Calculation of effect of cation disorder on silicate spinel phase boundaries     

Alexandra Navrotsky 《Earth and Planetary Science Letters》1977,33(3):437-442

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Footwall mineralization during Late Miocene extension along the West Cycladic Detachment System,Lavrion, Greece     

Alexandra Berger  David A. Schneider  Bernhard Grasemann  Daniel Stockli 《地学学报》2013,25(3):181-191

The West Cycladic Detachment System (WCDS) has been mapped from the western Cycladic islands to Lavrion (mainland Greece), where several top‐to‐SSW low‐angle normal faults at different structural levels are observed. Near the detachment horizon, hydrothermal fluid flow originating from Miocene intrusions lead to high‐temperature carbonate strata‐bound Pb–Ag–Zn ore deposits. Zircon (U–Th)/He cooling ages from the mineralized footwall are 7.1 ± 0.6 to 7.9 ± 0.6 Ma. Carbon‐ and O‐isotope analyses of the carbonate host indicate high water‐to‐rock ratios during brittle deformation‐induced metasomatism, and that this interaction, unrelated to proximity of the intrusions, plays the dominant role in the mineralization. The Late Miocene geology of Lavrion is strikingly similar to Serifos island on the SE termination of the WCDS, both characterized by strong localization of detachment faulting and the interaction of brittle deformation with the injection of fluids related to granitoid intrusions.  相似文献   

High-resolution record of uranium isotopic composition of Paleo-Baikal water during the last 100 ka     

N. A. Zhuchenko  E. P. Chebykin  E. L. Goldberg  O. G. Stepanova 《Doklady Earth Sciences》2007,414(1):630-633

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Radionuclides and mercury in the salt lakes of the Crimea     

Natalya Mirzoyeva  Larisa Gulina  Sergey Gulin  Olga Plotitsina  Alexandra Stetsuk  Svetlana Arkhipova  Nina Korkishko  Oleg Eremin 《中国海洋湖沼学报》2015,33(6):1413-1425

90 Sr concentrations,resulting from the Chernobyl NPP accident,were determined in the salt lakes of the Crimea(Lakes Kiyatskoe,Kirleutskoe,Kizil-Yar,Bakalskoe and Donuzlav),together with the redistribution between the components of the ecosystems.The content of mercury in the waters of the studied reservoirs was also established.Vertical distributions of natural radionuclide activities( 238 U,232 Th,226 Ra,210 Pb,40 K) and anthropogenic 137 Cs concentrations(as radiotracers) were determined in the bottom sediments of the Koyashskoe salt lake(located in the south-eastern Crimea) to evaluate the longterm dynamics and biogeochemical processes.Radiochemical and chemical analysis was undertaken and radiotracer and statistical methods were applied to the analytical data.The highest concentrations of 90 Sr in the water of Lake Kiyatskoe(350.5 and 98.0 Bq/m 3) and Lake Kirleutskoe(121.3 Bq/m 3) were due to the discharge of the Dnieper water from the North-Crimean Canal.The high content of mercury in Lake Kiyatskoe(363.2 ng/L) and in seawater near Lake Kizil-Yar(364 ng/L) exceeded the maximum permissible concentration(3.5 times the maximum).Natural radionuclides provide the main contribution to the total radioactivity(artificial and natural combined) in the bottom sediments of Lake Koyashskoe.The significant concentration of 210 Pb in the upper layer of bottom sediments of the lake indicates an active inflow of its parent radionuclide—gaseous 222 Rn from the lower layers of the bottom sediment.The average sedimentation rates in Lake Koyashskoe,determined using 210 Pb and 137 Cs data,were 0.117 and 0.109 cm per year,respectively.  相似文献   

Hyperspectral mapping of alteration assemblages within a hydrothermal vug at the Haughton impact structure,Canada          下载免费PDF全文

Rebecca N. Greenberger  John F. Mustard  Gordon R. Osinski  Livio L. Tornabene  Alexandra J. Pontefract  Cassandra L. Marion  Roberta L. Flemming  Janette H. Wilson  Edward A. Cloutis 《Meteoritics & planetary science》2016,51(12):2274-2292

Meteorite impacts on Earth and Mars can generate hydrothermal systems that alter the primary mineralogies of rocks and provide suitable environments for microbial colonization. We investigate a calcite–marcasite‐bearing vug at the ~23 km diameter Haughton impact structure, Devon Island, Nunavut, Canada, using imaging spectroscopy of the outcrop in the field (0.65–1.1 μm) and samples in the laboratory (0.4–2.5 μm), point spectroscopy (0.35–2.5 μm), major element chemistry, and X‐ray diffraction analyses. The mineral assemblages mapped at the outcrop include marcasite; marcasite with minor gypsum and jarosite; fibroferrite and copiapite with minor gypsum and melanterite; gypsum, Fe³⁺ oxides, and jarosite; and calcite, gypsum, clay, microcline, and quartz. Hyperspectral mapping of alteration phases shows spatial patterns that illuminate changes in alteration conditions and formation of specific mineral phases. Marcasite formed from the postimpact hydrothermal system under reducing conditions, while subsequent weathering oxidized the marcasite at low temperatures and water/rock ratios. The acidic fluids resulting from the oxidation collected on flat‐lying portions of the outcrop, precipitating fibroferrite + copiapite. That assemblage then likely dissolved, and the changing chemistry and pH resulting from interaction with the calcite‐rich host rock formed gypsum‐bearing red coatings. These results have implications for understanding water–rock interactions and habitabilities at this site and on Mars.  相似文献   

Temporal Evolution of a Rapidly-Moving Umbral Dot     

Hiroko Watanabe  Alexandra Tritschler  Reizaburo Kitai  Kiyoshi Ichimoto 《Solar physics》2010,266(1):5-16

We performed two-dimensional spectroscopic observations of the preceding sunspot of NOAA 10905 located off disk center (S8 E36, μ≈0.81) by using the Interferometric BI-dimensional Spectrometer (IBIS) operated at the Dunn Solar Telescope (DST) of the National Solar Observatory, New Mexico. The magnetically insensitive Fe I line at 709.04 nm was scanned in wavelength repetitively at an interval of 37 s to calculate sequences of maps of the line-wing and line-core intensity, and the line-of-sight Doppler velocity at different line depths (3% to 80%). Visual inspection of movies based on speckle reconstructions computed from simultaneous broadband data and the local continuum intensity at 709.04 nm revealed an umbral dot (UD) intruding rapidly from the umbral boundary to the center of the umbra. The apparent motion of this object was particularly fast (1.3 km s⁻¹) when compared to typical UDs. The lifetime and size of the UD was 8.7 min and 240 km, respectively. The rapid UD was visible even in the line-core intensity map of Fe I 709.04 nm and was accompanied by a persistent blueshift of about 0.06 km s⁻¹.  相似文献   

Kinetic oxygen isotope effects during dissimilatory sulfate reduction: A combined theoretical and experimental approach     

Alexandra V. Turchyn  Volker Brüchert  Gregory S. Engel  Daniel P. Schrag 《Geochimica et cosmochimica acta》2010,74(7):2011-6698

Kinetic isotope effects related to the breaking of chemical bonds drive sulfur isotope fractionation during dissimilatory sulfate reduction (DSR), whereas oxygen isotope fractionation during DSR is dominated by exchange between intercellular sulfur intermediates and water. We use a simplified biochemical model for DSR to explore how a kinetic oxygen isotope effect may be expressed. We then explore these relationships in light of evolving sulfur and oxygen isotope compositions (δ³⁴S_SO4 and δ¹⁸O_SO4) during batch culture growth of twelve strains of sulfate-reducing bacteria. Cultured under conditions to optimize growth and with identical δ¹⁸O_H2O and initial δ¹⁸O_SO4, all strains show ³⁴S enrichment, whereas only six strains show significant ¹⁸O enrichment. The remaining six show no (or minimal) change in δ¹⁸O_SO4 over the growth of the bacteria. We use these experimental and theoretical results to address three questions: (i) which sulfur intermediates exchange oxygen isotopes with water, (ii) what is the kinetic oxygen isotope effect related to the reduction of adenosine phosphosulfate (APS) to sulfite (SO₃²⁻), (iii) does a kinetic oxygen isotope effect impact the apparent oxygen isotope equilibrium values? We conclude that oxygen isotope exchange between water and a sulfur intermediate likely occurs downstream of APS and that our data constrain the kinetic oxygen isotope fractionation for the reduction of APS to sulfite to be smaller than 4‰. This small oxygen isotope effect impacts the apparent oxygen isotope equilibrium as controlled by the extent to which APS reduction is rate-limiting.  相似文献   

Deep structure of the region of the Uzon-Geyser volcanic-tectonic depression based on the data of microseismic sounding     

Yu. A. Kugaenko  V. A. Saltykov  A. V. Gorbatikov  M. Yu. Stepanova 《Doklady Earth Sciences》2010,435(1):1460-1465

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The Response of Estuarine Macrobenthic Communities to Natural- and Human-Induced Changes: Dynamics and Ecological Quality     

João Magalhães Neto  Heliana Teixeira  Joana Patrício  Alexandra Baeta  Helena Veríssimo  Rute Pinto  João Carlos Marques 《Estuaries and Coasts》2010,33(6):1327-1339

Anthropogenic activities are a disturbance factor of coastal systems and can be widely recognized as a major threat to the health of coastal systems. However, natural events cannot be disregarded from management issues because of their significant influence on the communities living in these areas. Based on long-term subtidal data from the Mondego Estuary (Portugal), the effects of natural events (e.g., floods and droughts) on macrobenthic communities were compared with the anthropogenic events. Sampling stations were grouped into characteristic zones (mouth, north arm, south arm) so the community dynamics of each of these estuarine areas could be followed over time. Environmental assessment was performed for stations using the Benthic Assessment Tool (BAT), and compared with the existing pressures. Human impacts persist over a number of years and gradually reduce ecosystem health, as discussed in the European Water Framework Directive. Paradoxically, natural events cause stronger impacts but are of a shorter duration, which allows for a faster recovery of macrobenthic communities. The study showed that caution should be taken when developing and implementing water policies so as not to disregard the importance of the different events (natural and human-caused) on the ecosystem health (e.g., community degradation and water quality and ecological quality status assessment).  相似文献   

Amphibole as an archivist of magmatic crystallization conditions: problems,potential, and implications for inferring magma storage prior to the paroxysmal 2010 eruption of Mount Merapi,Indonesia     

Saskia?ErdmannEmail author  Caroline?Martel  Michel?Pichavant  Alexandra?Kushnir 《Contributions to Mineralogy and Petrology》2014,167(6):1016

Amphibole is widely employed to calculate crystallization temperature and pressure, although its potential as a geobarometer has always been debated. Recently, Ridolfi et al. (Contrib Mineral Petrol 160:45–66, 2010) and Ridolfi and Renzulli (Contrib Mineral Petrol 163:877–895, 2012) have presented calibrations for calculating temperature, pressure, fO₂, melt H₂O, and melt major and minor oxide composition from amphibole with a large compositional range. Using their calibrations, we have (i) calculated crystallization conditions for amphibole from eleven published experimental studies to examine the problems and the potential of the new calibrations; and (ii) calculated crystallization conditions for amphibole from basaltic–andesitic pyroclasts erupted during the paroxysmal 2010 eruption of Mount Merapi in Java, Indonesia, to infer pre-eruptive conditions. Our comparison of experimental and calculated values shows that calculated crystallization temperatures are reasonable estimates. Calculated fO₂ and melt SiO₂ content yields potentially useful estimates at moderately reduced to moderately oxidized conditions and intermediate to felsic melt compositions. However, calculated crystallization pressure and melt H₂O content are untenable estimates that largely reflect compositional variation in the crystallizing magmas and crystallization temperature and not the calculated parameters. Amphibole from Merapi’s pyroclasts yields calculated conditions of ~200–800 MPa, ~900–1,050 °C, ~NNO + 0.3–NNO + 1.1, ~3.7–7.2 wt% melt H₂O, and ~58–71 wt% melt SiO₂. We interpret the variations in calculated temperature, fO₂, and melt SiO₂ content as reasonable estimates, but conclude that the large calculated pressure variation for amphibole from Merapi and many other arc volcanoes is evidence for thorough mixing of mafic to felsic magmas and not necessarily evidence for crystallization over a large depth range. In contrast, bimodal pressure estimates obtained for other arc magmas reflect amphibole crystallization from mafic and more evolved magmas, respectively, and should not necessarily be taken as evidence for crystallization in two reservoirs at variable depth.  相似文献