Radio VHF Precursors of Earthquakes

On the basis of our analysis of a number of continuous observations made by the monitoring network on Crete, we assume that electrically active clouds are produced in an atmosphere above the sea on the eve of earthquake. These electrically active clouds, which occur at heights of 0.1–10 km, create the conditions for electrical discharges in an atmosphere that may be the source of the very high frequency (VHF) radio-emissions registered on Crete. We further suggest possible mechanisms of thunder electricity generation. We present the model of convection transport in which the first condition in the generation of thunder electricity is an atmosphere with a horizontal gradient of temperature. Base on this model, the occurrence of electrical charges on the surface of the sea and their transportation further upwards to heights of up to 10 km is due to pollution energy allocated within the bottom of the sea as gases and heat injection. The average flux density and power estimations of the VHF precursors were made for the Crete net situation to compare with published VHF data and radio star sources.     

Constraints on the cooling history of the H‐chondrite parent body from phosphate and chondrule Pb‐isotopic dates from Estacado

Abstract— To constrain the metamorphic history of the H‐chondrite parent body, we dated phosphates and chondrules from four H6 chondritic meteorites using U‐Pb systematics. Reconnaissance analyses revealed that only Estacado had a sufficiently high ²⁰⁶Pb/²⁰⁴Pb ratio suitable for our purposes. The Pb‐Pb isochron date for Estacado phosphates is measured to be 4492 ± 15 Ma. The internal residue‐second leachate isochron for Estacado chondrules yielded the chondrule date of 4546 ± 18 Ma. An alternative age estimate for Estacado chondrules of 4527.6 ± 6.3 Ma is obtained from an isochron including two chondrules, two magnetically separated fractions, and four bulk chondrite analyses. This isochron date might represent the age of termination of Pb diffusion from the chondrules to the matrix. From these dates and previously established closure temperatures for Pb diffusion in phosphates and chondrules, we estimate an average cooling rate for Estacado between 5.5 ± 3.2 Myr/°C and 8.3 ± 5.0 Myr/°C. Using previously published results for Ste. Marguerite (H4) and Richardton (H5), our data reveal that the cooling rates of H chondrites decrease markedly with increasing metamorphic grade, in agreement with the predictions of the “onion‐shell” asteroid model. Several issues, however, need to be addressed before confirming this model for the H‐chondrite parent body: the discrepancies between peak metamorphic temperatures established by various mineral thermometers need to be resolved, diffusion and other mechanisms of element migration in polycrystalline solids must be better understood, and dating techniques should be further improved.     

A practical tool for simulating the presence of gas comae in thermophysical modeling of cometary nuclei

A longstanding problem in thermophysical modeling of cometary nuclei has been to accurately formulate the boundary conditions at the nucleus/coma interface. A correct treatment of the problem, where the Knudsen layer gas just above the cometary surface (which is not in thermodynamic equilibrium) is modeled in parallel with the nucleus, is extremely time-consuming and has so far been avoided. Instead, simplifying assumptions regarding the coma properties are used, e.g., the surface gas density is assumed equal to zero or set to the local saturation value, and the coma backflux is neglected or given some realistic but approximate value. The resulting inaccuracy regarding the exchange of mass, energy, and momentum between the nucleus and the coma, may introduce significant errors in the calculated nucleus temperature profiles, gas production rates, and momentum transfer efficiencies. In this paper, we present a practical, accurate, and time-efficient tool which makes it possible to consider the nucleus and the innermost coma of a comet (the former assumed to consist of a porous mixture of crystalline water ice and dust) as a coupled, physically consistent system. The tool consists of interpolation tables for the surface gas density and pressure, the recondensing coma backflux, and the cooling energy flux due to diffusely scattered coma molecules. The tables cover a wide range of surface temperatures and sub-surface temperature profiles, and can be used to improve the boundary conditions used in thermophysical models. The interpolation tables have been obtained by calculating the transmission distribution functions of gas emerging from sublimating porous ice/dust mixtures with various temperature profiles, which then are used as source functions in a Direct Simulation Monte Carlo model of inelastic intermolecular collisions in the Knudsen layer.     

Structural softening of the lithosphere

Structural softening is a decrease in the amount of stress needed to deform the lithosphere at a particular rate because of its structural reorganization while all true rheological properties remain constant. Structural softening is fundamentally different than material softening, where the decrease in stress is generated by a change in rheological properties with progressive deformation, such as grain size reduction resulting from large shearing strain. We study structural softening generated by folding of the crust-mantle boundary, which is a structural instability that inevitably develops during compression of the mechanically layered lithosphere. For ductile rheologies, the stress decrease represents a decrease of the effective lithospheric viscosity, which is proportional to the ratio of stress to lithospheric shortening strain rate. We present analytical and numerical results quantifying the decrease in stress and effective viscosity that occur during shortening at a constant rate. The decrease in effective viscosity can be up to 10-fold.     

A new model for the distribution of flare energies

A new model is presented for the distribution of solar (and stellar) flare energies. Flares are assumed to result from energy release in multiple reconnecting current sheets (RCSs) in the solar corona. The model takes into account both the dynamical evolution of separate RCSs and their interaction by virtue of coalescence. Thus an attempt is made to generalize previously suggested avalanche and RCS models for the flare frequency distribution. The number of RCSs is not specified and the strength of their interaction is governed by a coalescence parameter. Under quite general assumptions, the resulting distribution can be approximated by a power law with the slope 3/2 < < 7/4, as required by numerous observations. A softer distribution of numerous nanoflares, responsible for the coronal heating, is also possible under suitable choice of parameters.     

Nd and Sr isotopic geochemistry of mafic layered intrusions in the eastern Baltic shield: implications for the evolution of Paleoproterozoic continental mafic magmas

 Nd and Sr isotopic data are presented for the 2449–2441 Ma Olanga and Burakovka layered mafic complexes in the eastern Baltic Shield. These complexes have similar tectonic position, but differ in two aspects: the age of the enclosing crust and the post-crystallization metamorphic history. The Sm–Nd isotopic results for the Kivakka and Lukkulaisvaara intrusions, Olanga Complex, are consistent with the model of closed-system crystallization of a single magma without significant wallrock assimilation. The Rb–Sr systems of minerals were disturbed by late Rb addition during 1.75–1.50 Ga metamorphism. The Nd and Sr isotopic systems in the Burakovka complex show no metamorphic disturbance and indicate mixing of at least four isotopically distinct components. Isotopic variations in the Burakovka Complex can be explained by a 4–20 per cent contamination of a primary komatiitic or picritic magma with a Mesoarchean crust, similar to that exposed in the region. A similar model, applied to the Olanga Complex using a Neoarchean crustal isotopic composition, cannot reproduce the observed isotopic signature. The nearly uniform initial ɛ_Nd values between −1 and −2.3, observed in the Kivakka and Lukkulaisvaara intrusions of the Olanga Complex, as well as in the other 2.50–2.44 Ga layered mafic intrusions throughout the eastern Baltic Shield, are better explained by a mantle plume model with small amounts of crustal contamination and minor involvement of asthenospheric material. This model is also consistent with the geological observations and the temporal distribution of the Paleoproterozoic mafic magmatism in the eastern Baltic Shield. As an alternative, the enriched isotopic characteristics may be explained by melting of a metasomatically modified lithospheric mantle source. Received: 4 August 1994/Accepted: 5 April 1996