Splitting of dislocations in olivine,cross-slip-controlled creep and mantle rheology

The splitting of the [100] dislocations of forsterite Mg₂SiO₄ is investigated in a hard-sphere model. Glissile splittings exist in (001) and the most energetically favorable is the one that does not involve cutting of SiO₄ tetrahedra: $\text{[100] → [}\text{1}\text{6}\text{,}\text{1}\text{9}\text{,}\text{1}\text{6}\text{+ [}\text{2}\text{3}\text{, 0, 0] + [}\text{1}\text{6}\text{,}\text{1}\text{9}\text{,}\text{1}\text{6}$ [100] screw dislocations are shown to be able to split simultaneously on (001) and (010) according to the reaction: $\text{[100] = [}\text{1}\text{6}\text{,}\text{1}\text{36}\text{,}\text{1}\text{4}\text{] + [}\text{1}\text{6}\text{,}\text{1}\text{9}\text{,}\text{1}\text{6}\text{] + [}\text{1}\text{3}\text{, 0, θ] + [}\text{1}\text{6}\text{,}\text{1}\text{36}\text{,}\text{1}\text{4}\text{] + [}\text{1}\text{6}\text{,}\text{1}\text{6}\text{,}\text{1}\text{6}\text{]}$ This sessile configuration is analogous to the one found for screw dislocations of body-centered cubic metals. It accounts forthe long rectilinear sessile screw segments commonly observed, in experimentally and naturally deformed olivine. A new creep model for the upper mantle is proposed, where recovery is controlled by cross-slip of screw dislocations instead of climb of edge dislocations. The creep law, fitted on the experimental results of the literature is: $\text{?}\text{?}\text{= 1.2 · 10}{}^4\text{σ}{}^2\text{exp}\text{? [(125000 ? 11.7σ + PV}{}^1\text{/RT]>}$ (σ in bars) the activation volume for cross-slip is estimated and viscosity-depth curves are plotted. The proposed creep mechanism is grain-size independent and less non-Newtonian than the climb-controlled one; it is found to be dominant over the latter at stresses smaller than 100 bar.     

Beitrag zur feinstmahlung von industriemineralen

Industrial minerals of a high degree of fineness such as kaolinite, calcite and barite are used to an increasing extent in the chemical industries as filling material in the manu-facturing of paper, paints and plastics or as ingredients of ceramic products. Grains smaller than 5 μm can be produced by dry grinding in a jet mill or by wet grinding in drum mills. To avoid impurities due to wear, these mills are lined with rubber or plastic and ceramic balls are used as grinding bodies.Our investigations in a jet mill show that the fineness of the ground product depends on the feed rate and on the air volume. The grain sizes received in the drum mill are influenced by feed rate, pulp density, speed of rotation and by the grinding bodies (density, size).REM-photographs of mill discharge in the grain-size ranges 10-5 μm and below 5 μm indicate that in both the grains mainly show crystal planes as boundaries. After dry grinding the grains show smooth and clean faces; thus is traced back to the impact effect of the air streams. In wet grinding the fine grains are very prone to form agglomerates.     

Coulees volcaniques dans le paleozoique superieur des zones internes helleniques (Grece continentale) — environnement sedimentaire et caracterisation petrographique

Volcanic flows of Late Paleozoic age in the Pelagonian zone belonging to the internal Hellenic zones, have been studied, for the first time, from petrographical and geochemical viewpoints.

Relevant petrographical data and the geochemical analysis lead us to consider that:

1. (a) the basic lavas can be linked to tholeitic basalts.

2. (b) the acid lavas are associated to metarhyolites.

3. (c) the two magmatic flows do not originate from the same parental magma.

The geochemical results obtained, compared to those from other deposits of nearly the same age outcropping around the Mediterranean, indicate that the latest Hercynian volcanism has been affected by intra-plate distension phenomena preceding the great Lower Mesozoic break-up which caused the formation of the Tethys Ocean.

It is remarkable that in the Mesogean Basin the sedimentary environment is nearly similar to that described in this paper. The volcanic flows end up in a sialic sedimentary basin containing coarse detritic formations that resulted from erosion of the Variscan chain along the southern margin of the European continent.

Abstract

Dans la zone pélagonienne appartenant aux zones internes helléniques, des coulées volcaniques d'âge paléozoïque supérieur sont étudiées, pour la première fois, pétrographiquement et géochimiquement. Les différents agencements pétrographiques et les analyses géochimiques nous apprennent que:

1. (a) les laves basiques peuvent être rattachées á la famille des basaltes tholéitiques.

2. (b) les laves acides s'apparentent aux métarhyolites.

La quantification des différents éléments analyses nous amène á considérer que les deux lignées magmatiques ne sont pas issues d'un même magma parental.

La comparaison des résultats géochimiques avec d'autres gisements, sensiblement contemporains affleurant autour de la Méditerranée actuelle, nous indique que le volcanisme fini-hercynien est assujetti á des phénomènes de distension intraplaques, préparatoires á la grande fracturation mésozoïque inférieure ayant donné naissance á l'océan téthysien.

Ce qui est remarquable dans le bassin mésogéen, c'est que l'environnement sédimentaire est toujours, á peu près, comparable á celui que nous décrivons ici. En effet, les coulées volcaniques s'épanchent dans un bassin sédimentaire á fond sialique au sein de formations détritiques grossières nées de la destruction de la chaîne varisque sur la bordure méridionale du continent europeen.     

Oceanic island Pb: Two-stage histories and mantle evolution

Leads in basaltic suites from seven oceanic islands form linear arrays on²⁰⁶Pb/²⁰⁴Pb versus²⁰⁷Pb/²⁰⁴Pb diagrams. These arrays are more reasonably interpreted as secondary isochrons than as mixing lines, because of their systematic relationship. Separate two-stage histories calculated for the leads from each island indicate that the source materials for the magmas were derived from a single primary reservoir with present²³⁸U/²⁰⁴Pb of 7.91 ± 0.04 by secondary enrichment in U/Pb at different times from 2.5 to 1 Ga ago. This is confirmed by a plot of isochron slope versus intercept, on which the points describing each island's Pb-Pb array all lie very near a single straight line. The isochrons for the Canary Islands and Hawaii, at least, are significantly different. The²⁰⁸Pb/²⁰⁴Pb versus²⁰⁶Pb/²⁰⁴Pb relationships are less coherent. The lead isotopic characteristics are consistent with a model in which lead in the oceanic island magmas is derived from ancient subducted oceanic crust. In particular, this explains the close relationship between lead in mid-ocean ridge and oceanic island basalts without invoking mixing.     

Untersuchungen zur kreislaufmahlung in einer aufbereitung für komplexe sulfidische erze

Circuit grinding was investigated in the Rammelsberg ore dressing plant. From short-interval sampling it was possible to make a statement about the non-stationary and stationary state of grinding. The objective of these investigations was to establish the distribution curves of the quantitative flow, of the grain sizes and of the mineralogical components in the individual streams of the circuit grinding.     

Inner core anisotropy, anomalies in the time-averaged paleomagnetic field, and polarity transition paths

The diffusion of the dynamo-generated magnetic field into the electrically conducting inner core of the Earth may provide an explanation for several problematic aspects of long-term geomagnetic field behavior. We present a simple model which illustrates how an induced magnetization in the inner core which changes on diffusive timescales can provide a biasing field which could produce the observed anomalies in the time-averaged field and polarity reversals. The Earth's inner core exhibits an anisotropy in seismic velocities which can be explained by a preferred orientation of a polycrystalline aggregate of hexagonal close-packed (hcp) iron, an elastically anisotropic phase. Room temperature analogs of hcp iron also exhibit a strong anisotropy of magnetic susceptibility, ranging from 15 to 40% anisotropy. At inner core conditions the magnetic susceptibility of hcp iron is estimated to be between 10⁻⁴ and 10⁻³ SI. We speculate here that the anisotropy in magnetic susceptibility in the inner core could produce the observed anomalies in the time-averaged paleomagnetic field, polarity asymmetry, and recurring transitional virtual geomagnetic pole (VGP) positions.     

The 3100 BP eruption of the Soufrière of Guadeloupe

Fragments of unaltered andesite found at all levels in the deposits of the catastrophic flank-failure, directed-blast eruption of the Soufrière of Guadeloupe in 3100 bp are thought to be remnants of the cryptodome. They were observed in analytical transmission electron microscopy for clues to the evolution of the intrusion prior to the eruption. Several features that could potentially be used as temperature markers were identified, among which the angle between microexsolutions of magnetite in augite phenocrysts was used to find an upper boundary of the temperature of the intrusion before the eruption: 600–700°C. Calculation of the time a dyke or sphere-shaped intrusion may have taken to cool from the emplacement temperature down to the temperature of exsolution of the magnetite leads to an estimate of the time between emplacement of the intrusion and the eruption, which could not have been less than a few tens of years. It therefore seems probable that the emplacement of the magmatic intrusion was not the immediate cause of the flank destabilization and catastrophic eruption of the Soufrière in 3100 bp.     

Eddy fluxes of CO2, water vapor,and sensible heat over a deciduous forest

Fluxes of CO₂, latent heat and sensible heat were measured above a fully-leafed deciduous forest in eastern Tennessee with the eddy correlation technique. These are among the first reported observations over such a surface. The influences of solar radiation, vapor pressure deficit and the aerodynamic and canopy resistances on these mass and energy exchanges are examined. Following a concept introduced by McNaughton and Jarvis (1983), examination of our data suggest that the water vapor exchange of a deciduous forest is not as strongly coupled with net radiation as is that of agricultural crops. The degree of decoupling is smaller than in the case of a coniferous forest. This difference may be attributable in part to the greater aerodynamic resistance to water vapor transfer in a deciduous forest. It appears that the concept of decoupling may be extended to the CO₂ exchange of a deciduous forest as well.Published as Paper No. 7832, Journal Series, Nebraska Agricultural Research Division. ATDD Contribution No. 85-17.