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  • 1
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract.  Peridotites, dykes and gabbros from the 470–420 Ma Trinity Ophiolite Complex of northern California exhibit large geochemical rare earth element (REE) and Nd isotopic variations on the small scales which are indicative of a complex history. The Trinity Ophiolite, which covers an area of ≈1600 km2, consists of three distinct units: (1) a ∼2–4 km-thick sheet of plastically deformed peridotites, including various ultrabasic lithologies (plagioclase and spinel lherzolite, harzburgite, wherlite and dunite); the peridotite unit is a fragment of mantle lithosphere of oceanic affinity; (2) a series of small (∼1 km diameter) undeformed gabbroic massifs; (3) several generations of basic dykes. The peridotites display the largest geochemical and isotopic variations, with ɛNd(T) values ranging from +10 down to 0. In the gabbroic massifs and intrusive dykes, the variation in model ɛNd(T) values is reduced to 7 ɛNd units: 0 to +7. As a general rule, peridotites, gabbros and dykes with ɛNd(T) values around 0 or +3 give less depleted L(light)REE patterns than do those with ɛNd(T) values in the range +7 to +10. In the peridotites, the Nd isotopic variations take place over very short distances, with jumps as large as 7 ɛNd units occurring on scales of less than 20 m. Comparison with available age data indicates that the peridotites with ɛNd(T)≈+10 could be slightly older than the intrusive gabbro massifs and basic dykes (470 Ma vs. 420 Ma). Strontium isotopic data used in connection with Sm-Nd results demonstrate that the 10 ɛNd units variation displayed by the Trinity Peridotite is a primary feature and not an artefact due to REE mobility during seawater interaction. The variable Nd isotopic signatures and variable LREE patterns in the Trinity Peridotite cannot represent mantle source characteristics as there is evidence that this unit was partially melted when it rose as part of the upwelling convecting mantle. Field, petrographic, geochemical and isotopic data rather suggest that the observed heterogeneity is due to local reactions between a 470 Ma proto-peridotite with ɛNd(T)=+10 and younger (420 Ma) basaltic melts with lower ɛNd(T) values (i.e. the gabbroic massifs and the dykes). The gabbros and basic dykes of the Trinity Complex have geochemical and isotopic compositions similar to the arc basalts from the adjacent Copley Formation, so it is proposed that the younger melts are related to arc magmatism.
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  • 2
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  The Ordovician volcano-sedimentary succession of Erquy (northern Brittany) is made of immature sediments thermally metamorphosed at the contact of intruding basic sills. Pillow lavas constitute the upper part of the sequence. The trace element geochemistry of sills and pillow lavas suggests that they were derived from a tholeiitic source located beneath a passive margin. This volcanic sequence was metamorphosed under low to moderate greenschist facies conditions. In this study the direction and amplitude of chemical and isotopic fluxes in the basalt-sediment-water system were established and the oxygen and hydrogen isotope compositions of the aqueous fluid that reacted with the volcanic rocks were characterized. Cationic thermometry on chlorites and isotopic thermometry on plagioclase-chlorite pairs indicate closure metamorphic temperatures in the range 200–250°C for the basaltic sills. Stable isotope compositions of iron-rich chlorites (δ18O = 5.5‰; δD from –60 to –50‰) and plagioclases (δ18O from +9 to +10‰) reveal that the source of the fluid was certainly seawater. The δ18O variations within the sills are strongly correlated with the rate of progress of the main metamorphic reaction:clinopyroxene + plagioclase + ilmenite → chlorite + albite + epidote + quartz + sphene that produced major element mobility at the scale of the volcano-sedimentary sequence. Calculation of elemental fluxes by mass balance combined with oxygen isotopic compositions of basalts shows that the highest water-rock ratios (≥1) were at sill-sediment boundaries and within pillow lavas at the top of the pile. The volcanosedimentary sequence of Erquy was a net sink for Na and a source for Ca. No Mg uptake could be detected whereas the hydrothermal alteration of the sediments released Fe, Si, and K trapped by the volcanic rocks. The δ18O value of the fluid reacting with sills appears to have shifted no more than +4‰ after percolation at low temperature through immature sediments (δ18O ≈ +12‰). The Erquy volcano-sedimentary sequence represents a marine hydrothermal system dominated by low-temperature exchange which allowed a general 18O-enrichment of the volcanic rocks and a 18O-depletion of sediments.
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  • 3
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The Ordovician volcano-sedimentary succession of Erquy (northern Brittany) is made of immature sediments thermally metamorphosed at the contact of intruding basic sills. Pillow lavas constitute the upper part of the sequence. The trace element geochemistry of sills and pillow lavas suggests that they were derived from a tholeiitic source located beneath a passive margin. This volcanic sequence was metamorphosed under low to moderate greenschist facies conditions. In this study the direction and amplitude of chemical and isotopic fluxes in the basalt-sediment-water system were established and the oxygen and hydrogen isotope compositions of the aqueous fluid that reacted with the volcanic rocks were characterized. Cationic thermometry on chlorites and isotopic thermometry on plagioclase-chlorite pairs indicate closure metamorphic temperatures in the range 200–250°C for the basaltic sills. Stable isotope compositions of iron-rich chlorites (δ18O-5.5‰; δD from-60 to-50‰) and plagioclases (δ18O from +9 to +10‰) reveal that the source of the fluid was certainly seawater. The δ18O variations within the sills are strongly correlated with the rate of progress of the main metamorphic reaction:clinopyroxene+plagioclase+ilmenite → chlorite+albite+epidote+quartz+sphene that produced major element mobility at the scale of the volcanosedimentary sequence. Calculation of elemental fluxes by mass balance combined with oxygen isotopic compositions of basalts shows that the highest water-rock ratios (≥1) were at sill-sediment boundaries and within pillow lavas at the top of the pile. The volcanosedimentary sequence of Erquy was a net sink for Na and a source for Ca. No Mg uptake could be detected whereas the hydrothermal alteration of the sediments released Fe, Si, and K trapped by the volcanic rocks. The δ18O value of the fluid reacting with sills appears to have shifted no more than +4‰ after percolation at low temperature through immature sediments (δ18O≈12‰). The Erquy volcano-sedimentary sequence represents a marine hydrothermal system dominated by low-temperature exchange which allowed a general 18O-enrichment of the volcanic rocks and a 18O-depletion of sediments.
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  • 4
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    In:  Geological Society Special Publication 258: 83-100.
    Publication Date: 2007-10-08
    Description: New material from the Santa Marta Formation (late Coniacian-?early Maastrichtian) of James Ross Island contributes significantly to the current knowledge of Late Cretaceous Antarctic fish diversity. The taxon list for the Santa Marta Formation is extended, and new records of neoselachians and teleosts are reported. The stratigraphic ranges of some previously known taxa are enlarged, and the palaeobiogeography and palaeoecology of Late Cretaceous Antarctic fishes are discussed. Top predators that occupied the higher levels in the food chain along with marine tetrapods dominate the marine faunas from the Santa Marta and Lopez de Bertodano formations. The only fish adapted to crushing hard-shelled invertebrates were the chimeroids. Rays, an important component of marine fish associations, as well as fish from lower trophic levels, remain unknown from the Late Cretaceous of Antarctica.
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  • 5
  • 6
    Publication Date: 2007-03-01
    Print ISSN: 0883-1351
    Electronic ISSN: 0883-1351
    Topics: Geosciences
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  • 7
    Publication Date: 2016-07-22
    Description: We present a new and innovative way of determining the oxygen level of Earth’s past atmosphere by directly measuring inclusion gases trapped in halite. After intensive screening using multiple depositional, textural/fabric, and geochemical parameters, we determined that tectonically undisturbed cumulate, chevron, and cornet halite inclusions may retain atmospheric gas during crystallization from shallow saline, lagoonal, and/or saltpan brine. These are the first measurements of inclusion gas for the Neoproterozoic obtained from 815 ± 15–m.y.–old Browne Formation chevron halite of the Officer Basin, southwest Australia. The 31 gas measurements afford us a direct glimpse of the composition of the mid- to late Neoproterozoic atmosphere and register an average oxygen content of 10.9%. The measured p O 2 puts oxygenation of Earth’s paleoatmosphere ~100–200 m.y. ahead of current models and proxy studies. It also puts oxygenation of the Neoproterozoic atmosphere in agreement with time of diversification of eukaryotes and in advance of the emergence of marine animal life.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 8
    Publication Date: 2015-01-28
    Description: The widespread hypoxic conditions of the tumor microenvironment can impair the metabolism of bioessential elements such as copper and sulfur, notably by changing their redox state and, as a consequence, their ability to bind specific molecules. Because competing redox state is known to drive isotopic fractionation, we have used here...
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 9
    Publication Date: 2015-07-11
    Description: A bstract :  Porous micritic facies, either primary chalks or resulting from secondary destructive micritization, can constitute important hydrocarbon or water reservoirs. Characterization of reservoir properties and the understanding of factors which controlled the distribution of porosity are of primary interest to evaluate the prospective reserves. Middle and late Oxfordian limestones of the eastern Paris Basin show several horizons with porosities higher than 20%. The porosity is mainly microporous and located either within secondary micritized grains or in the micritic matrix. Using SEM, cathodoluminescence, as well as confocal microscopy, stable oxygen and carbon isotope ratios, and petrophysical measurements, a scenario for the evolution of the micropores is proposed. Lime mudstones to packstones constitute the majority of the high-porosity facies (HPFs). Inner lagoonal deposits are more micritized and thus more porous than grainstones, and facies rich in leiolitic oncoids and echinoid clasts are less impacted by micritization. Micritization was responsible for an increase of the intragranular porosity in most grain types. During both eogenesis and shallow burial, mineralogical stabilization dissolved aragonitic particles and allowed precipitation of calcite rhombs. This process was probably enhanced below surfaces of subaerial exposure. During burial, Ostwald ripening allowed the growth of larger micrite crystals at the expense of smaller ones during early Berriasian and late Aptian recharges of deep aquifers when the northern margin of the basin was exposed. Overgrowths on micrite crystals were more important in intervals strongly affected by chemical compaction, which favored oversaturation of waters with respect to calcite. In low-porosity horizons (LPFs), the dense micritic texture of oncoids and the monocrystalline architecture of echinoid clasts prevented an intense micritization, while the strong chemical compaction enhanced poronecrosis. Telogenetic fracturing created new fluid pathways that favored inputs of meteoric fluid in porous micrite and allowed the continuation of Ostwald ripening during Cenozoic times. As a whole, mesogenetic inputs of waters undersaturated with respect to calcite in deep aquifers during exposure of basin margins are a more efficient process than early subaerial exposure for enhancing aggrading neomorphism and appearance of microporous micrites. Initial mineralogical heterogeneities also impact the intensity of chemical compaction and thus the stratigraphical distribution of microporous limestones.
    Print ISSN: 1527-1404
    Topics: Geosciences
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  • 10
    Publication Date: 2016-01-27
    Description: Oxygen isotope compositions of tooth enamel increments in theropod dinosaurs are investigated as potential proxies of climate seasonality. Six teeth of large carnivorous theropods collected from four Cretaceous formations deposited under contrasted climates have been sampled. These teeth have been analyzed for the oxygen isotope compositions of their apatite phosphate ( 18 O p ) through incremental sampling performed along the major growth axis. Significant fluctuations in oxygen isotope compositions along the growth axis of each tooth are observed and interpreted as reflecting seasonality in ingested local surface water 18 O w values. Fluctuations in 18 O p values of theropod teeth from the Aptian of Thailand and Cenomanian of Morocco vary similarly to meteoric water 18 O mw values occurring today in sub-tropical regions subjected to large seasonal amounts of precipitations. A dinosaur tooth recovered from the more inland and mid-latitude Nemegt Formation of Mongolia shows a seasonal pattern similar to present-day cold temperate and continental climate. Finally, the high latitude and coastal Kakanaut Formation (Russia) experienced strongly dampened seasonal variations, most likely due to the influence of warm Pacific oceanic currents. Such conditions occur today in high latitude regions submitted to marine influence. These results further highlight the potential of using the oxygen isotope compositions of large theropod teeth to reconstruct past seasonal variations of terrestrial climates. Increased knowledge of past seasonality may help to better understand the complex interactions between climate and the dynamics of land biodiversity in terms of ecological adaptations, biogeography and the evolutionary history of organisms.
    Print ISSN: 0883-1351
    Electronic ISSN: 0883-1351
    Topics: Geosciences
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