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Marguerite Godard - One of the best experts on this subject based on the ideXlab platform.
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B, Sr and Pb isotope geochemistry of high-pressure Alpine metaperidotites monitors fluid-mediated element recycling during serpentinite dehydration in subduction melange (Cima di Gagnone, Swiss Central Alps)
Geochimica et Cosmochimica Acta, 2015Co-Authors: E. Cannao, S. Agostini, M. Scambelluri, S. Tonarini, Marguerite GodardAbstract:Tectonic mixing of slab- and mantle-derived materials at the interface between converging plates highly enhances fluid-mediated mass transfer from the slab to the overlying mantle. Subduction mélanges can provide information about the interaction among different slices accreted at plate interface domains, with implications on the tectonic and geochemical evolution of the plate-interface itself. At Cima di Gagnone, pelitic schists and gneiss enclose chlorite harzburgite and garnet peridotite lenses, like in subduction mélanges located in-between downgoing slabs and overlying mantle. These peridotites host MORB-type eclogite and metarodingite, and derive from dehydration of serpentinized mantle protoliths. Their enrichment in fluid-mobile B, As, Sb, U, Th is the result of an early-stage oceanic serpentinization, followed by interaction with host Metasediments during subduction burial. Here we define the element exchange process in the Gagnone mélange by means of the B, Sr and Pb isotope analysis of its main lithologies (ultramafic, mafic rocks and paragneiss). The 87Sr/86Sr and 206Pb/204Pb ratios of ultramafic rocks (0.7090–0.7124 and 18.292–18.837, respectively) show enrichments in radiogenic Sr and Pb after exchange with the host paraschist (up to 0.7287 87Sr/86Sr; 18.751 206Pb/204Pb). The δ11B values of peridotites (down to −10‰) point to a combined effect of (1) 11B release to deserpentinization fluids (serpentinized protoliths likely had positive δ11B and lower radiogenic Sr, Pb), and of (2) exchange with fluids from the surrounding Metasediments. The whole Gagnone rock-suite is finally overprinted by retrograde fluids that essentially bring to an increase in radiogenic Pb (about 19.0 206Pb/204Pb) and to values of 0.710 87Sr/86Sr and of −10‰ δ11B. The recognition of different stages of interaction between mantle rocks and sedimentary/crustal reservoirs allows us to define the geochemical effects related to the early coupling of such rocks along the plate-interface. Our study shows that ultramafic rocks involved in subduction-zone metamorphism and serpentinization uptake radiogenic Pb and Sr released by associated sedimentary reservoirs. The exchange process envisioned here is not only representative of subduction mélanges: it can also be a proxy of mass transfer between slab and serpentinized supra-subduction mantle, as occurs in forearcs. Dehydration of the Gagnone-type serpentinized mantle releases crust-derived components to arcs, without direct involvement of Metasediment dehydration and/or melting in subarc environments. The retention of appreciable amounts of fluid-mobile elements, radiogenic Pb and Sr in dehydrated Gagnone peridotites has implications on element recycling in the deep Earth’s mantle.
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high pressure serpentinites a trap and release system controlled by metamorphic conditions example from the piedmont zone of the western alps
Chemical Geology, 2013Co-Authors: Romain Lafay, Stéphane Guillot, Marguerite Godard, Stephane Schwartz, Baptiste Debret, Fabien Deschamps, Christian NicolletAbstract:We provide new insights into the geochemistry of serpentinites from the Alpine orogenic wedge representing a paleo-subduction zone. These serpentinites are derived from similar oceanic protoliths, but they have experienced different metamorphic conditions related to three different structural levels of the paleo-subduction zone ((1) obducted: Chenaillet ophiolite, (2) accretionary wedge: Queyras Schistes lustres complex and (3) serpentinite channel: Monviso ophiolite). Metamorphism undergone by these three units is well defined, increasing eastward from sub-greenschist to eclogite facies conditions, and allows us to examine trace element behavior from the oceanic ridge environment to subduction. Serpentinites first record moderate trace element enrichment due to seawater interaction resulting in the replacement of olivine and pyroxene by chrysotile and lizardite below 300 °C. In the sediment-dominated accretionary wedge, serpentinites are strongly enriched in fluid-mobile-elements (B, Li, As, Sb, and Cs) and act as a trapping system following the metamorphic gradient (from 300 to 390 °C) up to total replacement of the lizardite/chrysotile assemblage by antigorite. Under higher temperature conditions (T > 390 °C), no enrichment was observed, and some fluid-mobile elements were released (B, Li, Cs, and Sr). Moreover, in the serpentinite channel (T > 460 °C), most of the fluid-mobile elements are absent due to the scarcity of Metasediments which prevent geochemical exchange between Metasediments and serpentinites. This is also due to the onset of antigorite breakdown and the release of fluid-mobile elements. Thus, we emphasize that the geochemistry of Alpine serpentinites is strongly dependent on (1) the grade of metamorphism and (2) the ability of Metasediments to supply fluid-mobile elements. We conclude that serpentinites act as a trap-and-release system for fluid-mobile elements in a subduction context.
Axel Gerdes - One of the best experts on this subject based on the ideXlab platform.
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subduction factory in an ampoule experiments on sediment peridotite interaction under temperature gradient conditions
Geochimica et Cosmochimica Acta, 2018Co-Authors: Alan B Woodland, V K Bulatov, Gerhard P Brey, A V Girnis, Heidi E Hofer, Axel GerdesAbstract:Abstract To better understand processes above subducted oceanic slabs, we have undertaken experiments with juxtaposed sediment and peridotite layers at pressures of 7.5 and 10.5 GPa at a controlled temperature gradient from ∼100 to ∼500 °C per a sample length of ∼3 mm. The sediment starting material contains H 2 O (6.9 wt%) and CO 2 (5.9 wt%) and has a major-element composition similar to GLOSS (Plank and Langmuir, 1998) doped with trace elements at 10–100 ppm levels. Several experiments were conducted with ∼0.5 wt% Cl or F. The peridotite layer is composed of natural olivine (66 wt%), orthopyroxene (27 wt%) and garnet (7 wt%) mixed with ∼15 wt% graphite. Several experimental configurations were investigated, but the “basic” setup has the sediment layer at the bottom in the cold zone (400–1200 °C) overlain by peridotite at 900–1500 °C. The temperature distribution was determined by two thermocouples and orthopyroxene–garnet thermometry. Features common to many experiments are (1) the development of multiple layers of various lithologies and a pool of hydrous silicate or carbonate–silicate melt in the hottest part of the capsule; (2) replacement of olivine by orthopyroxene in the metaperidotite; (3) preservation and growth of garnet and local development of magnesite in the metaperidotite layer; (4) enrichment in garnet within the Metasediment layer at the contact with the metaperidotite; (5) formation of a clinopyroxene–garnet assemblage at the bottom (the coldest part); (6) presence of K-bearing phases (phlogopite or phengite) and carbonates in the Metasediment layer only at temperatures 2 compared to the starting composition, and the sediment gains MgO, FeO and Cr 2 O 3 . Potassium is fully extracted into the melt, while Na and Ca are largely retained in the coldest part of the Metasediment layer in clinopyroxene, Ca-rich garnet and aragonite. The melt is a product of interaction between partial melt or fluid from the sediment and peridotite. It has a silico-carbonatite composition with variable SiO 2 , MgO, FeO and CaO contents and low Al 2 O 3 . The addition of Cl has almost no effect on element distribution, whereas the addition of F results in the appearance of humite-group minerals containing significant amounts of Ti. Trace-element distribution is controlled by pressure, temperature and mineral assemblages. At low temperatures in the sediment layer (
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subduction factory in an ampoule experiments on sediment peridotite interaction under temperature gradient conditions
Geochimica et Cosmochimica Acta, 2018Co-Authors: Alan B Woodland, V K Bulatov, Gerhard P Brey, A V Girnis, Heidi E Hofer, Axel GerdesAbstract:Abstract To better understand processes above subducted oceanic slabs, we have undertaken experiments with juxtaposed sediment and peridotite layers at pressures of 7.5 and 10.5 GPa at a controlled temperature gradient from ∼100 to ∼500 °C per a sample length of ∼3 mm. The sediment starting material contains H2O (6.9 wt%) and CO2 (5.9 wt%) and has a major-element composition similar to GLOSS (Plank and Langmuir, 1998) doped with trace elements at 10–100 ppm levels. Several experiments were conducted with ∼0.5 wt% Cl or F. The peridotite layer is composed of natural olivine (66 wt%), orthopyroxene (27 wt%) and garnet (7 wt%) mixed with ∼15 wt% graphite. Several experimental configurations were investigated, but the “basic” setup has the sediment layer at the bottom in the cold zone (400–1200 °C) overlain by peridotite at 900–1500 °C. The temperature distribution was determined by two thermocouples and orthopyroxene–garnet thermometry. Features common to many experiments are (1) the development of multiple layers of various lithologies and a pool of hydrous silicate or carbonate–silicate melt in the hottest part of the capsule; (2) replacement of olivine by orthopyroxene in the metaperidotite; (3) preservation and growth of garnet and local development of magnesite in the metaperidotite layer; (4) enrichment in garnet within the Metasediment layer at the contact with the metaperidotite; (5) formation of a clinopyroxene–garnet assemblage at the bottom (the coldest part); (6) presence of K-bearing phases (phlogopite or phengite) and carbonates in the Metasediment layer only at temperatures
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combined u pb and hf isotope la mc icp ms analyses of detrital zircons comparison with shrimp and new constraints for the provenance and age of an armorican Metasediment in central germany
Earth and Planetary Science Letters, 2006Co-Authors: Axel Gerdes, Armin ZehAbstract:Abstract Uranium–lead ages obtained by LA-ICP-MS analyses of zircon cores from a high-grade Armorican Metasediment from the Mid-German Crystalline Rise, Central Germany, yield results which are identical to, but more precise than those previously obtained by SHRIMP dating. This is mainly due to the fact that SHRIMP analyses are more sensitive than LA-ICP-MS analyses to common Pb contamination on the surface of the grain mount. The new U–Pb ages, in combination with in-situ Hf isotope analyses of zircon, provide the first evidence that detrital zircons within Armorican sediments crystallized in both juvenile and evolved magmatic rocks during the Archaean at 2.7–2.9 Ga, the Palaeoproterozoic at 1.8–2.1 Ga, and the Neoproterozoic/Early Palaeozoic at 500–720 Ma. In addition, zircons were formed at ca. 1.0 Ga by remelting of Palaeoproterozoic crust during the Grenville orogeny. The U–Pb dataset shows an age gap between 1.8 and 1.0 Ga, which is characteristic of Armorican sediments, and indicates that the Metasediment protolith is younger than Late Cambrian. In addition, the data support previous conclusions that sediments constituting the Armorican terrane assemblage were derived from three crustal sources. Dominant sources were the Avalonian–Cadomian belt (ca. 45%), situated at the northern margin of Gondwana during the Neoproterozoic, and the West-African and/or eastern Amazonian cratons (ca. 50%). The Grenville belt was a minor source (
E. Cannao - One of the best experts on this subject based on the ideXlab platform.
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B, Sr and Pb isotope geochemistry of high-pressure Alpine metaperidotites monitors fluid-mediated element recycling during serpentinite dehydration in subduction melange (Cima di Gagnone, Swiss Central Alps)
Geochimica et Cosmochimica Acta, 2015Co-Authors: E. Cannao, S. Agostini, M. Scambelluri, S. Tonarini, Marguerite GodardAbstract:Tectonic mixing of slab- and mantle-derived materials at the interface between converging plates highly enhances fluid-mediated mass transfer from the slab to the overlying mantle. Subduction mélanges can provide information about the interaction among different slices accreted at plate interface domains, with implications on the tectonic and geochemical evolution of the plate-interface itself. At Cima di Gagnone, pelitic schists and gneiss enclose chlorite harzburgite and garnet peridotite lenses, like in subduction mélanges located in-between downgoing slabs and overlying mantle. These peridotites host MORB-type eclogite and metarodingite, and derive from dehydration of serpentinized mantle protoliths. Their enrichment in fluid-mobile B, As, Sb, U, Th is the result of an early-stage oceanic serpentinization, followed by interaction with host Metasediments during subduction burial. Here we define the element exchange process in the Gagnone mélange by means of the B, Sr and Pb isotope analysis of its main lithologies (ultramafic, mafic rocks and paragneiss). The 87Sr/86Sr and 206Pb/204Pb ratios of ultramafic rocks (0.7090–0.7124 and 18.292–18.837, respectively) show enrichments in radiogenic Sr and Pb after exchange with the host paraschist (up to 0.7287 87Sr/86Sr; 18.751 206Pb/204Pb). The δ11B values of peridotites (down to −10‰) point to a combined effect of (1) 11B release to deserpentinization fluids (serpentinized protoliths likely had positive δ11B and lower radiogenic Sr, Pb), and of (2) exchange with fluids from the surrounding Metasediments. The whole Gagnone rock-suite is finally overprinted by retrograde fluids that essentially bring to an increase in radiogenic Pb (about 19.0 206Pb/204Pb) and to values of 0.710 87Sr/86Sr and of −10‰ δ11B. The recognition of different stages of interaction between mantle rocks and sedimentary/crustal reservoirs allows us to define the geochemical effects related to the early coupling of such rocks along the plate-interface. Our study shows that ultramafic rocks involved in subduction-zone metamorphism and serpentinization uptake radiogenic Pb and Sr released by associated sedimentary reservoirs. The exchange process envisioned here is not only representative of subduction mélanges: it can also be a proxy of mass transfer between slab and serpentinized supra-subduction mantle, as occurs in forearcs. Dehydration of the Gagnone-type serpentinized mantle releases crust-derived components to arcs, without direct involvement of Metasediment dehydration and/or melting in subarc environments. The retention of appreciable amounts of fluid-mobile elements, radiogenic Pb and Sr in dehydrated Gagnone peridotites has implications on element recycling in the deep Earth’s mantle.
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B, Sr and Pb isotope geochemistry of high-pressure Alpine metaperidotites monitors fluid-mediated element recycling during serpentinite dehydration in subduction mélange (Cima di Gagnone, Swiss Central Alps)
'Elsevier BV', 2015Co-Authors: E. Cannao, S. Agostini, M. Scambelluri, S. Tonarini, M. GodardAbstract:Tectonic mixing of slab- and mantle-derived materials at the interface between converging plates highly enhances fluid-mediated mass transfer from the slab to the overlying mantle. Subduction m\ue9langes can provide information about the interaction among different slices accreted at plate interface domains, with implications on the tectonic and geochemical evolution of the plate-interface itself. At Cima di Gagnone, pelitic schists and gneiss enclose chlorite harzburgite and garnet peridotite lenses, like in subduction m\ue9langes located in-between downgoing slabs and overlying mantle. These peridotites host MORB-type eclogite and metarodingite, and derive from dehydration of serpentinized mantle protoliths. Their enrichment in fluid-mobile B, As, Sb, U, Th is the result of an early-stage oceanic serpentinization, followed by interaction with host Metasediments during subduction burial. Here we define the element exchange process in the Gagnone m\ue9lange by means of the B, Sr and Pb isotope analysis of its main lithologies (ultramafic, mafic rocks and paragneiss). The 87Sr/86Sr and 206Pb/204Pb ratios of ultramafic rocks (0.7090-0.7124 and 18.292-18.837, respectively) show enrichments in radiogenic Sr and Pb after exchange with the host paraschist (up to 0.7287 87Sr/86Sr; 18.751 206Pb/204Pb). The \u3b411B values of peridotites (down to -10\u2030) point to a combined effect of (1) 11B release to deserpentinization fluids (serpentinized protoliths likely had positive \u3b411B and lower radiogenic Sr, Pb), and of (2) exchange with fluids from the surrounding Metasediments. The whole Gagnone rock-suite is finally overprinted by retrograde fluids that essentially bring to an increase in radiogenic Pb (about 19.0 206Pb/204Pb) and to values of 0.710 87Sr/86Sr and of -10\u2030 \u3b411B. The recognition of different stages of interaction between mantle rocks and sedimentary/crustal reservoirs allows us to define the geochemical effects related to the early coupling of such rocks along the plate-interface. Our study shows that ultramafic rocks involved in subduction-zone metamorphism and serpentinization uptake radiogenic Pb and Sr released by associated sedimentary reservoirs. The exchange process envisioned here is not only representative of subduction m\ue9langes: it can also be a proxy of mass transfer between slab and serpentinized supra-subduction mantle, as occurs in forearcs. Dehydration of the Gagnone-type serpentinized mantle releases crust-derived components to arcs, without direct involvement of Metasediment dehydration and/or melting in subarc environments. The retention of appreciable amounts of fluid-mobile elements, radiogenic Pb and Sr in dehydrated Gagnone peridotites has implications on element recycling in the deep Earth's mantle
Yuanbao Wu - One of the best experts on this subject based on the ideXlab platform.
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cadomian ediacaran cambrian arc magmatism in the chahjam biarjmand metamorphic complex iran magmatism along the northern active margin of gondwana
Gondwana Research, 2015Co-Authors: Hadi Shafaii Moghadam, Mohsen Khademi, Zhaochu Hu, Robert J Stern, J F Santos, Yuanbao WuAbstract:Abstract The ChahJam–Biarjmand complex (CJBC), flanked by the Alborz Mountains in the north and the Lut–Tabas block to the south, is part of Central Iranian block, where the oldest continental crust of Iran is found. This complex contains granitic to tonalitic orthogneissic rocks (old plutons) and associated Metasediments, amphibolites and paragneisses. Metamorphosed granitic and granodioritic dikes intrude orthogneisses as well as Metasediments and are abundant close to the plutons (orthogneissic rocks). Based on the results of bulk rock trace and rare earth elements, the orthogneissic rocks are inferred to have crystallized from subduction-related melts. Amphibolites also have subduction-related signatures and are inferred to have formed both as metamorphosed volcanoclastic sediments and as attenuated basic dikes. The presence of para-amphibolites associated with paragneisses and Metasediments (mica schists) could represent a sedimentary basin filled with magmatic arc erosional products. U–Pb zircon dating of the ChahJam–Biarjmand rocks yielded 238U/206Pb crystallization ages of ca. 550 to 530 Ma (Ediacaran–early Cambrian). Sr–Nd isotope systematics on whole rocks (eNd(t) = − 2.2 to − 5.5) and zircon Hf isotope results indicate that CJBC Cadomian granitic rocks contain older, possible Mesoproterozoic, continental crust in their source. The ChahJam–Biarjmand granitic–tonalitic gneissic rocks are coeval with other similar-aged metagranites and gneisses within Iranian basement exposed in Central Iran, the Sanandaj–Sirjan Zone and the Alborz Mountains, as well as in the Tauride–Anatolide platform in western Anatolia and in NW Turkey. All these dispersed Cadomian basement rocks are interpreted to show fragments of Neoproterozoic–early Cambrian continental arcs bordering the northern active margin of Gondwana.
Stefan Schmid - One of the best experts on this subject based on the ideXlab platform.
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3-D assessment of peak-metamorphic conditions by Raman spectroscopy of carbonaceous material: an example from the margin of the Lepontine dome (Swiss Central Alps)
International Journal of Earth Sciences, 2011Co-Authors: Michael Wiederkehr, Romain Bousquet, Martin Ziemann, Alfons Berger, Stefan SchmidAbstract:This study monitors regional changes in the crystallinity of carbonaceous matter (CM) by applying Micro-Raman spectroscopy to a total of 214 Metasediment samples (largely so-called Bündnerschiefer) dominantly metamorphosed under blueschist- to amphibolite-facies conditions. They were collected within the northeastern margin of the Lepontine dome and easterly adjacent areas of the Swiss Central Alps. Three-dimensional mapping of isotemperature contours in map and profile views shows that the isotemperature contours associated with the Miocene Barrow-type Lepontine metamorphic event cut across refolded nappe contacts, both along and across strike within the northeastern margin of the Lepontine dome and adjacent areas. Further to the northeast, the isotemperature contours reflect temperatures reached during the Late Eocene subduction-related blueschist-facies event and/or during subsequent near-isothermal decompression; these contours appear folded by younger, large-scale post-nappe-stacking folds. A substantial jump in the recorded maximum temperatures across the tectonic contact between the frontal Adula nappe complex and surrounding Metasediments indicates that this contact accommodated differential tectonic movement of the Adula nappe with respect to the enveloping Bündnerschiefer after maximum temperatures were reached within the northern Adula nappe, i.e. after Late Eocene time.