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Now showing items 1 - 8 of 8

  • 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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  • Focused fluid transfer through the mantle above subduction zones

    Pirard, Cassian   Hermann, Joerg  

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  • Experimentally determined stability of alkali amphibole in metasomatised dunite at sub-arc pressures

    Pirard, Cassian   Hermann, J?rg  

    The phase and melting relationships of olivine mixed with 25 % of hydrous felsic slab melt have been determined in piston-cylinder experiments between 2.5 and 4.5 GPa and 800 to 1,050 degrees C to constrain metasomatic processes in the mantle wedge above subduction zones. At sub-solidus conditions, olivine, orthopyroxene, phlogopite, a Na-rich amphibole and an aqueous fluid are present. Na-rich amphibole is still observed at 950 degrees C at 4.5 GPa, providing evidence that this hydrous phase might be stable at sub-arc depths in an alkali-rich, Ca-poor mantle wedge. The maximum temperature stability is reached at 1,000 degrees C at 3.5 GPa, where amphibole coexists with hydrous melt. A sodium-rich phlogopite is stable over the whole range of P-T conditions investigated. At 2.5 GPa, 850 degrees C, aspidolite (Na analogue of phlogopite) has been observed as a sodium-bearing phase in the peridotite. The wet solidus in the metasomatised dunite lies between 850 and 900 degrees C at 2.5 GPa and between 950 and 975 degrees C at 3.5 GPa. At 4.5 GPa, melting relations are ambiguous and no clear solidus was found. The consumption of amphibole and minor phlogopite at the wet solidus produced Na- and H2O-rich phonolitic melts. The presence of phlogopite and sodic amphibole in the metasomatised dunite has implications on alkali and water storage in the part of the mantle wedge that is coupled to the down-going slab and might play a role on alkali and trace element recycling through arc magmatism.
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  • The sulfides and selenides of the Musonoi mine, Kolwezi, Katanga, Democratic Republic of Congo

    Pirard, Cassian   Hatert, Frederic  

    An investigation of ore minerals from the Thrust-Slice "2400"' of the Musonoi Extension mine, Katanga, Democratic Republic of Congo, has provided new insights into the origin of copper sulfides and selenides. The early Sulfides are pyrite, covellite, spionkopite, and digenite, which crystallize, with authigenic quartz, during diagenesis. Metasomatic enrichment ill U and Se, which is responsible for the unusual mineralogy of the Musonoi deposit, firstly gave rise to the crystallization of Cu-Pd-bearing trogtalite and various palladium selenides (oosterboschite, verbeekite and unidentified Pd-Cu-Pt-Se phases). Subsequently, Se-rich copper minerals, berzelianite and Se-bearing carrollite formed, followed by the crystallization of Se-bearing digenite. Temperature estimates based on these assemblages indicate at least 200 degrees C. Meteoric alteration affected these primary minerals and mainly resulted in the leaching of copper. Replacement phases are spionkopite, yarrowite, and athabascaite, which formed at temperatures below 100 degrees C. Electron-microprobe analyses show the existence of a complete solid-solution series between high digenite and berzelianite. The variation of the a unit-cell parameter along this solid solution follows the equation y = -0.3446x(3) + 0.5485x(2)-0.0358x + 5.5758, R-2 = 0.96. A similar mechanism of S-for-Se substitution accounts for the precipitation of Se-bearing spionkopite and S-bearing athabascaite. These observations provide new data on the sequence of transformation affecting copper sulfides and selenides at low-temperature, metoric conditions.
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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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  • Petrology and Geochemistry of the Crust-Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific

    Pirard, Cassian   Hermann, Joerg   O'Neill, Hugh St C.  

    The Massif du Sud ophiolite, New Caledonia, SW Pacific, is one of the largest exposed ultramafic bodies on Earth. The ophiolite consists of a mantle section of ultra-depleted tectonite harzburgite, overlain by a large dunite zone, which is separated by a transition zone from the gabbros at the top of the massif. Profiles through the stratigraphy of the mantle section show complex geochemical and petrological variations. The harzburgites are characterized by a high degree of partial melting and document a complex evolution from mantle exhumation towards a supra-subduction zone environment. Olivine and spinel compositions suggest that the harzburgites are residual after boninite-like melt extraction at 5-10 kbar. The lower dunite zone is analogous to the replacive dunite channels in the harzburgite, in that it is formed by pyroxene consumption. In contrast, the upper dunite zone has chemical and textural characteristics of cumulus olivine crystallized from primitive mafic melts. The upper dunite zone grades into a transition zone with pyroxenite cumulates that are intruded by gabbro sills. The crystallization sequence and mineral compositions indicate that pyroxenites and gabbros formed from hydrous, oxidized primitive basaltic magmas at similar to 1250 degrees C and 2-4 kbar. Their geochemistry indicates that these parental melts are transitional between boninites and primitive arc magmas and carry a fore-arc basalt signature. The Massif du Sud therefore represents a crust-mantle section in a nascent arc. The cryptic transition between residual mantle rocks and crustal cumulates highlights the difficulty of estimating the thickness and average composition of an arc crust by seismic methods.
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  • The zircon record of high-pressure metasedimentary rocks of New Caledonia:Implications for regional tectonics of the south-west Pacific

    Pirard, Cassian   Spandler, Carl  

    We report the U-Pb age, and trace element and hafnium isotope composition of zircons recovered from clastic metasedimentary rocks that span a range of metamorphic grades (prehnite-pumpellyite to eclogite facies) across the high-pressure metamorphic belt of northern New Caledonia. We use these data to evaluate the sedimentary source and environment of formation of these rocks, as well as their respective metamorphic evolution. Metasediments from the low-grade Koumac and blueschist-facies Diahot sequences contain zircon with age populations of 1800-1500 Ma, 700-500 Ma, 300-250 Ma and 140-80 Ma. These grains have highly variable Hf isotope compositions (epsilon Hf - 36 to + 12), and some have thin metamorphic rim zones dated to similar to 38 Ma. Zircon grains from the high-grade metamorphic section (Pouebo Eclogite Melange) are mostly Upper Cretaceous and lowermost Eocene in age, with similar to 40 Ma metamorphic overgrowth zones. The Upper Cretaceous zircons from across the belt have similar isotopic compositions, whereas the lowermost Eocene zircons have relatively high epsilon Hf of + 6 to + 13. The zircon record of the eclogites often lacks the diverse old detrital signature that is prevalent in the blueschist-facies rocks; rather the age populations of the eclogite grade zircons reflect derivation from nearby volcanic sources. We suggest that the bulk of Koumac and Diahot sequences represent the erosional products of the Norfolk Ridge that were transported eastwards and accumulated into the East New Caledonia Basin. Paleozoic and Precambrian zircon grains were originally sourced from Paleozoic orogenic belts of the eastern margin of Australia, and possibly from now-submerged ridges of continental character. We suggest that the Mesozoic zircon grains derived from volcanic arc activity on the eastern margin of Gondwana. Extension led to a reduced continental sediment input into the East New Caledonia Basin during the Cretaceous, which explains the limited amounts of ancient detritus in the mid-Cretaceous to Eocene sedimentary protoliths of the Pouebo Eclogite Melange. The combined Hf isotope and age data for our zircon record point to the existence of a west-dipping subduction zone active throughout the Late Triassic to the Late Cretaceous, possibly with some short-lived periods of volcanic hiatus. In the early Eocene, a new east-dipping subduction formed following a major change in plate kinematics, producing younger metavolcanic zircons and eventually leading to the metamorphism of sedimentary units in the middle Eocene. (C) 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
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  • The zircon record of high-pressure metasedimentary rocks of New Caledonia: Implications for regional tectonics of the south-west Pacific

    Pirard, Cassian   Spandler, Carl  

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