A close look at dihedral angles and melt geometry in olivine-basalt aggregates: a TEM study

Olivine-basalt aggregates sintered at high P/T have been used as a simplest approximation of partially molten upper mantle peridotite. In the past, geometry of partial melt in polycrystalline olivine (and other materials) has been characterised by dihedral (wetting) angles which depend upon surface free energy. However, since olivine (like most other crystalline materials) is distinctively anisotropic, the simple surface energy balance defining the dihedral angles cos(Θ/2)=_gb/2_sl is not valid and melt geometry is more complicated than can be expressed by a single dihedral angle value. We examine in detail melt geometry in aggregates held at high temperature and pressure for very long times (240–612 h). We show the simple dihedral angle concept to be invalid via transmission electron microscope images. Olivine-basalt interfaces are frequently planar crystal faces (F-faces) which are controlled by the crystal structure rather than the surface area minimisation used in the simple dihedral angle concept. Nevertheless, the dihedral angles may provide useful insights in some situations. They may give a rough estimation of the wetting behaviour of a system, or be used to approximate the melt distribution if F-faces are not present (possibly at large grain size and very low melt fraction). Our measurements, excluding F-faces, give a range of dihedral angle values from 0 to 10° which is significantly lower than reported previously (20–50°). The nature of 0° angles (films and layers up to 1 μm in thickness) is unclear but their frequency compared to dry grain boundaries depends on grain size and melt fraction (e.g. 70% for grain size 43 μm and melt fraction 2%). Received: 13 April 1997 / Accepted: 2 October 1997     

Die Matrix der Fische VIII. Röntgenfeinstruktur-Untersuchungen an Fischschuppen

Summary We have found that most of the inorganic components in the scales of different inland fish species consist of crystallized hydroxyl apatit (part of which contains adsorbed [PO₄], in analogy to the mineral constituents of the bones of other vertebrate animals.

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7. Mitteilung siehe diese Zeitschrift, S. 94.     

Die Matrix der Fische XIII. Vitamin B12 in der Leber vonSalmo irideus

Summary The content of the liver of 875 days old rainbow-trouts, fed exclusively with fresh beef-liver, was about 137γ of B₁₂/100 g dried material, whilst liver of one year old rainbow-trouts from fish-ponds contained only about 42γ B₁₂/100 g dry weight.

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12. Mitteilung siehe Verh. Schweiz. naturf. Ges., Davos (1950), im Druck.     

Seismic Properties of Anita Bay Dunite: an Exploratory Study of the Influence of Water

As a pilot study of the role of water in the attenuation ofseismic waves in the Earth's upper mantle, we have performeda series of seismic-frequency torsional forced-oscillation experimentson a natural (Anita Bay) dunite containing accessory hydrousphases, at high temperatures to 1300°C and confining pressure(P_c) of 200 MPa, within a gas-medium high-pressure apparatus.Both oven-dried and pre-fired specimens wrapped in Ni–Fefoil within the (poorly) vented assembly were recovered essentiallydry after 50–100 h of annealing at 1300°C followedby slow staged cooling. The results for those specimens indicatebroadly similar absorption-band viscoelastic behaviour, butwith systematic differences in the frequency dependence of strain-energydissipation Q^–1, attributed to differences in the smallvolume fraction of silicate melt and its spatial distribution.In contrast, it has been demonstrated that a new assembly involvinga welded Pt capsule retains aqueous fluid during prolonged exposureto high temperatures—allowing the first high-temperaturetorsional forced-oscillation measurements under high aqueousfluid pore pressure P_f. At temperatures >1000°C, a markedreduction in shear modulus, without concomitant increase inQ^–1, is attributed to the widespread wetting of grainboundaries resulting from grain-scale hydrofracturing and themaintenance of conditions of low differential pressure P_d =P_c – P_f . Staged cooling from 1000°C is accompaniedby decreasing P_f and progressive restoration of significantlypositive differential pressure resulting in a microstructuralregime in which the fluid on grain boundaries is increasinglyrestricted to arrays of pores. The more pronounced viscoelasticbehaviour observed within this regime for the Pt-encapsulatedspecimen compared with the essentially dry specimens may reflectboth water-enhanced solid-state relaxation and the direct influenceof the fluid phase. The scenario of overpressurized fluids andhydrofracturing in the Pt-encapsulated dunite specimen may havesome relevance to the high Q^–1 and low-velocity zonesobserved in subduction-zone environments. The outcomes of thisexploratory study indicate that the presence of water can havea significant effect on the seismic wave attenuation in theupper mantle and provide the foundation for more detailed studieson the role of water. KEY WORDS: seismic wave attenuation; water; dunite; hydrous mineral; shear modulus; viscoelasticity; olivine; grain-scale hydrofracturing     

Coastal sediments from the Algarve: low-latitude climate archive for the Aptian-Albian

The Late Aptian to Early Albian transition has previously been identified as a possible example of substantial climate cooling within the mid-Cretaceous greenhouse period. To study the response of continental weathering and terrestrial vegetation to this cooling episode at low- to mid-latitudes, marine nearshore deposits from the Algarve Basin (SW Portugal) have been investigated with a combined approach including palynology, clay mineralogy and bulk-rock geochemistry. In the Lower Aptian part of the succession, quartz-rich sandstone facies is accompanied by high abundances of early diagenetic kaolinite, which is interpreted to reflect episodes of enhanced humidity and high meteoric flow-through. In contrast, the Late Aptian to Early Albian deposits are characterized by high abundances of detrital clay minerals (mica and chlorite) indicating the dominance of physical weathering processes in the source area, most probably related to low precipitation rates in conjunction with tectonically enhanced erosion. Palynological data show a strong dominance of Classopollis pollen associated with low pteridophyte spore abundances, suggesting warm semi-arid to arid palaeoenvironments. Changes in sedimentation patterns from varicoloured lagoonal marls to thick-bedded shallow-water carbonates are neither expressed in the spore-pollen assemblages nor in the distributions of clay minerals which both remain essentially stable throughout the Late Aptian to Early Albian. These relatively stable patterns are in contrast with various lines of evidence, predominantly from high-latitude areas, that suggest a significant cooling during this time interval. Our study demonstrates that terrestrial environments of low- to mid-latitude regions were not significantly affected by the Late Aptian - Early Albian “cold snap”.     

HI observations ofHII-galaxies

High-resolution observations of the star forming dwarf galaxies Haro 21 and Markarian 314 are presented and are compared with a set of narrow band (H) CCD images. TheHi system related to Haro 21 measures about 12 kpc in diameter and is much more extended than the optical emission. The optical light follows that of an exponential disk and has a scale length of only 600 pc. TheHi distribution shows a lack of neutral gas near its centre where most of the H emission is found. The velocity field is regular and shows the characteristics of a disk in differential rotation. The rotation curve is rising out to the last measured point and the total mass is estimated to be about 1.5×10¹⁰ M . This implies that Haro 21 is a galaxy which is dominated by Dark Matter.Partly based on observations collected at the European Southern Observatory, La Silla, Chile.