Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p21e..02w&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P21E-02
Physics
[6045] Planetary Sciences: Comets And Small Bodies / Physics And Chemistry Of Materials, [6063] Planetary Sciences: Comets And Small Bodies / Volcanism, [6205] Planetary Sciences: Solar System Objects / Asteroids, [6024] Planetary Sciences: Comets And Small Bodies / Interiors
Scientific paper
A variety of meteorite groups represent samples of asteroids that formed while 26Al was still the dominant heat source in Solar System materials. These bodies differentiated to varying degrees beyond the temperature of FeNi-FeS melting, with sufficient silicate melting to allow metal core formation. The silicate melts segregated upward from the interiors to suffer various fates: intrusion at shallow levels, eruption onto the surface, or ejection into space in explosive eruptions in which the eruption speed exceeded the escape speed. These three styles of plutonic/volcanic activity were not mutually exclusive; their relative importance was a function of asteroid size and composition, with the major compositional factor being the total available volatile inventory. Much research has been concerned with whether silicate melts were extracted from the mantle during the period of mantle heating or while the mantle was cooling after reaching its peak temperature and degree of partial melting (a "magma ocean" stage). Traditionally, the relevant arguments have been based on the petrology and geochemistry of the meteorites sampling these bodies. Instead, we focus on the fluid dynamic aspects of eruption and intrusion processes and show how these impose additional limitations on various aspects of the igneous activity. For example, 40% melting of bodies the size of 4 Vesta (~250 km radius) and the Ureilite Parent Body (UPB, ~100 km radius) over the course of a 0.5 Ma heating period represent melt volume production rates of ~350 and 20 cubic meters per second, respectively, in each of what we demonstrate should have been ~4 volcanic provinces on each body. All differentiated asteroids must of necessity have had a surface layer ~10 km thick at sub-solidus temperatures controlled by conductive cooling. To erupt magma at the surface (or intrude magma at very shallow depth) through such a crust would have required the propagation of dikes within which the combination of dike width and magma flow rate was great enough to avoid excessive cooling. The critical minimal volume fluxes are 2200 and 2500 cubic meters per second, respectively, for Vesta and the UPB. Thus melt accumulation in the upper mantle or lower crust between volcanic episodes must have occupied at least ~85% of Vesta's active period and ~99% of the UPB's active life. The petrological and geochemical consequences of this extreme episodicity in melt movement, especially in small differentiated asteroids, need to be explored for all varieties of melt extraction models. Additional issues concern eruption styles at asteroid surfaces. The absence of atmospheres on small bodies means that, unless magmas approaching the surface are entirely devoid of volatiles, there is likely to be some kind of explosive aspect to the activity. Thus, what appear to be effusive lava flows may instead be the results of welding of pyroclasts falling from hot, optically-dense, fire fountains. The range of possibilities for forming both continuous and fragmental deposits on asteroid surfaces will be reviewed.
Keil Klaus
Wilson Leslie
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