Asteroid Differentiation: Pyroclastic Volcanism to Magma Oceans

Abstract— Asteroid differentiation was driven by a complex array of magmatic processes. This paper summarizes theoretical and somewhat speculative research on the physics of these processes. Partial melts in asteroids migrate rapidly, taking < 10⁶ years to reach surface regions. On relatively small (<100 km) asteroids with sufficient volatiles in partial melts (<3000 ppm), explosive volcanism accelerated melts to greater than escape velocity, explaining the apparent lack of basaltic components on the parent asteroids of some differentiated meteorites. Partial melting products include the melts (some eucrites, angrites), residues (lodranites, ureilites), and unfractionated residues (acapulcoites). The high liquidus temperatures of magmatic iron meteorites, the existence of pallasites with only olivine, and the fact that enstatite achondrites formed from ultramafic magmas argue for the existence of magma oceans on some asteroids. Asteroidal magma oceans would have been turbulently convective. This would have prevented crystals nucleated at the upper cooling surface (the only place for crystal nucleation in a low-pressure body) from settling until the magma became choked with crystals. After turbulent convection slowed, crystals and magma would have segregated, leaving a body stratified from center to surface as follows: a metallic core, a small pallasite zone, a dunite region, a feldspathic pyroxenite, and basaltic intrusions and lava flows (if the basaltic components had not been lost by explosive volcanism). The pallasite and dunite zones probably formed from coarse (0.5–1 cm) residual olivine left after formation of the magma ocean at >50% partial melting of the silicate assemblage. Iron cores crystallized dendritically from the outside to the inside. The rapid melt migration rate of silicate melts suggests that ²⁶Al could not be responsible for forming asteroidal magma oceans because it would leave the interior before a sufficient amount of melting occurred. Other heat sources are more likely candidates. Our analysis suggests that if Earth-forming planetesimals had differentiated they were either small (<100 km) and poor in volatiles (<1000 ppm) or they were rich in volatiles and large enough (>300 km) to retain the products of pyroclastic eruptions; if these conditions were not met, Earth would not have a basaltic component.     

Circumstellar dust shells around very young and massive stars

In this paper we investigate the properties of dust in circumstellar shells around very young massive compact IR sources (Becklin-Neugebauer objects).We found no correlation between the optical depth in the centre of the 10-m band and the 3.1-m ice band. An inverse correlation between the strength of the silicate feature and the colour temperature for the 8–13 m interval was detected. Our sample of BN objects extends this kind of relation already known for Mira stars and OH/IR stars to higher optical depths.We present a radiative transfer model for BN objects and discuss its main properties. Using this model, the interpretation of the observations led to the conclusion that the type of silicates present in the dust shells of very young stellar objects is different from that type around oxygen-rich giants and supergiants. These different silicates may be tentatively identified with pyroxenes and olivines, respectively.We studied the influence of the adopted dust model in deriving source parameters of BN objects. The object W3-IRS5 was discussed in some detail.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

Bücher- und Zeitschriftenschau

Ohne Zusammenfassung     

Alaska in den Jahren 1909 und 1910

Ohne Zusammenfassung     

Observationally-based infrared efficiencies and planck means for circumstellar dust grains

In this paper the mean relative efficiency factors for absorption of dust grains in the wavelength range from 8 to 22 m have been derived from IR spectra of the optically thin dust shells surrounding the O-rich supergiants Cep, Ori, X Her, and R Cas. The resulting absorption efficiency curve has been extrapolated into the near as well as the far infrared regions, having regard to restrictions set by experience. Planck mean efficiency factors have been calculated for the temperature range from 30 to 1500 K. The curveQ _P(T), which shows a maximum atT=300 K, has been approximated conveniently by a power law on either side of this maximum.     

Oberrheinisches Grundgebirge und Rheingraben

Zusammenfassung Anstelle des Rheingrabens befand sich während der variszischen Faltung eine von Blattverschiebungen durchsetzte Scherzone. Sie diente granitischen und gabbroiden Intrusiva sowie den Vulkaniten des Rotliegenden als bevorzugte Aufstiegsbahn. Vorzerklüftet und isostatisch unkompensiert behielt der Krustenspan bis zum Mitteldogger seinen kratonischen Zustand, gehalten von der Klammer variszischer Konsolidation. Vom Mitteleozän an hatten neuerliche Blattverschübe das basale Kluftgitter für die vertikale Grabentektonik vollends gangbar gemacht. Keilförmig trieb sich der Lineamentkörper in die Kruste vor, junge Füllmassen einsaugend und die Grabenschultern isostatisch aufstemmend. Die randlichen Widerlager wurden auseinandergetrieben, wodurch Raum für die sich steigernde Taphrogenese und den im Aquitan folgenden antithetischen Zerfall der Grabenscholle geschaffen wurde. Der dem aufwallenden basaltischen Substrat entspringende Grabenvulkanismus bildete den Höhepunkt krustaler Zerspaltung.

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The Rhine-Graben is preceded by Hercynian transcurrent faulting favouring intrusion and extrusion of granitic and gabbroide plutonites and lower Permian extrusives. Though fractured and isostatically out of balance, the Rhine-block remained cratonic until Middle-Dogger times on account of its "Hercynian" nature. From middle-Eocene times on, renewed transcurrent movements facilitated dip-slip movements along the graben-edge-faults. The Rhine block, sucked down the young fill and isostatically buckled the graben edges. Marginal buttresses were driven apart; they gave room for increasing thaphrogenesis and for anti-thetic collapse during Aquitanian times. Basaltic graben volcanism underlined the peak of tectonic activity.

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Résumé Lors du plissement varisque, il y eut à l'endroit du fossé rhénan, une zone d'écailles; celle-ci favorisa la montée de roches intrusives granitiques et gabbroïques ainsi que de volcanites du Rotliegend. Préalablement fracturée et dans un état isostatique non compensé, cette zone d'écaillés conserva son état cratonique jusqu'au Dogger moyen, enserrée entre des masses de consolidation varisque. A partir de l'Eocène moyen, de nouveaux déplacements des écailles préparèrent le réseau de fissures pour une tectonique verticale en fossé. Un segment linéamentaire s'enfonça en coin dans l'écorce, déterminant l'aspiration de nouvelles masses de remplissage et la montée isostatique des compartiments flanquant latéralement le fossé. Les contreforts ont été écartés les uns des autres, ce qui a fourni l'espace pour la taphrogenèse et pour la fragmentation antithétique du massif affaisé qui s'est produite à l'Aquitanien.La fissuration crustale a atteint son point culminant avec le volcanisme à caractère basique qui a fait éruption dans le fossé.

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