A rocking multianvil: elimination of chemical segregation in fluid-saturated high-pressure experiments

Fluid saturated high-pressure experiments often result in strongly zoned experimental charges, this hinders experimentation in chemically homogeneous systems which in turn has serious consequences on equilibration, reaction progress, and (apparent) phase stabilities. In order to overcome these problems, a 600-ton press accommodating either a multianvil or end-loaded piston cylinder module has been mounted in such a way that it can be turned by 180°, thus inverting its position in the gravity field. During turning, hydraulic pressure, heating power, and cooling water remain connected allowing fully controlled pressures and temperatures during experiments.A series of experiments at 13 GPa, 950°C, on a serpentine bulk composition in the MgO-SiO₂-H₂O system demonstrates that continuous turning at a rate of 2 turns/min results in a nearly homogeneous charge composed of phase E + enstatite. The same experiment at static conditions resulted in four mineral zones: quench phase E, enstatite, enstatite + phase E, and phase E + phase A. Phase A disappears in experiments at a turning rate ≥1 turn/min. A static 15-min experiment shows that zonation already forms within this short time span. Placing two short capsules within a single static experiment reveals that the fluid migrates to the hot spot in each capsule and is not gravitationally driven toward the top. The zonation pattern follows isotherms within the capsule, and the degree of zonation increases with temperature gradient (measured as 10 °C within a capsule) and run time.Our preferred interpretation is that Soret diffusion causes a density-stratified fluid within the capsule that does not convect in a static experiment and results in temperature dependant chemical zonation. The aggravation of zonation and appearance of additional phases with run time can be explained with a dissolution-reprecipitation process where the cold spot of the capsule is relatively MgO enriched and the hot spot relatively SiO₂ and H₂O enriched (at 13 GPa and 950°C). Rocking and tilting of a stratified fluid induces Rayleigh-Taylor instabilities, causing chemical rehomogenization. If turning is faster than the time required to build significant chemical potential gradients in the fluid, chemical zonation in the distribution of the solids is suppressed.     

A model for the numerical simulation of tephra fall deposits

A simple semianalytical model to simulate ash dispersion and deposition produced by sustained Plinian and sub-Plinian eruption columns based on the 2D advection–dispersion equation was applied. The eruption column acts as a vertical line source with a given mass distribution and neglects the complex dynamics within the eruption column. Thus, the use of the model is limited to areas far from the vent where the dynamics of the eruption column play a minor role. Vertical wind and diffusion components are considered negligible with respect to the horizontal ones. The dispersion and deposition of particles in the model is only governed by gravitational settling, horizontal eddy diffusion, and wind advection. The model accounts for different types and size classes of a user-defined number of particle classes and changing settling velocity with altitude. In as much as wind profiles are considered constant on the entire domain, the model validity is limited to medium-range distances (about 30–200 km away from the source).The model was used to reconstruct the tephra fall deposit from the documented Plinian eruption of Mt. Vesuvius, Italy, in 79 A.D. In this case, the model was able to broadly reproduce the characteristic medium-range tephra deposit. The results support the validity of the model, which has the advantage of being simple and fast to compute. It has the potential to serve as a simple tool for predicting the distribution of ash fall of hypothetical or real eruptions of a given magnitude and a given wind profile. Using a statistical set of frequent wind profiles, it also was used to construct air fall hazard maps of the most likely affected areas around active volcanoes where a large eruption is expected to occur.     

Verification of our previous definition of preferred earthquake nucleation areas in Kanto-Tokai, Japan

We have proposed that points of future initiation of rupture may be mapped, based on minima in local recurrence times, which are equivalent to local maxima in the probability for main shocks to occur. These minima are often controlled by anomalously low b-values (logN = a − bM). Of the Kanto-Tokai area, approximately 12% showed anomalously short recurrence times and was proposed as asperities, based on seismicity up to 1999. During the period 1999–2003.5, about 75% of the earthquakes with M ≥ 3.5 fell into the asperities, earlier defined (for example 19 out of 23 M ≥ 3.8 events). The probability for this to occur by chance is approximately 2 10^− 14. This supports our idea that the most likely volumes to produce main shocks may be mapped by minima in local recurrence times.     

Ocean floor basalt, not continental gabbro: a reinterpretation of the Hoher Bogen amphibolites, Teplá-Barrandian, Bohemian massif

At its southern margin along the Hoher Bogen mountain, the Teplá-Barrandian (Bohemian massif, Central Europe) is made up of a 1- to 4-km wide belt of amphibolites. An upper amphibolite/lower granulite facies Variscan metamorphism has brought forth coarse-grained, weakly foliated rocks with hbl+pl±cpx±opx±grt parageneses. Since the beginning of this century, these rocks, together with fine-grained or mylonitized amphibolites, have been regarded as metamorphic gabbros (gabbro amphibolites) of the Neukirchen-Kdyne igneous complex. Relics of magmatic textures, however, cannot be found anywhere. The amphibolites are therefore reinterpreted as metamorphic basalts. The Hoher Bogen amphibolites (HBA) derive from N-type MORB. The most primitive samples have Mg#s between 60 and 65. Locally occurring (garnet-)hornblendites and leucodioritic mobilisates are the products of partial melting of amphibolites during the Variscan metamorphism and do not belong to the primary magmatic rock association. Ultramafic rocks are tectonically emplaced between the HBA belt and the metapelitic rocks of the Moldanubian. At the very least, the metapyroxenites among them seem to have a cumulus origin. Together with the ultramafic rocks, the HBA belt may be regarded as a metaophiolite, comparable to the Mariánské Lazne complex. The reinterpretation of the former "gabbro amphibolites" as a metaophiolite has consequences for the geology of the Teplá-Barrandian: the size of the Neukirchen-Kdyne igneous complex is reduced. The HBA belt is a piece of oceanic crust which is possibly younger than the Precambrian metasedimentary/metavolcanic country rock of the Neukirchen-Kdyne igneous complex.     

Global and local controls influencing the deposition of the La Luna Formation (Cenomanian-Campanian), western Venezuela

Bulk and molecular geochemical, micropalaeontological, and carbon-isotopic data are used to address the different local and global factors influencing the environment of sedimentation of the La Luna Formation (Cenomanian-Campanian, approximate palaeolatitude 15°N) in a single section in western Venezuela. Based on the constructed chronostratigraphic framework, oxygen-depleted bottom-water conditions and black-shale deposition started in western Venezuela well before the widespread occurrence of organic-rich sediments in higher palaeolatitude regions such as the Tethys and the North Atlantic near or at the Cenomanian-Turonian boundary. In the La Luna Formation, palaeoenvironmental conditions that allowed the preservation of organic matter (mainly of marine origin), prevailed until Santonian times in a distal platform facies with very low siliciclastic input. Changes in lithology appear to reflect the local response to eustatic sea-level variations and the presence of a migrating upwelling belt affecting the bioproductivity of silica and carbonate. A marked δ¹³C_org isotopic excursion is recognised in the middle part of the section, and is apparently unrelated to local palaeoenvironmental changes in bioproductivity and oxygen depletion. Biological marker data show no variations in association with the isotopic excursion, being mainly controlled by local fluctuations in organic-matter input and preservation.     

Precursory quiescence before the August 1982 Stone Canyon,San Andreas fault,earthquakes

The Stone Canyon earthquake sequence started during August 1982 and lasted for about four months. It contained four mainshocks withM _L 4, each with an aftershock zone about 4 km long. These mainshocks, progressing from southeast to northwest, ruptured a segment of the fault approximately 20 km long leaving two gaps, which were later filled by theM _L =4.6 mainshocks of January 14, and May 31, 1986. The equivalent magnitude of the sequence isM _L =5.0.Precursory seismic quiescence could be identified in: (1) the northernmost 10 km of the aftershock zone which contained three of the mainshocks; and (2) the southern gap in the aftershock zone. The fault segment containing the first mainshock and its aftershocks did not show quiescence. This pattern of precursory quiescence is very similar to two cases in Hawaii where the rupture initiation points of the mainshocks (M _S =7.2 and 6.6, respectively) were located in volumes of constant seismicity rate, surrounded by volumes with pronounced precursory quiescence.The precursory quiescence before the August 1982 Stone Canyon earthquakes lasted for 76 weeks, amounted to a reduction in rate of about 60%, and could be recognized without any false alarms. That is, the anomaly was unique within the 60 km study segment of the fault and in the years 1975 through August 1982. Eighteen foreshocks occurred between July 27 and August 7, 1982. We conclude that the August 1982 mainshocks could have been predicted, based on seismic quiescence and foreshocks.     

A model for the emission spectrum of Active Galactic Nuclei

The overall spectrum of Active Galactic Nuclei is discussed in terms of a wind and shock model. The central object of the active galactic nuclei is the source of a strong supercritical wind. The interaction of the wind with the surrounding medium generates the non-thermal radiation observed in active galactic nuclei. The spectrum predicted by the wind and shock model is compared with the observations of the quasar 3C 273.     

Stochastic and dynamic temperature changes in the interplanetary gas

Exact solutions have been found to the Fokker-Planck equations, incorporating stochastic velocity changes and modelling particles moving in an inverse square central force field under an inverse square collision frequency. The solutions for the velocity distribution contain a combination of collisional and dynamical (reversible) heating. At a general position, there are two populations each with three distinct temperatures, one normal to the orbital plane and the others closely parallel and perpendicular to the mean orbit. Collisional heating is strong and most readily detected in the secondary component of gas which reaches upstream directions along indirect orbits (attractive central force). For interplanetary helium gas reaching 1 a.u., the collisional heating ranges from effective transverse increase of 200 K and radial increase of 1500 K in the downstream wake, to several thousand K increase in radial temperature of the secondary component transverse to the initial gas stream. In interpreting 584 Å sky background radiation observations, the dynamical changes in the velocity spread have to be taken into account for helium gas that is initially hot, when Doppler shifts relative to the solar emission line are significant; the present solutions being the thermal approximations to the distribution function reveal the appropriate radial temperature as a function of space.