Mantle rheology: radial and lateral viscosity variations inferred from microphysical creep laws     

Giorgio Ranalli   《Journal of Geodynamics》2001,32(4-5)

Inferences on the rheology of the mantle based on theoretical and experimental rate equations for steady state creep are discussed and compared with results from geophysical models. The radial increase of viscosity by one to three orders of magnitude across the mantle, required by inversion of postglacial rebound and geodynamic data, is confirmed by microphysical models based on the estimation of continuous and discontinuous changes of creep parameters with depth. The upper mantle (viscosity 10²⁰–10²¹ Pa s) is likely to show non-Newtonian rheology (power-law creep) for average grain sizes larger than 0.1 mm as an order of magnitude. Given the variability of both grain size and stress conditions, local regions of linear rheology can be present. The rheology of transition zone and lower mantle (viscosity 10²²–10²⁴ Pa s) cannot be definitely resolved at present. Estimation of creep parameters leads to possible nonlinear or mixed rheology, if grain sizes are not lower than 0.1 mm and flow conditions can be approximated by a constant strain rate of about 10⁻¹⁵ s⁻¹. This conclusion can be modified by different flow conditions (e.g. a decrease in strain rate or constant viscous dissipation). Furthermore, experiments on fine-grained garnetites and perovskite analogues have shown that diffusion creep is predominant at laboratory conditions. However, the pressure dependence of creep in these phases is unknown, and therefore direct extrapolation to lower mantle conditions is necessarily speculative. Lateral variations of viscosity, largest in the upper and lowermost mantle (up to 2–4 orders of magnitude) are predicted by models based on lateral temperature anomalies derived from seismic tomographic models.  相似文献   

The relationship between websterite and peridotite in the Balmuccia peridotite massif (NW Italy) as revealed by trace element variations in clinopyroxene   总被引：7，自引：0，他引：7  

Rivalenti  Giorgio  Mazzucchelli  Maurizio  Vannucci  Riccardo  Hofmann  Albrecht W.  Ottolini  Luisa  Bottazzi  Piero  Obermiller  Walter 《Contributions to Mineralogy and Petrology》1995,122(3):275-288

 Field, mineralogical and petrological data are presented on a newly found carbonatite occurrence associated with “kamafugite” lava at Cupaello, central Italy. This carbonatite occurrence is part of the Late Pleistocene Umbria-Latium ultra-alkaline district (ULUD) which extends southwards within the Apennines to Mount Vulture, delineating an important magmatic province along the most peripheral belt of the Tyrrhenian extensional tectonic system. This province is distinct, but probably related genetically with the more abundant and common leucite-bearing assemblages of the Roman Comagmatic Region and represents the first reported occurrence of carbonatite assemblages in the Mediterranean Basin. The Cupaello suite indicates that primary or near-primary mantle silicate melts of “kamafugitic” composition are transitional with Ca-carbonatite liquid and provides direct evidence of immiscibility of carbonatite from “kamafugite” magma. It is inferred that a primary mantle origin of Ca-carbonatites is conditional upon a potential silicate magma that may be coupled with the carbonatite, but may not have reached the surface. The data indicate a strong genetic link between ULUD Ca-carbonatites and some African analogues, supporting the view that their genesis depends on similar source and associated tectonic conditions. Received: 17 January 1995 / Accepted: 14 June 1995  相似文献   

Geochemical study of graywackes as a possible starting material of para-amphibolites     

Giorgio Rivalenti  Gian Paolo Sighinolfi 《Contributions to Mineralogy and Petrology》1969,23(3):173-188

The possibility of para-amphibolites originating from the metamorphic transformation of graywackes is investigated. A series of 35 samples of Vosges graywackes has been studied and the geochemical trends for major and trace elements have been determined to this end. The strong similarity of the geochemical characteristics of this group of graywackes to those of a series of para-amphibolites of the Western Alps conforms to the present hypothesis. By comparing sedimentary geochemical trends with igneous trends one is able to distinguish ortho- from para-derivatives.  相似文献   

Fourier spectrum of the seiches of an ideal rectangular basin with sinusoidally varying level     

Giorgio Manzoni 《Pure and Applied Geophysics》1969,72(1):148-154

Summary If the level of an ideal rectangular water basin varies sinusoidally with amplitude 2h ₁ and angular frequency ₀ the modes of the free oscillations of the basin split into multiplets with spacing ₀: the shape and the height of the peaks are given as functions of the mean levelh ₀ and of the lengthl of the basin and of the amplitude 2h ₁.Istituto di Geodesia e Geofisica, Universita' di Trieste, Italy.- This research has been supported by E.R.O.-U.S. Army, under contract DA-91-591-3538.  相似文献   

Radiometric age evidences of a Triassic Thermal Event in the Southern Alps     

Prof. Giorgio Ferrara  F. Innocenti 《International Journal of Earth Sciences》1974,63(2):572-581

In the Southern Alps several K/Ar age determinations point out the existence of a thermal event of Upper-Triassic age. Radiometric evidences concern overprinting on K/Ar ages of biotites belonging to hercynian periadriatic massives, and of muscovites of pegmatites in which also a Sr isotopic re-equilibration has taken place. Some evidences are also recognizable in ancient metamorphic basement. In the same time an extensive extrusive and intensive magmatic activity of alkaline affinity occurred. These facts are interpreted as predrift events shortly preceeding the set up of lower Jurassic oceanic conditions of alpine area.  相似文献   

Über einen Zusammenhang zwischen atmosphärischen und kosmischen Phänomenen und physikalisch-chemischen Prozessen     

Giorgio Piccardi 《Theoretical and Applied Climatology》1955,6(4):486-505

Zusammenfassung Sei langer Zeit wird ein Zusammenhang zwischen atmosphärischen und kosmischen Vorgängen und physikalisch-chemischen biochemischen und biologischen Prozessen angenommen. Für Forschungen auf diesem Gebiete hat der Verfasser eine ganz einfache, differentialle Methode ausgearbeitet. Es werden Serien-Fällungen von BiOCl gleichzeitig mit normalem und mit physikalisch aktiviertem Wasser bei identischen Bedingungen gemacht; dabei wird die Anzahl der schnelleren und die Anzahl der langsameren Fällungen im aktivierten Wasser gezählt. Die zeitlichen Änderungen des Prozentsatzes der schnelleren Fällungen werden verfolgt. Die Experimente wurden unter zwei verschiedenen Bedingungen ausgeführt: 1. in einem ganz metallfreien Raum und 2. in einem mit Metall abgeschirmten Raum. Die Unterschiede der Ergebnisse beweisen, daß die registrierten Variationen tatsächlich auf Einwirkungen äußerer Faktoren zurückzuführen sind.Es wurde festgestellt: eine säkuläre Variation, die möglicherweise in Zusammenhang mit der Sonnenaktivität steht; eine jährliche Variation, die man vielleicht mit der Erdbewegung in Verbindung bringen kann; und mehrere kleinere Variationen, die mit verschiedenen Elementen in Zusammenhang stehen können. Es werden Korrelationen mit dem Luftdruck, mit dem luftelektrischen Potentialgefälle und mit der kosmischen Strahlungsintensität festgestellt. Ferner zeigt sich, daß die Beschaffenheit des Metallschirmes von Bedeutung ist. Aus in Florenz, Brüssel und Wien durchgeführten Beobachtungen ließ sich ein Breiteneffekt feststellen. Die Untersuchung der Wirkung der Metallabschirmungan sich mit einem vereinfachten Test ohne Aktivierung ergab, daß im Allgemeinen die BiOCl-Fällung unter Abschirmung schneller erfolgt und daß ein Zusammenhang zwischen der spezifischen elektrischen Leitfähigkeit der verwendeten Metalle und dem Abschirmungseffekt besteht.

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Summary It is commonly believed for a long time that there exists a relation between atmospheric and cosmic phenomena and physico-chemical, biochemical and biological processes. The author has developed a simple and differential method for researches in this field. Series of precipitations with BiOCl are simultaneously made with normal and with activated water at identical conditions, whereby is counted the number of the faster and of the slower precipitations in the activated water; the temporal variations of the percentage of the faster precipitations are studied. The experiments have been carried out with the following two conditions: (1) in a room free of any metal, (2) in a room isolated with metal. The differences in the results prove that the variations obtained are due in fact to the influence of exterior factors.Has been ascertained: a secular variation which might be in connexion with the sunspot activity, an annual variation which might be in connexion with the earth's rotation, and several smaller variations which may be in relation with different elements. There are found correlations with the atmospheric pressure, with the potential gradient of atmospheric electricity and with the intensity of cosmic radiation. Is found, in addition, that the nature of the metal screen is of importance. It was determined also an effect of latitude from observations carried out at Florence, Brussels and Vienna. The investigation of the effect of the metal screen, by a simplified test without activation, has shown that in general the BiOCl precipitation is going on more rapidly when using the screen, and that there is a relation between the specific electrical conductivity of the metals used and the effect of screening.

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Résumé Depuis longtemps on a envisagé une relation entre les phénomènes atmosphériques, solaires et cosmiques et les phénomènes physicochimiques, biochimique set biologiques L'auteur a développé un test inorganique pour l'étude de cette relation. Il fait simplement précipiter par séries dans des conditions identiques, du BiOCl avec de l'eau normale et avec de l'eau activée. Le pourcentage des cas dans lesquels la précipitation marche plus vite dans l'eau activée (T%) est la réponse du test inorganique. L'auteur a conduit ses essais contemporairement au-dehors et au-dedans d'une chambre de cuivre. Les différences dans les résultats obtenus prouvent que les variations observées sont vraiment dûes aux influences exercées par des facteurs extérieurs.On a constaté qu'il existe a) une variation séculaire qui est probablement en rapport avec l'activité solaire, b) une variation annuelle qui peut être due au mouvement de la terre, c) plusieurs autres variations plus petites qui peuvent dépendre de différents éléments. On remarque des corrélations avec la pression atmosphérique, le gradient du potentiel électrique de l'air et l'intensité du rayonnement cosmique. D'autre part, on constate qu'il est important de tenir compte de la nature de l'écran métallique. On a aussi constaté qu'il existait un effet de latitude en comparant les observations faites à Florence, Bruxelles et Vienne. En utilisant un test simplifié sans activation (Test P) l'auteur a montré qu'en général la précipitation se produit plus rapidement au-dessous de l'écran et qu'il existe une relation entre la conductibilité électrique spécifique des métaux employés et l'effet observé.

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Mit 10 Textabbildungen.  相似文献   

Temperature,fluid content and rheology of localized ductile shear zones in subsolidus cooling plutons     

Alberto Ceccato  Philippe Goncalves  Giorgio Pennacchioni 《Journal of Metamorphic Geology》2020,38(8):881-903

Temperature and fluid content are critical parameters that control rock rheology and strain localization in the continental crust. Here, we determine by thermodynamic modelling the of localized ductile shearing during cooling of three different granitoid plutons: the Rieserferner and the Adamello plutons in the Italian Alps, and the Lake Edison pluton in the Sierra Nevada—USA. Shear zones exploited precursor joints, associated veins and alteration zones. and P−T phase diagram sections were computed with Perple_X in the system MnO−Na₂O−CaO −K₂O−FeO−MgO−Al₂O₃−SiO₂−H₂O−Fe₂O₃. The phase diagram sections show that the nucleation of the brittle precursors (joints, veins) occurred at T» 450°C at fluid-saturated conditions. Localized ductile shearing likely occurred at temperature ranging between 420 and 460°C evolving from initially fluid-saturated to fluid-undersaturated conditions in a closed system. In this temperature range, granitoid rocks are potentially subject to a series of retrograde metamorphic reactions replacing the load-bearing feldspars with weaker phyllosilicates. Metamorphic reactions occurred in spatial association with the precursory structures, leading to localized shearing. Decreasing temperature and fluid-undersaturated conditions likely hampered progressive strain accommodation in shear zones by slowing down metamorphic reactions, thermally activated dislocation creep processes, fluid-mediated deformation mechanisms and weakening mechanisms. Polyphase granitoid ultramylonite and mylonitic quartz veins have been affected differently by the fluid-undersaturated conditions of the system, as consequence of different dominant deformation mechanisms and syn-kinematic paragenesis during localized shearing. Localized ductile shearing in cooling plutons effectively occurs in a limited temperature range (420–460°C) in which the strain accommodation capacity of the shear zone is controlled by the negative feedback between the cooling rate, the kinetics of metamorphic reactions and deformation mechanisms, and the consumption of the limited amount of available fluids.  相似文献   

Chromite composition and platinum-group mineral assemblage in the Uktus Uralian-Alaskan-type complex (Central Urals,Russia)     

Giorgio?GarutiEmail author  Evgeny?V.?Pushkarev  Federica?Zaccarini  Roberto?Cabella  Elena?Anikina 《Mineralium Deposita》2003,38(3):312-326

Chromitite segregations in dunites of the Uktus Uralian-Alaskan-type complex (Central Urals, Russia) display large variation of the chromite composition: Cr/(Cr+Al)=0.46-0.77, Fe²⁺/(Fe²⁺+Mg)=0.28-0.66, and Fe³⁺/(Fe³⁺+Fe²⁺)=0.23-0.59. Three types of PGM assemblages have been recognized, varying in accordance with chromite composition: type I, dominated by Ru-Os-Ir (sulfides), is associated with magnesiochromite having Fe³⁺/(Fe³⁺+Fe²⁺)<0.30, in the southern dunite body. Type II, containing abundant Pt-Ir (alloys, minor sulfides), is found in magnesiochromite with Fe³⁺/(Fe³⁺+Fe²⁺)=0.40-0.44; type III, consisting of Ir-Rh-Pt-Pd (alloys, sulfarsenides, antimonides) in Fe-rich chromite having Fe²⁺/(Fe²⁺+Mg)=0.66 and Fe³⁺/(Fe³⁺+Fe²⁺)=0.59. Positive anomalies of Ir and Pt, and a negative peak of Ru characterize the PGE patterns of chromitites with type II and III PGM assemblages, whereas a positive Pt anomaly is observed in their dunite host. Intensive fractionation of Pt-Fe alloys in the Uktus chromitites reflects the anomalous behavior of Pt which is decoupled from Rh and Pd. Among other factors, the high iron activity and oxygen fugacity in the parent melt appear to exert a major control on precipitation of Pt-Fe alloys, below sulfur saturation. The strong Pt anomaly in chromitites from Uktus may indicate that Uralian-Alaskan-type magmas were derived from a Pt-rich mantle source.  相似文献   

Fault plane processes and mesoscopic structure of a strong-type seismogenic fault in tonalites (Adamello batholith, Southern Alps)   总被引：3，自引：3，他引：3  

Giulio Di Toro  Giorgio Pennacchioni   《Tectonophysics》2005,402(1-4):55

The Gole Larghe Fault is an exhumed paleoseismic fault crosscutting the Adamello tonalites (Italian Southern Alps). Ambient conditions of faulting were 9–11 km in depth and 250–300 °C. In the study area the fault accommodates 1100 m of dextral strike-slip over a fault thickness of 550 m. Displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons (faults_1–2–3). Fault displacement is in the range of few centimeters (faults₃) to a maximum of a few tens of meters in major faults₁. Faults_1–2 nucleated on pre-existing joints, whereas faults₃ are newly generated fractures produced during slip along faults_1–2. Each fault within the Gole Larghe Fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production. Pseudotachylytes are usually generated at the host rock/cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins. Pseudotachylytes and cataclasites have a similar chemical composition which is enriched in Loss On Ignition, K, Rb, Ba, U and Fe³⁺ compared to host rock.We envision two models for the evolution of the Gole Larghe Fault. In both models synkinematic fluid–rock interaction along a fault causes fault hardening by precipitation of abundant K-feldspar+epidote (and minor chlorite) in the cataclasite matrix conducive to final production of pseudotachylyte. In the first model, induration occurs progressively by differential precipitation related to fabric evolution in cataclasites. In the second model, induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir. Whatever the model, the Gole Larghe Fault represents a strong fault, where hardening processes resulted in a low displacement/fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement/fault thickness ratios.  相似文献   

Book Review: Geodynamics By D. L. Turcotte and G. Schubert, Cambridge University Press, New York, 2002, 456 p., U.S.110 (hb), 110 (hb), 45 (pb) ISBN 0-521-66186-2, 0-521-66624-4.     

Giorgio Ranalli 《Mathematical Geology》2005,37(1):125-126

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