Quantum mechanical potential surfaces and calculations on minerals and molecular clusters

Recently, ab-initio quantum mechanical potential surfaces calculated for silicate hydroxyacid molecules were used to extract covalent potentials for use in mineral physics calculations (Lasaga and Gibbs 1987). The calculations showed that these potentials are capable of generating the structure and physical properties of silicate minerals. In this paper we explore in more detail the suitability of various covalent potentials in mimicking the topography of the ab-initio potential surfaces. We also extend the use of such quantum-derived potentials in generating the structures of hydroxyacid dimers, trimers, and pentamers of silicate tetrahedra and in studying the structure and the dynamical properties of minerals and glasses.     

The objective determination of the instantaneous predominant frequency of seismic signals and inferences onQ of coda waves

A technique to detect spectrum variations versus time along seismic signals is applied to coda waves of local earthquakes (Friuli, Northern Italy). The technique consists of an autoregressive modeling and utilizes nonlinear spectral analysis where the spectrum of stochastic processes is estimated as the transfer function of the filter that whitens the process under analysis. This approach appears to be particularly well suited to those investigations where automatic measurements of the instantaneous frequency have to be carried out on digital data. The detection of variations of the instantaneous frequency along the coda allows computation of seismic-Q in the lithosphere and its frequency dependence: the result obtained is $$Q = 100f^{0.4} $$ which appears to be strongly consistent with that, based on the estimate of the coda amplitude decay in the band including the most significant frequencies of the signals under analysis.     

Site response using coda-wave techniques: Applications to short period data from central and southern Italy

The method of coda waves was applied to two different sets of data for the evaluation of the relative site response.The first set of data consists of low magnitude earthquakes with closely spaced locations, recorded at a small aperture array of velocimeters located in the Abruzzo region, central Italy. The second set of data is composed of events with epicentral distance ranging from 20 to 300 km, recorded at a seismological network with an aperture of about 100 km located in the Puglia region, southern Italy.Results show that the coda wave method furnishes stable estimates of the site effect. An amplification, relative to an arbitrary site, of a factor of about 2 occurs in the 1.7–6 Hz frequency band for two stations of the Abruzzo network, while an amplification factor of about 0.5 occurs in the whole frequency band (1–24 Hz) for one of the stations of the Puglia network. This station is located in an area which is correlated with a low macroseismic intensity anomaly.     

Association of subsurface radon changes in Alaska and the northeastern United States with earthquakes

Changes of radon concentration in the ground have been reported to correlate with seismic activity in many places over widely varying distances from the earthquake epicenters, sometimes over 100's and occasionally over 1000's of km depending on the magnitude M of the earthquakes. For proper use of such correlations in forecasting the location and size of impending earthquakes, it would be useful to establish a relation between M and the maximum distance x_M at which radon can be significantly altered by preseismic effects.We have monitored radon in the ground continuously at Blue Mountain Lake, NY starting Dec. 3. 1975 and at three locations in Alaska—Icy Bay, Yakataga, and Sand Point (starting Nov. 12, 1979; March 5, 1980; and June 29, 1980 respectively). Sample correlations from visual examination of the recent radon record and tentative use of a proposed scaling relation show a number of possible earthquake-related signals at these locations. The data are compatible with scaling relations that were derived from two separate models of premonitory elastic strains. In this work x_M = 10^0.48M, where x_M is in units of km and M ≥ 3. Since tilt and strain signals correlate similarly with magnitude and distance, it is likely that most earthquake-related radon signals are mechanically induced.     

Seismic Q for the lithosphere of the Montenegro region (Yugoslavia): frequency,depth and time windowing effects

The availability of accelerometric data for the Montenegro earthquake of 15th April 1979 makes it possible to investigate seismic Q of the lithosphere in that region, in particular, its dependence on frequency, on the depth reached by seismic waves, and on the length of time windows in which signals are processed. Two different spectral methods, S phase energy ratio and coda envelope decay, are applied, respectively, to direct and scattered shear waves. Similar results are obtained using different portions of the recordings, i.e., coda waves for the envelope decay fit and the S wave train, with a significant duration of ～ 10 s, for the energy ratios. The same apparent Q (Q ～ 40 f, where f is the frequency expressed in Hz) that is found for other neighbouring central Mediterranean regions (e.g., Ancona, on the central Italian Adriatic coast; Valnerina, in the central Apennines; Irpinia, in the southern Apennines) is also found for the southern Yugoslavian coast, in the band 1–25 Hz up to a maximum range of ～ 120 km from the focus. This strong frequency dependence is probably connected with the type of small-scale heterogeneity and the same geological age and level of tectonic activity peculiar to all these seismotectonic areas.In order to compare the apparent Q of the whole S wave train, ～ 10 s long, with the (intrinsic) apparent Q of the single direct S wave (usually 1 s or less), the maximum entropy method is applied in the energy spectrum computation for shorter wave trains. The use of shorter time windows does not reveal any significant variation in the tendency of Q to increase linearly with frequency as the length of the time window containing the sample of the S waves decreases. This seems to indicate that scattering-dependent Q is generally inseparable from intrinsic Q in the lithosphere when estimates based on variations with distance of the seismic signal spectrum are used. While the type of linear growth with frequency does not seem to undergo any variations (it remains of the Q = qf type), the data show there are a considerable decrease in the coefficient of proportionality Q with decreasing duration of the window of S waves analysed, probably as a result of variations in seismic attenuation with depth.     

Aeromagnetic evidence for the rotation of Sardinia (Mediterranean Sea): comparison with the paleomagnetic measurements

The aeromagnetic survey of Sardinia (western Mediterranean) delineated a large magnetic anomaly located in the western part of the island. The shape of the anomaly is improved and simplified by upward continuation, and modelling in terms of three-dimensional structure gives a westward remanent magnetization. Some tests have been carried out to suppress ambiguities on the magnetic dual aspect “magnetization distribution-shape of sources”. The optimal declination obtained after a few testings is 30° west which is ascribed to the rotation of Sardinia. This value confirms the results provided by paleomagnetic investigations and is significant because it characterizes a large and deep-rooted structure involved in the rotation of Sardinia. Our results thus illustrate another interesting application of aeromagnetic surveys.     

On granitoid emplacement and related structures. A review

Granitoid magmas are emplaced at the upper crust under a complex interaction of gravitational processes and horizontal tectonics. Natural intrusive bodies show a wide variety of structural patterns that must be strongly related with their dynamic history of ascent and emplacement. Comparisons of natural structural patterns with experimental and numerical models are necessary to interpret in a coherent fashion the emplacement mechanism involved.From natural, experimental, numerical and theoretical considerations one can reach important conclusions on the problem of ascent and final emplacement of granitoid magmas in the Earth's crust. The present paper reviews the principal emplacement mechanisms referred to in the literature; that is, doming, diapirism, ballooning, stoping, cauldron subsidence and dike propagation.Dike propagation is the most effektive process in magma transport from deep zones in the crust and upper mantle. Extensional fractures for dike propagation and magma ascent can be developed at depth in the crust according to the modern dike propagation theory. In orogenic domains, magmas reach the upper crust through narrow channels or dikes and are accumulated in a final reservoir in the form of a discordant supracrustal pluton or a ballooning pluton if, during emplacement, any regional deformation acts simultaneously.

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Zusammenfassung Die Intrusion granitischer Magmen in die obere Krust ist das Ergebnis eines komplexen Zusammenspiels von schwerkraftbedingten Prozessen und horizontalen, tektonichen Bewegungen. Die gro\e Vielfalt tektonischer Strukturen von granitischen Plutonen spiegelt die Geschichte der Dynamik, ihres Aufstieges und ihrer Platznahme wider. Um den jeweils vorhandenen Intrusionsmechanismus zusammenhängend deuten zu können, sind Vergleiche der natürlichen, tektonischen Elemente mit experimentellen und numerischen Modellen notwendig. Hieraus lassen sich gegebenenfalls weitreichende Schlüsse zum Problem des Aufstiegs und der endgültigen Platznahme der Magmen in der Erkruste ziehen.In dieser Arbeit werden die wesentlichen Aufstiegsmechanismen von Magmen aus der Literatur diskutiert. Im Einzelnen sind dieses: Aufwölbung, Diapirismus, Aufweitung, Abbau, Kraterbildung und Gangentwicklung.Das Vordringen von Gängen ist der wirksamste Proze\ des Magmentransports aus tieferen Zonen der Kruste und des oberen Mantels. Ausdehnungsklüfte lassen sich als Ursache für das Vordringen von Gängen und den Magmenaufstieg nach der heutigen Theorie über die Entwicklung von Gängen in der tiefen Kruste herleiten. In orogenen Zonen erreichen Magmen die obere Erdkruste durch enge Kanäle oder Gänge und reichern sich dort auch in Form eines dikordanten, subkrustalen oder aufgweiteten Plutons an, wenn gleichzeitig mit der Platznahme regionaltektonische Deformationen ablaufen.

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Resumen Los magmas graníticos se emplazan en la corteza superior bajo una compleja interaccion entre procesos gravitatorios y tectónica horizontal. Los cuerpos intrusivos naturales muestran una amplia variedad de patrones estructurales que pueden estar directamente relacionados con la dinámica de ascenso y emplazamiento. Para explicar de forma cohérente el mecanismo de emplazamiento implicado en un plutón es necesario establecer comparaciones entre patrones estructurales de plutones y modelos experimentales y numéricos.A partir de consideraciones naturales, experimentales, numéricas y teóricas, se pueden extraer importantes conclusiones sobre el problema del ascenso y emplazamiento de magmas en la corteza. En este trabajo se revisan los principales mecanismos de emplazamiento referidos en la literatura; es decir, doming, diapirismo, ballooning, stoping, cauldron subsidence y propagación de diques.Propagación de diques es el proceso más efectivo en el transporte de magmas desde zonas profundas en la corteza y manto superior. De acuerdo con la moderna teoría de propagación de diques, las fracuras extensionales pueden producirse en profundidad en la corteza. En dominios orogénicos, los magmas alcanzan la corteza superior a través de estrechos canales o diques y se acumulan en un reservorio final, bien en forma de plutones discordantes supracrustales, o bien en la forma de un »ballooning pluton« si durante el emplazamiento actûa alguna deformación regional.

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Kinetics of reactions between aqueous sulfates and sulfides in hydrothermal systems

The rates of chemical reactions between aqueous sulfates and sulfides are essentially identical to sulfur isotopic exchange rates between them, because both the chemical and isotopic reactions involve simultaneous oxidation of sulfide-sulfur atoms and reduction of sulfate-sulfur. The rate of reaction can be expressed as a second order rate law: R = k·[∑SO₄^2?]·[∑S^2?], where R is the overall rate, k is the rate constant and [∑SO₄^2?] and [∑S^2?] are molal concentrations. We have computed the rate constants from the available experimental data on the partial exchange of sulfur isotopes between aqueous sulfates and sulfides using the rate law established by us: $\text{ln(}\text{α}{}^{\mathrm{e}}\text{? α}\text{α}{}^{\mathrm{e}}\text{? α}{}^0\text{) = ? kt([∑SO}{}_4{}^{\mathrm{2?}}\text{] + [∑S}{}^{\mathrm{2?}}\text{])}$, where t is time and α⁰, α, and α^e are, respectively, the fractionation factors at t = 0 (the initial condition), at the end of experiment, and at equilibrium. The equilibrium fractionation factor can be expressed as: $\text{1000 ln α}{}^{\mathrm{e}}\text{=}\text{6.463 × 10}{}^6\text{T}{}^2\text{+ 0.56 (±.5)}$ (T in Kelvin).The rate constants are strongly dependent on T and pH, but not in as simple a manner as suggested by Igumnov (1976). Our rate constants in Na-bearing hydrothermal solutions decrease by 1 order of magnitude with an increase in pH by 1 unit at pH's less than ~3, remain constant in the pH range of ~4 to ~7, and again decrease at pH >7. The activation energy for the reaction also depends on pH: 18.4 ± 1 kcal/mole at pH = 2, 29.6 ± 1 kcal/mole at pH = 4 to 7, and between 40 and 47 kcal/mole at pH around 9. The observed pH dependence of the rate constant and of the activation energy can be best explained by a model involving thiosulfate molecules as reaction intermediates, in which the intramolecular exchange of sulfur atoms in thiosulfates becomes the rate determining step.The rate constants obtained in this study were used to compute the changes in the isotopic fractionation factors between aqueous sulfates and sulfides during cooling of fluids. Comparisons with data of coexisting sulfate-sulfide minerals in hydrothermal deposits, suggest that simple cooling was not a likely mechanism for coprecipitation of sulfate and sulfide minerals at temperatures below 350°C. Mixing of sulfide-rich solutions with sulfate-rich solutions at or near the depositional sites is a more reasonable process for explaining the observed fractionation.The degree of attainment of chemical equilibrium between aqueous sulfates and sulfides in a hydrothermal system, and the applicability of a_O2-pH type diagrams to mineral deposits, depends on the ∑S content and the thermal history of the fluid, which in turn is controlled by the flow rate and the thermal gradient in the system.The rates of sulfate reduction by non-bacterial processes involving a variety of reductants are also dependent on T, pH, [∑SO₄^2?], and [∑S^2?], and appear to be fast enough to become geochemically important at temperatures above about 200°C.     

Frequency relationship for seismic Qβ of central southern Italy from accelerograms for the Irpinia earthquake (1980)

The frequency dependence of Q_β for seismic waves in a distance range with a maximum of 150 km from the epicentre of the Irpinia earthquake of November 23, 1980 has been sought using displacement spectral ratios computed from strong-motion accelerograms recorded in the region. The method has been applied to calculate the behaviour of Q_β as a function of frequency in the band 0.1–25 Hz, and to investigate whether azimuthal variations appear in seismic Q_β for the lithosphere in central southern Italy. The same result is obtained using data from stations in western south Italy as using data from eastern south Italy, namely,

$\text{Q}{}_{\mathrm{\beta }}\text{(f) = 40f (}\text{Hz}\text{)}$

The linear relationship suggest that apparent Q_β depends more on the scale of heterogeneity of the lithosphere, affecting reflection and scattering mechanisms, than on intrinsic energy losses related to the anelasticity of the materials through which the seismic waves propagate.The existence of a peak in Q_β^?1 has been investigated in the low-frequency band (0.1–2.5 Hz) using a higher resolution power. A stable result in this low-Q_β zone is not possible on the basis of the available data: only in six Q_β(f) profiles does an evident minimum exist, between 0.2 and 1 Hz, while in nine cases the curves are monotonically increasing from the lowest observable frequencies; a further nine cases appear of uncertain interpretation.