Hi All, Tracy asked:
> We already assume that pallasites are from the core-mantle boundaries of > asteroids large enough to have undergone differentiation at one point in > their lives (before being turned into specimens in our collections!) > What other theories for the formation of mesosiderites, besides the > mashing together of a pallasite body and another rocky body, allow > for the incorporation of the occasional rogue olivine crystal? Alan Rubin devotes a chapter to mesosiderites in his very readable _Disturbing the Solar System_. In particular, he outlines a biography of the mesosiderite parent body, the central mystery of which is of course, "What happened to the olivine?" For a typical differentiated asteroid with a 200-km diameter, the iron-nickel core would be about 100-km in diameter, and the basaltic crust would be about 5 km thick. Thus the basalt and gabbro crustal portion of the asteroid is separated from the metallic core by a ~45-km thick shell of olivine-rich mantle. Yet, mesosiderites contain about a 50-50 split of silicates and Fe-Ni, with only about 1-2% olivine. A bit difficult to mix the metal and the silicates w/o also getting a lot more olivine. Perhaps the most obvious solution is to have the iron and silicates in mesosiderites come from different origins: start with a differentiated asteroid as before, but then impact it with a relatively low-velocity nickel-iron body. (A high-velocity impact won't work because the metal then becomes vaporized and too dispersed.) Low-velocity impacts (< 2 km/sec) between asteroids are rare today, but they would have been much more common in the early solar system. Alan has outlined a 5-stage process that ultimately results in mesosiderites here on earth. I won't spoil the fun ... buy the book and learn all the details. Less than $20 in paperback from Amazon, and about the same for a good quality used hardcover there. --Rob ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://six.pairlist.net/mailman/listinfo/meteorite-list
