Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.600w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 600
Mathematics
Logic
Asteroids, Vesta, Eruption, Basaltic, Intrusive Rocks, Lava, Silicate Melts
Scientific paper
Using current models of volcanic intrusion and eruption mechanisms, and compositional evidence from the howardite-eucrite-diogenite meteorites on densities and rheological properties of magmas produced on the asteroid 4 Vesta, we predict likely dimensions of dikes, sills and surface lava flows, as follows. Dikes carrying magma from partial melt zones in Vesta's mantle had vertical extents of 9 to 20 km, widths from 0.7 to 3 m and internal excess pressures between 1.2 and 2.4 MPa. These values are controlled by the apparent fracture toughness of country rocks, reflecting stress distributions and the presence or absence of gas-filled cavities, at dike tips. Magma cooling would have prevented dikes from erupting unless apparent fracture toughnesses of country rocks were greater than ~55 MPa ml/2. Eruptions typically lasted for 6 to 30 hours and had effusion rates ranging from ~0.05 to ~3 m3 s-1 per meter of horizontal surface fissure outcrop (basaltic eruptions on Earth have a similar range). Lava volumes ranged from ~zero (where magma stalled due to excessive cooling at shallow depths to form dike-like intrusions with widths of ~0.7 m and volumes of ~50 million m3) up to ~1 km3 (in which case cooling of the shrinking dike at the end of the eruption trapped ~7 million m3 of magma as a shallow dike ~0.6 m wide). Lava flow dimensions depended on effusion rates and mean surface slopes. Widths ranged from a few hundred meters to a few km, with lengths from a few km to several tens of km. Flow thicknesses were relatively insensitive to the controlling variables and ranged from ~5 to 20 meters. Intervals between successive eruptions in the same area were ~1000 years if Vesta accumulated a 20 km thick crust in ~1 Ma: since eucrite and howardite meteorites derived from these ~10 m thick flows were intensely brecciated, craters with diameters of several meters must have been forming at a rate of several tens per km2 per year during the period of active volcanism. Evolution of Vesta's crustal density structure (deeper layers compacting under the growing load) should have led to neutral magma buoyancy zones forming at depths from 30 to 80 km. Intrusions of magma trapped in these zones as dikes or sills had lengths from ~9 to 20 km, thicknesses from 0.5 to 3 m, and volumes from 50 to 900 million m3. There would have been at least 100 year intervals between successive intrusions in the same area, much longer than the time needed to cool a single intrusion (~1 year for a shallow 3 m thick dike or sill) and so large shallow magma reservoirs should not commonly have formed. The sizes of thermally metamorphosed zones adjacent to intrusions (a few meters) would have been too small to explain the prevalent thermal effects seen in the HED meteorites, suggesting that heat diffusing from the interior of Vesta following progressive brecciation and burial of surface eruptives is the metamorphic agent. Cooling rates implied by plagioclase crystal sizes in eucrites suggest that they cooled as parts of rock bodies 5 to 10 m thick, consistent with eucrites having been parts of surface lava flows predicted to be 5 to 20 m thick. Some eucrites may have formed parts of shallow dikes or sills, though such intrusions should generally have been less than 3 m thick.
Keil Klaus
Wilson Leslie
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