April 2, 2013: No magma ocean on Vesta (or elsewhere in the Asteroid Belt); volatile loss

There is moderately strong evidence that the Earth experienced one or more magma ocean episodes during which large-scale magmas were at or close to the surface. Because the rates of heat loss is very high, such episodes could only be powered by impacts and the periods during which magma was exposed were very brief. It has been popular to discuss magma oceans in the Asteroid Belt; in particular, several studies have inferred that the HED (howardite, eucrite, diogenite) meteorites formed in a magma ocean on Vesta. In fact, it appears to be impossible to create true magma oceans in the Asteroid Belt because impact heating liberates less heat and 26Al heats at much too low a rate. What the authors really are discussing is global magmas located beneath an insulating megaregolithic shell. All igneous models of asteroids use 26Al as the heat source even though the 26Al/27Al ratios in the chondrites in our museums are too low to produce appreciable melting. The general justification is that impact heating doesn’t work so it must have been 26Al heating. In effect, 26Al is treated as a free parameter. Until recently this was justified because Hf/W model ages for iron meteorites implied that the irons are older than chondrites. And modelers are now producing large melt fractions in porous asteroids at relatively low impact velocities (~5 km s-1). The most cited paper justifying a magma ocean on the HED parent asteroid is by Greenwood et al. (2005) who showed that most HEDs formed a very tight cluster. The cluster does indeed imply a single, well mixed magma but the data only require a relatively small magma. In fact, the D17O and e54Cr data favor a model in which the HEDs form on the same asteroid as the IIIAB magmatic irons (and thus not on Vesta). Richter and Drake (1997) suggested that a magma ocean was the best way to explain the low alkali contents in eucrites. However, internal heating by 26Al is not expected to result in much volatile loss because there is no carrier phase at volatilization temperatures . Impact heat is probably much more efficient at evaporating alkalis. The isotopic links between the HEDs and IIIAB irons seem stronger than the spectroscopic links between HEDs and the surface of Vesta. These groups probably formed in one or more regional magmas produced by minor impacts on a porous asteroid.

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