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Now showing items 1 - 16 of 51

  • Focused fluid transfer through the mantle above subduction zones

    Pirard, Cassian   Hermann, Joerg  

    Volcanic arcs above subduction zones are enriched in volatiles and fluid-mobile elements with respect to mid-oceanic ridge basalts. There is general consensus that this particular subduction zone signature is generated by fluid-induced extraction of these elements from subducted oceanic crust and its sedimentary cover. However, how these fluids are transferred through the mantle wedge to the locus of partial melting and what modification the fluids will experience is unresolved. Here we investigate the interaction of slab fluids with the mantle wedge through a series of high-pressure experiments. We explore two end-member processes of focused and porous reactive flow of hydrous slab melts through the mantle. Transfer by porous flow leads to the formation of hydrous minerals that sequester fluid-mobile elements and residual fluids characterized by trace element patterns inconsistent with typical arc lavas. In contrast, no hydrous minerals are formed in the reaction zone of experiments mimicking focused flow, and the typical trace element signature acquired during fluid extraction from the slab is preserved, indicating that this is an efficient process for element transfer through the mantle wedge.
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  • Age and thermal history of Eo- and Neohimalayan granitoids, eastern Himalaya

    Aikman, Amos B.   Harrison, T. Mark   Hermann, Joerg  

    Four distinct granitoid suites outcrop across a N-S transect in the eastern Himalaya between the Main Central Thrust (MCT) and the Indus Tsangpo suture. The Arunachal and Tsona leucogranites outcrop, respectively, in the MCT hanging-wall and adjacent the South Tibetan Detachment. Both crystallized at about 19 Ma, and appear to be lateral equivalents of the High Himalayan Leucogranites. In the central Tethyan Himalaya, the Dala granitoids form a suite of undeformed plutons emplaced into deformed Tethyan Himalayan Sequence metasediments. In the northern Tethyan Himalaya, the Yala-Xiangbo granitoids are part of an igneous complex in the core of the Yala-Xiangbo dome. Both the Dala and Yala-Xiangbo granitoids have Eocene magmatic ages (44.1 +/- 1.2 Ma and 42 +/- 5 Ma, respectively) that are notably older than the vast majority of post-collisional granitoids along the main Himalayan Arc. Pelitic units from the core of the Yala-Xiangbo dome record two episodes of amphibolite-grade metamorphism and monazite growth coinciding with well-documented episodes of Eohimalayan and Neohimalayan metamorphism. The thermochronological histories of the Data and Yala-Xiangbo units suggest that the eastern Tethyan Himalaya was tectonically quiescent from ca. 40 Ma until ca. 15 Ma. Miocene exhumation of the Dala and Yala-Xiangbo units, and all North Himalayan Domes studied to date, appears best explained by a structural control, such as concurrent north-directed motion on the Great Counter Thrust. We hypothesize that an extension of the Ninety East Ridge may have been responsible for Eocene high heat-flow beneath the eastern Himalaya and explain the localization of magmatism there. (C) 2012 Elsevier Ltd. All rights reserved.
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  • Fluid/mineral interaction in UHP garnet peridotite RID H-1802-2011

    Malaspina, Nadia   Hermann, Joerg   Scambelluri, Marco  

    We present two case studies of metasomatised garnet peridotite from the Sulu (Zhimafang) and of garnet orthopyroxenite from the Dabie Shan (Maowu) ultrahigh-pressure terranes (Eastern China). The mantle-derived peridotite from Zhimafang shows two ultrahigh-pressure (UHP) mineral assemblages. The older one is made of porphyroclastic garnet rich in inclusions (Grt(1)), coarse exsolved clinopyroxene (Cpx(1)) and coarse phlogopite flakes (Ph(1)). The younger paragenesis consists of fine-grained olivine+clinopyroxene (CPx(2))+orthopyroxene +/- magnesite +/- Phl(2) equilibrated with neoblastic garnet (Grt(2)). The inclusions inside porphyroclastic Grt(1) are polyphase secondary inclusions related to microfractures cutting the garnet core. They display irregular shapes and contain microcrystals of calcic-amphibole, chlorite, phlogopite and rare talc, associated with pyrite and/or spinel. The low Al(2)O(3) content(< 0.2 wt.%) in orthopyroxene coexisting with garnets and clinopyroxenes indicates equilibration at P=4.0-6.0 GPa and T=700-1000 degrees C. The trace element composition of Cpx(1) and Phl(1) combined with the petrologic and isotopic data of Yang and Jahn [Yang, J.J., Jahn, B.M., 2000. Deep subduction of mantle-derived garnet peridotites from the Su-Lu UHP metamorphic terrane in China. Journal of Metamorphic Geology 18,167-180.] suggests that the Zhimafang garnet peridotite experienced metasomatism by a melt with alkaline character at high-temperature conditions (T similar to 1000 degrees C and P > 5.0 GPa). The microtextural identification of pseudosecondary inclusions in the porphyroclastic garnet core and their geochemical characterisation indicate that an incompatible element-and silicate-rich fluid subsequently metasomatised the garnet peridotite and equilibrated with the newly formed Cpx(2) probably during Triassic UHP metamorphism. Ultramafic metasomatic layers at Maowu Ultramafic Complex (Dabie Shan) consist of layered websterite and orthopyroxenite which preserve an old olivine+orthopyroxene (Opx(1))+garnet (Grt(1))+/- Ti-clinohumite paragenesis, overgrown by poikilitic Opx(2). Grt(2) is associated with Opx(2)+phlogopite along the foliation, and fine-grained idiomorphic clinopyroxene also occurs. Grt(2) cores contain disseminated primary polyphase inclusions. The textural and geochemical analyses of the primary polyphase inclusions indicate that they derive from a homogeneous fluid characterised by high LILE concentrations with spikes in Cs. Ba. Pb and high U/Th. These inclusions are interpreted as remnants of the LILE- and LREE-enriched residual fluid produced when a crust derived Si-rich metasomatic agent reacted with a previous harzburgite to form garnet orthopyroxenite. The in-situ trace element analyses of the major phases garnet, clinopyroxene and phlogopite that formed at the same time as the polyphase inclusions at Maowu, permit the determination of empirical mineral/fluid partitioning at pressures relevant for element recycling in subduction zones. Our estimated D(Cpx/fluid) suggests that all LILE are highly incompatible, Th and U are moderately incompatible, Pb is close to unity and Sr is moderately compatible. Phlogopite preferentially incorporates Rb and K with respect to Ba and Cs, and Th with respect to U. The similarity between the residual Maowu fluid with the secondary inclusions in the UHP wedge-type garnet peridotite from Sulu, indicates tht the fluids produced from reactions at the slab-mantle interface may be effective metasomatic agents in the mantle wedge. Such reactions may produce phlogopite, which plays an important role in controlling the LILE characteristics of the slab-derived fluid in subduction zones. (C) 2008 Elsevier B.V. All rights reserved.
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  • Diffusion of (40)Ar in muscovite RID H-1802-2011

    Harrison, T. Mark   Celerier, Julien   Aikman, Amos B.   Hermann, Joerg   Heizler, Matthew T.  

    Hydrothermal treatment of closely sized muscovite aggregates in a piston-cylinder apparatus induced (40)Ar* loss that is revealed in (40)Ar/(39)Ar step heating spectra. Age spectra and Arrhenius data, however, differ from that expected from a single diffusion length scale. A numerical model of episodic loss assuming the presence of multiple diffusion domains yields excellent fits between synthetic and actual degassing spectra. We used this model to isolate (40)Ar* loss from the grains that remained intact during hydrothermal treatment at 10 kbar permitting calculation of diffusion coefficients in the temperature range 730-600 degrees C. Diffusion data generated in this manner yield an activation energy (F) of 63 +/- 7 kcal/mol and frequency factor (D(o)) of 2.3 (+70)(2.2) cm(2)/s. Experiments at 20 kbar yield diffusivities lower by about all order of magnitude and correspond to an activation Volume of similar to 14 cm(3)/mol. Together, these parameters predict substantially greater retentivity of Ar in muscovite than previously assumed and correspond to a closure temperature (T(c)) of 425 degrees C for a 100 mu m radius grain cooling at 10 degrees C/Ma at 10 kbar (T(c)-405 degrees C at 5 kbar. Age and log (r/r(o)) spectra for the run products show strong correlations indicating that muscovites can retain Ar diffusion boundaries and mechanisms that define their natural retentivity during vacuum step heating. This may permit the application of high resolution, continuous (40)Ar/(39)Ar thermochronology to low grade, regionally metamorphosed terranes. (C) 2008 Elsevier Ltd. All rights reserved.
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  • Partial melting of lower crust at 10-15 kbar: constraints on adakite and TTG formation

    Qian, Qing   Hermann, Joerg  

    The pressure-temperature (P-T) conditions for producing adakite/tonalite-trondhjemite-granodiorite (TTG) magmas from lower crust compositions are still open to debate. We have carried out partial melting experiments of mafic lower crust in the piston-cylinder apparatus at 10-15 kbar and 800-1,050 A degrees C to investigate the major and trace elements of melts and residual minerals and further constrain the P-T range appropriate for adakite/TTG formation. The experimental residues include the following: amphibolite (plagioclase + amphibole +/- A garnet) at 10-15 kbar and 800 A degrees C, garnet granulite (plagioclase + amphibole + garnet + clinopyroxene + orthopyroxene) at 12.5 kbar and 900 A degrees C, two-pyroxene granulite (plagioclase + clinopyroxene + orthopyroxene +/- A amphibole) at 10 kbar and 900 A degrees C and 10-12.5 kbar and 1,000 A degrees C, garnet pyroxenite (garnet + clinopyroxene +/- A amphibole) at 13.5-15 kbar and 900-1,000 A degrees C, and pyroxenite (clinopyroxene + orthopyroxene) at 15 kbar and 1,050 A degrees C. The partial melts change from granodiorite to tonalite with increasing melt proportions. Sr enrichment occurs in partial melts in equilibrium with < 20 wt% plagioclase, whereas depletions of Ti, Sr, and heavy rare earth elements (HREE) occur relative to the starting material when the amounts of residual amphibole, plagioclase, and garnet are > 20 wt%, respectively. Major elements and trace element patterns of partial melts produced by 10-40 wt% melting of lower crust composition at 10-12.5 kbar and 800-900 A degrees C and 15 kbar and 800 A degrees C closely resemble adakite/TTG rocks. TiO2 contents of the 1,000-1,050 A degrees C melts are higher than that of pristine adakite/TTG. In comparison with natural adakite/TTG, partial melts produced at 10-12.5 kbar and 1,000 A degrees C and 15 kbar and 1,050 A degrees C have elevated HREE, whereas partial melts at 13.5-15 kbar and 900-1,000 A degrees C in equilibrium with > 20 wt% garnet have depressed Yb and elevated La/Yb and Gd/Yb. It is suggested that the most appropriate P-T conditions for producing adakite/TTG from mafic lower crust are 800-950 A degrees C and 10-12.5 kbar (corresponding to a depth of 30-40 km), whereas a depth of > 45-50 km is unfavorable. Consequently, an overthickened crust and eclogite residue are not necessarily required for producing adakite/TTG from lower crust. The lower crust delamination model, which has been embraced for intra-continental adakite/TTG formation, should be reappraised.
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  • Sediment melts at sub-arc depths: An experimental study RID H-1802-2011

    Hermann, Joerg   Spandler, Carl J.  

    The phase and melting relations in subducted pelites have been investigated experimentally at conditions relevant for slabs at sub-arc depths (T 600-1050 degrees C, P 2.5-4.5 GPa). The fluid-present experiments produced a dominant paragenesis consisting of garnetphengiteclinopyroxenecoesitekyanite that coexists with a fluid phase at run conditions. Garnet contains detectable amounts of Na2O (up to 05 wt%), P2O5 (up to 056 wt%), and TiO2 (up to 09 wt) in all experiments. Phengite is stable up to 1000 degrees C at 4.5 GPa and is characterized by high TiO2 contents of up to 2 wt%. The solidus has been determined at 700 degrees C, 25 GPa and is situated between 700 and 750 degrees C at 3.5 GPa. At 800 degrees C, 4.5 GPa glass was present in the experiments, indicating that at such conditions a hydrous melt is stable. In contrast, at 700 degrees C, 3.5 and 4.5 GPa, a solute-rich, non-quenchable aqueous fluid was present. This indicates that the solidus is steeply sloping in P-T space. Fluid-present (vapour undersaturated) partial melting of the pelites occurs according to a generalized reaction phengite + omphacite + coesite + fluid = melt + garnet. The H2O content of the produced melt decreases with increasing temperature. The K2O content of the melt is buffered by phengite and increases with increasing temperature from 25 to 10 wt, whereas Na2O decreases from 7 to 23 wt. Hence, the melt compositions change from trondhjemitic to granitic with increasing temperature. The K2O/H2O increases strongly as a function of temperature and nature of the fluid phase. It is 0.0004-0.002 in the aqueous fluid, and then increases gradually from about 0.1 at 750-800 degrees C to about 1 at 1000 degrees C in the hydrous melt. This provides evidence that hydrous melts are needed for efficient extraction of K and other large ion lithophile elements from subducted sediments. Primitive subduction-related magmas typically have K2O/H2O of similar to 0.1-0.4, indicating that hydrous melts rather than aqueous fluids are responsible for large ion lithophile element transfer in subduction zones and that top-slab temperatures at sub-arc depths are likely to be 700-900 degrees C.
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  • Nanoparticles and nanostructures formed by laser: what can we learn from the modeling?

    Itina, Tatiana E.   Bouflous, Nawfel   Hermann, Joerg   Delaporte, Philippe  

    The processes involved in nanoparticle and nanostructure formation by laser are analyzed. Relative contributions of several mechanisms involved are compared. First, we consider the formation of "primary" particles and discuss the difference between femtosecond and nanosecond regimes. Then, "secondary" particle/aggregate formation is discussed. In particular, attention is focused on (i) direct cluster ejection from a target under rapid laser interaction; (ii) condensation/evaporation; (iii) fragmentation/aggregation processes during cluster diffusion; (iv) diffusion, aggregation, and/or coalescence. In addition, routes of control over particle size distribution are proposed. Possibility of formation of colloidal nanoparticles with very narrow size distribution is proven numerically. The role of such parameters as ablation yield, laser wavelength and laser fluence, and surface tension are examined. Finally, controlled nanoparticle self-assembly is discussed as a potential technique for future development of nanomaterials.
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  • Ediacaran 2,500-km-long synchronous deep continental subduction in the West Gondwana Orogen

    Ganade de Araujo, Carlos E.   Rubatto, Daniela   Hermann, Joerg   Cordani, Umberto G.   Caby, Renaud   Basei, Miguel A. S.  

    The deeply eroded West Gondwana Orogen is a major continental collision zone that exposes numerous occurrences of deeply subducted rocks, such as eclogites. The position of these eclogites marks the suture zone between colliding cratons, and the age of metamorphism constrains the transition from subduction-dominated tectonics to continental collision and mountain building. Here we investigate the metamorphic conditions and age of high-pressure and ultrahigh-pressure eclogites from Mali, Togo and NE-Brazil and demonstrate that continental subduction occurred within 20 million years over at least a 2,500-km-long section of the orogen during the Ediacaran. We consider this to be the earliest evidence of large-scale deep-continental subduction and consequent appearance of Himalayan-scale mountains in the geological record. The rise and subsequent erosion of such mountains in the Late Ediacaran is perfectly timed to deliver sediments and nutrients that are thought to have been necessary for the subsequent evolution of sustainable life on Earth.
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  • Organic and inorganic materials analysis by Laser-Induced Breakdown Spectroscopy

    Brygo, Francois   Hermann, Joerg  

    The present study deals with calibration-free analysis of materials by Laser-Induced Breakdown Spectroscopy (LIBS). A numerical code computes the spectral radiance emitted from a plasma in local thermal equilibrium. The numerical code includes the calculation of the plasma composition by solving the system of Saha-equations, mass conservation and neutrality equation. The chemical reactions within the plasma plume are considered, especially in case of organic materials ablation. The line intensities are then computed. In the present study, the lines are considered to be optically thin. The modeling of the chemical composition of an organic material and a steel sample is presented. Comparing the experimental spectra to the computed ones, it was possible to measure the elemental concentrations of the steel with good accuracy without any requirement of preliminary calibration.
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  • Mechanisms of small clusters production by short and ultra-short laser ablation

    Itina, Tatiana E.   Gouriet, Karine   Zhigilei, Leonid V.   Noel, Sylvie   Hermann, Joerg   Sentis, Marc  

    The mechanisms involved into the formation of clusters by pulsed laser ablation are studied both numerically and experimentally. To facilitate the model validation by comparison with experimental results, the time and length scales of the simulation are considerably increased. This increase is achieved by using a combination of molecular dynamics (MD) and the direct simulation Monte Carlo (DSMC) methods. The combined MD-DSMC model is then used to compare the relative contribution of the two channels of the cluster production by laser ablation: (i) direct cluster ejection upon the laser-material interaction, and (ii) collisional sticking and aggregation in the ablated gas flow. Calculation results demonstrate that both of these mechanisms play a role. The initial cluster ejection provides cluster precursors thus eliminating the three-body collision bottleneck in the cluster growth process. The presence of clusters thus facilitates the following collisional condensation and evaporation processes. The rates of these processes become considerable, leading to the modification of not only the plume cluster composition, but also the dynamics of the plume expansion. Calculation results explain several recent experimental findings. (c) 2007 Elsevier B.V. All rights reserved.
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  • Investigation of nanoparticle generation during femtosecond laser ablation of metals

    Noel, Sylvie   Hermann, Joerg   Itina, Tatiana  

    The production of nanoparticles via femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume. (C) 2007 Elsevier B.V. All rights reserved.
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  • Fractionation of Nb and Ta by biotite and phengite: Implications for the "missing Nb paradox"

    Stepanov, Aleksandr S.   Hermann, Joerg  

    The subchondritic Nb/Ta in both the continental crust and the depleted mantle remains enigmatic and is called the "missing Nb paradox." We present partitioning data between biotite and granitic melt for experimental and natural samples that provide evidence that Nb is compatible in biotite and phengite. Nb can thus be enriched in the residue during partial melting of crustal rocks. Additionally, biotite and phengite in equilibrium with granitic melts preferentially incorporate Nb over Ta. Therefore incipient partial melting of biotite-rich crustal rocks produces restites with high Nb/Ta. Progressive melting of such rocks leads to the consumption of biotite and the formation of peritectic rutile or ilmenite, which retain the high-Nb/Ta signature. We suggest that such mid to lower crustal granulites could represent an important Nb-rich reservoir with high Nb/Ta. Similarly, high-Ti phengite that is present in deeply subducted sediments preferentially incorporates Nb over Ta. High-pressure incipient partial melting in the presence of residual phengite thus also produces restites with high Nb/Ta that could be subducted to the deeper mantle.
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  • On the evolution of orogens: Pressure cycles and deformation mode switches RID H-1802-2011

    Beltrando, Marco   Hermann, Joerg   Lister, Gordon   Compagnoni, Roberto  

    Evidence of two burial-exhumation cycles that took place during a single orogeny has been found in rocks belonging to the Piemonte unit of the Western Alps. An early high pressure event, which resulted from tectonic burial down to pressures of 1.5 GPa, was followed by exhumation to ca. 0.20-0.35 GPa as a result of extensional deformation. Renewed shortening culminated in a second burial episode down to pressures of 0.65-0.80 GPa, before the final exhumation took place. Existing geochronological data allow only ca. 13-19 Ma for the completion of both burial-exhumation cycles. Therefore, we suggest that the evolution of orogens is characterized by multiple short-lived burial-exhumation cycles related to orogen-scale alternance between shortening and extensional deformation. (C) 2007 Elsevier B.V. All rights reserved.
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  • Coupled inter-site reaction and diffusion:Rapid dehydrogenation of silicon vacancies in natural olivine

    Jollands, Michael C.   Kempf, Elias   Hermann, Joerg   Muentener, Othmar  

    Natural olivine from Zermatt (Switzerland) and Almklovdalen (Norway) were dehydrogenated at 1 bar, 517-1009 degrees C at various oxygen fugacity conditions. Following experiments, H contents (either bulk, or core-rim profiles) were measured using Fourier transform infrared spectroscopy, and spectra were resolved into Gaussian peaks. The starting olivine contained similar to 150 and similar to 10 wt. ppm H2O for the Zermatt and Almklovdalen samples respectively, but importantly the initial H distribution in both corresponded to defects with 4H(+) occupying Si vacancies. Experiments in pure forsterite Padron-Navarta Hermann,(2017) showed that this defect diffuses very slowly relative to other H diffusion mechanisms in olivine. Conversely, we show that, in the Almklovdalen samples, H loss from the Si-vacancy defect can be extremely rapid, and approaches the fastest known mechanism of H diffusion (proton-polaron diffusion) in olivine. The rate of dehydrogenation from the Zermatt olivine is slightly slower, more consistent with the diffusivity of hydrogenated M-site vacancies. The sum of all defects, along with the integrated area of the main Si vacancy peak (3612 cm(-1)) generally shows simple diffusive behaviour, whereas profiles (or maps) resolved into individual peaks reveal complex profile shapes not consistent with a simple out-diffusion mechanism. This behaviour can be modeled as a combination of inter-site reaction and diffusion, whereby H leaves the tetrahedral site, moves into a faster diffusion pathway, then rapidly exits the crystal. The rate of H loss from, or gain into, olivine can therefore be either diffusion limited, reaction limited, or a combination of the two. This may explain current discrepancies between experiments conducted under different conditions and between experimental and natural data, including the recent observation that H stored in Si vacancies in metamorphic olivine may be retentive over millions of years, despite the capability to diffuse rapidly via a different mechanism. (C) 2019 Elsevier Ltd. All rights reserved.
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  • The infrared signature of water associated with trivalent cations in olivine RID G-2302-2011 RID H-1802-2011

    Berry, Andrew J.   O'Neill, Hugh St. C.   Hermann, Joerg   Scott, Dean R.  

    Forsterite crystals were synthesised under water saturated conditions at 1400 degrees C and 1.5 GPa doped with trace amounts of either B, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ga, Y, Zr, In, Sm, Gd, Dy, Tm, or Lu. The common and intense hydroxyl stretching bands in the infrared spectra of spinel peridotite olivine, at 3572 and 3525 cm(-1), were only reproduced in the presence of Ti. Those samples where the trace element substitutes as the trivalent cation on the Mg2+ site were identified from a systematic variation in concentration with the trivalent ionic radius. The hydroxyl region of all samples is essentially identical except for between 3300 and 3400 cm(-1). This region is characterised by one or more bands, with the energy of the most intense feature being correlated with the ionic radius of the trivalent cation. The integrated intensity of these hydroxyl bands also correlates with the concentration of the trivalent cation. These correlations provide unambiguous evidence that bands, or peaks, in this region correspond to water at defect sites associated with trivalent cations. "Trivalent peaks" are sometimes observed in samples of mantle olivine and most likely indicate water associated with Fe3+. The water at this site is not incorporated under normal mantle conditions and should not be included in estimates of the water capacity of mantle olivine. These results emphasise the importance of identifying the infrared signature of different water substitution mechanisms. (C) 2007 Elsevier B.V All rights reserved.
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  • Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust RID H-1802-2011

    Namur, Olivier   Charlier, Bernard   Pirard, Cassian   Hermann, Joerg   Liegeois, Jean-Paul   Vander Auwera, Jacqueline  

    The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100-500 m-thick) and (2) as cm-to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe-Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An(68)-An(60)), trace elements (Sr: 1023-1071 ppm; Ba: 132-172 ppm) and Sr isotopic ratios ((87)Sr/(86)Sr(i): 0.70356-0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An(60)-An(38)). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75 g/cm(3) during early fractionation of troctolites and then decreased continuously to 2.16 g/cm(3) with fractionation of Fe-Ti oxide-bearing gabbros. Plagioclase (An(69)-An(60)) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (
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