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Eve L. Berger Thomas J. Zega Dante S. Lauretta 《Geochimica et cosmochimica acta》2011,75(12):3501-3513
The discovery of nickel-, copper-, and zinc-bearing iron sulfides from comet 81P/Wild 2 (Wild 2) represents the strongest evidence, in the Stardust collection, of grains that formed in an aqueous environment. We investigated three microtomed TEM sections which contain crystalline sulfide assemblages from Wild 2 and twelve thin sections of the hydrothermally altered CI chondrite Orgueil. Detailed structural and compositional characterizations of the sulfide grains from both collections reveal striking similarities. The Stardust samples include a cubanite (CuFe2S3) grain, a pyrrhotite [(Fe,Ni)1−xS]/pentlandite [(Fe,Ni)9S8] assemblage, and a pyrrhotite/sphalerite [(Fe,Zn)S] assemblage. Similarly, the CI-chondrite sulfides include individual cubanite and pyrrhotite grains, cubanite/pyrrhotite assemblages, pyrrhotite/pentlandite assemblages, as well as possible sphalerite inclusions within pyrrhotite grains. The cubanite is the low temperature orthorhombic form, which constrains temperature to a maximum of 210 °C. The Stardust and Orgueil pyrrhotites are the 4C monoclinic polytype, which is not stable above ∼250 °C. The combinations of cubanite and pyrrhotite, as well as pyrrhotite and pentlandite signify even lower temperatures. The crystal structures, compositions, and petrographic relationships of these sulfides constrain formation and alteration conditions. Taken together, these constraints attest to low-temperature hydrothermal processing.Our analyses of these minerals provide constraints on large scale issues such as: heat sources in the comet-forming region; aqueous activity on cometary bodies; and the extent and mechanisms of radial mixing of material in the early nebula. The sulfides in the Wild 2 collection are most likely the products of low-temperature aqueous alteration. They provide evidence of radial mixing of material (e.g. cubanite, troilite) from the inner solar system to the comet-forming region and possible secondary aqueous processing on the cometary body. 相似文献
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Abstract— The properties of compound chondrules and the implications that they have for the conditions and environment in which chondrules formed are investigated. Formulae to calculate the probability of detecting compound chondrules in thin sections are derived and applied to previous studies. This reinterpretation suggests that at least 5% of chondrules are compounds, a value that agrees well with studies in which whole chondrules were removed from meteorites. The observation that adhering compounds tend to have small contact arcs is strengthened by application of these formulae. While it has been observed that the secondaries of compound chondrules are usually smaller than their primaries, these same formulae suggest that this could be an observation bias. It is more likely than not that thin section analyses will identify compounds with secondaries that are smaller than their primaries. A new model for chondrule collisional evolution is also developed. From this model, it is inferred that chondrules would have formed, on average, in areas of the solar nebula that had solids concentrated at least 45 times over the canonical solar value. 相似文献
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Francesco De Pascale Marcello Bernardo Francesco Muto Dante Di Matteo Valeria Dattilo 《Natural Hazards》2017,86(2):569-586
The last few years have seen the debate on the geoethics of environmental and climatic protection growing to include resilience as a central idea within this new discipline, which holds many similarities with geography. Resilience analysis often looks at the capacity to re-establish conditions of equilibrium within a system which has been hit by a serious shock, e.g. a natural or man-made disaster. Geoethics works, in tandem with geological analyses and the geography of risk, to inform a population and develop integrated risk management in such a way as to strengthen a community’s resilience. The aim of this work is to study some people’s capacity to overcome what was potentially a disastrous event and, through a process of reconstruction, turn it into an occasion for growth. The experiment, carried out in the primary and middle schools in Aiello Calabro (Calabria, southern Italy), was conducted on the basis of the belief that there is a close relationship between a population’s having a realistic understanding of the risk of such an event, e.g. an earthquake, and high levels of resilience. We also tried to gain an insight into the relationship that may exist between resilience in primary and secondary school children and methods of coping which give an appropriate management of seismic risk. To be more precise, we try to discover whether there is a link between good/appropriate resilience and good/appropriate risk management. 相似文献
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Michael G. Babechuk Balz S. Kamber Dante Canil 《Geochimica et cosmochimica acta》2010,74(4):1448-2136
Tungsten is a moderately siderophile high-field-strength element that is hydrophile and widely regarded as highly incompatible during mantle melting. In an effort to extend empirical knowledge regarding the behaviour of W during the latter process, we report new high-precision trace element data (W, Th, U, Ba, La, Sm) that represent both terrestrial and planetary reservoirs: MORB (11), abyssal peridotites (8), eucrite basalts (3), and carbonaceous chondrites (8). A full trace element suite is also reported for Cordilleran Permian ophiolite peridotites (12) to better constrain the behaviour of W in the upper mantle. In addition, we report our long-term averages for a number of USGS (BIR-1, BHVO-1, BHVO-2, PCC-1, DTS-1) and GSJ (JA-3, JP-1) standard reference materials, some of which we conclude to be heterogeneous and contaminated with respect to W. The most significant finding of this study is that many of the highly depleted upper mantle peridotites contain far higher W concentrations than expected. In the absence of convincing indications for alteration, re-enrichment or contamination, we propose that the W excess was caused by retention in an Os-Ir alloy phase, whose stability is dependent on fO2 of the mantle source region. This explanation could help to account for the particularly low W content of N-MORB and implies that the lithophile behaviour of W in basaltic rocks is not an accurate representation of the behaviour in the melt source. These findings then become relevant to the interpretation of W-isotopic data for achondrites, where the fractionation of Hf from W during melting is used to infer the Hf/W of the parent body mantle. This is exemplified by the differentiation chronology of the eucrite parent body (EPB), which has been modeled with a melt source with high Hf/W. By contrast, we explore the alternative scenario with a low mantle Hf/W on the EPB. Using available eucrite literature data, a maximum core segregation age of 1.2 ± 1.2 Myr after the closure of CAIs is calculated with a more prolonged time between core formation and mantle fractionation of ca. 2 Myr. This timeline is consistent with most recent published chronologies of the EPB differentiation based on the 53Mn-53Cr and 26Al-26Mg systems. 相似文献
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Robert K. Smither Khaliefeh Abu Saleem Dante E. Roa Mark A. Beno Peter Von Ballmoos Gerry K. Skinner 《Experimental Astronomy》2005,20(1-3):201-210
A major goal of the MAX program is to detect and measure gamma rays produced following the nuclear reactions that take place in a supernova explosion. To detect a reasonable number of supernovae, sensitivities of the order of a few times 10-7 γ cm-2sec-1 are needed – much better than possible with current instruments. The approach in the MAX program is to use a crystal diffraction lens to collect photons over a large area and concentrate them on a small well-shielded detector, greatly improving the signal to noise ratio. The crystals need to have both high diffraction efficiency and a relatively broad energy bandwidth. With mosaic crystals there is a trade-off between bandwidth and diffraction efficiency – one can have either high efficiency or large bandwidth, but not both without losing too much intensity through atomic absorption. A recent breakthrough in our understanding of crystal diffraction for high-energy gamma rays has made it possible to develop crystals that have both high diffraction efficiency and a relatively broad energy bandwidth. These crystals have near perfect crystal structure, but the crystalline planes are slightly curved. Such curved planes can be obtained in 3 different ways, by using mixed crystals with a composition gradient, by applying a thermal gradient, and by mechanically bending a near perfect crystal. A series of experiments have been performed on all three types of crystals using high-energy x-ray beams from the Advanced Photon Source at the Argonne National Laboratory. Experiments performed at 3 energies, 93 keV, 123 keV and 153 keV, with both the thermal gradient Si crystals and with the mechanically bent Si crystals, demonstrated that one can achieve diffraction efficiencies approaching 100% with moderate energy bandwidths (ΔE/E = 1.4%) and low atomic absorption (transmission = 0.65), in excellent agreement with theory. The use of this type of diffraction crystal is expected to increase the sensitivity of gamma ray telescopes by a factor of 5 over that possible with normal mosaic crystals. 相似文献
49.
DanteSalvini 《地球空间信息科学学报》2002,5(1):18-21
One type of aerial cableway consists of a continuous cable,which works at the same time as carrying cable and drawing cable.This kind of cableway is supported by many towers,which divide the total length in linear sections.even small deviations from the planed layout can increase the strain of the cable and the danger of derailments.The deviations of the position of the towers from the planed layout are due to local sliding or sinking of the foundation or other mechanical movements.Up to now the axis of the cableways were periodically measured by traditional methods. The goal of this project is to demonstrate the possibility of measuring the axis of a cableway with GPS techniques,while the cable is moving.The results show that the proposed measuring device provides precise results in a simple and reliable way. 相似文献
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