1,722,316 research outputs found
RELATIVE CONTRIBUTIONS OF PRECAMBRIAN METAMORPHIC ROCKS AND CRETACEOUS-TERTIARY IGNEOUS ROCKS TO OLIGOCENE AND HOLOCENE FLUVIAL SANDS AND THE UNROOFING OF A MAGMATIC ARC
No full textBreve introduzione alla petrologia della crosta lunare e all’origine della Luna.
No full textGalileo (1610) fu il primo scienziato, circa quattro secoli fa, ad iniziare una osservazione scientifica e sistematica della luna dalla terra. Il primo campione lunare fu portato sulla terra nel 1969 e da allora e` incominciata una nuova era di esplorazione geologica della luna. In questo lavoro si presenta una sintesi dell'attuale livello di conoscenza nel campo della petrologia della crosta lunare ed una rassegna delle teorie sull'origine di questo corpo celeste. La nostra speranza e` quella di incoraggiare la comunita` dei geologi italiani a prendere parte alla ricerca scientifica che riguarda la luna. I campioni lunari sono conservati alla NASA e disponibili per tutti gli scienziati qualificati che li volessero studiare.
Le aree chiare e quelle scure visibili sulla luna sono fisiograficamente rilievi montuosi e bacini (mari) rispettivamente; i primi sono costituiti da rocce anortositiche ricche in Al, i secondi da basalti ricchi in Fe. I campioni studiati hanno mostrato quattro principali clans di rocce nella crosta lunare: le anortositi ferrose (eta` media 4.4 miliardi di anni), le rocce plutoniche ricche in Mg (4.2- 4.45 miliadri di anni), i basalti KREEP (arricchiti in K, REE, P, eruttate tra 3.8 e 4.0 miliardi di anni) e i basalti di mare ricchi in Fe (3.2- 4.2 miliadri di anni). Un unico oceano magmatico (profondo tra i 300 km e i 1000 km) si differenzio` circa 4.5 miliadri di anni fa con la migrazione dei plagioclasi verso l'alto e la cristallizzazione di minerali femici sul fondo. La fusione parziale della parte interna ed eterogenea della luna cosi` differenziata genero` il vulcanesimo che erutto` basalti KREEP e basalti di mare. Confrontata alla terra e ad altri pianeti, la luna e` impoverita di elementi volatili e di ferro, mentre la composizione della sua massa e` probabilmente molto simile a quella del mantello terrestre. La luna e la terra hanno anche un'identico comportamento degli isotopi dell' ossigeno.
Le interpretazioni dei dati geologici (sensu lato) accettate correntemente da tutti hanno portato per l'origine della luna alla nuova Giant Impact Hypothesis. L'ipotesi suggerisce che dopo la differenziazione della terra a formare un nucleo a composizione Fe-Ni e un mantello silicatico, un corpo delle dimensioni di Marte cadde su di essa. Il corpo che la colpi` evaporò ed espulse, in una nube che entro` in orbita attorno alla terra, una grande quantita` di detrito silicatico riscaldato proveniente dal mantello terrestre. Questa nube orbitante attorno alla terra si aggregò molto velocemente per formare la luna. L' origine così violenta della luna puo` spiegare il suo impoverimento in elementi volatili e in ferro, il comportamento degli isotopi dell'ossigeno e la presenza di un unico oceano magmatico.
Galileo (1610) started a systematic earth-based scientific observation of the moon about four centuries ago. The first samples from the moon were brought back to the earth in 1969 and a new era of geological exploration of the moon began. We present a summary of the present state of knowledge about the petrology of the lunar crust and review the theories for the origin of the moon. Our hope is to encourage the Italian geologic community to participate in lunar science research. Lunar samples are available from NASA for research to qualified scientists all over the world.
Physiographically, the light and dark areas of the moon are highlands and basins (mare), and are made up of Al-rich anorthositic rocks and Fe-rich basalts respectively. Sample studies have shown that ferroan anorthosites (average age is 4.4 b.y.), Mg-rich plutonic rocks (4.2 - 4.45 b.y.), KREEP basalts (enriched in K, REE, and P; erupted between 3.8 and 4.0 b.y.), and Fe-rich mare basalts (3.2 - 4.2 b.y.) are the four principal clans of rock-types in the lunar crust. A deep (estimated from 300km to 1000km) global magma ocean on the moon differentiated about 4.5 b.y. ago to float plagioclases to the top and crystallized mafic minerals towards the bottom. Partial melting of the heterogeneous interior of the differentiated moon generated volcanism that erupted KREEP basalts and mare basalts. Relative to the earth and other terrestrial planets, the moon is depleted in all volatile elements and also in iron. Otherwise, the bulk composition of the moon is probably very similar to that of the mantle of the earth. The moon and the earth also have identical oxygen isotope systematics.
The facts and currently accepted interpretations of all geological data (sensu lato) have led to the new Giant Impact Hypothesis for the origin of the moon. The hypothesis states that after the early earth differentiated to form a Fe-Ni core and a silicate mantle, a mars-sized body impacted on it. The result was to vaporize the impactor and eject a large amount of heated silicate debris from the earth's mantle into an earth-orbiting cloud. This earth-orbiting cloud accreted very quickly to form the moon. Such a violent birth of the moon explains its depletion in volatile elements and in iron, its oxygen isotope systematics, and its global magma ocean
Toxic metals in Venice Lagoon sediments: Model, observation, and possible removal
No full textWe have modeled the distribution of nine toxic metals in the surface sediments from 163 stations in the Venice lagoon using published data (Donazzolo and others 1984). Three entrances from the Adriatic Sea control the circulation in the lagoon and divide it into three basins. We assume, for purposes of modeling, that Porto Marghera at the head of the industrial Zone area is the single source of toxic metals in the Venice lagoon.
In a standing body of lagoon-water, concentration of pollutants at distance x from the source (Co) may be given by C = Coe-kx where k is the rate constant of dispersal. We calculated k empirically using concentrations at the source, and those at farthest that is end-points of the lagoon. Average k (ppm/km) in the lagoon are : Zn 0.165; Cd 0.116; Hg 0.110; Cu 0.105; Co 0.072; Pb 0.058; Ni 0.008; Cr (0.011) and Fe (0.018 percent/km) have complex distributions. Given the k values, concentration at source (Co), and the distance x of any point in the lagoon from the source, we have calculated the model concentrations of the nine metals at each sampling station.
Tides, currents, floor morphology, additional sources, and continued dumping perturb model distributions causing anomalies (observed minus model concentrations). Positive anomalies are found near the source where continued dumping perturbs initial boundary conditions, and in areas of sluggish circulation.
Negative anomalies are found in areas with strong currents that may flush sediments out of the lagoon. We have
thus identified areas in the lagoon where higher rate of sediment removal and exchange may lessen pollution
Provenance characteristics of detrital opaque Fe-Ti oxide minerals.
No full textWe have investigated petrogenetic characteristics of detrital opaque Fe-Ti oxide minerals (DOPQ) in sand samples collected from Holocene streams draining exclusively igneous and metamorphic source rocks in the Rocky Mountain region under a semi-arid climate, and from the Appalachian Mountains under a humid climate. Our sampling strategy allows us to compare and contrast the properties of detrital mineral grains not only from igneous and metamorphic source rocks, but also those from dissected-arc and recycled-orogen provenances. Reflected-light petrography including extensive modal analysis and electron probe microanalysis of polished grain mounts were the two principal methods of data gathering. Between 25% and 50% of all DOPQ show oxidation exsolution and/or other lamellar intergrowth textures, i.e., they are poly-mineralic grains. Lamellar intergrowth texture is present in about twice as many grains derived from igneous source rocks as from metamorphic source rocks. Lamellae thinner than 2 µm are predominant in grains derived from dissected-arc provenance, whereas lamellae thicker than 10 µm are predominant in grains derived from recycled-orogen provenance. Exsolution lamellae along {111} planes of magnetite (three directions) and presumably along {0001} planes of ilmenite (one direction) characterize dissected-arc and recycled-orogen provenance, respectively. Detrital ilmenites derived from the igneous source rocks studied show a wide range of TiO2 content with a mode around 47%, whereas those from the metamorphic source rocks show a tight cluster around 52% TiO2 content. DOPQ derived from metamorphic source rocks contain < 0.5% MgO, whereas those from igneous rocks may contain up to ~1.2% MgO. Both MgO and MnO are low, 0.3% and 2.5%, respectively, in DOPQ derived from dissected-arc provenance, but may be relatively high, up to 1.2% and 8.0%, respectively, in grains derived from recycled-o.rogen provenance.
Whereas no single character of DOPQ may be diagnostic of provenance, discriminant function analysis of the complete set of data show that collectively they are diagnostic in 95% to 100% of cases. We recommend the use of 1) presence of exsolution texture, 2) minimum lamellae widths and the number of crystallographic orientation of the lamellae, and 3) chemical composition including minor elements of DOPQ as important properties far provenance identification. This should supplement and refine existing criteria far provenance determination
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Does the diversity of physical and chemical properties of detrital zircons and garnets decrease with increasing maturity of sediments?
No full textThe distribution of the compositions and shapes of detrital zircons and that of the compositions and colors of detrital garnets in the Oligocene Renova Formation (RE), Miocene Six Mile Creek Formation (SX), and Holocene sands (HO) in southwestern Montana have been analyzed statistically to understand the evolution of their diversity through lime. Field, stratigraphic, and provenance relations indicate that the cumulative maturity of the sediments increase in the order HO-RE-SX although Renova rocks had been buried most deeply. A multivariate analysis of chemical and petrographic data on detrital zircons and garnets from these sediments, all fluvial in origin, shows that the diversity of zircon properties decreases in the arder HO-RE-SX bui that of garnets does not show any systematic patter. In general, however, diversity of garnet properties seems to be least in the Renova. We conclude that in these sediments: 1) the inherited diversity of physical and chemical properties of detrital zircons decreases with increasing maturity of sediments: 2) not all zircons are as durable as commonly believed; and 3) the inherited diversity of physical and chemical properties of detrital garnets is affected more by diagenesis than other sediment-maturing processes
Saffronization of a Land: Is it possible to separate Hinduism from Hindutva in India?
No full textDatta A, Basu A. Saffronization of a Land: Is it possible to separate Hinduism from Hindutva in India? Transcience: A Journal of Global Studies. 2023;14(1 of 2):43-50
Déjà vu: A botched memory operation, illegitimate to start with
No full textRather than a natural product, a computational analysis leads us to characterize déjà vu as a failure of memory retrieval, linked to the activation in neocortex of familiar items from a compositional memory in the absence of hippocampal input, and to a misappropriation by the self of what is of others
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