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B. Cenki-Tok F. Chabaux D. Lemarchand M.-C. Pierret M.-L. Bagard 《Geochimica et cosmochimica acta》2009,73(8):2215-7724
This study aims to constrain the factors controlling the calcium isotopic compositions in surface waters, especially the respective role of vegetation and water-rock interactions on Ca isotope fractionation in a continental forested ecosystem. The approach is to follow changes in space and time of the isotopic composition and concentration of Ca along its pathway through the hydro-geochemical reservoirs from atmospheric deposits to the outlet of the watershed via throughfalls, percolating soil solutions and springs. The study is focused on the Strengbach catchment, a small forested watershed located in the northeast of France in the Vosges mountains. The δ44/40Ca values of springs, brooks and stream waters from the catchment are comparable to those of continental rivers and fluctuate between 0.17 and 0.87‰. Soil solutions, however, are significantly depleted in lighter isotopes (δ44/40Ca: 1.00-1.47‰), whereas vegetation is strongly enriched (δ44/40Ca: −0.48‰ to +0.19‰). These results highlight that vegetation is a major factor controlling the calcium isotopic composition of soil solutions, with depletion in “light” calcium in the soil solutions from deeper parts of the soil compartments due to preferential 40Ca uptake by the plants rootsystem. However, mass balance calculations require the contribution of an additional Ca flux into the soil solutions most probably associated with water-rock interactions. The stream waters are marked by a seasonal variation of their δ44/40Ca, with low δ44/40Ca in winter and high δ44/40Ca in spring, summer and autumn. For some springs, nourishing the streamlet, a decrease of the δ44/40Ca value is observed when the discharge of the spring increases, with, in addition, a clear covariation between the δ44/40Ca and corresponding H4SiO4 concentrations: high δ44/40Ca values and low H4SiO4 concentrations at high discharge; low δ44/40Ca values and high H4SiO4 concentrations at low discharge. These data imply that during dry periods and low water flow rate the source waters carry a Ca isotopic signature from alteration of soil minerals, whereas during wet periods and high flow rates admixture of significant quantities of 40Ca depleted waters (vegetation induced signal) from uppermost soil horizons controls the isotopic composition of the source waters. This study clearly emphasizes the potential of Ca isotopes as tracers of biogeochemical processes at the water-rock-vegetation interface in a small forested catchment. 相似文献
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Mete Çetinkaplan Osman Candan Roland Oberhänsli Masafumi Sudo Bénédicte Cenki-Tok 《Journal of Metamorphic Geology》2020,38(4):379-419
Eclogite and blueschist facies rocks occurring as a tectonic unit between the underlying Menderes Massif (MM) and the overlying Afyon Zone/Lycian Nappes and the Bornova Flysch Zone in western Anatolia represent the eastward continuation of the Cycladic Blueschist Unit (CBU) in Turkey. This high-P unit is attributed to the closure of the Pindos Ocean and consists of (a) a Triassic to Upper Cretaceous coherent series derived from passive continental margin sediments and (b) the tectonically overlying Upper Cretaceous Selçuk mélange with eclogite blocks embedded in a pelitic epidote-blueschist matrix. The coherent series has experienced epidote-blueschist facies metamorphism (490 ± 25°C/11.5 ± 1.5 kbar; 38 km depth). 40Ar/39Ar white mica and 206Pb/238U monazite dating of quartz metaconglomerate from coherent series yielded middle Eocene ages of 44 ± 0.3 and 40.1 ± 3.1 Ma for epidote-blueschist facies metamorphism, respectively. The epidote-blueschist facies metamorphism of the matrix of the Selçuk mélange culminates at 520 ± 15°C/13 ± 1.5 kbar, 43 km depth, and is dated at 57.5 ± 0.3–54.5 ± 0.1 Ma (40Ar/39Ar phengite). Eclogite facies metamorphism of the blocks (570 ± 30°C/18 ± 2 kbar, 60 km depth) is early Eocene and dated at 56.2 ± 1.5 Ma by 206Pb/238U zircon. Eclogites experienced a nearly isothermal retrogression (490 ± 40°C/~6 to 7 kbar) during their incorporation into the Selçuk mélange. The retrograde overprints of the coherent series (410 ± 15°C/7 ± 1.5 kbar from Dilek Peninsula and 485 ± 33°C/~6 to 7 kbar from Selçuk–Tire area) and the Selçuk mélange (510 ± 15°C/6 ± 1 kbar) are dated at 35.8 ± 0.5–34.3 ± 0.1 Ma by 40Ar/39Ar white mica and 31.6 ± 6.6 Ma by 206Pb/238U allanite dating methods, respectively. Regional geological constrains reveal that the contact between the MM and the CBU originally formed a lithosphere-scale transform fault zone. 40Ar/39Ar white mica age from the contact indicates that the CBU and the MM were tectonically juxtaposed under greenschist facies conditions during late Eocene, 35.1 ± 0.3 Ma. 相似文献
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Trap Pierre Roger Françoise Cenki-Tok Bénédicte Paquette Jean-Louis 《International Journal of Earth Sciences》2017,106(2):453-476
International Journal of Earth Sciences - Unravelling the detailed pressure–temperature–time-deformation (P–T–t-D) evolution of magmatic and metamorphic rocks provides... 相似文献
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Paola Manzotti Daniela Rubatto Michele Zucali Afifé El Korh Bénédicte Cenki-Tok Michel Ballèvre Martin Engi 《Swiss Journal of Geoscience》2018,111(1-2):79-97
In the Dent Blanche Tectonic System, the Mont Morion biotite-bearing granite is a km-scale intrusion preserved in a low-strain volume. Zircon saturation thermometry suggests that it crystallised from a melt that reached about 800 °C. U–Pb zircon and allanite geochronology indicates crystallization of the magma in the Permian (290 ± 3 Ma; 280 ± 8 Ma, respectively). Migmatitic biotite-gneiss and amphibolite are found as xenoliths within the Mont Morion granite and constitute its country-rocks. In two samples of migmatitic biotite-gneiss zircon has metamorphic overgrowths that yield U–Pb ages of 285 ± 3 Ma and 281 ± 4 Ma, and are thus contemporaneous with the intrusion of the granite. The Mont Morion granite with its country-rocks of migmatitic biotite-bearing gneiss and amphibolite was thus emplaced at middle crustal levels while amphibolite facies metamorphism affected its country rocks. The magmatic and metamorphic record in the Mont Morion area reflects the high-temperature regime and lithospheric thinning of the Adriatic continental margin during Permian. 相似文献
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Summary The coexistence of a colourless and a yellow garnet was observed in eclogite-facies manganese concentrations of the Mesozoic
ophiolitic Zermatt-Saas Unit, at the Praborna mine near Saint-Marcel, Val d’Aoste, Italy, and in the upper Maurienne Valley,
France. They occur both in oxidised metachert with hematite and braunite (+ minor Mn-pyroxenoid and tirodite, rare tiragalloite;
with ardennite or piemontite in distinct layers), and in more reduced, carbonate-rich boudins included in it. The co-occurrence
takes a variety of textural aspects, from coexisting euhedral garnets (10–100 μm in size for the calderite to mm-size for
spessartine) to sharp overgrowths of yellow calderitic garnet on colourless spessartine, to yellow cauliflower-like masses
(a few hundreds of μm in size) overgrowing colourless spessartine and showing evidence of oscillatory zoning, resorption stages
and resumed growth. Sector zoning and anisotropy are common, although not consistent features.
Compositions can be expressed to 95% in the quadrilateral system (Ca, Mn2+)3 (Al, Fe3+)2 Si3O12, with less than 1.0 wt% MgO and 0.8 wt% TiO2 in colourless spessartine, and less than 0.2 wt% MgO and 1.6 wt% TiO2 in yellow garnet. Calcium partitions into the ferric garnet. Coexisting pairs define two compositional gaps, bounded by values
of the Fe3+/(Al + Fe3+) ratio of 10 and 15% for the first one, of 40 and 65% for the other. The optically obvious discontinuity (colour change and
Becke’s line) corresponds to the narrower gap, between colourless spessartine and yellow spessartine, whereas the broad compositional
gap occurs within yellow garnet, between yellow spessartine and yellow calderite, and is only revealed by back-scattered electron
images. Only the latter can be a candidate for a miscibility gap, if any.
Present address: Centre de Géochimie de la Surface – EOST, 1 rue Blessig, 67083 Strasbourg Cedex, France 相似文献
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