New Calculation of the Electrical Microfield Distribution in Plasma

Recently new calculations for the probability field distribution in plasma generating a new expression for the Debye screening fields was carried out by Martin and Perez (1996). This field is a very accurate solution of the Poisson equation for a point test charge in a plasma described by a uniform charge density at Boltzmann thermal equilibrium. The method that we here present is based mainly on the introduction of this adequate field considering the dependence of the potential with the magnitude of the test charge in the calculus of the distribution. First we find the spectral function F(k) carrying out the Fourier transform of the field which is numerically calculated for each representative value of the variable in k space and then fitting these data in order to get F(k). Thus, after the usual numerical integration on the wave number space d³k, the field strength distribution is achieved. This approximation recovers the Holtsmark distribution in the limit of high temperature, and/or, small densities as also these where 0. As in previous works (Ecker, 1972; Spatschek, 1990) the most probable value of the field decreases as the temperature is reduced, and/or, the density increased.     

Abundance Gradients in the Outer Galactic Disk

Radial abundance gradients of the element ratios O/H, Ne/H, S/H, and Ar/H are determined for a sample of disk planetary nebulae, emphasizing the behaviour of the gradients at large galactocentric distances. This revised version was published online in July 2006 with corrections to the Cover Date.     

POPs and MOPs

 Principal oscillation pattern (POP) analysis fits a first order multivariate autoregressive model to a reduced subset of the variables of a complex system. It has been shown in the past that important modes of complex systems can be detected through the POP technique. In this note two problems with this method will be addressed. Firstly, the POP analysis may face difficulties if the reduced system is of higher order in time than first. An example from linear equatorial wave dynamics is given to illustrate this point. Autoregressive models of higher order (MOP-models) are shown to provide a partial solution in such situations but are not fully satisfactory either. Nonlinearity may cause problems as well. A nonlinear low-order model is used to discuss this point. Both the POP and the MOP scheme are applied without data reduction. The MOP approach is superior to the POP technique in that it detects oscillating patterns which elude the POP analysis. The results suggest that the MOP analysis may be a valuable extension of the POP approach. Received: 29 July 1998 / Accepted: 10 February 1999     

Subduction Influence on Oxygen Fugacity and Trace and Volatile Elements in Basalts Across the Cascade Volcanic Arc

Fluids or melts derived from a subducting plate are often citedas a mechanism for the oxidation of arc magmas. What remainsunclear is the link between the fluid, oxygen fugacity, andother major and trace components, as well as the spatial distributionof the impact of those fluids. To test the potential effectsof addition of a subduction-derived fluid or melt to the sub-arcmantle, olivine-hosted melt inclusions from primitive basalticlavas sampled from across the central Oregon Cascades (43°–45°N)have been analyzed for major, trace and volatile elements andfO₂. Oxygen fugacity was determined in melt inclusions fromsulfur speciation determined by electron microprobe and fromolivine–chromite oxygen geobarometry. The overall rangein fO₂ based on sulfur speciation measurements is from <–0·25log units to + 1·9 log units (FMQ, where FMQ is fayalite–magnetite–quartzbuffer). Oxygen fugacity is positively correlated with fluid-mobiletrace element and light rare earth element contents in basaltsgenerated by relatively low-degree partial melting. Establishinga further correlation between fO₂ and fluid-mobile trace elementabundances with position along the arc requires the basaltsto be subdivided into shoshonitic, calc-alkaline, low-K tholeiiteand enriched intraplate basalt groups. Melt inclusions fromenriched intraplate and shoshonitic lavas show increasing fO₂and trace element abundances closer to the trench, whereas calc-alkalinemelt inclusions exhibit no significant across-arc variations.Low-K tholeiitic melt inclusions record an increase in incompatibletrace elements closer to the trench; however, there is no correlatedincrease in fO₂. The correlation observed in enriched intraplateand shoshonitic melt inclusions is interpreted to reflect aprogressively greater proportion of a fluid-rich, oxidized subductioncomponent in magmas generated nearer the subduction zone. Significantly,calc-alkaline melt inclusions with high ratios of large ionlithophile elements to high field strength elements, characteristicof ‘typical’ arc magmas, have oxidation states indistinguishablefrom low-K tholeiite and enriched intraplate basalt melt inclusions.The lack of across-arc geochemical variation in calc-alkalinemelt inclusions may suggest that these basalts are not necessarilythe most appropriate magmas for examining recent addition ofa subduction component to the sub-arc mantle. Flux and batchmelt model results produce a wide range of predicted amountsof melting and subduction component added to the mantle source;however, general trends characterized by increased melting andproportion of the subduction component from enriched intraplate,to low-K tholeiite, to calc-alkaline are robust. The model resultsdo not require enriched intraplate, low-K tholeiite and calc-alkalinemagmas to be produced from the same more fertile mantle source.However, enriched intraplate magmas, in contrast to calc-alkalineand low-K tholeiite magmas, cannot be generated from a depletedmantle source. Flux or batch melting of either the more fertileor depleted mantle sources used to generate the low-K tholeiite,calc-alkaline, and enriched intraplate magmas cannot reproduceshoshonitic compositions, which require a significantly depletedmantle source strongly metasomatized by a subduction component.The potential mantle source for shoshonitic basalts has a predictedfO₂ (after oxidation) from + 0·3 to + 2·4 logunits (FMQ) whereas the mantle source for low-K tholeiite, calc-alkaline,and enriched intraplate magmas may range from –1·1to + 0·7 log units (FMQ). KEY WORDS: basalt; Cascades; melt inclusions; oxidation state; volatiles     

Typical basin-fill sequences and basin migration in Yanshan, North China--Response to Mesozoic tectonic transition

Basin-fill sequences of Mesozoic typical basins in the Yanshan area, North China may be divided into four phases, reflecting lithosphere tectonic evolution from flexure (T3), flexure with weak rifting (J1+2), tectonic transition (J3), and rifting (K). Except the first phase, the other three phases all start with lava and volcaniclastic rocks, and end with thick coarse clastic rocks and/or conglomerates, showing cyclic basin development rather than simple cyclic rift mechanism and disciplinary basin-stress change from extension to compression in each phase. Prototype basin analysis, based on basin-fill sequences, paleocurrent distribution and depositional systems, shows that single basin-strike and structural-line direction controlling basin development had evidently changed from east-west to northeast in Late Jurassic in the Yanshan area, although basin group still occurred in east-west zonal distribution. Till Early Cretaceous, main structural-line strike controlling basins just turned to northeast by north in the studied area.     

Frostquakes in New England

Frostquakes are generated by tensional fracturing of the ground due to relatively rapid freezing. They are often accompanied by explosive noise and may reach Intensity V effects, yet they are only felt in neighborhoods and are not recorded on seismic networks. Frostquakes may crack macadam, concrete slabs, shallow soil pipe and some foundation materials along the line of the fracture. They occur in the early hours of the morning in mid-winter during major cold snaps. The ones investigated occurred in glacial sand and silt and none were found on till or bedrock. The recognition of frostquakes is important in correcting the records for fault-caused earthquakes. Frostquakes in the historic record complicate the difficulty of predicting large earthquakes from small ones.     

NASA's Astrophysics Data Archives

The NASA Office of Space Science has established a series of archivalcenters where science data acquired through its space science missionsis deposited. The availability of high quality data to the generalpublic through these open archives enables the maximization of sciencereturn of the flight missions. The Astrophysics Data Centers CoordinatingCouncil, an informal collaboration of archival centers, coordinates datafrom five archival centers distiguished primarily by the wavelength rangeof the data deposited there. Data are available in FITS format. Anoverview of NASA's data centers and services is presented in this paper.A standard front-end modifyer called Astrowbrowse is described. Othercatalog browsers and tools include WISARD and AMASE supported by theNational Space Scince Data Center, as well as ISAIA, a follow on toAstrobrowse.