排序方式： 共有16条查询结果，搜索用时 71 毫秒
1.
Effect of heat treatment on tourmaline from Brazil 总被引：4，自引：0，他引：4
Cristiane Castañeda Sigrid G. Eeckhout Geraldo Magela da Costa Nilson F. Botelho Eddy De Grave 《Physics and Chemistry of Minerals》2006,33(3):207-216
The crystal-chemical behaviour of tourmaline from Araçuaí, Minas Gerais state, Brazil, when subjected to heating in air atmosphere has been studied by several techniques, including EMPA, UV–Vis, TGA, and Mössbauer spectroscopy. The tourmaline samples are typically intermediate members of the elbaite-schorl series. The origin of colour and of its change after treatment has been discussed in terms of local disorder, presence of metal transition elements, oxidation of ferrous iron at the octahedral site, and simultaneous trap of the excess electron. These findings may be used to enhance the colour in tourmaline crystals or generate wanted colour changes. 相似文献
2.
Marc Peters Harald Strauss James Farquhar Charlotte Ockert Benjamin Eickmann Cristiane L. Jost 《Chemical Geology》2010,269(3-4):180-196
The role of sulfur in two hydrothermal vent systems, the Logatchev hydrothermal field at 14°45′N/44°58′W and several different vent sites along the southern Mid-Atlantic Ridge (SMAR) between 4°48′S and 9°33′S and between 12°22′W and 13°12′W, is examined by utilizing multiple sulfur isotope and sulfur concentration data. Isotope compositions for sulfide minerals and vent H2S from different SMAR sites range from + 1.5 to + 8.9‰ in δ34S and from + 0.001 to + 0.051‰ in Δ33S. These data indicate mixing of mantle sulfur with sulfur from seawater sulfate. Combined δ34S and Δ33S systematics reveal that vent sulfide from SMAR is characterized by a sulfur contribution from seawater sulfate between 25 and 33%. This higher contribution, compared with EPR sulfide, indicates increased seawater sulfate reduction at MAR, because of a deeper seated magma chamber and longer fluid upflow path length, and points to fundamental differences with respect to subsurface structures and fluid evolution at slow and fast spreading mid-ocean ridges.Additionally, isotope data uncover non-equilibrium isotopic exchange between dissolved sulfide and sulfate in an anhydrite bearing zone below the vent systems at fluid temperatures between 335 and 400 °C. δ34S values between + 0.2 to + 8.8‰ for dissolved and precipitated sulfide from Logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at SMAR. δ34S values between ? 24.5 and + 6.5‰ and Δ33S values between + 0.001 and + 0.125‰ for sulfide-bearing sediments and mafic/ultramafic host rocks from drill cores taken in the region of Logatchev indicate a clear contribution of biogenic sulfides formed via bacterial sulfate reduction. Basalts and basaltic glass from SMAR sites with Δ33S = ? 0.008‰ reveal lower Δ33S lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials.We conclude that the combined use of both δ34S and Δ33S provides a more detailed picture of the sulfur cycling in hydrothermal systems at the Mid-Atlantic Ridge and uncovers systematic differences to hydrothermal sites at different mid-ocean ridge sites. Multiple sulfur isotope measurements allow identification of incomplete isotope exchange in addition to isotope mixing as a second important factor influencing the isotopic composition of dissolved sulfide during fluid upflow. Furthermore, based on Δ33S we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when δ34S were identical. 相似文献
3.
4.
Gonçalves Leonardo Alkmim Fernando F. Pedrosa-Soares Antônio Gonçalves Cristiane C. Vieira Valter 《International Journal of Earth Sciences》2018,107(1):337-358
International Journal of Earth Sciences - The Araçuaí-West Congo orogen (AWCO) is one of the various components of the Brasiliano/Pan-African orogenic network generated during the... 相似文献
5.
6.
Michel Brossard Gustavo Adolfo de Freitas Fregonezi Maria de Fátima Guimarães Cristiane de Conti Medina Amarildo Pasini Lourival Vilela Miguel Angel Ayarza 《Comptes Rendus Geoscience》2012,344(1):41-49
Ferralsols are characterized by poorly-defined horizons, weak macrostructure and strong development of a fine granular structure comprising subangular micro-aggregates. In this study, the morphological and physical modifications caused by earthworm activity in a clayey ferralsol were analysed. After describing soil structures, undisturbed samples were taken for evaluating aggregates and determining clod bulk density and particle density. Soil water retention properties were measured and an inventory of soil invertebrate macrofauna was created. The structural and porous transformations were due to aggregates created by earthworm activity. Changes in bulk density can be associated with pore modifications caused by a change in the proportions of aggregate types, and a notable reduction of total porosity was measured, tending to increase soil volume with dense aggregates. Structural modifications affected the topsoil down to 0.5 m and water retention between −1 and −33 kPa, the principal water compartment of these soils. 相似文献
7.
This article reviews the literature involving benthic invertebrates that are cited in association with hull fouling, reporting the species that occur on the Brazilian coast and evaluating the importance of this vector for the introduction of nonindigenous and cryptogenic invertebrates in Brazil. It discusses some of the strategies that were used by the species that allowed for their overseas transport and made it easier to cross natural barriers that otherwise would have been obstacles to their dispersion. The compiled data list 343 species (65% nonindigenous and 35% cryptogenic), mainly from the northwestern Atlantic Ocean. The traveling fauna, composed mostly of cosmopolitan species (70.3%), is primarily euryhaline and marine stenohaline, with sessile and sedentary habits. After delineating the shipborne species’ ecological profiles and traveling strategies and evaluating their overlapping vectors, we concluded that hull vessels were the main vector of introduction to the Brazilian coast for 89.8% of the compiled species. 相似文献
8.
The Pitman Canyon paleoseismic record: A re-evaluation of southern San Andreas Fault segmentation 总被引：2，自引：0，他引：2
Correlation of three well-resolved paleoseismic records, including the Pitman Canyon site with its emerging record, presents a new possibility to understand fault segmentation. To be a useful concept, fault segment boundaries must be relatively stationary over multiple seismic cycles and must appear frequently in limiting the rupture extent of earthquakes; thus, sites on the same segment should share more paleoseismic events than those on different segments. A conclusive event correlation between sites is difficult or impossible due to dating uncertainties. However, often the data are adequate to preclude correlation and thus provide firm limits on rupture extent for those events. Thus clear non-correlations provide more information about segmentation than do unprovable potential correlations.The southern end of the most recent rupture in 1857, between Wrightwood and Pitman Canyon, is often used to define a segment boundary. However, there is an absence of significant non-correlation between the previous five Pitman Canyon events and the Wrightwood events. While both Pallett Creek and Wrightwood ruptured in 1857, only two of the previous five Wrightwood events can correlate with Pallett Creek events, which may or may not indicate that they actually do. These paleoseismic records do not support the existence of a segment boundary between Wrightwood and Pitman Canyon as defined by the 1857 rupture extent, suggesting a reevaluation of southern San Andreas Fault segmentation, and using historic ruptures to define segments in general. 相似文献
9.
Cristiane Bahi dos Santos-Fischer Jair Weschenfelder Iran Carlos Stalliviere Corrêa Jeffery Robert Stone Beatriz Appel Dehnhardt Eduardo Calixto Bortolin 《Estuaries and Coasts》2018,41(6):1601-1625
Drowning of the coast was initiated by a marine inundation after the Last Glacial Maximum (marine isotope stage (MIS) 2) and has continued during the mid-Holocene and highstand (MIS 1). Detailed analyses of two previously examined core stratigraphy and seismic profiles combined with new grain-size and detailed diatom analyses are used to study the history of the Barra Falsa paleochannel over the last 11,000 years BP and to document the peculiar deposition within the channel fill. A rapid sea-level rise was responsible for flooding the coast in 11,180–10,780, 8420–7930, 8150–7870, and 7640–7430 cal years BP, infilling a low topographic back-barrier region. High deposition rates suggest a rapid filling of the channel, which coincides with an accelerated period of sea-level rise, closely linked to the global 8.2-ka event. The morphology of the channel is recognized by facies units in the underlying strata related to one episode of cut and fill during a single cycle of base-level fall and rise. An overall transgressive sequence above the regional surface is related to marine and marine-brackish sediments, which corresponds to a seaward/central basil fill of a wave-dominated estuary. 相似文献
10.
This study tested the hypothesis that the community composition of Chironomidae will change according to the duration of the exposure of plant leaves in a subtropical stream. In addition, we expected the chemical quality of the detritus to affect the association of Chironomidae with decaying plant materials. The chironomid colonisation of leaf litter from Campomanesia xanthocarpa and Sebastiania brasiliensis was assessed in a subtropical Atlantic forest stream. Litter bags containing leaves from these two species were incubated in the stream for 22 days. We identified 7245 invertebrates associated with the leaf detritus of both plant species (5110 were Chironomidae, 70.7% of the total). Thirteen genera of Chironomidae were identified, with Rheotanytarsus, Thienemanniella and Corynoneura the most abundant (94% of all Chironomidae collected). Chironomidae assemblages changed with the duration of incubation in the stream. The dissimilarity and the functional composition of the Chironomidae communities were related to the chemical composition of the detritus. The chemical components that best explained the composition of the fauna were tannins, the C:N ratio, calcium, and magnesium. However, the functional feeding groups were explained by the chemical variables organic matter and C:N. This study found that the exposure time of the detritus and therefore its level of degradation and chemical composition are important factors determining the characteristics of the Chironomidae assemblage in subtropical streams. 相似文献