Origin of main-group pallasites

pallasite

X-ray map of the Brahin pallasite. Dark angular grains: olivine; yellow: troilite; pink; schreibersite; bluish purple: kamacite; pinksih purple: taenite.

Pallasites are meteorites composed mainly of large olivine crystals suspended in a metallic Fe-Ni matrix. Traditionally, they are thought to represent samples from the core-mantle boundaries of differentiated asteroids. Paleomagnetic evidence, however, suggests a different origin: an impact between two differentiated asteroids that mixed metal and olivine at shallow depths in the target body. An important factor in discriminating between these scenarios is whether metal and olivine are in equilibrium. If cooled at the core-mantle boundary, metal and olivine must have equilibrated because the temperatures are high and cooling rates are slow (on the order of tens of degrees per million year). Conversely, if metal and olivine mixtures cooled at shallow depths, following an impact event, the much faster cooling rates expected may prohibit metal-olivine equilibrium.

I measured the iron isotope compositions of the bulk metal and olivine in main-group pallasites (MGP are thought to have come from the sample parent body). To better interpret these data, I collaborated with Dr. Neil Bennett, who ran a series of piston cylinder experiments, and with Prof. Edwin Schauble, who performed ab initio calculations, to determine the equilibrium olivine-metal iron isotope fractionation at high temperature. The results of these experimental and theoretical studies are in excellent agreement and, in concert with my measurements on natural samples, prove that metal and olivine in main-group pallasites have never been in equilibrium. This provides robust geochemical support for an impact-origin of main-group pallasites, in agreement with the paleomagnetic data.

References:

  1. Sio, C.K., Bennett, N.R., Schauble, E., Edwards, P., Lesher, C.E., Wimpenny, J., Shahar, A. (In Prep). An impact origin for main-group pallasites revealed by iron isotopes.
  2. Tarduno, J. A., Cottrell, R. D., Nimmo, F., Hopkins, J., Voronov, J., Erickson, A., Blackman, E. Scott, E. R. D. & McKinley, R. (2012). Evidence for a dynamo in the main group pallasite parent body. Science, 338(6109), 939-942.