries, the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the Nuggihalli schist belt to be intense, pervasive and affected by later low-grade metamorphism. The primary composition of chromite has been used to compute the nature of the parental melt. The parental melt calculations indicate derivation from a high-Mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the Western Dharwar Craton. Tectonic discrimination diagrams using the primary composition of chromites indicate a supra-subduction zone setting (SSZ) for the Archean chromitites of Nuggihalli and derivation from a boninitic magma. The composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back-arc rift settings. Formation of the massive chromitites in Nuggihalli may be due to magma mixing process involving hydrous high-Mg magmas or may be related to intrusions of chromite crystal laden magma however, there is little scope to test these models because the host rocks are highly altered, serpentinized and deformed. The present configurations of the chromitite bodies are related to the multistage deformation processes that are common in Archean greenstone belts. Author Affiliation: (1) Department of Geological Sciences, Jadavpur University, Kolkata, 700032, India (2) Nordic Center for Earth Evolution, Natural History Museum of Denmark, University of Copenhagen, Aster Voldgade 5-7, 1350, Copenhagen, Denmark (3) Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West@79th Street, New York, NY, 10024, USA (4) Institute of Geography and Geology, University of Copenhagen, Aster Voldgade 10, 1350, Copenhagen, Denmark Article History: Registration Date: 03/03/2010 Received Date: 06/10/2009 Accepted Date: 02/03/2010 Online Date: 20/03/2010 Article note: Communicated by T. L. Grove. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-010-0511-5) contains supplementary material, which is available to authorized users.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.jenvman.2010.06.029 Byline: Vasilios Kapsimalis, Ioannis Panagiotopoulos, Theodore Kanellopoulos, Ioannis Hatzianestis, Panayota Antoniou, Christos Anagnostou Abstract: A multi-criteria approach was applied for the disposal into the sea of [approximately equal to]1a100a000m.sup.3 of sediment, dredged from a coastal area in the northeastern part of the Thermaikos Gulf. This sediment (classified as muddy) is distributed vertically into two distinct Layers (A and B) with the thickness of the surficial sedimentary unit ranging from 7 to 54cm. Its geochemistry reveals increased Cr and Ni concentrations, which may be attributed to natural enrichment through the erosion of the adjacent igneous and metamorphic rocks. In addition, a low to moderate contamination from urban-originated heavy metals, like Cu, Pb and Zn as well as from aliphatic and polycyclic hydrocarbons was identified for the upper Layer A. However, the limited proportion (5.5%) of the polluted Layer A in the total volume of the dredged material could not affect the good quality (assessed by the Sediment Quality Guidelines) of the bulk sediment. The identification of the optimum marine dumping site was based on (a) the physicochemical similarity (detected by the application of a cluster analysis) of the dredged material with the surficial deposits of potential dumping sites in the Outer Thermaikos Gulf, and (b) the consideration, based on previous studies, of various criteria related to the disposal area such as deep-water circulation, influence on living resources, impact on economical (aquaculture, fishing, navigation), recreational (fishing) and military activities. Author Affiliation: Hellenic Centre for Marine Research (HCMR), Institute of Oceanography, 46.7th km Athens-Sounion Ave., 19013 Anavyssos, Attica, Greece Article History: Received 17 November 2009; Revised 27 May 2010; Accepted 29 June 2010

Byline: Lyderic France (1,2,4), Juergen Koepke (2), Benoit Ildefonse (1), Sarah B. Cichy (2), Fabien Deschamps (3) Keywords: Mid-ocean ridge; Axial magma chamber; Hydrothermal system; Sheeted dike complex; Partial melting; Experimental petrology; Oceanic plagiogranite; Granoblastic dikes Abstract: In ophiolites and in present-day oceanic crust formed at fast spreading ridges, oceanic plagiogranites are commonly observed at, or close to the base of the sheeted dike complex. They can be produced either by differentiation of mafic melts, or by hydrous partial melting of the hydrothermally altered sheeted dikes. In addition, the hydrothermally altered base of the sheeted dike complex, which is often infiltrated by plagiogranitic veins, is usually recrystallized into granoblastic dikes that are commonly interpreted as a result of prograde granulitic metamorphism. To test the anatectic origin of oceanic plagiogranites, we performed melting experiments on a natural hydrothermally altered dike, under conditions that match those prevailing at the base of the sheeted dike complex. All generated melts are water saturated, transitional between tholeiitic and calc-alkaline, and match the compositions of oceanic plagiogranites observed close to the base of the sheeted dike complex. Newly crystallized clinopyroxene and plagioclase have compositions that are characteristic of the same minerals in granoblastic dikes. Published silicic melt compositions obtained in classical MORB fractionation experiments also broadly match the compositions of oceanic plagiogranites however, the compositions of the coexisting experimental minerals significantly deviate from those of the granoblastic dikes. Our results demonstrate that hydrous partial melting is a likely common process in the root zone of the sheeted dike complex, starting at temperatures exceeding 850degC. The newly formed melt can either crystallize to form oceanic plagiogranites or may be recycled within the melt lens resulting in hybridized and contaminated MORB melts. It represents the main MORB crustal contamination process. The residue after the partial melting event is represented by the granoblastic dikes. Our results support a model with a dynamic melt lens that has the potential to trigger hydrous partial melting reactions in the previously hydrothermally altered sheeted dikes. A new thermometer using the Al content of clinopyroxene is also elaborated. Author Affiliation: (1) Geosciences Montpellier, CNRS, Universite Montpellier 2, CC60, 34095, Montpellier Cedex 05, France (2) Institut fur Mineralogie, Leibniz Universitat Hannover, Callinstrasse 3, 30167, Hannover, Germany (3) LGCA UMR CNRS 5025, Universite Joseph-Fourier, BP 53, 38041, Grenoble cedex, France (4) Geosciences et Environnement Cergy, Universite Cergy-Pontoise, 5 mail Gay Lussac, Neuville sur Oise, 95031, Cergy-Pontoise cedex, France Article History: Registration Date: 10/02/2010 Received Date: 06/08/2009 Accepted Date: 10/02/2010 Online Date: 09/03/2010 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-010-0502-6) contains supplementary material, which is available to authorized users.

Byline: Donna L. Whitney (1), Nicholas C. A. Seaton (1) Keywords: Garnet; EBSD; Crystallization; Polycrystal Abstract: Polycrystalline garnets are common in metamorphic rocks and may form as a result of close spacing of nuclei (if clustering is early) or impingement of larger grains (if clustering occurs later in the growth history). The timing of clustering relative to garnet growth is relevant to understanding the formation and evolution of porphyroblasts and evaluating the significance (if any) of clustering. Electron backscattered diffraction (EBSD) analysis of garnet-bearing metamorphic rocks reveals the presence of polycrystalline garnet in nine localities examined in this study: the northern Appalachians (Vermont, Maine, New York, USA) North American Cordillera (North Cascades Range, Washington Snake Range, Nevada, USA) western Rocky Mountains (British Columbia, Canada) southern Menderes Massif (Turkey) Santander Massif (Colombia) and the Sanandaj--Sirjan zone (Hamadan, Iran). In some samples, polycrystals comprise ~20--30% of garnets analyzed, and chemical and textural evidence suggests that early coalescence of garnet polycrystals is common. Some early-coalescing polycrystals exhibit growth zoning that is concentric about the geometric center of the polycrystal. In thin section, these garnets may be undetectable as polycrystals based on morphology or zoning. In some polycrystals, zoning is unrelated to the location of internal grain boundaries in others, Fe--Mn--Mg zoning has a different pattern than that of Ca zoning patterns may vary on the scale of a single thin section. In addition, some polycrystals are characterized by high-angle misorientation boundaries that may be in special (non-random) orientations, an observation that indicates that these polycrystals are not random clusters of grains. The presence of internal grain boundaries may affect diffusion pathways and length scales, and may facilitate communication of porphyroblast interiors with matrix phases, thereby influencing reaction history of the rock and the composition/zoning of garnet. Author Affiliation: (1) Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN, 55455, USA Article History: Registration Date: 27/01/2010 Received Date: 15/09/2009 Accepted Date: 27/01/2010 Online Date: 19/02/2010 Article note: Communicated by J. Blundy.

Byline: Ben Buse (1), John C. Schumacher (1), R. Stephen J. Sparks (1), Matthew Field (2) Keywords: Kimberlite; Hydrothermal metamorphism; Xenolith; Serpentinisation; Post-emplacement Abstract: Metamorphic assemblages within Karoo basalt xenoliths, found within volcaniclastic kimberlite of the B/K9 pipe, Damtshaa, Botswana, constrain conditions of kimberlite alteration. Bultfonteinite and chlorite partially replace the original augite-plagioclase assemblage, driven by the serpentinisation of the kimberlite creating strong chemical potential gradients for Si and Mg. Hydrogarnet and serpentine replace these earlier metamorphic assemblages as the deposits cool. The bultfonteinite (ideally [Ca.sub.2]SiO.sub.2[OH,F].sub.4) and hydrogarnet assemblages require a water-rich fluid containing F.sup.-, and imply hydrothermal alteration dominated by external fluids rather than autometamorphism from deuteric fluids. Bultfonteinite and hydrogarnet are estimated to form at temperatures of ca. 350--250degC, which are similar to those for serpentinisation. Alteration within the B/K9 kimberlite predominantly occurs between 250 and 400degC. We attribute these conditions to increased efficiency of mass transfer and chemical reactions below the critical point of water and a consequence of volume-increasing serpentinisation and metasomatic reactions that take place over this temperature range. A comparison of the B/K9 kimberlite with kimberlites from Venetia, South Africa suggests that the composition and mineralogy of included xenoliths affects the alteration assemblages within kimberlite deposits. Author Affiliation: (1) Department of Earth Sciences, University of Bristol, Bristol, UK (2) DiaKim Consulting Limited, Wookey Hole, Wells, UK Article History: Registration Date: 19/01/2010 Received Date: 11/08/2009 Accepted Date: 19/01/2010 Online Date: 10/02/2010 Article note: Communicated by M. W. Schmidt.

Byline: Guillaume Girard (1), John Stix (1) Keywords: Crystal mush; Intraplate volcanism; Faulting; Low-[delt[a].sup.18]O magmas; Crystal residence times Abstract: The Central Plateau Member rhyolites have been erupted between 173 and 70 ka and are the youngest Yellowstone intracaldera rhyolites. They mostly comprise very voluminous lava flows totaling ~600 km.sup.3 in volume. Their eruptive vents define two NNW-trending lineaments which are aligned with regional faults. We present new whole rock, glass, and mineral analyses and propose a petrogenetic and volcano-tectonic model for these rhyolites. At a caldera-wide scale, there is a temporal enrichment in elements such as Nb, Y and HREE, and a depletion in Sr, Ba, and Ce/Yb. Simultaneously, clinopyroxene becomes less magnesian while Ti contents in quartz decrease. By contrast, quartz in all rhyolites is rounded and bears long glass re-entrants, suggesting heating. Based on these data and observations, we propose that the Central Plateau Member rhyolites have been generated as follows. A hydrothermally altered low-[delt[a].sup.18]O rhyolitic protolith beneath the Mallard Lake Resurgent Dome in the southwestern part of the caldera started to melt at ~250 ka. Repeated heating pulses caused the melting front to expand radially, and a large crystal mush formed beneath much of the caldera. The mush was able to differentiate but not erupt due to its high crystallinity and viscosity. Further inputs of heat and silicic magma in this mush increased the degree of melting, forming crystal-poor magma batches which erupted a few hundred to a few thousand years later through regional faults to form the Central Plateau Member rhyolites. Author Affiliation: (1) Earth and Planetary Sciences, McGill University, 3450 University street, Montreal, QC, H3A 2A7, Canada Article History: Registration Date: 31/12/2009 Received Date: 13/05/2009 Accepted Date: 30/12/2009 Online Date: 03/02/2010 Article note: Communicated by T. L. Grove. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0487-1) contains supplementary material, which is available to authorized users.

Byline: D. J. Cherniak (1), E. B. Watson (1) Keywords: Lithium; Zircon; Diffusion; Nuclear reaction analysis Abstract: Diffusion of Li under anhydrous conditions at 1 atm and under fluid-present elevated pressure (1.0--1.2 GPa) conditions has been measured in natural zircon. The source of diffusant for 1-atm experiments was ground natural spodumene, which was sealed under vacuum in silica glass capsules with polished slabs of zircon. An experiment using a Dy-bearing source was also conducted to evaluate possible rate-limiting effects on Li diffusion of slow-diffusing REE.sup.+3 that might provide charge balance. Diffusion experiments performed in the presence of [H.sub.2]O--CO.sub.2 fluid were run in a piston--cylinder apparatus, using a source consisting of a powdered mixture of spodumene, quartz and zircon with oxalic acid added to produce [H.sub.2]O--CO.sub.2 fluid. Nuclear reaction analysis (NRA) with the resonant nuclear reaction 7.sup.Li(p,I[3).sup.8]Be was used to measure diffusion profiles for the experiments. The following Arrhenius parameters were obtained for Li diffusion normal to the c-axis over the temperature range 703--1.151degC at 1 atm for experiments run with the spodumene source: D.sub.Li = 7.17 10.sup. - 7 ( - 275 [+ or -] 11 kJmol.sup. - 1 /RT)m.sup.2 s.sup. - 1. Diffusivities are similar for transport parallel to the c-axis, indicating little anisotropy for Li diffusion in zircon. Similar Li diffusivities were also found for experiments run under fluid-present conditions and for the experiment run with the Dy-bearing source. Li diffusion is considerably faster than diffusion of other cations in zircon, with a smaller activation energy for diffusion. Although Li diffusion in zircon is comparatively rapid, zircons will be moderately retentive of Li signatures at mid-crustal metamorphic temperatures, but they are unlikely to retain this information for geologically significant times under high-grade metamorphism. Author Affiliation: (1) Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Science Center 1W19, 110 8th St., Troy, NY, 12180, USA Article History: Registration Date: 31/12/2009 Received Date: 08/08/2009 Accepted Date: 30/12/2009 Online Date: 10/01/2010 Article note: Communicated by T. L. Grove.

Byline: Matthijs A. Smit (1), Erik E. Scherer (1), Michael Brocker (1), Herman L. M. Roermund (2) Keywords: Eclogite; Geochronology; Lu--Hf; Sm--Nd; Caledonides; Norway Abstract: The Scandinavian Caledonides contain several non-cogenetic eclogite-bearing terranes that were metamorphosed before the main orogenic stage in Scandian time (430--395 Ma). Although petrological and geochronological data from these terranes have provided essential information on the geodynamic history of the Caledonian orogenic cycle, the general picture is still patchy. To refine existing geodynamic models, we have dated the eclogite occurrence in the JA|ren nappe, SW Norway, by Lu--Hf and Sm--Nd geochronology. Five out of the six studied samples provide a weighted mean Lu--Hf age of 469.9 +- 1.2 Ma (+-2[sigma]). One sample provided a significantly younger age of 457.9 +- 2.4 Ma. Garnet from the younger sample grew exclusively at eclogite facies conditions. In contrast, garnet from the other samples comprises prograde cores and peak metamorphic rims. Age estimates that take Lu-contributions of each zone into account provide an age of 471.0 +- 0.9 Ma for the cores and suggest a ca. 455 Ma age for the rims, which is identical to the bulk-garnet age of the younger sample. The same pattern is indicated by Sm--Nd ages, although these are relatively imprecise and reflect isotopic disturbance during thermal overprinting upon exhumation. The data define a new high-pressure age population for the Scandinavian Caledonides, which allows more detailed insight into the subduction history that affected the Baltoscandian margin before Scandian continental collision. Furthermore, this study highlights the potential complexities involved in garnet geochronology and shows the strength of Lu--Hf dating for unraveling the geochronological record of HP rocks. Author Affiliation: (1) Institut fur Mineralogie, Westfalische Wilhelms-Universitat, Corrensstrasse 24, 48149, Munster, Germany (2) Faculteit Geowetenschappen, Universiteit Utrecht, Budapestlaan 4, 3584 CD, Utrecht, The Netherlands Article History: Registration Date: 10/08/2009 Received Date: 22/04/2009 Accepted Date: 10/08/2009 Online Date: 28/08/2009 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0440-3) contains supplementary material, which is available to authorized users.

Byline: M. Santosh (1,2), Toshiaki Tsunogae (3), Hisako Shimizu (3), Jean Dubessy (4) Keywords: Eclogite; High pressure granulite; Mineral phase equilibria; Fluid inclusions; CO.sub.2; Southern India; Gondwana Abstract: Eclogite-facies rocks and high-pressure granulites provide windows to the deeper parts of subduction zones and the root of mountain chains, carrying potential records of fluids associated with subduction-accretion-collision tectonics. Here, we report petrological and fluid inclusion data on retrogressed eclogite and high-pressure granulite samples from Sittampundi, Kanji Malai and Perundarai in southern India. These rocks occur within the trace of the Cambrian collisional suture which marks the final phase of amalgamation of the Gondwana supercontinent. The garnet--clinopyroxene assemblage in the eclogites preserves relict omphacite, whereas the high-pressure granulites are characterized by an assemblage of garnet and clinopyroxene in the absence of omphacite and with minor plagioclase, orthopyroxene, and quartz. Phase relations computed for the eclogite assemblage yield peak P--T conditions of 19 kbar and 1,010degC. The mafic granulites also preserve the memory of high to ultrahigh-temperature metamorphism followed by an isothermal decompression. Systematic fluid inclusion optical, microthermometric and laser Raman spectroscopic studies were conducted in garnet and plagioclase from the eclogite--high pressure granulite suite. The results suggest that the early fluids were a mixture of CO.sub.2, CH.sub.4 and N.sub.2 probably derived from decarbonation and devolatilization reactions in a subduction setting during the prograde stage. The later generation inclusions, which constitute the dominant category in all the samples studied, are characterized by a near-pure CO.sub.2 composition with moderate to high densities (up to 1.154 g/cm.sup.3). The highest density fluid inclusions recorded in this study occur within the mafic granulites from Sittampundi (0.968--1.154 g/cm.sup.3) and Kanji Malai (1.092--1.116 g/cm.sup.3). In some cases, carbonate minerals such as dolomite and calcite are associated with the CO.sub.2-rich fluid inclusions. The composition and densities of the later generation fluids closely match with those of the CO.sub.2-bearing fluid inclusions reported from ultrahigh-temperature granulites occurring proximal to the eclogite--high pressure granulite suite within this suture zone, and suggest a common tectonic link for the fluid regime. We evaluate the fluid characteristics associated with convergent plate margin processes and propose that the early aqueous fluids probably associated with the eclogites were consumed during the formation of the retrograde hydrous mineral assemblages, whereas the fluid regime of the high-pressure and ultrahigh-temperature granulites was mostly CO.sub.2-dominated. The tectonic setting of the rocks along a collisional suture marking the trace along which crustal blocks were welded through subduction--collision process is in favor of a model involving the derivation of CO.sub.2 from sub-lithospheric sources such as a carbonated tectosphere invaded by hot asthenosphere, or underplated mafic magmas. Author Affiliation: (1) Faculty of Science, Kochi University, Akebono-cho, Kochi, 780-8520, Japan (2) Department of Earth and Atmospheric Sciences, Center for Environmental Sciences, Saint Louis University, St. Louis, MO, 63108, USA (3) Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki, 305-8572, Japan (4) G2R(UMR 7566), Nancy Universite and CNRS, BP70239, 54506, Vandoeuvre-les-Nancy Cedex, France Article History: Registration Date: 16/07/2009 Received Date: 22/10/2008 Accepted Date: 16/07/2009 Online Date: 30/07/2009 Article note: Communicated by J.L.R. Touret. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0431-4) contains supplementary material, which is available to authorized users.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.08.025 Byline: Laura Schaefer, Bruce Fegley Keywords: Asteroids Abstract: We used chemical equilibrium calculations to model thermal metamorphism of ordinary chondritic material as a function of temperature, pressure, and trace element abundance and use our results to discuss volatile mobilization during thermal metamorphism of ordinary chondrite parent bodies. We compiled trace element abundances in H-, L-, and LL-chondrites for the elements Ag, As, Au, Bi, Br, Cd, Cs, Cu, Ga, Ge, I, In, Pb, Rb, Sb, Se, Sn, Te, Tl, and Zn, and identified abundance trends as a function of petrographic type within each class. We calculated volatility sequences for the trace elements in ordinary chondritic material, which differ significantly from the solar nebula volatility sequence. Our results are consistent with open-system thermal metamorphism. Abundance patterns of Ag and Zn remain difficult to explain. Author Affiliation: Planetary Chemistry Laboratory, Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130-4899, United States Article History: Received 4 January 2008; Revised 23 July 2009; Accepted 1 August 2009

Geological mapping and U-(Th)-Pb geochronology from the Karakoram Metamorphic Complex and Baltoro granite batholith has resulted in more detailed timing constraints on tectonic evolution of Asian crust. During and following collision and accretion of the Kohistan Arc and the Indian plate to the southern margin of Asia, crustal thickening along the Karakoram resulted in polyphase deformation, metamorphism and melting. U-Pb zircon and monazite ages show that kyanite- and sillimanite-grade metamorphism in the Baltoro region peaked during Oligocene-Early Miocene (c. 28-22 Ma) times. At structurally deeper levels sillimanite-grade migmatites of the Dassu dome have Pliocene (c. 5.4-3.5 Ma) ages. New U Pb ages from crustal melt biotite + muscovite + garnet leucogranites from the Trango Towers, Cathedral peak and Paiyu peak across the Baltoro batholith range from 19.8 [+ or -] 0.5 Ma to perhaps as young as 14 Ma. Using data from both the Hunza and Baltoro regions we suggest that regional high-grade metamorphism was diachronous in space and time and could have lasted continuously for at least 37 Ma (c. 50-13 Ma) or as much as 50 Ma (c. 63-13 Ma) along the southern margin of the Asian plate in the Karakoram. Crustal melting along the Baltoro granite batholith spanned at least 13 Ma (26.4-13 Ma). Supplementary material: Analytical methods and description of U-(Th)-Pb data of samples are available at http://www.geolsoc.org.uk/SUP18382. doi: 10.1144/0016-76492009-043

Byline: Jose Alberto Padron-Navarta (1), Jorg Hermann (2), Carlos J. Garrido (3), Vicente Lopez Sanchez-Vizcaino (4), Maria Teresa Gomez-Pugnaire (5) Keywords: Antigorite dehydration; Silica metasomatism; Orthopyroxene-bearing serpentinite Abstract: Piston cylinder experiments were performed to constrain the pressure and temperature conditions for two high-pressure antigorite dehydration reactions found in silica-enriched serpentinites from Cerro del Almirez (Nevado--Filabride Complex, Betic Cordillera, southern Spain). At 630--660degC and pressures greater than 1.6 GPa, antigorite first reacts with talc to form orthopyroxene +- chlorite + fluid. We show that orthopyroxene + antigorite is restricted to high-pressure metamorphism of silica-enriched serpentinite. This uncommon assemblage is helpful in constraining metamorphic conditions in cold subduction environments, where antigorite serpentinites have no diagnostic assemblages over a large pressure and temperature range. The second dehydration reaction leads to the breakdown of antigorite to olivine + orthopyroxene + chlorite + fluid. The maximum stability of antigorite is found at 680degC at 1.9 GPa, which also corresponds to the maximum pressure limit for tremolite coexisting with olivine + orthopyroxene. The high aluminium (3.70 wt% [Al.sub.2]O.sub.3) and chromium contents (0.59 wt% [Cr.sub.2]O.sub.3) of antigorite in the investigated starting material is responsible for the expansion of the serpentinite stability to 60--70degC higher temperatures at 1.8 GPa than the antigorite stability calculated in the Al-free system. The antigorite from our study has the highest Al--Cr contents among all experimental studies and therefore likely constraints the maximum stability of antigorite in natural systems. Comparison of experimental results with olivine--orthopyroxene--chlorite--tremolite assemblages outcropping in Cerro del Almirez indicates that peak metamorphic conditions were 680--710degC and 1.6--1.9 GPa. Author Affiliation: (1) Departamento de Mineralogia y Petrologia, Facultad de Ciencias, Universidad de Granada, Avda Fuentenueva s/n, 18002, Granada, Spain (2) Research School of Earth Sciences, The Australian National University, Canberra, 0200, ACT, Australia (3) Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC & UGR, 18002, Granada, Spain (4) Departamento de Geologia, Escuela Politecnica Superior, Universidad de Jaen, Alfonso X El Sabio 28, 23700, Linares, Spain (5) Departamento de Mineralogia y Petrologia, Facultad de Ciencias, Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada - CSIC, 18002, Granada, Spain Article History: Registration Date: 29/05/2009 Received Date: 13/02/2009 Accepted Date: 29/05/2009 Online Date: 20/06/2009 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0414-5) contains supplementary material, which is available to authorized users.

Byline: Estelle Auzanneau (1,2), M. W. Schmidt (1), D. Vielzeuf (3), J. A. D Connolly (1) Keywords: Phengite; Titanium; Thermodynamic modelling; Thermobarometry; UHP metamorphism Abstract: Phengite chemistry has been investigated in experiments on a natural SiO.sub.2--TiO.sub.2-saturated greywacke and a natural SiO.sub.2--TiO.sub.[2--Al.sub.2]SiO.sub.5-saturated pelite, at 1.5--8.0 GPa and 800--1,050degC. High Ti-contents (0.3--3.7 wt %), Ti-enrichment with temperature, and a strong inverse correlation of Ti-content with pressure are the important features of both experimental series. The changes in composition with pressure result from the Tschermak substitution (Si + R.sup.2+ = Al.sup.IV + Al.sup.VI) coupled with the substitution: Al.sup.VI + Si = Ti + Al.sup.IV. The latter exchange is best described using the end-member Ti-phengite (KMgTi[[Si.sub.3]Al]O.sub.10(OH).sub.2, TiP). In the rutile-quartz/coesite saturated experiments, the aluminoceladonite component increases with pressure while the muscovite, paragonite and Ti-phengite components decrease. A thermodynamic model combining data obtained in this and previous experimental studies are derived to use the equilibrium MgCel + Rt = TiP + Cs/Qz as a thermobarometer in felsic and basic rocks. Phengite, rutile and quartz/coesite are common phases in HT-(U)HP metamorphic rocks, and are often preserved from regression by entrapment in zircon or garnet, thus providing an opportunity to determine the T--P conditions of crystallization of these rocks. Two applications on natural examples (Sulu belt and Kokchetav massif) are presented and discussed. This study demonstrates that Ti is a significant constituent of phengites that could have significant effects on phase relationships and melting rates with decreasing P or increasing T in the continental crust. Author Affiliation: (1) Institute for Mineralogy and Petrology, ETH, 8092, Zurich, Switzerland (2) Laboratoire Magmas et Volcans, OPGC, CNRS et Universite Blaise Pascal, 5 rue Kessler, 63038, Clermont-Ferrand, France (3) CINaM, CNRS, Aix-Marseille University, Campus de Luminy, 13288, Marseille, France Article History: Registration Date: 13/05/2009 Received Date: 04/07/2008 Accepted Date: 13/05/2009 Online Date: 12/06/2009 Article note: Communicated by J. Blundy. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0412-7) contains supplementary material, which is available to authorized users.

Byline: Giovanna T. Sapienza (1,3), Marco Scambelluri (2), Roberto Braga (1) Keywords: Dolomite-bearing peridotites; Mantle wedge; C--O--H fluid; Subduction fluids; Ulten Zone Abstract: We document the presence of dolomite +- apatite in orogenic peridotites from the Ulten Zone (UZ, Italian Alps), the remnants of a Variscan mantle wedge tectonically coupled with eclogitized continental crust. These dolomite peridotites are associated with dominant carbonate-free amphibole peridotites, which formed in response to infiltration of aqueous subduction fluids lost by the associated crustal rocks during high-pressure (HP) metamorphism and retrogression. Dolomite-free and dolomite-bearing peridotites share the same metamorphic evolution, from garnet- (HP) to spinel-facies (low-pressure, LP) conditions. Dolomite and the texturally coexisting phases display equilibrium redistribution of rare earth elements and of incompatible trace elements during HP and LP metamorphism clinopyroxene and amphiboles from carbonate-free and carbonate-bearing peridotites have quite similar compositions. These features indicate that the UZ mantle rocks equilibrated with the same metasomatic agents: aqueous CO.sub.2-bearing fluids enriched in incompatible elements released by the crust. The P--T crystallization conditions of the dolomite peridotites (outside the field of carbonatite melt + amphibole peridotite coexistence), a lack of textures indicating quench of carbonic melts, a lack of increase in modal clinopyroxene by reaction with such melts and the observed amphibole increase at the expense of clinopyroxene, all suggest that dolomite formation was assisted by aqueous CO.sub.2-bearing fluids. A comparison of the trace element compositions of carbonates and amphiboles from the UZ peridotites and from peridotites metasomatized by carbonatite and/or carbon-bearing silicate melts does not help to unambiguously discriminate between the different agents (fluids or melts). The few observed differences (lower trace element contents in the fluid-related dolomite) may ultimately depend on the solute content of the metasomatic agent (CO.sub.2-bearing fluid versus carbonatite melt). This study provides strong evidence that C--O--H subduction fluids can produce 'carbonatite-like' assemblages in mantle rocks, thus being effective C carriers from the slab to the mantle wedge at relatively low P--T. If transported beyond the carbonate and amphibole solidus by further subduction, dolomite-bearing garnet + amphibole peridotites like the ones from Ulten can become sources of carbonatite and/or C-bearing silicate melts in the mantle wedge. Author Affiliation: (1) Dipartimento di Scienze della Terra e Geologico-Ambientali, Universita di Bologna, Bologna, Italy (2) Dipartimento per lo Studio del Territorio e delle sue Risorse, Universita di Genova, Genova, Italy (3) D&M Schlumberger Italiana S.p.A., Via Bondi 32, 48100, Ravenna, Italy Article History: Registration Date: 29/01/2009 Received Date: 07/07/2008 Accepted Date: 29/01/2009 Online Date: 03/03/2009 Article note: In memory of Lauro Morten 1941--2006. Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0389-2) contains supplementary material, which is available to authorized users.

Problem statement: The Muteh mining district is located in 70 km northeast of Golpaygan city within the Sanandaj-Sirjan metamorphic belt. There are 2 gold mines, 7 gold occurrences and numerous mineral indices in the Muteh gold district. There are few researches on Muteh gold district, but a detailed model is not clear yet. The aim of this study is to determine mineralogy of gold-bearing rocks and the role of these rocks in concentration of gold and to improve our knowledge about Muteh model. Approach: Detailed fieldwork carried out at different scales at the Muteh district. About 50 outcrops samples examined petrographically. Fifteen samples containing veinlets of sulfides and quartz selected for H, O and S stable isotope analysis. Petrography characterized by optical microscopy, Scanning Electron Microscopy (SEM) and X-Ray powder Diffraction (XRD) analysis. Results: Geological studies indicated that the study area show a major poly-phase metamorphism. Since the latest metamorphism was weaker than the earlier ones, the older rocks were affected by more intense metamorphism. The metamorphic rocks mainly consist of schists, quartzite, marble, amphibolite and gneisses. These rocks show two foliations ([S.sub.1] and [S.sub.2]). The [S.sub.2] foliation is the major phase in the metamorphic rocks. Pyrite is the most abundant and the important gold-bearing mineral at the study area. Based on evidences of deformation ([S.sub.2]) and crystallization, three main types of pyrites can be distinguished in the Muteh deposit: (1) pre-tectonic or gold bearing pyrite (2) syn-tectonic or disseminated pyrites along the foliation of the host rocks. (3) pyrite aggregates in the host rocks or in the metamorphic segregation quartz veins crosscutting the foliation of the host rocks. The sulfur isotope studies were carried out on pyrites within quartz veinlets, biotite schist and meta-volcanic rocks at the Muteh deposit. Five available data are highly variable even from the same types of hosted rocks and their [[delta].sup.34]S are +2.2, 6.6, 9.1, 13.9 and 16.9%[??]. Conclusion: There are three generations of pyrite in Muteh gold district. According to isotope data, it seems that source of sulfurs were not homogenous. The values showed more than one geological event for generation of pyrites in the study area. Compositions of sulfur isotopes indicated several different sources or processes for the sulfide fluids. The sulfur of these pyrites might have derived either directly from regional metamorphism that produced the metamorphic fluid or through dissolution and leaching of pre-existing sulfide-bearing minerals. Key words: Pyrites, Muteh gold district, foliation, metamorphic rocks, sulfur isotope, INTRODUCTION There are two gold mines, seven gold occurrences and numerous mineral indices in the Muteh gold district, which covers 160 [km.sup.2] in northeast of the metamorphic basement within Sanandaj-Sirjan [...]

Byline: John R. Bowman (1), John W. Valley (2), Noriko T. Kita (2) Abstract: Oxygen isotope ratios have been measured by ion microprobe and millimeter-scale dental drill along detailed sampling traverses across the boundary between periclase-bearing ([delt[a].sup.18]O = 11.8[per thousand]) and periclase-free ([delt[a].sup.18]O = 17.2[per thousand]) marble layers in the periclase (Per) zone of the Alta Stock aureole, Utah. These data define a steep, coherent gradient in [delt[a].sup.18]O that is displaced a short distance (~4 cm) into the periclase-free (Cal + Fo) layer. SEM and ion microprobe analyses show two isotopically and texturally distinct types of calcite at the grain scale. Clear (well polished) calcite grains are isotopically homogeneous (within analytical uncertainty +-0.27[per thousand], 2SD). More poorly polished (pitted), texturally retrograde 'turbid'-looking calcite has lower and more variable [delt[a].sup.18]O values, and replaces clear calcite along fractures, cleavage traces or grain boundaries. Despite significant lowering of the [delt[a].sup.18]O values in calcite throughout both layers during prograde metamorphism, ion microprobe analyses indicate that individual clear calcite grains are now isotopically homogeneous across the entire gradient in [delt[a].sup.18]O. Diffusion calculations indicate that conservative time scales required for isotopic homogenization of calcite grains by volume diffusion, 30,000--62,000 years at 575--600degC, exceed significantly the timescale (~1,250 years) estimated for the prograde development of the [delt[a].sup.18]O gradient at the boundary between these two marble layers. The ion microprobe data and these diffusion calculations suggest instead that surface reaction mechanisms accompanying recrystallization are responsible for the observed oxygen isotope homogeneity of these calcite grains. Thus, the ion microprobe data are consistent with the formation of calcite in oxygen isotope exchange equilibrium with infiltrating fluid during prograde reaction and recrystallization. Author Affiliation: (1) Department of Geology and Geophysics, University of Utah, Salt Lake City, UT, 84112, USA (2) Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI, 53706, USA Article History: Registration Date: 20/06/2008 Received Date: 26/01/2008 Accepted Date: 20/06/2008 Online Date: 20/08/2008 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-008-0321-1) contains supplementary material, which is available to authorized users.

Byline: B. L. A. Charlier (1), C. J. N. Wilson (2), J. P. Davidson (3) Keywords: Oruanui eruption; Sr isotopes; 87.sup.Sr/ [.sup.86]Sr; Microsampling; Magmatic processes; Zircon Abstract: Rhyolitic pumices in the 26.5 ka Oruanui eruption (Taupo volcano, New Zealand) contain an average of 10 wt% crystals. About 2 wt% of the crystal population is feldspar crystals that display bluish--grey cloudy cores, the colour being imparted by exsolved needles of rutile. The volume of cloudy-cored feldspars thus amounts to ~1.0 km.sup.3 in a total magma volume of ~530 km.sup.3. The cored feldspars show great variability in detail, but in general have a rounded cloudy core bounded by a zone rich in glass and mineral inclusions, that was then overgrown by a euhedral clear rim. Sr-isotopic variations in eight representative crystals were measured on micromilled samples of selected growth zones in the cores and rims, and linked to feldspar compositions through microprobe traverses. The cloudy cores range from 87.sup.Sr/ [.sup.86]Sr = 0.70547 to 0.70657, with compositions of An.sub.43 to An.sub.78. The overgrowth rims display wider variations: inner parts show extreme ranges in composition (maxima 87.sup.Sr/ [.sup.86]Sr = 0.70764 and An.sub.78), while outer parts in seven of eight crystals are zoned, with outward-decreasing Sr-isotopic and An values to figures that are in accord with the bulk pumice and other, clear-feldspar values, respectively. The three parts of the crystals represent distinct regimes. The cloudy cores are inherited from an intermediate plutonic protolith that has been subjected to melting. The inner overgrowth rims were crystallised from a high temperature, relatively radiogenic melt derived from Mesozoic-Palaeozoic metasedimentary rocks ('greywacke'). The outer euhedral rims reflect mixing into and continued growth within the growing Oruanui magma body. The cloudy-cored feldspars also contain rare zircon inclusions. Twenty one zircons were recovered by HF digestion of a bulk sample of cloudy feldspars and analysed by SHRIMP for U--Th isotopes with which to calculate model ages. Eighteen of 21 crystals returned finite ages, the model-age spectrum of which is similar to the age spectra from free zircons in Oruanui pumices. Assembly of the Oruanui magma body was not only rapid (over ~40 kyr, as shown by other data) but involved a wide open system, with significant contributions from partly-melted intermediate-composition igneous intrusions (cloudy cores) and greywacke melts (inner overgrowths) being introduced into the magma body up to the point of eruption. Such open system behaviour contrasts with that proposed in models for comparably voluminous silicic magmas derived dominantly by fractionation (such as the Bishop Tuff) where the magma and its crystal cargo were better insulated thermally and chemically from country-rock interaction. Author Affiliation: (1) Department of Earth and Environmental Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA, UK (2) Geology-SGGES, and Institute of Earth Science and Engineering, University of Auckland, PB 92019 Auckland Mail Centre, Auckland, 1142, New Zealand (3) Department of Earth Sciences, University of Durham, South Road, Durham, DH1 3LE, UK Article History: Registration Date: 26/05/2008 Received Date: 04/01/2008 Accepted Date: 26/05/2008 Online Date: 13/06/2008 Article note: Communicated by J. Blundy.

Byline: Thorsten Beitter (1), Thomas Wagner (2), Gregor Markl (1) Keywords: Kyanite; Aluminum solubility; Aluminum mobility; Alpe Sponda; Metamorphism Abstract: In this study, we have investigated the formation of quartz--kyanite veins of the Alpe Sponda, Central Alps, Switzerland. We have integrated field observations, fluid inclusion and stable isotope data and combined this with numerical geochemical modeling to constrain the chemical processes of aluminum transport and deposition. The estimated P--T conditions of the quartz--kyanite veins, based on conventional geothermometry (garnet--biotite, white mica solvus and quartz--kyanite oxygen isotope thermometry) and fluid inclusion data, are 550 +- 30degC at 5.0 +- 0.5 kbar. Geochemical modeling involved construction of aqueous species predominance diagrams, calculation of kyanite and quartz solubility, and reaction--path simulations. The results of the modeling demonstrate that (1) for the given chemical composition of the vein-forming fluids mixed Al--Si aqueous species are dominant in transporting Al, and that (2) fluid cooling along a small temperature gradient coupled with a pH decrease is able to explain the precipitation of the quartz--kyanite assemblages in the proportions that are observed in the Alpe Sponda veins. We conclude that sufficient amounts of Al can be transported in typical medium- to high-grade regional metamorphic fluids and that immobile behavior of Al is not very likely in advection--dominanted fluid--rock systems in the upper and middle crust. Author Affiliation: (1) Institut fur Geowissenschaften, Arbeitsbereich Mineralogie und Geodynamik, Wilhelmstr. 56, 72074, Tubingen, Germany (2) Institute of Isotope Geochemistry and Mineral Resources, ETH Zurich, NW F 82.4, Clausiusstrasse 25, 8092, Zurich, Switzerland Article History: Registration Date: 07/05/2008 Received Date: 05/11/2007 Accepted Date: 07/05/2008 Online Date: 27/05/2008 Article note: Communicated by J. Hoefs.

Southwest Spitsbergen, Wedel Jarlsberg Land, consists of two Proterozoic crustal blocks with differing metamorphic histories. Both blocks experienced Caledonian greenschist-facies metamorphism, but only the southern block records an earlier pervasive M1 amphibolite-facies metamorphism and strong deformational fabric. In situ EMPA total-Pb monazite geochronology from both matrix and porphyroblast inclusion results indicate that the older M1 metamorphism occurred at 643 [+ or -] 9 Ma, consistent with published cooling ages of c. 620 Ma (hornblende) and 580 Ma (mica) obtained from these same rocks. This region thus contains a lithostratigraphic profile and metamorphic history which are unique within the Svalbard Archipelago. Documentation of a pervasive late Neoproterozoic Barrovian metamorphism is difficult to reconcile with a quiescent non-tectonic regime typically inferred for this region, based on the occurrence of rift-drift sequences on the Baltic and Laurentian passive margins. Instead, our new metamorphic age implies an exotic origin of the pre-Devonian basement exposed in SW Spitsbergen and supports models of terrane assembly postulated for the Svalbard Archipelago. Keywords: Svalbard, Caledonides, terranes, geochronology, tectonics.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.04.018 Byline: Konrad J. Kossacki (a), Jacek Leliwa-KopystyAski (a)(b), Slawomira Szutowicz (b) Keywords: Comets; composition; Comets; nucleus; Ices Abstract: This work is intended to investigate the influence of temperature-dependent metamorphism of ice on the shape of small depressions in the surface of cometary nuclei. We are mainly interested in the role of initial cohesivity of a nucleus. For this purpose we simulate sublimation of ice from the facets of initially cylindrical depressions in ice of different initial structure. The simulations account for the diurnal and orbital changes of insolation and its dependence on the current shape of the depressions. Our model includes heat transport in the cometary material and metamorphism of ice. We present the results obtained for the nucleus of the Comet 67P/Churyumov-Gerasimenko, target of the ESA cornerstone mission Rosetta. Author Affiliation: (a) Institute of Geophysics of Warsaw University, Pasteura 7, 02-093 Warsaw, Poland (b) Space Research Center of Polish Academy of Sciences, Bartycka 18a, Warsaw, Poland Article History: Received 26 July 2005; Revised 11 April 2006 Article Note: (miscellaneous) Combi

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2006.02.001 Byline: R.J. Pankhurst (a), C.W. Rapela (b), C.M. Fanning (c), M. Marquez (d) Keywords: Patagonia; Gondwana; Sierras Australes; Cape fold belt; continental collision; U-Pb zircon Abstract: A review of the post-Cambrian igneous, structural and metamorphic history of Patagonia, largely revealed by a five-year programme of U-Pb zircon dating (32 samples), geochemical and isotope analysis, results in a new Late Palaeozoic collision model as the probable cause of the Gondwanide fold belts of South America and South Africa. In the northeastern part of the North Patagonian Massif, Cambro-Ordovician metasediments with a Gondwana provenance are intruded by Mid Ordovician granites analogous to those of the Famatinian arc of NW Argentina; this area is interpreted as Gondwana continental crust at least from Devonian times, probably underlain by Neoproterozoic crystalline basement affected by both Pampean and Famatinian events, with a Cambrian rifting episode previously identified in the basement of the Sierra de la Ventana. In the Devonian (following collision of the Argentine Precordillera terrane to the north), the site of magmatism jumped to the western and southwestern margins of the North Patagonian Massif, although as yet the tectonics of this magmatic event are poorly constrained. This was followed by Early Carboniferous I-type granites representing a subduction-related magmatic are and Mid Carboniferous S-type granites representing crustal anatexis. The disposition of these rocks implies that the North Patagonian Massif was in the upper plate, with northeasterly subduction beneath Gondwana prior to the collision of a southern landmass represented by the Deseado Massif and its probable extension in southeastern Patagonia. This 'Deseado terrane' may have originally rifted off from a similar position during the Cambrian episode. Intense metamorphism and granite emplacement in the upper plate continued into the Early Permian. Known aspects of Late Palaeozoic sedimentation, metamorphism, and deformation in the Sierra de la Ventana and adjacent Cape Fold Belt of South Africa are encompassed within this model. It is also compatible with modern geophysical and palaeomagnetic data that do not support previous hypotheses of southward-directed subduction and collision along the northern limit of Patagonia. Subsequent Permian break-off of the subducted plate, perhaps with delamination of the lower part of the upper plate, allowed access of heat to the overlying Gondwana margin and resulted in voluminous and widespread silicic plutonism and volcanism throughout Permian and into Triassic times. Thus the new model addresses and attempts to explain three long-standing geological enigmas -- the origin of the Gondwanide fold belts, the origin of Patagonia, and the cause of widespread Permian silicic magmatism (Choiyoi province) in southern South America. Differing significantly from previous models, it has new implications for the crustal structure, mineral resources, and plant and animal distribution in this part of Gondwana, since the southern landmass would have had an independent evolution throughout the Early Palaeozoic. Author Affiliation: (a) Visiting Research Associate, NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK (b) Centro de Investigaciones Geologicas, CONICET-UNLP, Calle 1 No. 644, 1900 La Plata, Argentina (c) Research School of Earth Sciences, The Australian National University, Mills Road, Canberra, ACT 200, Australia (d) Universidad Nacional de la Patagonia San Juan Bosco, Departamento de Geologia, Kilometro 4, 9000 Comodoro Rivadavia, Argentina Article History: Received 13 December 2005; Accepted 18 February 2006

U-Pb sensitive high resolution ion microprobe (SHRIMP) zircon geochronology, combined with REE geochemistry, has been applied in order to gain insight into the complex polymetamorphic history of the (ultra) high pressure [(U)HP] zone of Rhodope. Dating included a paragneiss of Central Rhodope, for which (U)HP conditions have been suggested, an amphibolitized eclogite, as well as a leucosome from a migmatized orthogneiss at the immediate contact to the amphibolitized eclogite, West Rhodope. The youngest detrital zircon cores of the paragneiss yielded ca. 560 Ma. This date indicates a maximum age for sedimentation in this part of Central Rhodope. The concentration of detrital core ages of the paragneiss between 670-560 Ma and around 2 Ga is consistent with a Gondwana provenance of the eroded rocks in this area of Central Rhodope. Metamorphic zircon rims of the same paragneiss yielded a lower intercept [sup.206]pb/[sup.238]U age of 148.8 [+ or -] 2.2 Ma. Variable post-148.8 Ma Pb-loss in the outermost zircon rims of the paragneiss, in combination with previous K-Ar and SHRIMP-data, suggest that this rock of Central Rhodope underwent an additional Upper Eocene (ca. 40 Ma) metamorphic/fluid event. In West Rhodope, the comagmatic zircon cores of the amphibolitized eclogite yielded a lower intercept [sup.206]pb/[sup.238]U age of 245.6 [+ or -] 3.9 Ma, which is interpreted as the time of crystallization of the gabbroic protolith. The metamorphic zircon rims of the same rock gave a lower intercept [sup.206]Pb/[sup.238]U age of 51.0 [+ or -] 1.0 Ma. REE data on the metamorphic rims of the zircons from both the paragneiss of Central Rhodope and the amphibolitized eclogite of West Rhodope show no Eu anomaly in the chondrite-normalized patterns, indicating that they formed at least under HP conditions. Flat or nearly flat HREE profiles of the same zircons are con sistent with the growth of garnet at the time of zircon formation. Low Nb and Ta contents of the zircon rims in the amphibolitized eclogite indicate concurrent growth of rutile. Based on the REE characteristics, the 148.8 [+ or -] 2.2Ma age of the garnet kyanite paragneiss, Central Rhodope and the 51.0 [+ or -] 1.0 Ma age of the amphibolitized eclogite, West Rhodope are interpreted to reflect the time close to the (U)HP and HP metamorphic peaks, respectively, with a good approximation. The magmatic zircon cores of the leucosome in the migmatized orthogneiss, West Rhodope, gave a lower intercept [sup.206]pb/[sup.238]U age of 294.3 [+ or -] 2.4 Ma for the crystallization of the granitoid protolith of the orthogneiss. Two oscillatory zircon rims around the Hercynian cores, yielded ages of 39.7 [+ or -] 1.2 and 38.1 [+ or -] 0.8 Ma (2 [sigma] errors), which are interpreted as the time of leucosome formation during migmatization. The zircons in the leucosome do not show the 51 Ma old HP metamorphism identified in the neighboring amphibolitized eclogite, possibly because the two rock types were brought together tectonically after 51 Ma. If one takes into account the two previously determined ages of ca. 73 Ma for (U)HP metamorphism in East Rhodope, as well as the ca. 42 Ma for HP metamorphism in Thermes area, Central Rhodope, four distinct events of (U)HP metamorphism throughout Alpine times can be distinguished: 149, 73, 51 and 42 Ma. Thus, it is envisaged that the Rhodope consists of different terranes, which resulted from multiple Alpine subductions and collisions of micro-continents, rather similar to the presently accepted picture in the Central and Western Alps. It is likely that these microcontinents were rifted off from thinned continental margins of Gondwana, between the African and the European plates before the onset of Alpine convergence.

This paper describes local mass transfer and fluid-rock interaction in epidote-blueschist subfacies basic schist around retrograde dolomite + calcite veins in the Nishisonogi Metamorphic Rocks, southwest Japan. The veins have alteration zones on both sides. The mineralogical and chemical changes during alteration were documented along two sample traverses normal to the veins. Approaching the veins, the peak metamorphic assemblage (winchite + epidote + chlorite + calcite + albite + quartz) has been modified by progressive breakdown of amphibole, epidote, and calcite to illite and dolomite. Mass-balance calculations using the isocon method reveal that [CO.sub.2] and K were added to the basic schists, and Ca was removed during alteration. The mass balance does not hold for these components between the vein + alteration zone and protolith as a whole. These findings suggest that the vein formation and metasomatism represent a process driven by fluid flow and long-distance element transport. Possible reaction paths between the vein-fluids and basic schists were estimated from the progressive change of mineral assemblages in the alteration zones. The phase relations on ionic activity diagrams indicate an increase in [logf.sub.[CO.sub.2]] and decreases in log([a.sub.[Ca.sup.2+]]/[[sigma].sub.[Ca.sup.2]]+[a.sup.2.sub.H+]) and log([a.sub.[Mg.sup.2+]]/[[sigma].sub.[Mg.sup.2+]][a.sup.2.sub.H+]) with progress of the alteration. Although a change in log([a.sub.K+]/[[sigma].sub.K+][a.sup.2.sub.H+]) is poorly constrained on the activity diagrams, the mass balance suggests a concentration gradient of K across the alteration zones. Hence, the reaction paths and mass balance calculations suggest that a combination of the concentration gradients of C[O.sub.2], Ca, Mg, and K produced by the vein-fluids drove the alteration process.