Silicate liquid immiscibility in impact melts          下载免费PDF全文

Christopher Hamann  Agnese Fazio  Matthias Ebert  Lutz Hecht  Richard Wirth  Luigi Folco  Alex Deutsch  Wolf Uwe Reimold 《Meteoritics & planetary science》2018,53(8):1594-1632

We have investigated silicate emulsions in impact glasses and impact melt rocks from the Wabar (Saudi Arabia), Kamil (Egypt), Barringer (USA), and Tenoumer (Mauritania) impact structures, and in experimentally generated impact glasses and laser-generated glasses (MEMIN research unit) by scanning electron microscopy, electron microprobe analysis, and transmission electron microscopy. Textural evidence of silicate liquid immiscibility includes droplets of one glass disseminated in a chemically distinct glassy matrix; sharp phase boundaries (menisci) between the two glasses; deformation and coalescence of droplets; and occurrence of secondary, nanometer-sized quench droplets in Si-rich glasses. The compositions of the conjugate immiscible liquids (Si-rich and Fe-rich) are consistent with phase separation in two-liquid fields in the general system Fe₂SiO₄–KAlSi₃O₈–SiO₂–CaO–MgO–TiO₂–P₂O₅. Major-element partition coefficients are well correlated with the degree of polymerization (NBO/T) of the Si-rich melt: Fe, Ca, Mg, and Ti are concentrated in the poorly polymerized, Fe-rich melt, whereas K, Na, and Si prefer the highly polymerized, Si-rich melt. Partitioning of Al is less pronounced and depends on bulk melt composition. Thus, major element partitioning between the conjugate liquids closely follows trends known from tholeiitic basalts, lunar basalts, and experimental analogs. The characteristics of impact melt inhomogeneity produced by melt unmixing in a miscibility gap are then compared to impact melt inhomogeneity caused by incomplete homogenization of different (miscible or immiscible) impact melts that result from shock melting of different target lithologies from the crater's melt zone, which do not fully homogenize and equilibrate due to rapid quenching. By taking previous reports on silicate emulsions in impact glasses into account, it follows that silicate impact melts of variable composition, cooling rate, and crystallization history might readily unmix during cooling, thereby rendering silicate liquid immiscibility a much more common process in the evolution of impact melts than previously recognized.  相似文献   

Characteristics of quadrant and octant advection schemes in wave models     

Luigi Cavaleri  Mauro Sclavo 《Coastal Engineering》1998,34(3-4)

We point out one problem of the grid advection schemes when used in wave models in coastal areas. The deficiency of the schemes is investigated by means of the ‘third' generation WAM wave model, in which the wave energy is advected by a first order upwind scheme. Two similar, alternative modifications of this scheme are analyzed, the second of which is shown to solve most of the problems encountered with advection along the co-ordinate axes.  相似文献   

Mass function of dormant black holes and the evolution of active galactic nuclei     

Paolo Salucci  Ewa Szuszkiewicz  Pierluigi Monaco  Luigi Danese 《Monthly notices of the Royal Astronomical Society》1999,307(3):637-644

Under the assumption that accretion on to massive black holes (BHs) powers active galactic nuclei (AGNs), the mass function (MF) of the BHs responsible for their past activity is estimated. For this, we take into account not only the activity related to the optically selected AGNs, but also that required to produce the hard X-ray background (HXRB). The MF of the massive dark objects (MDOs) in nearby quiescent galaxies is computed by means of the most recent results on their demography. The two mass functions match well under the assumption that the activity is concentrated in a single significant burst with λ L L _Edd being a weakly increasing function of luminosity. This behaviour may be indicative of some level of recurrence and/or of accretion rates insufficient to maintain the Eddington rates in low-luminosity/low-redshift objects. Our results support the scenario in which the early phase of intense nuclear activity occurred mainly in early-type galaxies (E/S0) during the relatively short period in which they still had an abundant interstellar medium. Only recently, with the decline of the quasi-stellar object (QSO) luminosities, did the activity in late‐type galaxies (Sa/Sab) become statistically significant.  相似文献   

Sequence stratigraphy based on high-resolution seismic profiles in the late Pleistocene and Holocene deposits of the Venice area   总被引：2，自引：0，他引：2  

Massimo Zecchin  Luca Baradello  Giuliano Brancolini  Federica Donda  Federica Rizzetto  Luigi Tosi 《Marine Geology》2008,253(3-4):185-198

The sequence-stratigraphic investigation by Very High-Resolution (VHR) seismic profiles allowed recognition of the detailed architecture of the late Pleistocene and Holocene succession of the Venice area. In this way deposits previously known by the analyses of scattered cores, mainly taken along the lagoon margin and the littoral strips, have been correlated at regional scale including the near offshore sector and the result has pointed out the lateral variability of the stratal architecture. Late Pleistocene deposits consist of an aggrading floodplain and fluvial channel fills accumulated during decreasing eustatic sea level, and they are coeval with offlapping forced regressive marine wedges in the Central Adriatic basin. The Holocene sequence is composed of three main seismic units separated by major stratal surfaces. Unit 1 (up to 9 m thick) is formed by channelized deposits separated by areas showing sub-horizontal and hummocky reflectors, and is bounded at the base by a surface that records prolonged conditions of subaerial exposure and at the top by a flatter surface resulting from erosion by marine processes. Deposits of Unit 1 are interpreted as estuarine and distributary channel fills, and back-barrier strata. Unit 2 is well distinguishable from Unit 1 only in the offshore area and at the barrier island bounding the Venice Lagoon, and is composed of a prograding marine wedge (up to 10 m thick) that interacts laterally with ebb tidal deltas. Unit 3 consists of a tidal channel complex and inlet deposits, which testify the evolution of the lagoon area. Tidal channels are entrenched in the lagoon mud flat (coeval with Units 1–2) and cut the Pleistocene–Holocene boundary in several places.Following current sequence-stratigraphic concepts, the Holocene sequence is composed of a paralic transgressive systems tract (TST) (Unit 1) overlying a sequence boundary (the Pleistocene–Holocene boundary) and overlain by a marine highstand systems tract (HST) (Unit 2) in seaward locations and by highstand lagoonal deposits landwards. TST and HST are separated by a downlap surface that is amalgamated with a wave ravinement surface in several places. Unit 3 is coeval with the upper part of Unit 2, and its development has been favoured by human interventions, which led to a transgression limited to the lagoon area.Local factors during the deposition, i.e. subsidence, sediment supply, physiography, and current/wave regimes, led to a significant lateral variability in the architecture of the Holocene sequence, as evidenced by the extreme thickness variation of the TST along both depositional strike and dip. The HST, instead, shows less pronounced strike variations in the stratal architecture. Also, present data clearly evidence that the human impact has a great relevance in influencing the late Holocene sedimentation.  相似文献   

Comparison between epiphyte assemblages of leaves and rhizomes of the seagrass Posidonia oceanica subjected to different levels of anthropogenic eutrophication     

David Balata  Iacopo Bertocci  Luigi Piazzi  Ugo Nesti 《Estuarine, Coastal and Shelf Science》2008,79(3):533-540

This paper aimed to compare epiphyte assemblages of leaves and rhizomes of Posidonia oceanica exposed to different levels of concentration of nutrients. The same design including a potentially impacted meadow and two reference meadows was used in each of two locations, characterized by the presence of a city or of suspended cages of a fish farm, respectively. This allowed to test for the consistency of responses of epiphytic assemblages to different sources of eutrophication. In both studies, results documented differences in patterns of composition and abundance of epiphytic assemblages on leaves between disturbed and reference meadows, while assemblages on rhizomes did not appear sensitive to this kind of disturbance. Moreover, in potentially impacted meadows, both assemblages showed different patterns of spatial variability compared to reference assemblages. Species composition and abundance of epiphyte assemblages seemed suitable for detecting moderate nutrient increases, even if adequate sampling designs are needed to separate patterns related to the large natural spatial variability of these systems from those related to changes in environmental conditions.  相似文献   

Jonathan I. Lunine,Earth: Evolution of a Habitable World     

Foschini  Luigi 《Earth, Moon, and Planets》1998,81(2):177-177

Earth, Moon, and Planets -  相似文献   

Geochemical trends in the alkali basaltic suite of the Assab range (Ethiopia)     

Lucia Civetta  Maria De Fino  Luigi La Volpe  Lucio Lirer 《Chemical Geology》1974,13(3):149-162

Two groups of rocks are distinguished in the Assab volcanic range (Afar) on the basis of their petrology. The first one is a typical alkaline suite with high Li, Ba, Rb, Sr, Th and U concentrations. A second subalkaline group may be related to a parental basalt characterized by low trace-element concentrations and intermediate in character between alkali and tholeiitic basalt.U and Th concentrations of the analyzed Assab basalts, as well the reported values for the alkali and transitional basalts of the East African Rift and the Afar depression, are higher than values reported for similar basalts from oceanic and continental environments.  相似文献   

On the shape and orientation of the tail of Comet Kohoutek (1973f)     

Angelo Bernasconi  Giuseppe Forti  Luigi Pansecchi 《Icarus》1977,31(3):401-411

Twenty-two photographs of Comet Kohoutek (1973f) have been projected onto the comet's orbital plane under the assumption of a flat tail confined in that plane. The comet has a mixed-type tail; therefore the results concerning Type I and Type II components of the tail are presented separately. The axis of the Type I tail appears to sweep back and forth with respect to the prolonged radius vector in a rather periodical way. Interpretations advanced by some authors for the analogous case of Comet Burnham (1959k) are mentioned briefly. For the Type II tail, a comparison with a theoretical tail model by Sekanina has allowed us to establish when the onset of appreciable dust production occurred. Finally, mention is made of the fact that some peculiarities of the dust tail might be also explained by assuming a three-dimensional model.  相似文献   

Catchment‐wide denudation rates at the margin of NE Tibet from in situ‐produced cosmogenic 10Be     

Luigi Palumbo  Ralf Hetzel  Mingxin Tao  Xiaobin Li 《地学学报》2011,23(1):42-48

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An integrated stratigraphic record of the Palaeocene–lower Eocene at Gubbio (Italy): new insights into the early Palaeogene hyperthermals and carbon isotope excursions     

Rodolfo Coccioni  Giuseppe Bancalà  Rita Catanzarit  Eliana Fornaciari  Fabrizio Frontalini  Luca Giusberti  Luigi Jovane  Valeria Luciani  Jairo Savian  Mario Sprovieri 《地学学报》2012,24(5):380-386

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